We all need essential amino acids, carbohydrate, essential fatty acids, and 28 vitamins and minerals to sustain life and health. However, nutritional needs vary from one life stage to another. During intrauterine development, infancy, and childhood, for example, recommended intakes of macronutrients and most micronutrients are higher relative to body size, compared with those during adulthood. In elderly persons, some nutrient needs (eg, vitamin D) increase, while others (eg, energy, iron) are reduced.
The National Academy of Sciences has published recommendations for Dietary Reference Intakes (DRI)1 that are specific for the various stages of life. It should be noted, however, that the DRIs are not designed for individuals who are either chronically ill or who are at high risk for illness due to age, genetic, or lifestyle factors (eg, smoking, alcohol intake, strenuous exercise). Clinicians must make their own judgments regarding nutrient requirements in such cases based on available information (see table).
In this chapter, we will examine nutrient needs throughout the life cycle. Two major themes emerge:
First, the predominant nutritional problem in developed countries is overnutrition. It has led to unprecedented epidemics of obesity and chronic diseases. Clinicians can assist patients in making the dietary shifts necessary to prevent overnutrition and its sequelae.
Second, a renewed emphasis on vegetables, fruits, whole grains, and legumes can help prevent weight problems and chronic illnesses, including cardiovascular disease,2 diabetes,3 and cancer,4 among others.5 Plant-based diets meet or exceed recommended intakes of most nutrients, and have the advantage of being lower in total fat, saturated fat, and cholesterol than typical American diets,6 with measurable health benefits.7
Adolescence and Adulthood
The Institute of Medicine recommends higher intakes of protein and energy for growth in the adolescent population. For most micronutrients, recommendations are the same as for adults. Exceptions are made for certain minerals needed for bone growth (eg, calcium and phosphorus).38 However, these recommendations are controversial, given the lack of evidence that higher intakes are an absolute requirement for bone growth. Evidence is clearer that bone calcium accretion increases as a result of exercise rather than from increases in calcium intake.39 Since weight gain and atherosclerosis often begin during these years, adolescents and young adults must establish healthy eating and lifestyle habits that reduce the risk for chronic disease later in life.
Micronutrient needs in adults 19 to 50 years of age differ slightly according to gender. Males require more of vitamins C, K, B1, B2, and B3; choline; magnesium; zinc; chromium; and manganese. Menstruating females require more iron, compared with males of similar age.
Excess Calorie Intakes: A Risk Factor Common to All Age Groups
The major nutritional problem encountered in developed countries is excess macronutrient intake (especially saturated fat, protein, and sugar) and insufficient intake of the fiber and micronutrients provided by vegetables, fruits, grains, and legumes.
Overnutrition begins early. Pregnant and lactating women are encouraged to eat more because they are "eating for two." While it is true that an expectant mother must provide nutrition for both herself and her developing baby, the increased energy requirement of pregnancy amounts to no more than about 300 calories per day.1 Excessive nutrient intake may result in excessive weight gain, conferring a greater risk for cesarean section and other complications of pregnancy and delivery.8
Overfed infants and children may develop dietary habits and perhaps even metabolic characteristics that have lifelong consequences.9,10,11 Higher-than-recommended energy intakes at 4 months of age have been shown to predict greater weight gain before 2 years and risk for obesity in childhood and adulthood.12, 13 Therefore, caretakers should select foods conducive to healthy body weights and restrain their desire to promote child growth through overfeeding.
Adolescents face a similar problem. Many teens consume higher-than-recommended amounts of fat, saturated fat, sodium, and sugars, thereby increasing the risk for adolescent and adult obesity, among other health problems.14 The increased prevalence of excess body weight in adolescents is correlated with escalating risk for type 2 diabetes in this population.15 This does not mean that adolescents are well nourished, however. In spite of their higher energy intake, adolescents frequently fail to achieve required intakes of essential micronutrients (eg, vitamins A and C).16 This problem is compounded by the fact that roughly 60% of female and more than 25% of male adolescents are dieting to lose weight at any given time, and between 1% and 9% report using maladaptive habits, such as purging, to do so.17
Adults in developed countries are at particular risk from excess energy intake. While a significant percentage of Americans (5% to 50%) consume less than half the recommended intake of micronutrients,18 energy balance is typically far in excess of needs. In Western countries, dietary staples (eg, meat, dairy products, vegetable oils, and sugar) are more energy dense than in traditional Asian or African cultures, where grains, legumes, and starchy vegetables are larger parts of the diet. This problem is aggravated by increases in food portion sizes and in the availability and consumption of calorie-dense, nutrient-poor fast foods.19 As a result, this age group is experiencing an epidemic of obesity-related diseases, including coronary heart disease, hypertension, diabetes, and cancer. Metabolic syndrome, often triggered by obesity, is a common problem in elderly persons, one associated with greater risk for premature mortality.20 These circumstances indicate a need for diets that are micronutrient dense, while modest in fat and energy.
