NUTRITIONAL NEEDS AND DIETARY INTAKE
Energy and nutrient requirements are high during adolescence to support the rapid growth and development just described. The dietary reference intakes (DRIs) are a set of four reference values useful for planning and evaluating the nutrient intakes of healthy people. While most DRIs are not based on actual experimental studies in adolescents, the reference values provide the best estimates for nutritional requirements of adolescent females. Reference values for adolescents were extrapolated based on data from experimental studies in adults or derived from population intake levels. Recommended dietary allowances (RDAs), adequate intakes (AIs), and tolerable upper intake levels (ULs) are useful for evaluating the diets of individuals and estimated average requirements (EARs) are best used to evaluate the diets of populations. RDAs represent daily intake levels sufficient to meet the needs of nearly all persons (97%–98%) in a particular life-stage and gender group. AIs are used in place of RDAs and EARs when there is not a sufficient amount of scientific evidence to support their establishment.ULs represent the highest level of usual intake that is likely to pose no risk of adverse health effects for nearly all (97%–98%) individuals in a life stage and gender group. However, as intakes of a nutrient increase above the UL, the potential increases for adverse health effects to occur.
In addition to these four reference values for nutrient intakes, the Institute of Medicine has developed estimated energy requirement (EER) levels for planning and evaluating calorie intakes based on age, sex, and physical activity level. Finally, acceptable macronutrient distribution ranges (AMDRs) were established to provide guidance on the proportion of total calories that should be consumed as protein, carbohydrate, and fat to prevent chronic disease and ensure sufficient nutrient intake. These reference values are categorized according to chronological age rather than stage of maturational development; therefore, practical application of the values in relation to adolescents should be informed by an understanding of how nutritional needs parallel rates of growth. Because of the rapid physiologic growth that occurs during adolescence, total energy and nutrient requirements are greater than during any other period in the life cycle. The greatest nutrient demands occur during the period of peak growth velocity. As reviewed in the first section of this chapter, the period of peak growth and peak nutrient requirements tend to occur a few years earlier among females than among males. Also, as adolescent males experience greater increases in height, weight, and lean body mass than do females, the magnitude increase in nutritional requirements is smaller for females than for males. While the micronutrient needs of females and males of the same age are very similar prior to the arrival of puberty, notable gender differences emerge in these requirements during adolescence. Sex differences in requirements are driven by sexspecific changes in body composition and the onset of menstruation in females during puberty. For example, menstruation creates an additional demand for iron.
Percentage of females and males aged 9–13 years whose usual dietary intake is less than the estimated average requirement (EAR). EARs correspond to daily intake levels sufficient to meet the needs of 50% of persons in a particular life-stage and gender group. The percentage of a population having intakes of a given nutrient below the EAR can be taken to represent the prevalence of inadequacy for that nutrient. and females have greater requirements than males after menarche. In contrast, females generally have lower requirements than males for the nutrients magnesium, zinc, niacin, riboflavin, thiamine, vitamin A, and vitamin B6. Relative to these requirements, national nutrition data indicate that many adolescents have marginal or inadequate intakes for several nutrients. These data further suggest that females more often have poor nutritional intake than do males, particularly for phosphorus (9–18 years), magnesium (9–18 years), zinc (9–18 years), iron (14–18 years), vitamin C (14–18 years), vitamin B6 (14–18 years), vitamin A (9–13 years), and folate (14–18 years). The 2005 Dietary Guidelines for Americans (Dietary Guidelines) and the US Department of Agriculture (USDA) food guidance system, MyPyramid, provide mainly food-based recommendations complimentary to the reference values and ranges for nutrient intake (i.e., DRIs, AMDRs) developed by the Institute of Medicine. The Dietary Guidelines are a set of science-based recommendations established on the principle that nutritional needs should be met primarily from foods and beverages. Recommendations of greatest relevance for adolescent females are outlined in Table 3.1. MyPyramid also provides guidance in the form of meal patterns to help individuals meet their total nutrient needs without consuming calories or other dietary components (e.g., trans fat) in excess. This guidance is based on the energy and nutrients provided by foods from five groups, including (1) grains, (2) vegetables, (3) fruits, (4) milk, and (5) meats and beans. National nutrition data indicate many adolescents consume less than the recommended number of servings from these food groups. Although oils, solid fats, and added sugars are not considered to be part of any food group, MyPyramid additionally provides guidance for these dietary components.
ENERGY AND DISCRETIONARY CALORIES
EERs for adolescents represent the amount of energy they require to support activity and normal pubertal growth and development. Individuals’ energy requirement is influenced by their basal metabolic rate, which is closely associated with lean body mass. Separate EER equations have been developed for males and females because males experience greater increases in lean body mass, height, and weight during adolescence. The equation developed for estimating energy requirements of adolescent females (Equation 3.1) includes an allowance for growth of 25 Cal (kcal) per day. Average energy requirements of adolescent females that were calculated using an average height for age and healthy weight for height are provided in Table 3.2.
Selected Recommendations from the 2005 Dietary Guidelines for Americans
Consume adequate nutrients within calorie needs
Meet recommended intakes within energy needs by adopting a balanced eating pattern
Females of childbearing age. Eat foods high in heme iron and consume iron-rich plant foods or iron fortified foods with an enhancer of iron absorption, such as vitamin C-rich foods. Consume adequate synthetic folic acid daily (from fortified foods or supplements) in addition to food forms of folate from a varied diet
Achieve and maintain a healthy weight
Maintain body weight in a healthy range by balancing calories from foods and beverages with calories expended
Those who need to lose weight.
