Diet recommendations for adolescents cannot be improved until scientists have more information about nutrient intakes, body compositions, and energy expenditures for adolescents.
Basic Nutritional Requirements of Adolescents
Most recommendations have relied on body measurements to index growth. To date, nutrient requirements for growth during adolescence have been estimated from studies of fetuses or adults. Because tissue composition during growth differs among different age groups, it is not appropriate to measure growth in teenagers using techniques that were developed to measure growth in adults and fetuses. In addition, most studies have been done on Caucasians; very few have been conducted on individuals of other ethnic groups. Data in the literature suggest that there are differences among ethnic groups in growth rate, metabolic rate, body composition, and onset of fertility.
To assess the nutritional needs of teenagers (including obese or pregnant teens), scientists must collect data on nutrient intakes, body composition, and energy expenditure from healthy adolescents who represent different ethnic groups.
Adolescence is the transition period between childhood and adulthood. The growth spurt that occurs during adolescence includes rapid bone growth, increased muscle mass, and increased body fat. The growth and development of adolescents reflect their genetic background and their dietary history during infancy and childhood. The food choices made by adolescents affect not only their growth and development during puberty but also their reproductive capacity and susceptibility to degenerative diseases when they become adults.
Nutritional Problems of Adolescents
Because of the high fat content of the American diet, obesity affects 10 to 35 percent of adolescents. Researchers have shown that infant weight correlates strongly with adult weight and that children who are overweight before puberty have a 75 percent chance of becoming obese adults. It has also been shown that those who are overweight after puberty have a 95 percent chance of becoming obese adults. It is not surprising, therefore, that data from the Second National Health and Nutrition Examination Survey (1976-80) indicated that approximately 34 million American adults (25.7 percent) are overweight.
Although fat deposition during puberty is essential for teenage girls in preparation for reproduction and lactation, the incidence of obesity in the United States is unfortunately high, especially in adult women. Obesity is associated with increased risk of diabetes, high blood pressure, heart disease, gallbladder disease, colon cancer, postmenopausal breast cancer, and menstrual irregularities. Unless obese women eat a prudent diet during pregnancy, they have a great risk of complications.
Pregnancy and Osteoporosis
The National Center for Health Statistics recently documented the sharpest rise in teenage pregnancies in 15 years; the annual incidence in the United States is now 1 in 10. The effects of pregnancy during adolescence on long-term health status are not completely understood. A female adolescent generally attains 99 percent of her mother’s bone size and 80 to 95 percent of her mother’s bone mass and bone density by 14 years of age. The growth spurt and onset of fertility that occur during puberty impose substantial nutrient demands on female adolescents.
The consequences of these changes during puberty depend on individual nutritional status, level of sexual maturity, and genetic background. An increasing health problem in the United States is posed by pregnant and lactating teenaged girls, who must accommodate the additional nutrient demands of reproduction. These girls are at high risk of nutrient deficiencies, such as calcium deficiency, and thus have a higher risk of developing osteoporosis later in life. There are many nutrition questions to be answered about pregnancy during adolescence.
Cholesterol and Atherosclerosis
Cholesterol is an essential component of cell membranes and is required for cell growth, replication, and maintenance. Plasma cholesterol concentrations are known to be higher in breast-fed infants than in formula-fed infants, perhaps because human milk has a higher cholesterol content than formula. Although cholesterol causes atherosclerosis in humans, animal studies have shown that a diet high in cholesterol early in life may protect against diets high in cholesterol later in life. The protective mechanism, however, has not been documented in humans.
Nutrition Research at the Children’s Nutrition Research Center
The Children’s Nutrition Research Center (CNRC) has developed numerous noninvasive techniques to measure lean body mass, fat mass, bone mineral content and density, basal metabolic rate, and total energy expenditure. Several studies are in progress at the CNRC to assess the nutrient requirements of adolescents.
