Nutrition in Adolescence1



Nutrition in Adolescence1


Marie-Pierre St-Onge

Kathleen L. Keller





GENERAL DEFINITIONS AND OVERVIEW


What Is Adolescence?

Adolescence is the transition period between childhood and adulthood, typically ranging from ages 11 to 21 years. The growth in physical size that occurs during this period is second only to the amount of growth experienced in the first year of life. In addition, dramatic psychologic, cognitive, reproductive, and behavioral growth occurs during this time as well. These rapid changes require sufficient energy and macro- and micronutrients to allow for maximal growth potential. However, adolescents often test limits and engage in risk-taking behaviors that can contribute to suboptimal nutrition. In addition, metabolic disorders such as type 2 diabetes (T2D), which were previously found only in adults, are increasingly occurring during adolescence (1). Adolescents need appropriate levels of support to help them learn to make healthful lifestyle choices so that they can reach their growth potential and avoid future chronic diseases. The following learning objectives are covered in this chapter.



  • Define the primary physical, physiologic, and psychosocial changes that occur during adolescence and review the implications for each of these on the ability to meet nutritional requirements.


  • Review the dietary reference intakes (DRIs) for adolescents.


  • Define the major trends in eating behavior in the United States and discuss how these behaviors affect nutritional status and growth during adolescence.


  • Review special considerations for adolescents whose circumstances require unique nutritional approaches, including eating disorders, pregnancy, obesity, and the teenage athlete.

The beginning of adolescence corresponds to the start of puberty for many, although puberty has increasingly been occurring at earlier ages, particularly for girls (2). Puberty refers to the period of adolescence in which secondary sex characteristics develop and an individual becomes capable of sexual reproduction. The pattern and timing of physical changes that occur are different for boys and girls, primarily because of the differential effects of the androgen hormones, estrogen and testosterone.


Hormonal Changes that Occur during Adolescence

The changes that signal the onset of puberty involve complex coordination of somatotrophic peptides (e.g., growth hormone and insulinlike growth factors [IGFs]), gonadotrophic hormones (e.g., estrogen and testosterone) and adipostatic hormones (e.g., leptin) (3) in the hypothalamus (4). During childhood, the central nervous system suppresses activity of the hypothalamic-pituitary-gonadal axis. At puberty, excitatory neurotransmitters signal the release of gonadotropin-releasing hormone in the hypothalamus, which subsequently stimulates secretion of the gonadotropins, luteinizing hormone (LH) and follicle-stimulating hormone (FSH). Increased concentrations of LH and FSH
stimulate maturation of the gonads and production of the sex steroid hormones, testosterone, responsible for the development of secondary sex characteristics that occur in boys; and estrogen, responsible for the development of secondary sex characteristics in girls. Estrogen and progesterone (synthesized in the corpus luteum and released in response to LH) control the menstrual cycle and development of secondary sex characteristics in girls.

Undernutrition has long been recognized as a key regulator of sexual maturation in both males and females (5). The discovery of leptin, a peptide hormone secreted from adipocytes, has shed light on this relationship (6). Animals and humans with a defect in the gene that secretes leptin are both extremely obese and infertile. This is in part because leptin is necessary for function of the gonadotropin- releasing hormone pulse generator that is responsible for pulsatile secretion of sex hormones (7). Undernourished boys and girls experience delays in sexual maturation that can in part be explained by reductions in leptin as a response to low body fat. In contrast, the relationship between overnutrition (e.g., in obesity) and sexual maturation is less clear. Although several studies suggested that obesity is associated with early sexual maturation in girls (2), this relationship has proved more complicated in boys. Some studies identified that higher levels of body fat are associated with later puberty in boys (8, 9), whereas others showed that the age of pubertal onset had declined by 3 months since the 1990s (10). Because the prevalence of obesity has increased in children over this period, investigators have hypothesized that increasing adiposity may positively influence the onset of puberty in boys by stimulating the hypothalamic-pituitary axis, similar to what has been proposed in girls. The extent to which puberty onset is the result of obesity itself or to the effects of hormones released from adipocytes is unclear.


