‘Skinny Fat’ May Carry Hidden Heart Risks That BMI Misses, Study Finds

Why a Normal BMI Doesn’t Guarantee a Healthy Heart, According to New Research

A person can step on the scale, see a “normal” number, and feel reassured about their heart health. New research suggests that reassurance can be dangerously wrong. A large study tracking more than 259,000 adults for two decades found some people with a healthy BMI carry substantially higher heart attack and stroke risk than that number suggests, tied closely to fat around the midsection. The scale may miss what the waist reveals.

Published in the Journal of the American College of Cardiology, the study pulled data from 15 long-running research groups, including the Framingham Heart Study and the Multi-Ethnic Study of Atherosclerosis. Researchers measured waist circumference and waist-to-hip ratio, two simple ways to estimate belly fat, then compared those against BMI to see which better predicted heart trouble over 20 years of follow-up. A tape measure, it turns out, may catch something the bathroom scale never will.

Among people with a normal BMI, roughly 1 in 20 still had a waist circumference large enough to be flagged as high risk, and nearly 1 in 5 had a concerning waist-to-hip ratio. Those numbers describe the “skinny fat” body type: a normal weight on paper, hiding a dangerous amount of fat around the midsection. Among people classified with obesity, about 9% had a waist size in the low-risk range, and 45% had a waist-to-hip ratio in the lower-risk category. Where fat sits on the body may matter just as much as how much a person weighs.

Nearly 260,000 Adults Had Their Waist Size and Heart Disease Risk Tracked

This study is part of the Cross-Cohort Collaboration, pooling data from more than a dozen long-term projects funded largely by the National Institutes of Health. Researchers gathered information on 259,388 adults without a history of coronary heart disease at the start, following them for outcomes ranging from heart attacks and strokes to heart failure, irregular heartbeat, and death.

Participants had height and weight recorded, along with waist circumference, waist-to-hip ratio, or both, then were sorted into normal weight, overweight, or obesity categories. “High” waist circumference meant above 88 centimeters (about 34.5 inches) for women and 102 centimeters (about 40 inches) for men. “High” waist-to-hip ratio meant above 0.85 for women and 0.90 for men.

Women made up most of the sample, a strength researchers noted since past studies leaned heavily male. Participants were around 60 years old on average, and the group included White, Black, Hispanic, Asian, and American Indian adults. Researchers adjusted for other known risk factors, including smoking, diabetes, and high blood pressure, to see whether waist measurements still carried added information.

‘Skinny Fat’ Carried Significantly Higher Heart Risk

One clear finding centered on normal-BMI people with high central fat, the classic “skinny fat” profile. Compared to normal-weight people with a slim waist, those with a large waist or high waist-to-hip ratio faced 15% to 50% higher risk across most of the nine heart outcomes tracked, including heart attacks, strokes, heart failure, and death.

That pattern flipped at the higher end of the BMI scale. Compared with normal-weight people who also had a smaller waist, people with obesity but a low waist circumference generally did not show significantly higher risk, with one exception: overall death risk ran lower, possibly reflecting better fitness or muscle mass in that group.

Waist-to-hip ratio told a different story for women. Those with obesity but a low ratio still faced higher risk for every outcome studied versus normal-weight women with a low ratio, though risk stayed lower than for women with both obesity and a high ratio. Men with obesity and a low ratio showed no added risk, aside from atrial fibrillation.

Researchers also estimated how much of the cardiovascular burden was statistically associated with excess belly fat. Among people with obesity, elevated waist size was linked to nearly half of the estimated burden of heart failure and atrial fibrillation, the largest share of any central fat measure studied.

Waist Size Predicts Heart Disease Risk Better Than BMI Alone

Body fat isn’t distributed evenly, and where it accumulates seems to matter more than total weight suggests. Fat around the abdomen and organs behaves differently than fat elsewhere, releasing inflammatory substances into the blood that can raise blood pressure, worsen cholesterol, and increase insulin resistance, setting the stage for heart disease over time. That’s also why two people with the same BMI can have completely different health outlooks. BMI, a simple ratio of weight to height, cannot tell whether that weight is muscle or fat, let alone where the fat sits. A bodybuilder and a person carrying excess belly fat might share the same BMI despite very different heart risks.

That gap in what BMI can see may also explain a puzzle that has confused cardiologists for years, sometimes called the “obesity paradox“: heart disease patients with a moderately elevated BMI who fare better than expected. Factor in fat distribution, and that apparent protective effect may partly reflect a lower-risk fat pattern hiding underneath a higher number on the scale.

Major medical organizations have already started shifting course. Groups including the International Atherosclerosis Society and a recent Lancet Commission have called for waist measurements alongside BMI, filling gaps BMI cannot see. This study suggests heart disease risk has less to do with the scale and more with the tape measure.

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Disclaimer: This article is based on findings from a peer-reviewed study and is intended for general informational purposes only. It is not a substitute for professional medical advice, diagnosis, or treatment. Anyone with concerns about their heart health or body composition should talk to a qualified healthcare provider.

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