With commentary by lead study author Pam Hinton, PhD, associate professor of nutrition and exercise physiology at the University of Missouri.
It was once thought that carrying some extra weight actually helped build stronger bones. The theory went that bones get stronger when they bear weight (as they do when you do weight bearing exercises or strength train). But more recently, studies have revealed that being obese or having type 2 diabetes can actually lead to an increased risk of low bone density and fractures.
Research has shown that obese people have reduced bone density relative to their body weight, as well as an increased risk of fractures even in those with normal bone density. In addition, type 2 diabetes and insulin resistance are also associated with an increased risk of fractures.
A new animal study published in the journal Metabolism confirmed these results, showing that obesity and type 2 diabetes led to weakened bones. But the study also found that exercise not only prevented weight gain and diabetes in the obese, but it increased bone strength. In other words, the exercise helped prevent low bone density even in the obese animals.
Pam Hinton, PhD, associate professor of nutrition and exercise physiology at the University of Missouri, and the lead author of the study, wanted to look at how obesity and type 2 diabetes affect bone structure, bone formation and strength. When her team compared obese rats that were forced to be sedentary with those made to exercise, they found that the sedentary rats accumulated less bone mass relative to their body weight than the exercising rats.
The sedentary obese rats had decreased bone formation, loss of bone mass and decreased bone strength. But this wasn’t the case for the exercising obese rats. “Exercise completely prevented the negative effects of obesity and diabetes on bone,” says Hinton. “In some instances, the rats in the exercise group had better bones than the lean controls that did not exercise.”
Numerous mechanisms have been hypothesized to explain why excess weight might hurt bone quality, including the effects on the osteoblast cells that build bone and the osteoclast cells the break down bone. Bones are continually renewing themselves, being built up and broken down, but differences in the speed of these two processes can lead to bone loss. “We found that bone formation was reduced in the obese rats in the early stages of diabetes, while bone breakdown was increased three-fold in obese and diabetic rats,” says Hinton. “Over time, these changes would lead to loss of bone mass and weakening of the bones.”
“What we’ve come to realize is that the bone of people with obesity and type 2 diabetes isn’t good quality bone,” says Hinton. These individuals have an increased risk of fractures, so that extra body weight isn’t protective.”
It’s already known that exercise strengthens bone. This study shows that exercise can help prevent not only the onset of diabetes in the obese, but poor bone quality and density.
The best type of exercise for bones is weight bearing activities like jogging or weight lifting.