Overweight persons with chronic diseases or on medication. Consult a healthcare provider about weight-loss strategies prior to starting a weight-reduction program to ensure appropriate management of other health conditions
Consume adequate amounts from food groups that contribute to health
Consume a sufficient amount of fruits and vegetables while staying within energy needs
several times a week
Consume whole-grain products often; at least half the grains should be whole grains, with the rest of the recommended grains coming from enriched or whole-grain products
Consume fat in moderation and make wise choices
When selecting and preparing meat, poultry, dry beans, and milk or milk products, make choices that are lean, low-fat, or fat-free
Limit intake of fats and oils high in saturated or trans fatty acids, and choose products low in such fats and oils
Choose carbohydrates wisely
Prevent dental caries by practicing good oral hygiene and consuming sugar- and starch-containing foods and beverages less frequently
Consume sodium in moderation and choose potassium-rich foods
Source: From U.S. Department of Health and Human Services, U.S. Department of Agriculture. Dietary GuidelinesforAmericans2005.http:==www.healthierus.gov=dietaryguidelines(accessedNov2006).
Percentage of female and male adolescents aged 12–19 years with food group intakes less than recommended based on energy intake and age. (Data from Cook, A. and Friday, J., Pyramid Servings Intakes in the United States 1999–2002, 1 Day, Agricultural
Research Service, U.S. Department of Agriculture, Beltsville, MD, 2005.)
EER for female adolescent 9–18 years is
EER ¼ 135:3 30:8 Age (years) þ PA [10:0 Weight (kg) þ 934
Height (m)] þ 25 (kcal=day for energy deposition) (3:1)
Physical activity coefficient (PA):
⦁ Sedentary¼1.00, less than 30 min a day of moderate physical activity in addition to daily activities.
⦁ Low active ¼1.16, at least 30 min and up to 60 min of daily moderate physical activity in addition to daily activities.
⦁ Active¼1.31, at least 60 min of daily moderate physical activity in addition to daily activities.
Accurately assessing the energy needs of adolescents is important as energy intake must be balanced with energy expenditure to support growth and avoid excessive weight gain or weight loss. If adolescents chronically consume an inadequate level of energy intake, linear growth may be compromised and sexual maturation delayed. Alternatively, if energy intake chronically exceeds requirements, then excess weight gain will occur. National data indicate 16% of adolescent females are overweight and are therefore likely consuming excess calories. The top sources of energy in the United States are high-calorie, low-nutrient foods and beverages such as sweets and carbonated soft drinks, indicating efforts to prevent overweight should focus on helping adolescents meet their nutritional needs with more nutrient-dense choices (e.g., lean meats, fat-free milk). The MyPyramid food guidance system recommends that individuals choose mostly nutrient-dense options when selecting foods and beverages within each food group. Recommended meal patterns are based on the nutrients and energy that would be provided by consuming foods and beverages in this form. Meal patterns appropriate for adolescent females with different energy needs are shown in Table 3.3. A specific number of discretionary calories are also recommended, which are equal to the difference between an individual’s EER and the calories provided by the recommended servings of grains, vegetables, fruits, milk, meats and beans, and oils on their meal pattern. Discretionary calories might be consumed in one of several ways, including (1) eating more nutrient-dense foods from within any food group; (2) selecting some foods that are in a form containing additional fat or sugar (e.g., 2% milk, sweetened cereal); (3) adding solid fats (e.g., butter) or sweeteners (e.g., syrup, sugar) to foods and beverages; or (4) eating or drinking items that provide only fat or sugar (e.g., candy, soft drinks).
Protein requirements are increased during adolescence to support the accrual of lean body mass as well as the maintenance of existing lean body mass. Peak periods of linear growth and weight gain require the greatest protein intake. Therefore, protein requirements of females per unit of height tend to be greatest between the ages of 11–14 years and for males tend to be greatest between the ages of 15–18 years. The protein RDA for preadolescent and adolescent ages 9–13 years is 0.95 g=kg (EAR¼0.76 g=kg) and for adolescent ages 14–18 years is 0.85 g=kg (EAR¼0.71 g=kg). The AMDR for protein recommends that calories from protein should represent 10%–30% of total energy intake. While most adolescents in the United States consume approximately twice the recommended level of protein, 14% of females aged 14–18 years have intakes less than the EAR. It may be more difficult for adolescents living in food-insecure households, following a vegetarian or vegan diet, or severely restricting their energy intake to achieve AIs of protein. The RDA for carbohydrate throughout adolescence is 130 g (EAR¼100 g) and the AMDR recommends that 45%–65% of total calories be consumed as carbohydrate. An additional recommendation for carbohydrate is to limit added sugars to no more than 25% of total energy intake. National nutrition data indicate that adolescent females consume 55% of their energy intake as carbohydrate and the majority of females (>97%) consume more than adequate amounts of carbohydrate. However, a large proportion (20%) of carbohydrate intake comes from sweeteners and added sugars with little nutritional value. Conversely, intakes of carbohydrate-rich foods that provide fibre and other important nutrients (e.g., fruit, vegetables, whole grains, and legumes) are less than recommended. Less than 3% of adolescent females have fibre intakes that exceed the adequate intake level of 26 g. In contrast to the MyPyramid meal patterns for adolescent females (Table 3.3), which include three to four servings of fruit and four to six servings of vegetables per day, approximately one-third of adolescent females consume less than one serving of fruit daily and 57% consume less than one serving of vegetables.
