CONCEPTS IN BIOLOGY

PART VI. PHYSIOLOGICAL PROCESSES

25. Nutrition. Food and Diet

25.7. Nutrition Through the Life Cycle

Nutritional needs vary throughout life and are related to many factors, including age, sex, reproductive status, and level of physical activity. Infants, children, adolescents, adults, and the elderly all require essentially the same types of nutrients but have special nutritional needs related to their stage of life, requiring slight adjustments in the kinds and amounts of nutrients they consume.

Infancy

A person’s total energy requirements per kilogram are highest during the first 12 months of life: 100 Calories per kilogram of body weight per day; 50% of this energy is required for an infant’s basal metabolic rate. Infants triple their weight and increase their length by 50% during their first year; this is their so-called first growth spurt. Because they are growing so rapidly, they require food that contains adequate proteins, vitamins, minerals, and water. They also need food that is high in Calories. For many reasons, the food that most easily meets these needs is human breast milk (table 25.7). Even with breast milk’s many nutrients, many physicians strongly recommend multivitamin supplements as part of an infant’s diet.

Studies have shown that kids under the age of 2 are “self regulating” when it comes to food intake. They eat only the amount they need. However, as they enter childhood, other factors begin to affect diet and they are likely to follow the eating patterns of their family.

TABLE 25.7. Comparison of Human Breast Milk and Cow's Milk*

* All milks are not alike. Each milk is unique to the species that produces it for its young, and each infant has its own growth rate. Humans have one of the slowest infant growth rates, and human milk contains the least amount of protein. Because cow’s milk is so different, many pediatricians recommend that human infants be fed either human breast milk or formulas developed to be comparable to breast milk during the first 12 months of life. The use of cow’s milk is discouraged. This table lists the relative amounts of the nutrients in human breast milk and cow’s milk.

Childhood

As infants reach childhood, their dietary needs change. The rate of growth generally slows between 1 year of age and puberty, and girls increase in height and weight slightly faster than boys.

During childhood, the body becomes leaner and the bones elongate; the brain reaches 100% of its adult size between the ages of 6 and 10. To meet children’s growth and energy needs adequately, protein intake should be high enough to take care of the development of new tissues. Minerals, such as calcium, zinc, and iron, as well as vitamins, are necessary to support growth and prevent anemia. Many parents continue to give their children multivitamin supplements, but this should be done only after a careful evaluation of their children’s diets. Three groups of children are at risk for vitamin deficiency and should receive such supplements:

1. Children from deprived families and those suffering from neglect or abuse

2. Children who have anorexia nervosa or poor eating habits or who are obese

3. Children who are strict vegetarians

During childhood, eating habits are very erratic and often cause parental concern. Children often limit their intake of milk, meat, and vegetables while increasing their intake of sweets. To get around these problems, parents can provide calcium by serving cheeses, yogurt, and soups as alternatives to milk. Meats can be made more acceptable if they are in easy-to-chew, bite-sized pieces, and vegetables might be more readily accepted if smaller portions are offered on a more frequent basis. Steering children away from high- fat foods (e.g., french fries, potato chips) and sugar (e.g., cookies, soft drinks) by offering healthy alternatives can lower their risk for potential health problems. For example, sweets in the form of fruits can help reduce dental caries (carrion = rotten). Parents can better meet the dietary needs of children by making food available on a more frequent basis, such as every 3 to 4 hours. Obesity is an increasing problem among children. Parents sometimes encourage this by insisting that children eat everything served to them. Before the age of 2, most children automatically regulate the food they eat to an appropriate amount. After that age, parents should be concerned about both the kinds and the amounts of food children eat.

Adolescence

The nutrition of an adolescent is extremely important because, during this period, the body changes from nonreproductive to reproductive. Puberty is usually considered to last between 5 and 7 years. Before puberty, males and females have similar proportions of body fat and muscle. Both body fat and muscle make up between 15% and 19% of the body’s weight. Lean body mass, primarily muscle, is about equal in males and females. During puberty, female body fat increases to about 23%, and in males it decreases to about 12%. Males double their muscle mass in comparison with females. The changes in body form that take place during puberty constitute the second growth spurt. Because of their more rapid rate of growth and unique growth patterns, males require more of certain nutrients than females (protein, vitamin A, magnesium, and zinc). During adolescence, youngsters gain as much as 20% of their adult height and 50% of their adult weight, and many body organs double in size. Nutritionists have taken these growth patterns and spurts into account by establishing Dietary Reference Intakes for males and females 10 to 20 years old, including requirements at the peaks of growth spurts. Dietary Reference Intakes at the peak of the growth spurt are much higher than they are for adults and children.