Sexual Maturity Ratings: Tanner Stages

Although the timing of major pubertal milestones in girls and boys varies substantially, the sequence of events that occur during puberty are consistently observed. To illustrate the importance of this variation for determining nutrient needs, one can compare two girls, both 12 years of age, but girl A has completed her growth spurt whereas girl B is still in the prepubertal stage. Girl A would likely require less energy for growth, but she may require additional micronutrients, such as iron, to account for blood losses during menstruation, compared with girl B. As one quickly realizes, chronologic age is irrelevant because of the substantial variation in onset and timing of pubertal events. During puberty, sexual maturation is more important in assessing nutrient needs, growth, and development than chronologic age.

One of the most common methods by which health professionals and researchers assess development is by evaluating Tanner stages, named for James Tanner, the pediatrician who first described these stages (11). These scales rate pubertal development based on secondary sex characteristics: testicular and penile development and appearance of pubic hair in boys and breast development and appearance of pubic hair in girls. Tanner stage 1 signifies the prepubertal stage, Tanner stages 2 to 5 depict various stages of puberty, and Tanner stage 5 indicates the completion of puberty (Table 55.1).








TABLE 55.1 TANNER STAGING IN ADOLESCENT GIRLS AND BOYS





































































GIRLS


STAGE


BREAST DEVELOPMENT



PUBIC HAIR GROWTH


1


Prepubertal; nipple elevation only



Prepubertal; no pubic hair


2


Small, raised breast bud



Sparse growth of hair along labia


3


General enlargement of/raising of breast and areola



Pigmentation, coarsening and curling, with an increase in amount


4


Further enlargement with projection of areola and nipple as secondary mound



Hair resembles adult type, but not spread to medial thighs


5


Mature, adult contour, with areola in same contour as breast and only nipple projecting



Adult type and quantity, spread to medial thighs


BOYS


STAGE


GENITAL DEVELOPMENT



PUBIC HAIR GROWTH


1


Prepubertal; no change in size or proportion of testes, scrotum, and penis from early childhood



Prepubertal; no pubic hair


2


Enlargement of scrotum and testes; reddening and change in texture in skin of scrotum; little or no penis enlargement



Sparse growth of hair at base of penis


3


Increase first in length, then width of penis; growth of testes and scrotum



Darkening, coarsening and curling, increase in amount


4


Enlargement of penis with growth in breadth and development of glands; further growth of testes and scrotum, darkening of scrotal skin



Hair resembles adult type, but not spread to medial thighs


5


Adult size and shape genitalia



Adult type and quantity, spread to medial thighs


Reprinted with permission from Tanner JM. Growth at Adolescence. 2nd ed. Oxford: Blackwell Scientific, 1962.



The onset and length of the Tanner stages have ethnic variations, particularly in girls. Non-Hispanic black girls enter puberty at an earlier age than non-Hispanic white girls (12). Nearly 50% of non-Hispanic black girls are reported to be in Tanner stage 2 by 8 years of age. However, by menarche, the age of the first menstrual period, the differences between non-Hispanic black and non-Hispanic white girls are less pronounced (12). Early puberty is a risk factor for future development of insulin resistance (IR), cardiovascular disease (CVD), and other chronic diseases (13); determining the reasons for early puberty therefore has clinical relevance. Another consideration in accurate determination of Tanner stage is the increasing prevalence of obesity. When obesity coincides with accumulation of fat in the breasts, self-assessment of Tanner stage can be compromised (14).


Body Composition Changes

Marked changes occur in both height and body composition during adolescence, both of which have major implications for determining energy and nutrient requirements and may subsequently affect body image and food choices. During this time, boys’ muscle mass increases and shoulders broaden, whereas girls increase body fat and develop rounder hips and smaller waists. The pattern and rate of development in body composition differ in boys and girls. Girls attain peak height growth velocity at a younger age than boys, at 11.5 years of age versus 13.5 years of age (3), but boys attain a higher maximal height growth velocity and increased height for a longer period of time.