Fat and Cholesterol
Given a lack of evidence to suggest that a defined intake level of fat is needed to prevent obesity or chronic diseases, no RDA or EAR has been set for total fat intake. However, the AMDR for young people aged 4–18 years recommends that 25%–35% of total calories be consumed as fat and AIs have been established for the essential fatty acids, linoleic acid (9–13 years¼10 g=day; 14–18 years¼11 g=day) and a-linolenic acid (9–13 years¼1.0 g=day; 14–18 years¼1.1 g=day) that are consistent with median intakes in populations free of deficiency symptoms. Additional macronutrient recommendations further provide guidance for cholesterol, saturated fat, and trans fat intake. To reduce chronic disease risk it is recommended that these dietary components be kept to a minimum while maintaining a nutritionally adequate diet. The Dietary Guidelines specifically recommend limiting saturated fat to less than 10% of total calories and cholesterol to less than 300 mg=day. Given a lack of evidence to suggest that a defined intake level of fat is needed to prevent obesity or chronic diseases, no RDA or EAR has been set for total fat intake. However, the AMDR for young people aged 4–18 years recommends that 25%–35% of total calories be consumed as fat and AIs have been established for the essential fatty acids, linoleic acid (9–13 years¼10 g=day; 14–18 years¼11 g=day) and a-linolenic acid (9–13 years¼1.0 g=day; 14–18 years¼1.1 g=day) that are consistent with median intakes in populations free of deficiency symptoms. Additional macronutrient recommendations further provide guidance for cholesterol, saturated fat, and trans fat intake. To reduce chronic disease risk it is recommended that these dietary components be kept to a minimum while maintaining a nutritionally adequate diet. The Dietary Guidelines specifically recommend limiting saturated fat to less than 10% of total calories and cholesterol to less than 300 mg=day.
In addition to the macronutrients, several micronutrients are important for adolescent growth and development. Reference intakes for micronutrients increase steadily through childhood and into adolescence and during periods of rapid growth there is a particular demand for iron, zinc, calcium, and folate. Relative to current recommendations, national nutrition data indicate several problem nutrients, especially among those aged 14–18 years. Data indicate poor intakes of folate, vitamins A, B6, C, and E, calcium, iron, zinc, magnesium, and phosphorus among many adolescent females. Male adolescents have similarly poor intakes of vitamins A and E, but in regard to most nutrients, females are more likely to have poor intakes. Requirements for nutrients of particular importance during periods of rapid growth and population intakes of these nutrients are discussed in greater detail below.
The calcium AI level established for adolescents aged 9–18 years is 1300 mg. Adequate calcium intake is especially important during adolescence to support the rapid expansion of bone mass. Starting around 12.5 years, adolescent females accumulate 40%–45% of peak bone mass over a period of 3–4 years. Although the period of peak accrual for adolescent males starts nearly 2 years later (around 14 years), males gain a similar proportion of their peak bone mass during adolescence. Observational research suggests that achieving adequate calcium intake during this period of peak accrual may reduce the risk of fractures in adolescents. If adequate calcium intake is further maintained throughout adolescence and bone mass accretion is optimized, then osteoporosis in adulthood will also be reduced. The skeleton stores the majority (99%) of total body calcium; therefore, the increased calcium required for expansion of bone mass cannot be supplied by body stores and must be met entirely by dietary intake. In the absence of an established EAR it is more difficult to determine the prevalence of inadequate calcium intake; however, national nutrition data indicate less than 10% of adolescent females have intakes greater than the AI value. A greater proportion of adolescent males than females achieve intakes greater than the AI value, but the proportion of males meeting the recommendation is still less than 50%. Good dietary sources of calcium include milk, other dairy products, dark green vegetables, and fortified foods such as juice, breakfast cereals, and bread. While calcium is best absorbed from dietary sources, especially dairy products, adolescents who are unable to achieve the recommendation may benefit from taking supplements. Calcium supplements (e.g., calcium carbonate, calcium citrate) are readily available and absorption is improved when taken with food in doses no greater than 500 mg.
Iron and Zinc
Consuming adequate amounts of iron and zinc is important during adolescence to support the expansion of blood volume as well as gains in lean muscle and bone tissue. Females further require additional iron after menarche to cover menstrual losses. Therefore, the established RDA for iron is higher for adolescent females aged 14–18 years (RDA¼15 mg; EAR¼7.9 mg) than for males aged 14–18 years (RDA¼11 mg; EAR¼7.7 mg) or females aged 9–13 years (RDA ¼8 mg; EAR¼5.7 mg). The recommended amount for a female should be adjusted up (2.5 mg) if she starts menstruating before the age of 14 years and likewise adjusted down if she does not start menstruating by 14 years. The RDA for zinc is 8 mg (EAR¼7 mg) for females aged 9–13 years and 9 mg (EAR ¼7.3 mg) for females 14–18 years. Consequences of poor iron intake include anaemia, poor exercise tolerance, and impaired cognitive abilities while poor zinc intake may delay growth and sexual maturation. Although iron needs may be partially met by mobilizing any body stores, national nutrition data indicate poor iron intake may be a problem for 16% of adolescent females aged 14–18 years. Poor zinc intake may be a problem for an even greater proportion of adolescent females aged 14–18 years (26%) as well as females aged 9–13 years (10%) but is uncommon among adolescent males aged 9–18 years (4%). Meat, fish, poultry, legumes, nuts, seeds, green leafy vegetables, whole grains, and fortified cereals are all good sources of iron. However, the nonheme iron found in plant sources has a much lower bioavailability than the heme iron found predominantly in meat, fish, and poultry. To enhance the absorption of nonheme iron from plant foods, they should be consumed with sources of heme iron or vitamin C. Good sources of zinc include red meat, shellfish, whole grains, and fortified breakfast cereals.
