In a retrospective study published in Osteoporosis International, researchers found that mothers who breastfed in total more than 33 months had greater bone strength than mothers who had breastfed less than 12 months. This improved strength was not attributed to a greater bone density later in life but to a larger bone size.
Bone remodeling activity increases during pregnancy and results in a lowering of the mothers bone density by approximately 5%. This loss of bone density is typically regained after weaning and does not affect bone density later in life. What is fascinating is that lactation actually causes bones to grow wider. Estrogen levels drop during lactation similar to those seen in women when they go through menopause. This drop in estrogen promotes outer cortical bone growth leading to an enlargement in bone diameter. Add diameter to bone and you get greater strength.
To illustrate this concept we can look at bicycle frames and to see why size is correlated with strength. In order to reduce frame weights, bicycle manufacturers have enlarged the tubes of bikes allowing them to make the tubes thinner and lighter. The larger the tube the better the tube is at resisting bending and breaking. The same is true when bone enlarges, it becomes stronger. This is exactly what happens during lactation and breastfeeding.
The authors of this Finish study concluded that lactation-associated estrogen deficiency causes bone enlargement and that the duration of breastfeeding impacts both size and strength of a mother’s bones later in life.
Wiklund, et al. 2012. Lactation is associated with greater maternal bone size and bone strength later in life. Osteoporosis International 23:1939-1945.