Girls gain fat mass (FM) steadily through age 16. Boys have an initial increase in FM between age 8 and 14 years, then a decline between ages 14 and 16 years, followed by a plateau (15). The distribution of FM also changes: in boys, increased deposition of subcutaneous adipose tissue (SAT) occurs in the trunk area, whereas in girls, SAT is deposited in the gluteal-femoral region. This results in the characteristic body composition patterns of adult men and women in which men have more upper body fat and women have wider hips and more lower body fat. Patterns of change in fat-free mass (FFM) also differ: girls increase in FFM until age 15 years, and boys increase in FFM through age 18 years, with the most rapid increase occurring between 12 and 15 years (15). The composition of FFM also changes during this time, from 80% water in young childhood to approximately 73% water by ages 10 to 15 years (16). The rise in density of FFM is caused by accretion of protein and minerals in the FFM compartment during growth.

For girls, a negative relationship has been observed among age at menarche, body mass index (BMI), and body fatness (15). Girls who are more pubertally advanced tend to be taller and have more FM, bone mineral content, and FFM compared with same-age girls at a lower stage of pubertal development (15). Girls with early growth spurts reach Tanner stage 2 and menarche earlier than girls with average and late growth spurts (17). Further, girls with early menarche are fatter at the end of puberty than girls with late menarche. This is of concern because tracking of FM into adulthood is strong: girls in the highest FM category have a 55% chance of staying within that category 10 years later, whereas girls in the lowest FM category in adolescence have a 77% change of staying within their category (18).

Bone’s response to forces and its capacity for growth are greatest during adolescence. Endogenous estrogens and androgens independently exert effects on bone acquisition and upkeep. Estrogen lowers the bone remodeling threshold, and girls experience greater gains in bone mass during puberty than boys (19). Bone acquisition and metabolism are influenced by hormonal, dietary, and lifestyle factors. In addition to the sex hormones, growth hormone, IGF-I, cortisol, thyroid hormones, parathyroid hormone, vitamin D, and leptin may influence bone metabolism during puberty (20). Physical activity during puberty has positive effects on bone accrual and turnover. Increased lean mass improves bone mass strength, and bone metabolism is influenced by dietary intakes of highquality protein, calcium, magnesium, phosphorus, and vitamins D, K, and C (20).

Changes in body composition that occur during adolescence guide nutrient recommendations: growth increases energy demands and protein requirement, and bone accrual requires protein, minerals, and vitamins. The adolescent period and the changes in body composition that occur can pose emotional and psychologic distress, which can lead to unhealthy eating patterns, affect subsequent health in adulthood, and set the stage for increased metabolic risk.


DAILY RECOMMENDED INTAKES FOR ADOLESCENTS


Dietary Guidelines for Americans

The Dietary Guidelines for Americans (DGA) were updated in 2010 (21). The main concept included in those guidelines was that Americans of all ages should balance calories to maintain and sustain a healthy body weight. For children and adolescents, this is defined as sex-specific BMI for age between the 5th and 85th percentile. For those who are overweight and obese, BMI for age between the 85th and 95th percentile and 95th percentile or greater, respectively, recommendations are to reduce calorie intakes from foods and beverages, increase physical activity, and reduce sedentary behavior. Other specific recommendations include limiting sodium intakes to less than 2300 mg/day or less than 1500 mg/day for non-Hispanic blacks and adolescents with hypertension, T2D, or chronic kidney disease; and limiting intakes of solid and trans-fats, added sugars, and foods with refined grains, particularly those that also contain solid fats, added sugars, and sodium.



Dietary Reference Intakes for Adolescents

The DRIs are established and published by the US Department of Agriculture. Committees consisting of US and Canadian experts on specific nutrients review the scientific literature, consider the roles of nutrients in reducing disease risk, evaluate indicators of adequacy, and estimate average requirements for each nutrient. This information is interpreted in light of current intakes by various North American population groups.

The DRIs consist of four types of reference values. The estimated average requirement (EAR) is the amount of a nutrient that would meet the requirements of 50% of healthy individuals of different sex and age groups. This value is used for calorie and macronutrient recommendations. The recommended dietary allowance (RDA) is calculated from the EAR to meet the needs of 97% to 98% of healthy individuals. The adequate intake (AI) is established when an EAR cannot be determined from available data. It is based on experimental data or determined from estimated intakes of a group of healthy individuals. The underlying assumption is that the amount of the nutrient consumed by these people is adequate to sustain health. The tolerable upper intake level is the highest amount of a nutrient that can be consumed without posing a risk of adverse side effects for almost all individuals.