Another micronutrient of particular significance during periods of rapid growth is folate. Folate is required for reactions that synthesize nucleotides and proteins and plays an important role in promoting the normal formation of red blood cells. The RDA for folate is 300 mg dietary folate equivalents (EAR ¼250 mg) for females aged 9–13 years and 400 mg dietary folate equivalents (330 mg) for females aged 14– 18 years. However, as considerable evidence has linked maternal folate intake early in pregnancy with neural tube defects among her offspring, it is recommended that after menarche all females should consume 400 mg dietary folate equivalents from supplements or fortified foods (in addition to the folate consumed from food sources) to ensure proper nutrition in the event of a pregnancy. Although recommendations for folate intake are similar for females and males, national nutrition data indicate that females 14–18 years more often have poor intakes than males in the same age group. Nearly 20% of adolescent females aged 14–18 years have intakes less than the EAR. Good sources of folate include dark green leafy vegetables, fruits, legumes, and fortified grains.
COMMON EATING BEHAVIORS
Micronutrient intake varies by eating habits. Eating behaviours influence the types and amounts of foods eaten and therefore impact micronutrient intakes. As a group, adolescent females engage in certain eating behaviours more often than younger children, including dieting, eating away from home, meal skipping, and snacking. These common eating behaviours and family meal patterns have several implications for dietary intake.
DIETING AND WEIGHT-CONTROL PRACTICES
Dieting and other weight-control practices are particularly common behaviours among adolescent females. In a recent national survey of high school students, 46% reported they were trying to lose weight. More females than males reported they were dieting to lose or maintain weight (55% vs. 27%), and females were also more likely to report the use of extreme methods to lose weight, such as taking diet pills (8% vs. 5%), taking laxatives, or vomiting (6% vs. 3%) in the past month. Changes in body shape and size during adolescence may lead to increased body dissatisfaction and thereby prompt efforts to lose weight. In addition, research indicates several other factors may play a role in the decision to attempt weight loss during this period of life. Factors that have been associated with increased dieting and use of weight control behaviours among adolescents include peer dieting, parental weight concerns, weight teasing, and media exposure. Media that discusses dieting or weight loss is particularly problematic as adolescent females may not have developed the skills to critically respond to the persuasive techniques of weight-loss advertisements that are not obviously deceptive. Some weight-control behaviours have been related to improved dietary intake; however, dieting and the use of weight-control practices are also strongly related to several undesirable outcomes. Females who report using moderate or more healthful dieting practices such as exercising, eating more fruits and vegetables, eating fewer sweets, and eating fewer high-fat foods may have a better diet quality than no dieters or females using unhealthful weight-control practices. In contrast, unhealthful dieting and weight-control practices (e.g., skipping meals, taking diet pills) are related to poor intakes of fruit, vegetables, grains, and several micronutrients (i.e., calcium, iron, vitamins A, C, and B6, folate, and zinc). Further, dieting during adolescence predicts the onset of depression; the initiation of extremely unhealthy weight-control behaviours (e.g., self-induced vomiting, laxative use); the onset of binge eating and other eating disorders; and excess weight gain or the onset of obesity. Compared with adolescent females who reported no dieting, dieters in one study were found to increase their body mass index (BMI) by an extra 0.6 units over a relatively short period of 5 years.
EATING AWAY FROM HOME
Although US adolescents consume the majority of their total energy intakes at home, national nutrition surveys suggest that on any given day more than 50% of adolescents eat something from a fast-food restaurant. Fast-food restaurants are favoured by adolescents because they provide (1) relatively inexpensive menu options; (2) an informal social setting for visiting with friends; (3) menu options that can be easily transported on hectic days; and (4) employment for many young people. Fast-food restaurants are often visited by male and female adolescents, but females tend to report less frequent consumption of fast-food than do males. Adolescents who report a lower socioeconomic status, playing a team sport, working at least 10 h a week, and a lack of time to eat healthy foods tend to report more frequent consumption of fast-food. As fast-food menu options tend to be less healthful than foods consumed at home, frequent consumption of fast-food may negatively impact on nutritional intake and health. More frequent consumption of fast-food by adolescents has been related to greater intakes of total fat, saturated fat, sodium, and high-fat, high-sugar foods.
