Infancy, considered as the time of birth until erect posture is assumed, is a highly vulnerable period of life, especially where nutrition is concerned. Infants have high nutrient requirements, are unable to secure food for themselves, and have immature digestive and absorptive functions. In the narrow sense, the focus of nutrition is on meeting nutritional needs to ensure health of the infant. In fact, growth is well recognized as a sensitive, but not specific, indicator of the overall health and nutritional status of infants. It is likely, however, that infant nutrition has long-term health effects. Some parameters that may be affected by nutrition in infancy include cardiovascular health, blood pressure, bone mineralization, low-density lipoprotein cholesterol, split pro-insulin, and cognitive development. Although these observations are tantalizing, they are observational: a causal relationship has not been established. It is likely that genetics and environmental factors also have an effect on health parameters, but present knowledge does not permit us to understand the relative importance of these factors or how they might interact. This chapter uses the definition of a nutrient requirement enunciated by Fomon, that because of practical difficulties in determining the influence of diet on the achievement of optimal health, the requirement for a nutrient usually is defined in a much more limited context: the quantity of the nutrient that will prevent all evidence of undernutrition attributable to the deficiency of the nutrient. Even this limited definition is problematic, because it is not always possible to factor out influences of the environment, genetics, nutrient–nutrient interactions, or nutrient–infant/child interactions.
Several approaches have been used to determine nutritional requirements. These include direct experimental evidence, extrapolation from experimental evidence relating to human subjects of other ages, analogy with the breast-fed infant, metabolic balance studies, clinical observations, and theoretically based calculations. Most recently, in setting the Dietary Reference Intakes (DRIs), the Institute of Medicine (IOM) relied heavily on clinical trials including dose–response, balance, depletion–repletion, prospective observational, case–control studies, and clinical observations in humans. Greater emphasis was placed on studies that measured actual dietary and supplement intake than those that depended on self-reported food and supplement intake. All studies were published in peer-reviewed journals. Nevertheless, for some nutrients, the available data did not provide a basis for proposing different requirements for various life stages or gender groups, most notably children younger than 6 months of age. For infants 0 to 6 months of age only, Adequate Intakes (AIs) ( Table 85-1 ) exist. The AI is based on the reported intake of human milk (780 mL/day), determined by test-weighing of full-term infants in three studies and by the reported average human milk concentration of a specified nutrient after 1 month of lactation. Although this is an intuitively logical approach, it provides information only for breast-fed infants. Human milk is a matrix of interacting factors, and each factor may be more or less biologically available in this matrix compared with the biologic availability of the factor when not in the human milk matrix. This means that there are no reference values applicable to non–breast-fed infants ( Tables 85-2 and 85-3 ). The AIs, based solely on estimates of nutrients in human milk, will result in frank deficiency for some nutrients if those nutrients are fed to non–breast-fed infants at the level of AIs. Furthermore, this approach assumes that the mother has no nutrient deficiency, that all events surrounding the birth were optimal (cord clamping, etc.), and that the mother’s milk has at least the average amount of nutrients. If any of these is not optimal and the infant is not supplemented, nutrient deficiency can occur.
TABLE 85-1
DIETARY REFERENCE INTAKES
Term
Abbreviation
Definition
Estimated Average Requirement
EAR
The average daily nutrient intake level estimated to meet the requirement of half the healthy individuals in a particular life stage and gender group
Recommended Dietary Allowance
RDA
Average daily nutrient intake level sufficient to meet the nutrient requirement of nearly all (97% to 98%) healthy individuals in a particular life stage and gender group
Adequate Intake
AI
Recommended average daily nutrient intake level based on observed or experimentally determined approximations or estimates of nutrient intake by a group (or groups) of apparently healthy people that are assumed to be adequately used when an RDA cannot be determined
Tolerable Upper Intake Level
UL
The highest average daily nutrient intake level likely to pose no risk of adverse health effects to almost all individuals in the general population. As intake increases above the UL, the potential risk of adverse effects increases
Acceptable Macronutrient Distribution Range
AMDR
Range of macronutrient intakes for a particular energy source that are associated with reduced risk of chronic disease while providing adequate intakes of essential nutrients
TABLE 85-2
ESTIMATED ENERGY REQUIREMENTS FOR INFANTS (KCAL/DAY)
