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.
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.
“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.”
“I don’t think adults should be under 1,200 calories a day,” agrees Beatrice Edwards, MD, MPH, associate professor of medicine and director of the Bone Health and Osteoporosis Center at Northwestern University’s Feinberg School of Medicine. “Below that, you’re putting yourself at risk.”
Even if you don’t have an eating disorder, you could be damaging your bones with “disordered eating,” Edwards says.
“I know a lot of women like this. They have very hectic lifestyles -- maybe they’re on the phone with Japan trading stocks and they have a power bar for breakfast, a cup of coffee for lunch, and a Lean Cuisine for dinner,” she says. “It’s not a ‘syndrome,’ but it’s happening, and people who do that are losing not only bone but muscle structure.”
How Can You Lose Weight and Preserve Bone Health at the Same Time?
Try the old-fashioned way, Edwards says. “No grapefruit diets!” She recommends balanced meal plans like those of Weight Watchers and Jenny Craig. “I particularly like Weight Watchers, because they say that after 50, women need three dairy servings a day and calcium supplements.”
If you’re restricting calories over a longer period of time (more than a few weeks), it’s particularly important to pay careful attention to the nutrients you’re getting, Kitchin says. “If you’re cutting out calcium with the calories you cut, that could definitely be an independent risk factor for osteoporosis.”
Whatever your diet, you should be getting 1,000 mg of calcium and 400-800 IU of vitamin D daily if you’re under 50. If you’re over 50, you need 1,200 mg of calcium and 800-1,000 IU of vitamin D daily.
Fortunately, there are plenty of good sources of calcium that won’t blow your diet:
- low-fat dairy products
- dark green, leafy vegetables
- calcium-fortified whole grain cereals
- calcium-fortified juices
- calcium supplements
If you’re a chronic dieter, you may be running low on these nutrients, so it’s even more important to take a calcium supplement.