Menu Options at Fast Food Restaurants
High-Calorie Menu Items Lower Calorie Alternatives
Big Mace (560 calories, 30 g fat) Cheeseburger (310 cal, 12 g fat)
Premium crispy chicken club (680 cal, Premium grilled chicken classic (420 cal,
29 g fat) 9 g fat)
Large French fries (570 cal, 30 g fat) Small French fries (250 cal, 13 g fat)
Large soft drink (310 cal, 0 g fat) 8 oz, 1% white milk jug (100 cal, 2.5 g fat)
12 oz Triple Thick Chocolate Shake (440 cal, Apple dippers and low-fat caramel dip
10 g fat) (100 cal, 1 g fat)
6’’ Personal pepperoni pan pizza (640 cal, 1 slice, 12’’ Fit n’ Delicious Pizzab (160 cal,
29 g fat) 4 g fat)
Hot wings and ranch dipping sauce (320 cal, Hot wings and lite ranch dressing (180 cal,
28 g fat) 13 g fat)
Double Decker Taco Supreme (380 cal, ‘‘Fresco Style’’ crunchy tacod (150 cal,
18 g fat) 7 g fat)
Nachos Bell Grande (790 cal, 44 g fat) Nachos (320 cal, 20 g fat)
Fiesta Taco Salad (860 cal, 46 g fat) Fiesta Taco Salad, no shellc (490 cal,
25 g fat)
Grilled stuff steak burrito (690 cal, 27 g fat) ‘‘Fresco Style’’ Steak Burrito Supreme
(350 cal, 9 g fat)
Source: From McDonald’s USA Nutrition Information, http:==www.mcdonalds.com=usa=eat=nutrition_info. html (accessed Nov 2006); Pizza Hut Nutrition Information, http:==www.pizzahut.com=menu= nutritioninfo.asp (accessed Nov 2006); and Taco Bell Printable Nutrition Guide, http:==www.yum. com=nutrition=documents=tb_nutrition.pdf (accessed Nov 2006).
Registered trademark of the McDonald’s Corporation, Oak Brook, Illinois.
Registered trademark of Pizza Hut, Inc., Dallas, Texas.
Registered trademark of the Taco Bell Corporation, Irvine, California.
d Item topped with a mix of diced tomatoes, onions, and cilantro instead of sauce or cheese.
such as French fries and soft drinks. In addition, more frequent consumption of fast-food is related to lower intakes of healthful foods (e.g., fruits, vegetables, and milk) and health-promoting food components (e.g., calcium, vitamins A and C, and fibre). Because fast-food menu options are often high in fat or sugar and offered in large portion sizes, menu options are often high in calories (Table 3.4). Healthful food choices (e.g., low-fat milk, fruit) and lower calorie options are sometimes available but, fast-food meals may easily provide all or nearly all of the energy an adolescent female requires for the entire day at just one meal. Adolescents may not compensate for the additional calories consumed from fast-food meals at other meals throughout the day; therefore, if they regularly consume fast-food, the development of overweight will be promoted. Research in adolescent females has shown fast-food consumption is positively associated with increases in weight over time.
In contrast to the negative implications of frequent fast-food consumption for nutritional intake, having regular meals with family (i.e., family meals) is associated with better diet quality and improved psychosocial health. When parents and adolescents regularly eat together, adolescents are more likely to have diets of higher nutritional quality. Frequency of family meals is related to higher intakes of fruits, vegetables, grains, calcium-rich foods, protein, fibre, and several key micronutrients (i.e., calcium, iron, vitamins A, C, E, and B6, and folate). In addition, frequent family meals are associated with lower intakes of soft drinks, saturated fat, and trans fat. Having regular family meals is associated with the possession of more developmental assets and less frequent engagement in high-risk behaviours, even after taking into account the influence of family relationships and communication. In a nationally representative sample, adolescents who indicated they had regular family dinners (i.e., 5–7 per week) were more likely than adolescents who indicated they had infrequent family dinners (i.e., one or fewer meals per week) to report parents and teachers had high expectations for them, positive self-esteem, a sense of purpose, a strong commitment to learning, and the possession of social resistance skills. Moreover, adolescents who indicated they had regular family dinners were less likely than adolescents who indicated infrequent family dinners to report substance use, school problems, violent or antisocial behaviour, sexual intercourse, depressive symptoms, or attempted suicide. Family meals also appear to protect female adolescents from engaging in chronic dieting, unhealthy weight-control behaviours, and binge eating or purging. Despite the many benefits associated with family meals, there is great variability among adolescents in the frequency of having meals with family. On average, adolescents report having four to five meals with their family in a week; however, about one third of adolescents report having less than three meals per week and a similar proportion report having meals with their family at least one time per day. Family meals tend to occur more frequently for adolescents when their family socioeconomic status is higher and when their mother does not work for pay. Younger adolescents attending middle school are more likely to report having frequent family meals than older adolescents attending high school. The majority of adolescents indicate that they enjoy and value eating meals with their family, but conflicting schedules of parents and adolescents present a major barrier to having regular meals together.
Skipping meals can also have an impact on nutritional intake. The frequency of skipping meals increases with age as young people progress through childhood and adolescence. Breakfast is the most frequently skipped meal. National nutrition data indicate that breakfast is skipped by 9% of school-aged children (6–8 years), 15% of preadolescents (9–13 years), and approximately one-third of adolescents (14–18 years). Among adolescents, skipping breakfast is more often reported by females than by males, adolescents with high levels of perceived life stress, and adolescents of lower socioeconomic status. Adolescents report lack of time and not being hungry in the morning as barriers to eating breakfast. Several other unhealthy behaviours are also related to meal skipping, including smoking, using unhealthy weight-control methods, a sedentary lifestyle, and high media use. Among a large sample of secondary school students, those students who watched four or more hours of television daily were 7 times more likely to skip a meal to watch television, and those who played computer games at least 4 times a week were 9 times more likely to skip a meal to play computer games. The consequences of skipping meals, especially breakfast, include lower daily intakes of micronutrients and compromised academic performance. In general, those who skip breakfast do not consume the micronutrients they fail to consume in the morning at other meals during the day. The nutrients most often reduced when breakfast is skipped include those essential for proper growth and development during adolescence—calcium, iron, and zinc.