Adulthood

People who have completed the changes associated with adolescence are considered to have entered adulthood. During adulthood, the body enters a plateau phase, and diet and nutrition focus on maintenance and disease prevention. Nutrients are used primarily for tissue replacement and repair, and changes such as weight loss occur slowly. Because the BMR slows, as does physical activity, the need for food energy decreases from about 2,900 Calories in average young adult males (ages 20 to 40) to about 2,300 for elderly men. For women, the corresponding numbers decrease from 2,200 to 1,900 Calories. Protein intake for most U.S. citizens is usually in excess of the recommended amount. The Dietary Reference Intakes standard for protein is about 56 grams for men and 46 grams for women each day. About 25-50% should come from animal foods to ensure intake of the essential amino acids. The rest should be from plant-protein foods, such as whole grains, legumes, nuts, and vegetables.

Old Age

As people move into their sixties and seventies, the digestion and absorption of all nutrients through the intestinal tract slows down. The number of cells undergoing mitosis is reduced, resulting in an overall loss in the number of body cells. With age, complex organs, such as the kidneys and brain, function less efficiently, and protein synthesis becomes inefficient. With regard to nutrition, energy requirements for the elderly decrease as the BMR slows, physical activity decreases, and eating habits change.

The change in eating habits is significant, because it can result in dietary deficiencies. For example, linoleic acid, an essential fatty acid, may fall below required levels as an older person reduces the amount of food he or she eats. The same is true for some vitamins and minerals. Therefore, it may be necessary to supplement the diet daily with 1 tablespoon of vegetable oil. Vitamin E, multiple vitamins, or a mineral supplement may also be necessary. The loss of body protein means that people must be certain to meet their daily Dietary Reference Intakes for protein and participate in regular exercise to prevent muscle loss. As with all stages of the life cycle, regular exercise is important in maintaining a healthy, efficiently functioning body.

The two minerals that demand special attention are calcium and iron, especially for women. A daily intake of 1,200 milligrams of calcium should prevent calcium loss from bones (figure 25.12), and a daily intake of 15 milligrams of iron should allow adequate amounts of hemoglobin to be manufactured to prevent anemia in women over 50 and men over 60. In order to reduce the risk for chronic diseases such as heart attack and stroke, adults should eat a balanced diet, participate in regular exercise programs, control their weight, avoid tobacco and alcohol, and practice stress management.

FIGURE 25.12. Osteoporosis

(a) Healthy bone and (b) a section of bone from a person with osteoporosis. This calcium-deficiency disease results in a change in the density of the bones as a result of the loss of bone mass. Bones that have undergone this change look “lacy” or like Swiss cheese, with larger than normal holes. (c) A few risk factors associated with this disease are being female and fair-skinned; having a sedentary lifestyle; using alcohol, caffeine, and tobacco; and having reached menopause.

Pregnancy and Lactation

The period of pregnancy and milk production (lactation) requires that special attention be paid to the diet to ensure proper fetal development, a safe delivery, and a healthy milk supply. Studies have shown that an inadequate supply of essential nutrients can result in infertility, spontaneous abortion, and abnormal fetal development.

The daily amount of essential nutrients must be increased, as should Caloric intake. Calories must be increased by 300 per day to meet the needs of an increased BMR; the development of the uterus, breasts, and placenta; and the work required for fetal growth. Some of these Calories can be obtained by drinking milk, which simultaneously supplies the calcium needed for fetal bone development. Women who cannot tolerate milk should consume supplementary sources of calcium. In addition, their protein intake should be at least 71 grams per day; however, as mentioned earlier, most people in developed countries consume much more than this per day. Two essential nutrients, folic acid and iron, should be obtained through prenatal supplements, because they are essential to cell division and the development of the fetal blood supply.

The mother’s nutritional status affects the developing baby in several ways (figure 25.13). If she is under 15 years of age or has had three or more pregnancies in a 2-year period, her nutritional stores are inadequate to support a successful pregnancy. The use of drugs, such as alcohol, caffeine, nicotine, and “hard” drugs (e.g., heroin), can result in decreased nutrient exchange between the mother and fetus. In particular, heavy smoking can result in low birth weight, and alcohol abuse is responsible for fetal alcohol syndrome (FAS). Children with FAS may show the characteristics such as: small size for their age, facial abnormalities, poor coordination, hyperactive behavior; learning disabilities; developmental disabilities (e.g., speech and language delays); mental retardation or low IQ.

FIGURE 25.13. Diet Is Important During Pregnancy

During pregnancy diet is extremely important. Whatever the mother eats can influence the development of the embryo. A healthy diet assures that the embryo will get the nutrients it needs. The use of drugs and alcohol during pregnancy can have severe effects on the developing embryo.

25.7. CONCEPT REVIEW

13. During which phase of the life cycle is a person’s demand for Calories per unit of body weight the highest?

14. What changes need to be made to the diet of the elderly?

15. What changes should be made in the diet of pregnant and nursing mothers?