Milk fat provides infants with the essential fatty acids important for the growth of their neurological tissue and cell membranes. It also provides about 50 percent of their energy needs. Scientists need to understand how dietary fat is used to produce human milk, so food recommendations can be formulated for breastfeeding mothers.

Maternal Nutrition and Lactation

Most of the dietary fatty acids consumed by lactating women are (1) converted to energy through oxidation, (2) secreted into milk, or (3) stored in maternal adipose tissues. The Children's Nutrition Research Center has studied what happens to dietary fats in lactating, well-nourished women who consume either a low-fat diet or a high-fat diet. The diets were randomly assigned to 16 women who were nursing their infants.

The results of the study indicate that women on the low-fat diet had a lower concentration of fat in their milk but produced greater amounts of milk. Thus, their daily secretion of milk fat did not change, but their total carbohydrate production increased. The study also indicates that women with more body fat are better able to store dietary lipids and consequently may have difficulty losing weight.

In contrast, Otomi Indian women living in rural Mexico who consume a low-fat diet and have low body fat may produce milk that has a low fat content. Lactation was studied in these women because their infants were growing poorly. The Otomi women consume a low-fat, corn-predominant diet. Although their milk production rates were actually 15-20 percent higher than rates reported for well-nourished women, the concentrations of fat and energy in their milk were lower and may have contributed to the poor growth of their infants.

Infant Nutrition

Growth standards for infants from the National Center for Health Statistics (NCHS), Washington, DC, were derived primarily from formula-fed infants studied 20-50 years ago. Efforts are now under way to revise growth standards for infants. In a study at the Children's Nutrition Research Center, the growth of breast-fed and formula-fed infants was monitored for 9 months. Investigators found that formula-fed infants gained more weight after 3 months than breast-fed infants of the same age.

Until recently, most nutrition studies in infants relied on measurements of weight and length to estimate growth and body composition. Today, however, scientists have new techniques, such as total body electrical conductivity, to measure lean and fat body mass. Surprisingly, initial studies suggest that breast-fed infants may have more body fat than formula-fed infants. Other methods make it possible to measure how infants use the nutrients in the food they eat. Using indirect calorimetry and the doubly labeled water method, investigators have shown that breast-fed infants not only consume fewer calories than formula-fed babies but also expend fewer calories.

Besides differences in how breast-fed and formula-fed infants use calories, there are important differences in their biochemical makeup. Breast-fed infants have higher plasma cholesterol concentrations than formula-fed infants, presumably because of the higher cholesterol content of human milk than formula. The synthesis of cholesterol in breast-fed infants is one-third that of formula-fed infants. Scientists are now trying to determine how cholesterol intake in infancy affects cholesterol levels in adulthood.

Nutrition of the Preterm Infant

The survival rate of preterm infants has increased dramatically in the last several years; at least 90 percent of infants born prematurely survive. Scientists are studying differences in the composition of human and cow's milk to determine the levels of nutrients most suitable for preterm infants. Feeding human milk to preterm infants appears to be advantageous, because (1) the fat content of human milk is more appropriate than that of cow's milk for infant brain development, and (2) the levels of immunoglobulins in human milk may increase an infant's ability to defend against infection. Human milk, however, is "designed" by nature for full-term infants, whose bones are more fully developed, so investigators at the Children's Nutrition Research Center are studying how much additional calcium and phosphorus must be added to human milk to ensure healthy bone growth in preterm infants.

Studying Postnatal Growth and Development

Some infant nutrition questions must be studied in animals, such as the infant pig, to avoid the possibility of harming human subjects. Scientists at the Children's Nutrition Research Center are studying genetically lean and obese piglets to learn how fat and cholesterol are used in early life. They have found that cholesterol levels in genetically obese piglets continue to rise when their diets contain cholesterol, suggesting that these piglets may not be able to shut down cholesterol synthesis when cholesterol is provided in the diet. Studies have also shown that piglets with low levels of plasma cholesterol grow more slowly than piglets with higher levels. Because infant formula has very little cholesterol compared with human milk, both findings are important in designing human infant formulas, whether for full-term or preterm infants.

Rats are also studied to learn more about human infants. Because they are particularly immature at birth, rats are relevant to studies of preterm infants. Investigators have found that before 10 days of age, the weight gain of rat pups consists almost entirely of protein, with very little increase in body fat. This phenomenon can also be seen in preterm infants. From the studies in rats, scientists have learned that adequate nutrition immediately after birth is very important to ensure normal maturation.

Continuing Research

Nutrient utilization in lactating females and their offspring is being investigated through noninvasive techniques and animal models. We have yet to fully understand how maternal diet and nutritional status affect milk composition and how dietary manipulation of infant weight gain, body composition, and serum cholesterol affects health later in life. Studies to ensure that preterm infants get adequate nutrition will enable normal growth and development and promote optimal cognitive and immune functions. Both short- and long-term effects of infant nutrition on the developing organism are important in defining the nutritional requirements of infants.

Mike Espy

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