The contribution of snacks, foods, and beverages consumed between meals to the diets of adolescents is considerable and has increased over the past 30 years. Although national nutrition data indicate that slightly more males than females snack, more than 85% of adolescent females consume snacks. Among those who snack, an average of two snacks is consumed per day, representing an increase of 24% in daily snacking occasions since the late 1970s. More frequent snacking is related to meal skipping, more television viewing, lower levels of physical activity, and higher levels of perceived life stress. Foods and beverages consumed as snacks provide an average of 612 cal=day or approximately one-fourth of daily energy intake. While snacks tend to be more energy dense than meals, snacks tend to be less dense in the key nutrients calcium, iron, zinc, and folate. Given the contribution of snacks to the diets of adolescents, eating healthful foods and beverages (e.g., fruits, vegetables, and low-fat milk) for snacks is of importance. Focus group research in adolescents suggests making healthful options appealing, convenient, inexpensive relative to less nutritious options, and ready availability at home and school promotes their selection.
SPECIAL CONDITIONS AND NUTRITION CONCERNS
Many factors influence the nutritional needs of adolescent females. Overweight, diabetes, eating disorders, iron deficiency anaemia, pregnancy, and lactation are common conditions that influence nutrition during adolescence.
The growing prevalence of overweight among adolescents in the United States represents a serious threat to the immediate and long-term health of young females. Adolescent overweight, defined here as adolescents who have a BMI equal to or greater than the 95th percentile of the age- and gender-specific Centres for Disease Control (CDC) BMI charts, has more than doubled over the past three decades since the 1970s. In 2003–2004, ~16% of adolescent females (12–19 years) were overweight. In a society that stigmatizes persons who are overweight, overweight female adolescents are at increased risk for low self-esteem, depression, and using unhealthy weight-control behaviours (e.g., smoking cigarettes, vomiting, taking diuretics). Overweight is also associated with increased risk for several adverse chronic health problems, including hypertension, cardiovascular disease, metabolic syndrome, and Type 2 diabetes. While many factors are related to the development of overweight, eating behaviours play a fundamental role in regulating the balance between energy intake and expenditure. Choosing a diet that follows the Dietary Guidelines, reducing the number of meals eaten away from home, and structuring eating times can help to prevent the development of overweight in adolescents along with the inclusion of at least 60 min of daily moderate to vigorous physical activity. See Chapter 7 for a full discussion of nutritional interventions appropriate for the prevention and treatment of overweight.
Overweight status increases risk for Type 2 diabetes mellitus and a dramatic increase in the disease has occurred concomitantly with increases in the prevalence of adolescent overweight. In contrast, Type 1 diabetes mellitus is immune-mediated and its etiology is unrelated to weight status. Both forms of diabetes are caused by a disorder of insulin regulation and lead to disturbances in carbohydrate, protein, and fat metabolism. However, Type 1 diabetes is generally characterized by absolute insulin deficiency while persons with Type 2 diabetes often continue to produce some insulin. A 2001 population-based study of physician diagnosed diabetes in young people estimated the crude prevalence of diabetes to be 2.80 cases=1000 adolescents 10–19 years. Type 1 diabetes represented the majority of cases, but Type 2 diabetes, formerly found only in adults, accounted for 0.42 cases=1000 adolescents. The nutritional management of diabetes typically involves carbohydrate counting, and for adolescents with Type 2 diabetes may also encourage weight loss. If diabetes is not closely managed, diabetic ketoacidosis or hypoglycaemia may result. Long-term consequences of poor glycaemic control include increased risk for heart, eye, and kidney damage. See Chapter 8 for a full discussion of diabetes mellitus.
Although full-syndrome eating disorders are less prevalent than overweight, dieting behaviours, and other nutritional concerns of adolescents, they are associated with the highest morbidity and mortality rates among psychiatric disorders. Diagnostic criteria have been established by the American Psychiatric Association for three full syndrome eating disorders: anorexia nervosa, bulimia nervosa, and binge eating disorder. Anorexia nervosa is characterized by severe weight loss and an intense, irrational fear of becoming fat even though underweight. The disorder occurs in ~1% of adolescents, being more common in females than in males. Related medical complications of the condition include cardiac arrhythmias, compromised linear growth, and impaired bone mineral accretion. Bulimia nervosa is characterized by recurrent, uncontrolled eating episodes involving the consumption of large amounts of food in a short time and followed shortly by compensatory behaviours (e.g., self-induced vomiting, laxative or diuretic abuse, enemas, fasting, and intense exercise). The disorder affects 1%–3% of adolescents. Like anorexia nervosa, bulimia nervosa primarily affects females and may lead to serious cardiovascular complications. Binge eating disorder is also characterized by recurrent, uncontrolled eating episodes, which involve the consumption of large amounts of food, but the eating episodes are not followed by compensatory behaviours. Despite the significance of this disorder associated with the development of overweight and considerable psychiatric distress, few studies have assessed its prevalence among adolescents. It is likely that most eating disorders have a multifactorial aetiology, but dieting appears to be a common precipitating behaviour for all three of these disorders. The impact of dieting and restrictive eating habits on nutritional intake is discussed in Section 3.4.1. For a complete discussion of eating disorders see Chapter 17.
