Does exercise intensity or diet influence lactic acid accumulation in breast milk? Med. Sci. Sports Exerc., Vol. 31, No. 1, pp. 105-110, 1999.
Purpose: This study examined the relationships among diet, exercise intensity, and breast milk composition in lactating women.
Methods: Twelve lactating women were randomly assigned to either a high (N = 6; 5.03 g carbohydrate (CHO)·kg body mass (BM)−1) or moderate (N = 6; 3.89 g CHO·kg BM−1) carbohydrate diet. Milk and blood samples were collected before and after a nonexercise session (control) and maximal, lactic acid-threshold (LAT), and 20% below the LAT (LAT-20) intensities.
Results: The 30-min exercise LAT bout was more stressful than the 30-min LAT-20 bout (rating of perceived exertion (RPE) = 15 vs 12, respectively, P < 0.05). Milk LA was significantly higher at 0 min following maximal exercise in the high and moderate CHO groups (1.27 ± 0.56 and 1.52 ± 0.49 mM, respectively) and following LAT exercise (0.19 ± 0.16 and 0.25 ± 0.12 mM, respectively), when compared with the control session (0.08 ± 0.03 and 0.09 ± 0.05 mM, respectively). This was not observed following the LAT-20 exercise in the high and moderate CHO groups (0.11 ± 0.04 and 0.12 ± 0.08 mM, respectively). Elevated milk LA persisted in the 30-min collection point after maximal exercise only. There was no significant effect of dietary treatment on milk or blood LA at any of the collection points.
Conclusions: In lactating women whose caloric needs are being met: 1) dietary CHO intake, within a practical range, does not influence LA levels in breast milk at rest or after exercise; 2) LA appearance in the milk is a function of exercise intensity; and 3) moderate intensity exercise (RPE = 12) will not increase breast milk LA levels.