For setting nutrient intake guidelines, the Institute of Medicine defines adolescence as ages 9 to 18 years. DRIs for adolescents account for variability in requirements related to growth rates. Table 55.2 shows the nutritional goals set forth by the DGA based on DRI and dietary guidelines recommendations.


DIETARY BEHAVIORS OF ADOLESCENTS


Skipping Meals

The transition from childhood to adolescence is a time when eating habits are changing, and the patterns developed during adolescence tend to continue into adulthood (22).
Data from the National Longitudinal Study of Adolescent Health (23) showed that regular breakfast consumption in adolescence significantly predicted young adulthood breakfast patterns. Breakfast is a common meal skipped by many adolescents, and its consumption tends to decrease with age during adolescence (24). In the National Health and Nutrition Examination Survey (NHANES) from 1999 to 2006, 20% of children 9 to 13 years old skipped breakfast compared with 32% of 14 to 18 year olds (25). This finding has implications for the health of adolescents because low meal frequency, breakfast skipping, and high consumption of sugar-sweetened beverages (SSBs) have been identified as factors associated with obesity (26).








TABLE 55.2 NUTRITIONAL GOALS FOR AGE-GENDER GROUPS BASED ON DIETARY REFERENCE INTAKES AND DIETARY GUIDELINES RECOMMENDATIONS





















































































































































































































































NUTRIENT (UNITS)


SOURCE OF GOAL


FEMALE 9-13 y


MALE 9-13 y


FEMALE 14-18 y


MALE 14-18 y


Protein (g)


RDA


34


34


46


52


% of calories


AMDR


10-30


10-30


10-30


10-30


Carbohydrates (g)


RDA


130


130


130


130


% of calories


AMDR


45-65


45-65


45-65


45-65


Total fiber (g)


IOM


22


25


25


31


Total fat (% of calories)


AMDR


25-35


25-35


25-35


25-35


Saturated fat (% of calories)


DG


<10


<10


<10


<10


Linoleic acid (g)


AI


10


12


11


16


% of calories


AMDR


5-10


5-10


5-10


5-10


α-Linolenic acid (g)


AI


1.0


1.2


1.1


1.6


% of calories


AMDR


0.6-1.2


0.6-1.2


0.6-1.2


0.6-1.2


Cholesterol (mg)


DG


<300


<300


<300


<300


Calcium (mg)


RDA


1,300


1,300


1,300


1,300


Iron (mg)


RDA


8


8


15


11


Magnesium (mg)


RDA


240


240


360


410


Phosphorus (mg)


RDA


1,250


1,250


1,250


1,250


Potassium (mg)


AI


4,500


4,500


4,700


4,700


Sodium (mg)


UL


<2,200


<2,200


<2,300


<2,300


Zinc (mg)


RDA


8


8


9


11


Copper (μg)


RDA


700


700


890


890


Selenium (μg)


RDA


40


40


55


55


Vitamin A (μg RAE)


RDA


600


600


700


900


Vitamin D (μg)


RDA


15


15


15


15


Vitamin E (mg AT)


RDA


11


11


15


15


Vitamin C (mg)


RDA


45


45


65


75


Thiamin (mg)


RDA


0.9


0.9


1.0


1.2


Riboflavin (mg)


RDA


0.9


0.9


1.0


1.3


Niacin (mg)


RDA


12


12


14


16


Folate (μg)


RDA


300


300


400


400


Vitamin B6 (mg)


RDA


1.0


1.0


1.2


1.3


Vitamin B12 (μg)


RDA


1.8


1.8


2.4


2.4


Choline (mg)


AI


375


375


400


550


Vitamin K (μg)


AI


60


60


75


75


AI, adequate intake; AMDR, acceptable macronutrient distribution range; AT, α-tocopherol; DG, dietary guidelines; IOM, Institute of Medicine; RAE, retinoic acid equivalents; RDA, recommended dietary allowance; UL, upper limit.


Adapted with permission from US Departments of Agriculture and of Health and Human Services. Report of the Dietary Guidelines Advisory Committee on the Dietary Guidelines for Americans, 2010. Washington, DC: US Government Printing Office, 2010.

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