or no impact on milk volume, but most studies have been too small to test the hypothesis adequately and lacked the design needed for causal inference. Studies of animals indicate that there may be a threshold below which energy intake is insufficient to support normal milk production, but it is likely that most studies in humans have been conducted on women with intakes well above this postulated threshold.
The weight loss ordinarily experienced by lactating women has no apparent deleterious effects on milk production. Although lactating women typically lose 0.5 to 1 kg (˜1 to 2 lb) per month, some women lose as much as 2 kg (˜4 lb) per month and successfully maintain milk volume. Regular exercise appears to be compatible with production of an adequate volume of milk.
The influence of maternal intake of specific nutrients on milk volume has not been investigated satisfactorily. Early studies in developing countries suggest a positive association of protein intake with milk volume, but those studies remain inconclusive. Fluids consumed in excess of thirst do not increase milk volume.
The composition of human milk is distinct from the milk of other mammals and from infant formulas ordinarily derived from them. Human milk is unique in its physical structure, types and concentrations of macronutrients (protein, fat, and carbohydrate), micronutrients (vitamins and minerals), enzymes, hormones, growth factors, host resistance factors, inducers/modulators of the immune system, and anti-inflammatory agents.
A number of generalizations can be made about the effects of maternal nutrition on the composition of milk (see also Table 1-1):
Even if the usual dietary intake of a macronutrient is less than that recommended in Recommended Dietary Allowances (NRC, 1989), there will be little or no effect on the total amount of that nutrient in the milk. However, the proportions of the different fatty acids in human milk vary with maternal dietary intake.
The concentrations of major minerals (calcium, phosphorus, magnesium, sodium, and potassium) in human milk are not affected by the diet. Maternal intakes of selenium and iodine are positively related to their concentrations in human milk, but there is no convincing evidence that the concentrations of other trace elements in human milk are affected by maternal diet.
The vitamin content of human milk is dependent upon the mother's current vitamin intake and her vitamin stores, but the strength of the relationships varies with the vitamin. Chronically low maternal intake of vitamins may result in milk that contains low amounts of these essential nutrients.