IRON DEFICIENCY ANEMIA
During the rapid growth of adolescence, females are at particular risk for the development of iron deficiency anaemia. Iron requirements increase dramatically to support increases in lean body mass, the expansion of total blood volume, and the onset of menses. Iron deficiency, a condition defined by the absence of bone marrow iron stores, progresses to iron deficiency anaemia when the deficiency impairs blood production and the concentration of red blood cells in haemoglobin falls (Table 3.5). As a result of increased iron requirements and poor intakes, the prevalence of iron deficiency (16%) is higher among females aged 16–19 years than any other group. Among females 12–15 years, the prevalence of iron deficiency (9%) is lower but still greater than the prevalence among adolescent males (5%). The prevalence of iron deficiency anaemia is 2% among females 12–19 years. Risk factors for the development of anaemia include following a restrictive vegetarian diet, participating in a strenuous or endurance sport, skipping meals, and following a restrictive weight control diet. To help prevent iron deficiency, adolescent females should be encouraged to include iron-rich foods such as meat and fortified cereals in their usual diet. Treatment for iron deficiency will typically also include iron supplements of 60–120 mg elemental iron=day. The consequences of iron deficiency for adolescent growth and development are great. Iron deficiency may delay or impair physical growth and compromise cognitive achievements. In addition, TABLE 3.5
Maximum Haemoglobin Concentrations and
Haematocrit Values for Female Adolescents
Age (Years) Hemoglobin (<g=dL) Hematocrit (<%)
12 to <15 11.8 35.7
15 to <18 12 35.9
18 12 35.7
Source: From Centres for Disease Control and Prevention, MMWR, 47, 1998.
a No pregnant or lactating females.
iron deficiency and anaemia can increase fatigue, impair physical activity and work capacity, and lower resistance to infection. For adolescents who are pregnant, iron deficiency during the first two trimesters further increases risk for a preterm delivery and delivery of a low-birth weight infant.
PREGNANCY AND LACTATION
Births to adolescent females have decreased markedly since 1980. However, adolescent childbearing is not uncommon; the annual rate of births in 2002 was 43 per 1000 adolescent females. To ensure the delivery of a healthy infant and provide for the needs of mothers who may themselves still be growing, a nutritionally adequate diet is critical. Because of poor diet and high nutritional requirements, adolescent females are at increased risk for delivering low-birth weight infants having high rates of mortality during the first year of life. The Institute of Medicine has determined equations for estimating energy intakes that will support the need so f adolescent mothers and healthy fatal development. As changes in total energy expenditure and weight gain are minor during the first trimester, no increase in energy intake is recommended. EERs during the second and third trimesters of pregnancy are determined by summing requirements during the no pregnant state, a median change in total energy expenditure of 8 Cal (kcal) per week, and the energy deposition during pregnancy of 180 Cal=day. Energy requirements during lactation take into account the energy required for milk output(500cal=day) and energy mobilization from tissue stores during the first 6 months post partum (170 Cal=day). EERs for lactating women beyond 6 months postpartum assume weight stability (0 Cal mobilized from energy stores) and reduced milk production requiring 400 Cal=day. Higher intakes of protein and micronutrients are also required to support fatal development and milk production. An additional 25 g (EAR¼21 g) of protein per day is recommended for pregnant and lactating adolescents. Vitamin and mineral requirements of pregnant and lactating adolescents are also increased for folate, niacin, riboflavin, thiamine, vitamins A, B6, B12, and E, iron, magnesium, and zinc. To meet these requirements, adolescents may require low-dose or prenatal vitamin–mineral supplements. A full discussion of nutritional requirements and concerns during pregnancy and lactation can be found in Chapter 5.
PROMOTING HEALTHY EATING
This chapter has reviewed considerable gaps between the current dietary practices of female adolescents and nutritional recommendations for health. Efforts are needed to promote healthy eating behaviours and improve the dietary intake of female adolescents. Adolescence is a particularly important time to intervene on dietary intake because adequate nutrition is essential to support rapid growth and eating behaviours established during this period often track into adulthood. Interventions to promote healthy eating in adolescents need to consider the multiple environmental contexts that influence their eating behaviours. Home, school, and community environments each influence on the eating behaviours of adolescent females and offer various opportunities for promoting good nutrition.
It is important that nutrition intervention efforts address the family and home environments of adolescents because this context has a particularly strong influence on eating and weight-related behaviours. Parents can positively influence the behaviour of their son or daughter by (1) modelling healthy eating and weight-related behaviours; (2) providing an environment that makes it easy to choose nutritious foods and beverages; and (3) avoiding negative comments about weight. The food choices of adolescents are related to the choices made and modelled by their parents; when parents consume more healthy foods such as fruits, vegetables, and dairy foods, their adolescents also consume greater amounts of the same foods. However, it has also been found that adolescents practice unhealthy weight-related behaviours modelled by their parents. When adolescent females observe their parents dieting, they are more likely themselves to use unhealthful weight-control behaviours and to worry about weight gain. As adolescents tend to choose snacks that are readily accessible and convenient, parents can also help to improve their adolescent’s dietary intake by keeping the kitchen stocked with healthful foods and beverages that are simple to prepare and take ‘‘on-the-go’’ (i.e., prewashed fruit and vegetables, string cheese). Having regular family meals and involving adolescents in meal preparation are opportunities for parents to not only provide a healthful meal but also to model cooking skills and the consumption of nutrient-dense foods and beverages. Teasing about weight has been related to increases in binge eating, dieting, and unhealthy weight-control behaviours. Therefore, it is important that parents establish rules to eliminate weight teasing in the home and alternatively focus on encouraging their daughter or son to adopt healthful eating behaviours.
