Obesity is associated with an increased risk of premature death, but which part of the body carries the added fat can make a big difference. Extra weight in some places can actually lower the risk.
Researchers who wrote in BMJ reviewed 72 prospective studies that enrolled more than two and a half million participants with data on body fat and mortality. They found that central obesity – a large waistline – was consistently associated with a higher risk for all-cause mortality. In pooled data from 50 studies, every 4 inch increase in waist size was associated with an 11 percent increased relative risk of premature death. The association was significant after adjusting for smoking, physical activity, and alcohol consumption.
The waist size is an indicator of the amount of visceral fat, or fat, that is stored in the abdomen around the internal organs. This type of fat is linked to an increased risk of heart disease, type 2 diabetes, cancer, and Alzheimer's disease.
However, an increase in fat in two places appears to be associated with a lower risk of death. Three studies showed that every two-inch increase in thigh circumference was associated with an 18 percent lower risk for all-cause mortality. In nine studies involving nearly 300,000 participants, increasing a woman's hip circumference by 4 inches was associated with a 10 percent lower risk of death.
"Thigh size is an indicator of the amount of muscle that is protective," said Tauseef Ahmad Khan, co-author of the review, 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 body measurements reveals even more information about the risk of premature death. If two people have the same hip size, the person with the larger waist is at greater risk of premature death.
For example, imagine a man with a waist of 34 inches and hips of 37 inches and another with the same hip size but a waist of 41 inches. The latter's relative risk of death was nearly 50 percent higher, according to the researchers. 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 the waist-to-hip ratio was with a 20 percent higher relative risk of death and association associated in women than in men.
It is unclear whether there is a risk of a waist that is too small. "There is an area in these measures," said Dr. Khan, "an area where these numbers are beneficial." Above this range there is more risk, but more research needs to be done on lower ranges. "
Losing excess weight is, of course, desirable, but there is likely no way to redistribute weight or lose weight at the waist alone. "It doesn't work that way," said Dr. Khan. "You have to reduce the overall weight, and that also reduces the central fatness."
There is a way to combine many of these different factors into a single measurement by using a formula called A.B.S.I. or a body shape index is used. This calculation does not only include weight and height like B.M.I. or body mass index, but also age, gender and waist size. It could provide a more accurate estimate of the risk, say the authors.
A.B.S.I. is mainly used as a research tool, but anyone can calculate it here: https://www.fatcalc.com/absi. Any increase in the A.B.S.I. by 0.005 units. was associated with a 15 percent higher risk for all-cause mortality.
"The takeaway message is, watch your waist size," said Dr. Khan. "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 you are at high risk."