Moderate exercise isn't thought to affect breast milk quantity or quality, or your baby's growth. If you're breastfeeding, it's important to stay hydrated. Keep a water bottle handy during your workout, and drink plenty of fluids throughout the day.
Some research suggests that high-intensity exercise might cause lactic acid to accumulate in breast milk and produce a sour taste a baby might not like, but this is likely rare.
If vigorous exercise is a priority during the first few months of breastfeeding, consider feeding your baby before your workout or pumping before your workout and feeding your baby the pumped breast milk afterward. Alternatively, exercise first and then take a shower, express a few milliliters of breast milk and, after a half-hour or an hour, offer the breast.