Aspects of the school environment with the potential to impact eating behaviours of female adolescents include (1) school food services; (2) school food policies; and (3) classroom nutrition education. Meals for students are provided through the National School Lunch Program (NSLP) and School Breakfast Program (SBP). These federally sponsored programs, administered by the USDA in conjunction with state and local education agencies, allow students who live in households with incomes between 130% and 185% of the poverty level to receive meals at reduced prices and those from households with incomes 130% of the poverty level or below to receive school meals free of charge. Foods and beverages provided to students through these programs must meet nutritional standards. Meals must provide no more than 30% of energy from fat and less than 10% of energy from saturated fat. In addition, regulations require school lunch meals to provide one-third of the RDAs for protein, vitamin A, vitamin C, iron, calcium, and energy and breakfasts to provide one-fourth of the RDA for these same nutrients. Research evaluating the nutritional impact of these programs has found that participants have higher intakes of most vitamins and minerals than nonparticipants and consume less added sugar. Federal regulations for foods and beverages sold beyond these programs (i.e., competitive foods) are minimal and their availability is high; in a nationally representative survey, 97% of middle schools and 99% of high schools were found to sell foods and beverages through a la carte, vending machines, school stores, or multiple venues. Only certain foods and beverages of minimal nutritional value (e.g., carbonated soft drinks, chewing gum, water ices, and hard candy) are prohibited if they are sold in the foodservice area during school meal periods. Unless a school determines more restrictive guidelines as part of its own school wellness policy, the nutritional quality of competitive foods and beverages are largely unregulated. Expert groups have developed sample policies for encouraging healthy eating that schools can use as a model in drafting their own wellness policies. The majority of foods and beverages sold through a la carte programs, vending machines, and school stores (i.e., competitive foods) are high-fat or high-sugar items such as salty snacks, sweet baked goods, sugared soft drinks, and candy. Although some nutritious options (e.g., low-fat milk, vegetables, fruit) are available through these venues, they are less likely to be selected when the environment provides ready access to many foods of limited nutritional value. Multiple research studies have found that the availability of competitive food and beverage items may negatively impact the eating habits of young people. School food practices (e.g., foods and beverages are allowed in classrooms; foods and beverages are used as rewards or incentives) allowing greater access to foods and beverages beyond the meal period have been further related to higher BMI values in secondary students.
Teens Eating for Energy and Nutrition
To promote healthy eating, it is recommended that schools develop nutrition guidelines for foods and beverages sold beyond school meals, limit the periods during which students have access to these products, and prohibit their use as incentives for students. In addition, nutrition education is essential to teach the skills necessary for selection of a healthful diet. Students should be educated on using recommendations for healthful eating (e.g., the 2005 Dietary Guidelines for Americans) and preparing both nutritious meals and snacks. To enhance effectiveness, school-based nutrition interventions for adolescents should optimally address all aspects of the school environment and whenever possible also address the home environment. Moreover, involving students in the promotion of healthful foods at school is important to address the influence of peer norms. Common elements of effective programs identified through critical reviews of intervention efforts are summarized in Table 3.6 along with illustrative examples from the Teens Eating for Energy and Nutrition at School (TEENS) intervention. TEENS was a multicomponent intervention developed for students in the seventh and eighth grades with the goal of increasing fruit and vegetable intake and decreasing fat intake to reduce future cancer risk.
The physical neighbourhood environment and community resources also have the potential to greatly impact adolescents’ food choices. For example, fast-food restaurants, convenience stores, and vending machines can provide access to high-fat, high-sugar snack foods and beverages. These food outlets are frequently located within a short walking distance of school buildings and community recreation centres making them particularly convenient for adolescents. While few interventions have assessed the impact of modifying the availability of foods and beverages in neighbourhood community settings on intakes of adolescents, some work in schools suggests that modifying the availability and price structure of foods can have a considerable influence on food selection. In addition, community resources such as after-school programs, community sport teams, and WIC can provide nutrition education and access to healthful foods for adolescents. The physical environment and resources available to adolescents might be addressed by a community coalition or task force. Key members of a community representing restaurants, groceries, retail outlets, recreation facilities, religious organizations, youth groups, libraries, and families can be invited to participate. Once formed, a coalition or task force evaluates needs of the community and works collaboratively for improvements or to develop nutrition programs for adolescents.
Adolescent females are at risk for poor nutritional intakes compared with their male counterparts and other age groups in spite of the fact that adolescence is a time when a healthful diet is necessary for optimal growth. Western culture promotes unhealthy eating habits and weight-loss practices, which contribute to nutritional deficiencies and health conditions. Special attention to the promotion of adequate nutrition is needed for adolescent females in all of their surrounding environments, including home, school, and the community.