The role of nutrition in fertility has been the subject of a limited body of research focusing particularly on the role of antioxidants, other micronutrients, and alcohol. However, while nutritional and lifestyle factors may affect fertility directly, they also influence risk for several diseases that impair fertility, including polycystic ovarian syndrome, endometriosis, and uterine fibroids (see relevant chapters).
In females, some studies suggest a potential role for high-dose (750 mg/day) vitamin C and combinations of antioxidants, iron, and arginine supplements in achieving pregnancy.21 Celiac disease, an immune-mediated condition triggered by gluten, can also impair fertility in women by causing amenorrhea, inducing malabsorption of nutrients needed for organogenesis, and resulting in spontaneous abortion. In affected individuals, fertility may be improved by a gluten-free diet.22
In males, infertility may occur by disruption of the normal equilibrium between the production of reactive oxygen species by semen and oxygen-radical scavengers. This may occur through smoking, infection of the reproductive tract, varicocele,23 and perhaps through poor diet as well. The result is oxidative damage to sperm. Controlled studies of high-dose combinations of supplementary antioxidants (vitamins C, >200 mg/day; vitamin E, 200-600 IU/day; selenium, 100-200 μg/day) improved sperm motility and morphology and increased pregnancy rates, particularly in former smokers.23
Carnitine is concentrated within the epididymis and contributes directly to the energy supply required by sperm for maturation and motility.24 Treatment with carnitine or acetylcarnitine (1.0-2.0 g/day) increases the number and motility of sperm, and the number of spontaneous pregnancies.23,24
Alcohol consumption is associated with decreased fertility in both women25 and men.26 In males, alcohol consumption contributes to impotence and also to a reduction of blood testosterone concentrations and impairment of Sertoli cell function and sperm maturation.26
Infancy and Early Childhood
Requirements for macronutrients and micronutrients are higher on a per-kilogram basis during infancy and childhood than at any other developmental stage. These needs are influenced by the rapid cell division occurring during growth, which requires protein, energy, and nutrients involved in DNA synthesis and metabolism of protein, calories, and fat. Increased needs for these nutrients are reflected in DRIs for these age groups,1 some of which are briefly discussed below.
Energy. While most adults require 25 to 30 calories per kg, a 4 kg infant requires more than 100 kcals/kg (430 calories/day). Infants 4 to 6 months who weigh 6 kg require roughly 82 kcals/kg (490 calories/day). Energy needs remain high through the early formative years. Children 1 to 3 years of age require approximately 83 kcals/kg (990 kcals/day). Energy requirements decline thereafter and are based on weight, height, and physical activity.
As an energy source, breast milk offers significant advantages over manufactured formula. Breast-feeding is associated with reduced risk for obesity, 34 allergies, hypertension, and type 1 diabetes; improved cognitive development; and decreased incidence and severity of infections. It is also less costly than formula feeding.35
Water. Total water requirements (from beverages and foods) are also higher in infants and children than for adults. Children have larger body surface area per unit of body weight and a reduced capacity for sweating when compared with adults, and therefore are at greater risk of morbidity and mortality from dehydration.36 Parents may underestimate these fluid needs, especially if infants and children are experiencing fever, diarrhea, or exposure to extreme temperatures (eg, in vehicles during summer).
Essential fatty acids. Requirements for fatty acids on a per-kilogram basis are higher in infants than adults (see below). Through desaturation and elongation, linolenic and alpha-linolenic acids are converted to long-chain fatty acids (arachidonic and docosahexanoic acids) that play key roles in the central nervous system. Since both saturated fats and trans fatty acids inhibit these pathways,37 infants and children should not ingest foods that contain a predominance of these fats.
Due to reductions in lean body mass, metabolic rate, and physical activity, elderly persons require less energy than younger individuals. Some DRIs for elderly persons differ from those of younger adults. For example, in order to reduce the risk for age-related bone loss and fracture, the DRI for vitamin D is increased from 200 IU/day to 400 in individuals 51 to 70 years of age and from 200 IU/day to 600 IU/day for those >70 years of age. Suggested iron intakes drop from 18 mg per day in women ages 19 to 50 to 8 mg/day after age 50, due to iron conservation and decreased losses in postmenopausal women, compared with younger women.1
Some elderly persons have difficulty getting adequate nutrition because of age- or disease-related impairments in chewing, swallowing, digesting, and absorbing nutrients.40 Their nutrient status may also be affected by decreased production of digestive enzymes, senescent changes in the cells of the bowel surface, and drug-nutrient interactions40 (see Micronutrients). The results can be far-reaching. For example, a study in elderly long-term care residents demonstrated frequent deficiency in selenium, a mineral important for immune function.40 In turn, impaired immune function affects susceptibility to infections and malignancies. The role of vitamin B6 in immunity also presents a rationale for higher recommended intakes for elderly persons.41
Nutritional interventions should first emphasize healthful foods, with supplements playing a judicious secondary role. Although modest supplementary doses of micronutrients can both prevent deficiency and support immune function (see Upper Respiratory Infection), overzealous supplementation (eg, high-dose zinc) may have the opposite effect and result in immunosuppression.42 Multiple vitamin-mineral supplements have not been consistently shown to reduce the incidence of infection in elderly individuals.43 The effects of multiple vitamin-mineral supplementation on cancer risk may be mixed, with some studies showing benefit,44 and others showing increased cancer risk related to supplement use (eg, increased risk for prostate cancer45 and non-Hodgkin-s lymphoma in women).46 Risks may be specific to certain nutrients. For example, high calcium intake has been associated with prostate cancer risk (see Prostate Cancer), while other micronutrients have protective effects.
