Recommendations on dietary nutrient intakes for lactating women by different scientific authorities are typically based on the estimated total amount of each nutrient secreted daily into breast milk, taking into account, where known, the efficiency of milk synthesis and the bioavailability of the nutrient in the maternal diet. This estimate for each nutrient is then added to the recommended nutrient intake for non-pregnant, non-lactating women.
The onset of lactation after parturition is brought about by the major hormonal changes that occur in this period. During the first 2-7 days post-partum a thick yellow fluid (colostrum) is secreted. With the progress of lactation, the volume of milk secreted increases and its nutrient composition changes, with an increase, decrease or no change in concentration, depending on the nutrient. After about 21 days the milk secreted is considered mature milk. The volume of breast milk secreted daily increases rapidly in the first post-partum days, being ^500 ml on day 5, ^650 ml at 1 month, and ^750 ml at 3 months, remaining relatively stable during full lactation but decreasing during weaning. In industrialized countries, the average volume of breast milk produced is 750-800 ml day-1 in the first 4-5 months postpartum and decreases to 600 ml day-1 during 6-12 months after delivery. In this period, the volume of milk produced may be even lower and more variable, depending on the weaning practices adopted.
The FAO/WHO and DRI committees considered 750 and 780 ml, respectively, as the average milk volume produced during full lactation and the basis for recommendations. For most nutrients, average concentration in mature milk multiplied by the average milk volume was used to estimate the total amount of nutrient secreted daily into breast milk. A correction factor was then applied to account for the nutrient bioavailability in the maternal diet and, where known, for the anabolic cost of milk synthesis, and the final value was added to the recommended intake of nonpregnant, nonlactating women. The stage of lactation was considered to be a factor for some nutrients and, where applicable, separate values were given according to the period of time post-partum.
The volume of milk secreted during lactation is not influenced by maternal nutritional status, unless maternal undernutrition is severe. The composition of breast milk for most nutrients is adequate to support infant growth and development in a wide range of maternal nutritional status. However, maternal diet and nutritional status do have an influence on the concentration of some micro-nutrients such as vitamin A, thiamin, riboflavin, vitamins B6 and B12, iodine, and selenium. Also, the fatty acid composition of breast milk can be affected by maternal diet.
An important step taken by the DRI committees when setting recommendations was taking maternal age into account, thus giving separate values for adolescent (<18 years) and adult (19-50years) lac-tating women. For some nutrients, adolescent lactat-ing women may have greater requirements than adult women because they are still growing and they need to cover their own nutrient demands. Recommendation of intakes during lactation of calcium, phosphorus, magnesium, iron, and zinc are higher for adolescent than for adult women.
In general, there is considerable uncertainty in establishing dietary nutrient recommendations for lactation due to high intra- and interindividual variability in breast milk volume output and in several specific nutrient concentrations in breast milk, and to temporal changes in milk volume and nutrient concentrations during the lactation period. The composition of breast milk is affected by several factors depending on the nutrient, such as stage of lactation, changes during nursing, diurnal rhythm, maternal diet, gestational age at birth, and parity. Moreover, the total amount of nutrients secreted into breast milk depends on the extent and duration of breast feeding. In addition, physiological adaptation to the increased nutrient lactation demands such as increased nutrient absorption and conservation, and use of maternal nutrient stores, which are quite specific for each nutrient and not easily quantified, contributes to the degree of uncertainty. Maternal age and maternal nutritional status during pregnancy and lactation may influence the homeostatic adaptations during lactation such as the efficiency of nutrient absorption and the degree of mobilization of maternal nutrient stores. These factors are not well known and are difficult to quantify.
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