There is scarce research on the acute effects of exercise on breast milk composition and currently few exercise guidelines for lactating people. The most recent recommendations on postpartum physical activity from the American College of Obstetricians and Gynaecologists merely states that regular aerobic exercise in lactating women has been shown to improve maternal cardiovascular fitness without affecting milk production, composition, or infant growth.32 These recommendations are based on observational studies33, 34 and one small RCT35 from the 1990s analysing total concentrations of lipids, protein, lactose, and some minerals in breastmilk after exercise.
In our study, we found statistically significant increased (⁓ 22%) concentrations of breast milk adiponectin 1 hour after high-intensity training (HIIT) in exclusively breastfeeding people, compared with a day with no exercise (REST), with tendencies of elevated concentrations at all other time-points after both moderate intensity continuous training (MICT) and HIIT. These findings suggest that it may take some time before an exercise-induced increase in breast milk adiponectin concentration is evident, and that HIIT is a more potent stimulus for such increase than MICT.
There could be an alternative explanation for the presented findings of increased adiponectin concentration 1 hour after HIIT. Contrary to our expectation, we observed higher average morning concentration at the day of the REST condition despite randomising the order of HIIT, MICT and REST conditions. Thus, the increase we observed at the MICT and HIIT conditions could have been due to regression to the mean. However, circulating adiponectin concentrations (in serum) have shown to be elevated after exercise. While this is the first study on the acute effects of exercise on adiponectin concentrations in breast milk, breast milk adiponectin concentrations have been reported to correlate with concentrations in blood.9 Presuming that there is a correlation between concentrations in these two body fluids, we may compare our findings to studies on acute effect of endurance training on circulating adiponectin levels.
Elevated blood adiponectin concentrations have been observed after different forms of endurance exercise including rowing, running, cycling, and step-aerobic.36–40 In agreement with our findings, Jürimäe and colleagues found that plasma adiponectin concentrations were unchanged immediately after a maximal 6,000 m rowing ergometer test (lasting on average 20 min) in highly trained male rowers.36 However, circulating concentrations of adiponectin were unchanged immediately after the exercise bout but increased by ⁓ 20% after 30 min of recovery. In contrast to both Jürimäe and colleagues and our present study, Schön and colleagues reported ⁓ 10% elevated serum adiponectin concentrations immediately after a 90-min run at 75–80% of heart rate maximum, followed by a return to baseline concentrations after 60 min of recovery in healthy young individuals.37 There are also studies that suggest no alterations in circulating adiponectin levels actuely after exercise.41–44 The reasons for these diverging findings may include sex-differences, differences in the type of exercise, duration and intensity of exercise, as well as methodological differences in analyses and in time-points of sampling.
As studies report that increased concentrations of adiponectin in breast milk may play a role in protection against early rapid weight gain in infancy14–17, we argue that our findings indicate that maternal exercise during lactation can be a strategy to help prevent childhood obesity. We therefore suggest further studies investigating both the immediate influence of a single exercise session on breast milk composition, as well as chronic adaptations with regular exercise training. We also propose that further research should take into account the whole breast milk matrix, which is composed of many bio-active compounds. We envison that more research on the detailed effects of maternal exercise on breast milk composition will provide an evidence-base for more detailed guidelines from the American College of Obstetricians and Gynaecologists and other organisations.