Weight loss and bone loss can sometimes go hand in hand.
Doctors know that women with anorexia, who severely restrict calories for a long time, are at increased risk for osteoporosis. The eating disorder interferes with hormones needed to maintain bone, not to mention the foods people need to build bone.
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But what if you don’t have anorexia? What’s the relationship between osteoporosis and normal dieting? How do you know if you’re at risk for bone loss? What kind of dieting is safe for your bones?
Those are hard questions to answer, says Felicia Cosman, MD, medical director of the Clinical Research Center at Helen Hayes Hospital in Haverstraw, N.Y., and an editor of Osteoporosis: An Evidence-Based Guide to Prevention and Management.
When Does a Diet Put You at Risk for Osteoporosis?
Overall, the best way to protect your bones while losing weight is to exercise regularly and eat healthy foods. What if you don’t want to take the slow but sure route to weight loss?
“[If you are] crash dieting for a few weeks for your wedding or some other big event, there probably won’t be any major impact -- although I would always advise making sure you get adequate calcium and vitamin D during that time period,” Cosman says.
But people who try to maintain a very thin weight throughout their lifetime are definitely at higher risk of osteoporosis. What’s “thin?” Experts define it as a body mass index of 18.5 or below.
“BMI seems to have the strongest relationship with bone,” says Beth Kitchin, MS, RD, assistant professor in the department of nutrition sciences at the University of Alabama-Birmingham. “Below a normal BMI, that’s where you see an impact on bone density. It’s a continuum, of course: someone with a BMI of 19 or 20 still might have lower bone mass than average, even though their BMI is within the healthy range.”
Cosman suggests that weight -- independent of BMI -- is a predictor of bone loss. “Being thinner than about 127 pounds, in many studies, does seem to be associated with some risk of osteoporosis-related fracture,” Cosman says.
She hastens to add that she’s not suggesting people become overweight to prevent bone loss.
“It’s just important to know that, if you have a thin frame, you need to pay particular attention to building and maintaining healthy bone.”
The Impact of Crash Dieting on Bone Health
Even if you’re not “small-boned” or particularly thin, long-term “crash” dieting can have an impact on your bone health, Cosman says.
“If you go for six months or so eating 800 or 900 calories a day, that’s likely to be bad for your bones. I would say that a threshold of at least 1,200 calories per day is about what’s needed to maintain your bones and tissues. If your caloric intake is significantly below that for an extended period of time, you’re probably doing damage.”