The relationship between belly size and hip size appears to be a more useful measure of health risk than the widely used body-mass index, or BMI.

According to a study published in The Lancet, a calculation comparing waist circumference to hip circumference is a better predictor of heart attack risk than BMI, a measure of weight relative to height. The results have implications for those seeking to assess their health for the new year: a measuring tape may be more useful than a scale.

Body-mass index is often used to screen for obesity and to assess risk for a variety of diseases and conditions, including diabetes, metabolic syndrome and heart attack.

But the Lancet study, described by the authors as the largest and most conclusive to date, found that “BMI is a very weak predictor of the risk of a heart attack,” said Salim Yusuf, lead author and director of the Population Health Research Institute at McMaster University in Hamilton, Ontario. “Measuring the girth of the waist and [the] girth of the hip is far more powerful.”

The authors suggested people forgo calculating BMI. “I’d say just do the waist-to-hip ratio,” Yusuf said. “There really is no additional value [in] doing the BMI.”

But a commentary in the same journal urged caution, noting that the study doesn’t prove a high waist-to-hip ratio causes risk, and it’s not clear how a higher number would lead to a heart attack.

Using any of the methods to assess obesity and risk — BMI, waist-to-hip ratio, or any other measurement — shouldn’t be done without “considering the overall health of the individual,” according to the National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). “However defined, overweight and obesity contribute to the development of a number of debilitating diseases, including arthritis, heart disease and diabetes.”

Good news for large hips

People — even lean people — whose waists are wider than their hips may carry much of their fat in the abdomen, close to organs such as the heart, stomach, liver and kidneys. Why this poses a greater health risk than fat concentrated around the hips is not completely understood, but some experts say they believe the heightened risk has to do with fat so closely surrounding the liver and other organs. Other recent studies have linked waist-to-hip ratio to risk of diabetes and hypertension.

Men with waist-to-hip ratios greater than 1.0 are generally considered to have “excess fat” and be at higher risk for disease, according to the NIDDK. For women, the number is 0.8.

The Lancet study finds a similar link between ratio and risk, but uses a different cutoff point for men to distinguish degree of risk for heart attack.

The findings suggest that men with waist-to-hip ratios greater than 0.95 are at heightened risk for a heart attack; women with ratios above 0.8 are at increased risk, said study co-author Arya Sharma, director of the Canadian Obesity Network. That risk “rose progressively with increasing values for waist-to-hip ratio, with no evidence of a threshold,” according to the study.

A man with a 36-inch waist and 35-inch hips has a waist-to-hip ratio of 1.03 and, according to the Lancet findings, an elevated risk for heart attack. If he were to reduce his waist to less than 33 inches (assuming his hip measure remained unchanged), his ratio would drop to 0.94, putting him at lower risk.

A woman with a 37-inch waist and 39-inch hips would have a waist-to-hip ratio of 0.95, putting her at higher risk, according to Lancet data. Reducing her waist to 31 inches would reduce her ratio to 0.79, taking her out of the high risk group, provided her hip size didn’t increase.

There may also be another, more surprising way to reduce risk: increase hip circumference. The study found a “protective effect” tied to a larger hip measurement; other, smaller studies have noted a similar phenomenon.

The waist is key

“The whole concept is central obesity — a beer belly or a paunch,” said Wm. James Howard, an endocrinologist and vice president for academic affairs at Washington Hospital Center. “You can almost be certain [that] if you have an increased waist circumference, that it’s at least partially … fat deeper in the abdomen, around the intestines and the liver,” he said. Weight carried near the hips is “not negative like the fat within the abdomen,” meaning it doesn’t appear to heighten risk for obesity-linked diseases, Howard continued.

The problem with BMI, experts said, is that it doesn’t take into account where weight is carried.

That means that someone carrying the bulk of their weight at the hips might have as high a BMI as someone carrying a similar amount of weight at the waist.

And someone with a BMI that is “quite low [may] still have increased risk for heart attack based on [the presence of] abdominal fat,” Sharma said.