How does breastfeeding protect the mother's bones?
Every time that I look up breastfeeding benefits for the mom I get these main 3 along with others:
Decreased chance of breast cancer
Decreased chance of ovarian cancer
Decreased chance of osteoporosis(put another way, increased bone density)
This seems counter intuitive though, not the cancer protection but the osteoporosis protection. Your body takes calcium from your bones when calcium is in increased demand. Typically a mom won't eat enough calcium to prevent this bone breakdown. In pregnant women this is significant because you have an unborn baby inside you whose bones are being ossified.
But it would seem to me that breastfeeding would also increase the bodily demand for calcium because an ounce of breastmilk contains 10 mg of calcium and a breastfeeding woman can produce up to a liter of breastmilk a day. That is 33.814 ounces or 338 mg of calcium for enough breastmilk to feed the mom's baby/babies. That is a lot of calcium just for breastfeeding. Not to mention that the needed intake for anyone between 18-50 is 1000 mg of calcium. This means with the calcium needed for breastmilk production, you get a net intake of around 750 mg of calcium. That is a net deficiency so you get bone breakdown.
So it would make sense then that the ideal calcium intake for a breastfeeding woman would be 1350 mg of calcium due to the amount of calcium in breastmilk and the fact that a woman can produce up to a liter of breastmilk in 1 day. That way you don't end up with bone breakdown.
So if up to 338 mg of calcium goes into making breastmilk, why do breastfeeding women with an intake of 1000 mg of calcium which is what is needed for the average 18-50 year old human, not get osteoporosis but instead increased bone density? Do oxytocin and prolactin somehow divert more calcium towards bone(like do they increase active vitamin D levels) to offset the fact that about a third of the calcium that is needed is diverted to the breasts for milk production?
Because I don't see how you could possibly end up with an increase in bone density when 1/3 of your required calcium is diverted into breastmilk for the baby.
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I can understand your confusion, since the literature has confounding data. However, I would suggest that you expand your reasoning beyond only thinking about calcium demand and consider hormonal changes as well. Regardless, a newer paper that recently came out from Hwang et al. (2016) has some great information. They cite the papers that have shown protective effects as well as the papers that show deleterious effects. It is a complex issue, since the length of time of the breastfeeding matters as well as past and present medical history; it would be important to correct for differences in exercise, smoking, and diet; and age of breastfeeding as well as age of menarche are important as well.
References
Hwang, I. R., Choi, Y. K., Lee, W. K., Kim, J. G., Lee, I. K., Kim, S. W., & Park, K. G. (2016). Association between prolonged breastfeeding and bone mineral density and osteoporosis in postmenopausal women: KNHANES 2010-2011. Osteoporosis International, 27(1), 257–265. doi.org/10.1007/s00198-015-3292-x
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