Effects on long-chain polyunsaturated fatty acid concentrations in breast milk

Nutrition Research Newsletter, August, 2005

It is well-established that breast milk is the optimal food for infants, providing all the nourishment they need to grow physically and mentally for the first 6 months of life. Long-chain polyunsaturated fatty acids (LC-PUFA), specifically a-linolenic acid (LNA, 18:3 n-3), linoleic acid (LA, 18:2 n-6), docosahexaenoic acid (DHA, 22:6 n-3), eicosapentaenoic acid (EPA, 20:5 n-3), and araehidonic acid (AA, 20:4 n-6) are some of the many components in human milk that promote infant growth and development. LNA and LA are essential fatty acids that can be desaturated and elongated into the longer chain fatty acids EPA, DHA, and AA. DHA is an important nutrient in breast milk that is crucial for neural and visual development of the infant. During lactation, 70 mg to 80 mg of DHA is utilized per day for breast milk production. AA is necessary for infant growth and development. Intake of LC-PUFA by the breast-fed infant depends on maternal diet, body fat stores, and possibly also the mother's activity level. Dietary fatty acids of lactating women can be utilized in three ways: (a) stored in the adipose tissue, (b) transferred to the mammary gland for incorporation into milk, and/ or (e) used for energy, especially during exercise. Exercise mobilizes fatty acids from body stores for energy. Increased metabolism during exercise may lead to a decrease in concentrations of LC-PUFA available for incorporation into breast milk. However, the increased loss due to oxidation of the fatty acids may be countered by an increase in fatty acid mobilization from adipose stores. This phenomenon has not been investigated in lactating women.

Current recommendations for the general population from the CDC and the American College of Sports Medicine are to exercise moderately 30 minutes per day, every day of the week, whereas the Institute of Medicine recommends incorporating 60 minutes of moderate exercise per day to achieve optimal health benefits. In addition, the American Academy of Pediatrics and the American Dietetic Association recommend that all women breastfeed their infants during the first year of life. However, there is a paucity of research about the effects of exercise by lactating women on the composition of their breast milk. Therefore, the purpose of this study was to investigate the effects of chronic and acute exercise on plasma and breast milk LC-PUFA. Another objective was to determine if breastfeeding women consume adequate amounts of LC-PUFA to compensate for those used for energy during exercise.

Healthy--absence of chronic disease, nonsmoking, exclusively breastfeeding women were recruited from prenatal classes and obstetricians' offices. Participants were eligible if: (a) their BMI was between 20 and 30, (b) their infants' birth weight was more than 2500 g, and (c) there were no birth complications. Mothers were assigned to one of two groups based on their self-reported exercise history. The exercise group consisted of mothers who exercised at least 30 min 3 days/week for the past 6 weeks (n= 30). Women were assigned to the sedentary group if they exercised once per week or less (n= 23). A subsample (n= 14) of women from the exercise group completed a second study examining effects of acute exercise, in which subjects participated in an exercise session and a rest session.

Long-chain-PUFA are essential for infant growth and development. The amount of LC-PUFA in breast milk depends on maternal diet and body stores. Because exercise increases mobilization and utilization of fatty acids, maternal activity may also influence the amount of LC-PUFA in breast milk. LC-PUFA in plasma and breast milk were measured at 12 weeks postpartum, in exercising and sedentary women. Dietary intake was recorded for 3 days. A subsample of women participated in exercise and rest sessions to examine the acute effects of exercise on breast milk LC-PUFA.

There were no differences in dietary intake between the two groups. Mean intake of LA was 11.05 [ or -] 1.39 and 9.34 [ or -] 0.97 and LNA was 0.96 [ or -] 0.12 and 0.824 [ or -] 0.09 g/day by the sedentary and exercise groups, respectively. These amounts are close to the adequate intakes of LA and LNA for lactation (13 g/day and 1.3 g/day, respectively). No differences were found in LC-PUFA in plasma and breast milk between groups. After 30 minutes of exercise, there was a trend for an increase in LA and LNA concentrations in breast milk, with no change in DHA, EPA, and AA concentrations.

In this study, dietary intake and exercise did not significantly affect the plasma and breast milk of lactating women. However, there was a trend toward an increase in LA and LNA after an acute exercise bout. With the exception of AA intake, there were no differences in the diets of the women in this study. On average, these women ate diets sufficient in energy, protein, and fat, and were similar to dietary intakes of lactating women reported by other researchers. It is possible that the observation of no effect of diet or exercise on the LC-PUFA concentrations (except AA) in breast milk was due to insufficient variation in intakes among the women.