Being overweight is linked to an increased risk for premature death, but which part of the body carries the added fat could make a big difference. Extra weight in some places may actually lower the risk.

Researchers, writing in BMJ, reviewed 72 prospective studies that included more than two and a half million participants with data on body fat and mortality. They found that central adiposity — a large waist — was consistently associated with a higher risk of all-cause mortality. In pooled data from 50 studies, each four-inch increase in waist size was associated with an 11 percent increased relative risk for premature death. The association was significant after adjusting for smoking, physical activity and alcohol consumption.

Waist size is an indicator of the amount of visceral fat, or fat stored in the abdomen around the internal organs. This kind of fat is associated with an increased risk for heart disease, Type 2 diabetes, cancer and Alzheimer’s disease.

But increased fat in two places appears to be associated with a lower risk of death. Three studies showed that each two-inch increase in thigh circumference was associated with an 18 percent lower risk of all-cause mortality. In nine studies involving almost 300,000 participants, a four-inch increase in a woman’s hip circumference was associated with a 10 percent lower risk of death.

“Thigh size is an indicator of the amount of muscle, which is protective,” said a co-author of the review, Tauseef Ahmad Khan, a postdoctoral fellow at the University of Toronto. “And hip fat is not visceral fat, but subcutaneous fat, which is considered beneficial.”

Comparing waist size with other bodily measurements reveals still more information about the risk for premature death. Given two people with the same hip size, the person with the larger waist is at higher risk for premature mortality.

For example, consider one man with a 34-inch waist and 37-inch hips, and another with the same hip size but a 41-inch waist. The latter’s relative risk of death, the researchers found, was almost 50 percent higher. Small changes in this waist-to-hip ratio make a big difference: In 31 studies that reported the ratio, each 0.1 unit increase in waist-to-hip ratio was associated with a 20 percent higher relative risk of death, with a stronger association in women than in men.

It is unclear if there is a risk in having too small a waist. “There is a range in these measures,” Dr. Khan said, “a range in which these numbers are beneficial. Above that range, there is higher risk, but more research has to be done about lower ranges.”

Losing excess weight is of course desirable, but there is probably no way to redistribute weight, or lose weight in the waist alone. “It doesn’t work that way,” said Dr. Khan. “You have to reduce overall weight, and that also reduces central fatness.”

There is a way to put many of these various factors together in a single measurement, using a formula called A.B.S.I., or a body shape index. This calculation includes not only weight and height, like B.M.I., or body mass index, but also age, sex and waist circumference. It may produce a more accurate estimate of risk, the authors say.

A.B.S.I. is used mostly as a research tool, but anyone can calculate it here: https://www.fatcalc.com/absi. Each 0.005 unit increase in A.B.S.I. was associated with a 15 percent higher risk of all-cause mortality.

“The takeaway message is watch your waist size,” Dr. Khan said. “It’s more important than a simple measure of weight. You can have a normal weight and B.M.I., but if your waist is large, that puts you at high risk.”