Bone Mineral Density and Post natal Physical Activity
A limited number of studies have commented on the combined effect of physical activity and lactation on bone mineral density (BMD). A randomised-control trial of small sample size published by Lovelady et al, (2009) concluded that 16 weeks of a prescribed, graded, resistance and aerobic exercises minimised the losses of lumbar-spine BMD during lactation in healthy, non-obese, exclusively breastfeeding women from 2-3 weeks postpartum when compared to controls (p < 0.05).  The clinical significance of this difference is unclear.
A similar study published by the same group in 2012 concluded that moderate energy restriction, walking, and resistance training are safe methods for weight loss in overweight fully breast-feeding women with no adverse effects on BMD and infant growth. 
In comparison, data from a small observational study published in 1998 suggests that regular, self-selected, recreational exercise has no impact on early postpartum lactation-induced BMD loss.  It is hypothesised that this was in part due to the comparative lack of resistance exercise chosen by the study participants.
Further research is needed to examine the effects of the intensity, duration, and frequency of exercise on bone density in non-overweight and overweight/obese breastfeeding women.
Quality of evidence
Strength of recommendation
Based on the limited available evidence, it is recommended that participation in moderate-intensity physical activity does not appear to have a deleterious effect on BMD in breastfeeding mothers, and that participation in a graded resistance programme may even be protective of lumbar spine lactation-associated BMD loss.
- Lovelady, C. A., Bopp, M. J., Colleran, H. L., Mackie, H. K., & Wideman, L. (2009). Effect of exercise training on loss of bone mineral density during lactation. Med Sci Sports Exerc, 41(10), 1902-7.
- Colleran, H. L., Wideman, L., & Lovelady, C. A. (2012). Effects of energy restriction and exercise on bone mineral density during lactation. Med Sci Sports Exerc, 44(8), 1570-1579.
- Little, K. D. (1998). Self-selected recreational exercise has no impact on early postpartum lactation-induced bone loss. Medicine and science in sports and exercise, 30(6), 831-836.