In one cross-sectional study, nutrient intakes, cardiorespiratory fitness, body composition, energy expenditure, plasma lipid profile, and plasma iron status are being measured in 200 healthy female adolescents between 10 and 16 years of age. Equal numbers of adolescents representing four ethnic groups (white, black, Hispanic, and Asian) are being studied. Nutrient intakes for a 3-day period are being analyzed from food records. Cardiorespiratory fitness is being evaluated by determining the maximal heart rate and maximum oxygen utilization while the subject is walking and running on a motorized treadmill. Body density is being measured by weighing the subject in and out of the water. Because fat is less dense than water, lower body density reflects higher body fat. Lean body mass is being estimated in a total body electrical conductivity (TOBEC) machine. The TOBEC machine has been approved for use on infants, children, and adults by the Food and Drug Administration.
Total body bone mineral content and density are being measured by a technique called dual-energy x-ray absorptiometry. The subject lies in a supine position during the whole-body scan, which takes approximately 15 minutes.
The energy required for daily activities, including sleep, is being measured while the subject is inside a whole-body indirect calorimeter for 24 hours. The whole-body calorimeter is similar in size to a small bedroom and has a bed, table, chair, television, VCR, stereo system, exercise bicycle, telephone, and toilet facilities. The subject also eats breakfast, lunch, and dinner in the calorimeter. The 24-hour energy expenditure measured reflects the caloric needs of the subject while being confined in the chamber. Activities inside the calorimeter, however, are limited. Because the subject is being monitored constantly inside the chamber, behavior and activity patterns are not typical. The whole-body calorimeter provides important information on basic energy needs but does not provide an accurate estimate of the caloric needs of the subject in a free-living environment.
The energy needs of free-living subjects are being estimated using the doubly labeled water method. After a baseline saliva sample is collected, each subject drinks a known amount of water containing the stable isotopes deuterium and oxygen-18. The energy expenditures of the free-living subjects are calculated from the rates of disappearance of these isotopes in subsequent saliva samples. A blood sample is collected from each subject after the subjects have fasted overnight. Each sample is tested for plasma total cholesterol, triglyceride, high-density and low-density lipoprotein cholesterol, apolipoproteins A-1 and B, total iron, and ferritin. The relationships of nutrient intake, body composition, caloric needs, cardiorespiratory fitness, plasma lipid profile, and iron status to each other and to sexual maturity and race are being evaluated.
Another study underway at the CNRC will establish a body composition database for both men and women. Results from this study will define the relationships between nutrient intake and body composition of healthy children from infancy through adolescence for different ethnic groups. The data will also establish acceptable upper and lower limits within which to evaluate children who are ill. Approximately 1,000 healthy children under 18 years of age from 3 ethnic groups (white, black, and Hispanic) are being studied in the Metabolic Research Unit and Body Composition Laboratory at the CNRC.
Two studies at CNRC are designed to define the nutrient needs of pregnant and lactating teenagers. One longitudinal study will estimate the nutrient intakes, changes in body composition, and changes in energy expenditure of 20 healthy pregnant teenagers under 17 years of age. These teenagers are being studied four times between 8 and 40 weeks of gestation and immediately after delivery. The other study will measure the dietary intakes, milk production, and changes in body composition and growth in lactating adolescents. Preliminary results indicate that body weight, lean body mass, and body fat in lactating adolescents are maintained during the first 3 months after giving birth. This conservation of body mass may occur at the expense of milk production in the adolescent mother.
To test the hypothesis that early neonatal ingestion of large quantities of cholesterol protects the infant from high-cholesterol diets later in life, the CNRC is planning to study the effects of dietary cholesterol on in vivo cholesterol synthesis. The studies will be conducted in 4-month-old infants: 12 exclusively breast-fed, 12 formula-fed, and 12 formula-fed with added cholesterol. Later, all infants will be fed a formula with low cholesterol levels. The same procedure will be repeated at 11 months of age. The infants will then be fed a diet high in cholesterol, and the effects of the diet on the cholesterol levels in the infants will be evaluated.
The United States is confronted with phenomenal economic costs from obesity, adolescent pregnancy, poor birth outcomes, and cardiovascular heart diseases related to atherosclerosis. The data collected in the CNRC studies will provide important information for the formulation of food recommendations for healthy teenagers as well as obese and pregnant teenagers. We hope these recommendations, besides helping to provide adequate education, will also help reduce nutrition-related health costs in our society.
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