The lure of "thinner thighs" has been used to market countless books, DVDs, and diets over the years. If new research published today is confirmed by other studies, though, perhaps there should be an addendum: " . . . but not too thin." Writing in the medical journal BMJ, Danish researchers who followed 2,800-plus people for longer than a decade reported that—believe it or not—a smaller thigh circumference is associated with a bigger risk of heart disease and premature death. This effect was independent of body fat percentage and BMI.
Before you back up your pickup truck to the Ben & Jerry's factory, however, consider the details. First, it's not the case that the bigger the thighs, the lower the risk, no matter what. People with thighs measuring less than 60 centimeters (a little more than 23½ inches) around were at higher risk of heart problems and early death than those with larger thighs, but no extra benefit accrued to either men or women whose thighs were more than 60 centimeters around. (And that's not that big—go measure yourself, just below the point where your seat meets your legs.)
Even if your thighs are smaller than the cutoff, you do not have permission to eat until they reach that milestone. This study—as well as similar research that found an association between wider hips and a lower rate of heart disease and early death—doesn't pinpoint the mechanism that might be behind the link.
It might be that thigh and hip fat protects against heart disease. "Fat tissue in the lower body has a greater capacity for producing substances beneficial to health," such as anti-inflammatory compounds, says Berit Heitmann, director of research at the Research Unit for Dietary Studies at Copenhagen University Hospital's Institute of Preventive Medicine. Heitman was an author of the new study.
But another hypothesis is that larger muscles—in the thighs and in the rear end, in the case of hip circumference—are the protective element. Other research shows that low amounts of muscle mass in the lower body may mean the body is less able to use insulin to help process glucose, possibly leading to type 2 diabetes and perhaps, eventually, heart disease. It may not be an either-or thing, says Heitmann; the mechanism might differ by gender. The muscle hypothesis might be further teased out by looking at whether people who strength-train and therefore tend to develop more muscle mass are at a lower risk of death than people who do an equivalent amount of aerobic exercise, says Francisco Lopez-Jimenez, a cardiologist at the Mayo Clinic in Minnesota. In order to play a useful role in assessment of heart disease risk, in any case, the thigh association has to be confirmed by other studies and be shown to add something to the calculus already used to estimate heart disease risk .
In the meantime, heart disease experts like the American Heart Association advise aiming for a healthy BMI of between 18.5 and 24.9, which is an admittedly imperfect but still useful tool to gauge your body fat. Other measures of fatness include your waist circumference (larger than 40 inches for a man or 35 for a woman is no good) and your waist-to-hip ratio; both of those point at the notion that fat around the waist is what really matters in terms of disease risk. And even without proof that muscle mass protects against heart disease, there's no downside to strength-training your lower body (and the rest of you), since its benefits are legion, including staving off sarcopenia, an age-related decline in muscle mass and function. As my colleague Deborah Kotz reported last week, building strong thighs also may prevent the pain of arthritis. Here are some exercises to get you started.