Alcohol intake can be a serious problem in elderly persons. The hazards of excess alcohol intake include sleep disorders, problematic interactions with medications, loss of nutrients, and a greater risk for dehydration, particularly in those who take diuretics. Roughly one-third of elderly persons who overuse or abuse alcohol first develop their drinking problems after the age of 60 years.47
Pregnancy and Lactation
Pregnant and lactating women have increased requirements for both macronutrients and micronutrients. The failure to achieve required intakes may increase risk for certain chronic diseases in their children, sometimes manifesting many years later.10,11 For instance, studies of the Dutch famine during World War II (in which rations were progressively cut from 1,400 kcal/day in August 1944 to 1,000 kcal/day in December, and ultimately to 500 kcal/day) found that undernutrition during mid- to late pregnancy increased the risk for glucose intolerance and resulted in greater progression of age-related hypertension.27 Malnutrition of women during early pregnancy correlated with higher body weights of their offspring as adults, along with increased risk for coronary heart disease and certain central nervous system anomalies.10,27,28
Protein requirements in pregnancy rise to 1.1 g/kg/day (71 g), amounting to more than a 50% increase in protein intake to allow for fetal growth and milk production. The source of protein may be as important as the quantity, however. Some evidence suggests that protein requirements can be more safely met by vegetable than by animal protein. Meat is a major source of saturated fat and cholesterol; it is also a common source of ingestible pathogens29 and a rich source of arachidonic acid, a precursor of the immunosuppressive eicosanoid PGE2.
Pregnant women also should not meet their increased need for protein by the intake of certain types of fish, such as shark, swordfish, mackerel, and tilefish, which often contain high levels of methylmercury, a potent human neurotoxin that readily crosses the placenta.30 Other mercury-contaminated fish, including tuna and fish taken from polluted waters (pike, walleye, and bass), should be especially avoided.31 There is no nutritional requirement for fish or fish oils. Vegetable protein sources, aside from meeting protein needs, can help meet the increased needs for folate, potassium, and magnesium and provide fiber, which can help reduce the constipation that is a common complaint during pregnancy.
Pregnant and/or lactating women also require increased amounts of vitamins A, C, E, and certain B vitamins (thiamine, riboflavin, niacin, pyridoxine, choline, cobalamin, and folate). Folate intake is especially important for the prevention of neural tube defects and should be consumed in adequate amounts prior to conception; evidence shows that average intakes are only ~60% of current recommendations.32 Folate intakes were noted to be poorest in women eating a typical Western diet and highest in women eating vegetarian diets.33 Pregnant women also require increased amounts of calcium, phosphorus, magnesium, iron, zinc, potassium, selenium, copper, chromium, manganese, and molybdenum.1 Prenatal vitamin-mineral formulas are suggested to increase the likelihood that these nutrient needs will be met.
Requirements for energy and micronutrients change throughout the life cycle. Although inadequate intake of certain micronutrients is a concern, far greater problems come from the dietary excesses of energy, saturated fat, cholesterol, and refined carbohydrate, which are fueling the current epidemics of obesity and chronic disease. Clinicians can assist patients in choosing foods that keep energy intake within reasonable bounds, while maximizing intakes of nutrient-rich foods, particularly vegetables, fruits, legumes, and whole grains.
Change in Nutrient Needs
Increased requirements: energy, protein, essential fatty acids, vitamin A, vitamin C, B-vitamins (B1, B2, B3, B5, B6, B12, folate, choline) & calcium, phosphorus**, magnesium, potassium, iron, zinc, copper, chromium, selenium, iodine, manganese, molybdenum
Increased requirements: vitamins A, C, E, all B-vitamins, sodium, magnesium**
Decreased requirements: iron
Increased requirements: energy, protein, essential fatty acids
Increased requirements: energy, protein, calcium, phosphorus, magnesium, zinc (females only)
Early adulthood (ages 19-50)
Increased requirements for males compared with females: vitamins C, K; B1, B2, B3, and choline; magnesium, zinc, chromium, manganese
Increased requirements for females compared with males: iron
Middle age (ages 51-70)*
Increased requirements: vitamin B6, vitamin D
Elderly (age 70+)*
Increased requirements: vitamin D
Decreased requirements: energy; iron (females only)
* Relative to adult requirements for those 19-50 years of age (and on a per-kg basis for macronutrients).
** Applies only to individuals under age 18.
For detailed nutrient recommendations, see chapters on Macronutrients and Micronutrients.
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