Study of nearly 260,000 people finds that excess abdominal fat raised cardiovascular risk even at normal or overweight BMI; Sheba cardiologist Dr. Anat Berkovich says where fat is stored may matter as much as how much there isFor decades, body mass index, or BMI, has served as a convenient shorthand for assessing obesity and the health risks that come with it. But a large study published in the Journal of the American College of Cardiology suggests the measure may leave out an important part of the picture: where fat is stored in the body.Among people classified as having normal weight or overweight by BMI, those with a high waist circumference or waist-to-hip ratio had a 15% to 50% higher risk for most of the cardiovascular outcomes examined, including heart attack, stroke, heart failure and atrial fibrillation.GalleryNot just how much fat, but also where it accumulates(Photo: Shutterstock)The findings suggest that cardiovascular risk assessment should take into account not only body weight, but also the distribution of body fat.BMI has long been used as a simple, accessible tool for an initial assessment of weight status and the health risks associated with it. “BMI has repeatedly been shown to be an independent predictor of cardiovascular disease,” said Dr. Anat Berkovich, deputy director of the Cardiology Division at Sheba Medical Center. “Across most weight ranges, an increase in BMI is associated with an increase in cardiovascular risk. Based on the result, patients are classified as normal weight, overweight, obese or severely obese.”The problem is that BMI cannot tell the whole story. It measures weight relative to height, but does not distinguish between fat and muscle or reveal where fat is concentrated in the body.“We’ve known for years that BMI has limitations,” Dr. Berkovich said. “For some people, it does not accurately reflect body fat. A muscular athlete, for example, may have a high BMI because of muscle mass despite having very little body fat. So BMI is not equally reliable for everyone.”Where fat accumulates also matters. Abdominal fat includes visceral fat, which surrounds internal organs and is more metabolically active than subcutaneous fat, the fat stored beneath the skin. “It has a very significant metabolic role,” Berkovich said. “It secretes adipokines, cytokines and other biologically active substances that affect metabolism and inflammatory processes and increase cardiovascular risk. In the past, we may have thought of it as simply something sitting there. Today, we understand that it is highly active tissue.”Waist circumference does not directly measure visceral fat, but it serves as a useful indirect marker of abdominal fat accumulation.Medical guidelines also recognize that a single number cannot tell the whole story. “The 2021 ESC guidelines already emphasized the need to include measures of central obesity, such as waist circumference, and the latest guidelines place even greater emphasis on this,” Dr. Berkovich said. “Sports cardiology guidelines also note that in some cases BMI alone is not enough and should be combined with other measures of body composition and central obesity.”Abdominal fat can reveal risk that BMI does not always capture(Photo: Shutterstock)According to ESC guidelines, a healthy waist circumference is considered up to 94 centimeters (about 37 inches) for men and 80 centimeters (about 31.5 inches) for women. Cardiovascular risk rises above those levels and is particularly high above 102 centimeters (about 40 inches) for men and 88 centimeters (or about 34.5 inches) for women.That is where the new study comes in. Rather than simply dividing participants into normal weight, overweight and obesity groups, researchers examined people whose BMI and abdominal-fat measurements told different stories.They compared, for example, people with a normal BMI but a high waist circumference with those whose BMI suggested obesity but whose waist measurements were relatively low.The analysis pooled data from 15 prospective cohort studies involving 259,388 men and women who did not have coronary heart disease at the outset. Most had waist-circumference data, while more than 80% also had waist-to-hip measurements.Participants were followed for a median of 20 years. “That is an exceptionally long median follow-up and one of the study’s major strengths,” Dr. Berkovich said.Dr. Anat BerkovichPhoto: Sheba Medical CenterResearchers examined nine outcomes, including fatal and nonfatal heart attack and stroke, heart failure, atrial fibrillation, coronary heart disease, cardiovascular disease overall, coronary heart disease death, cardiovascular death and death from any cause.“The association with abdominal-fat measures was highly significant across the different categories,” Berkovich said. “The pattern was consistent across the outcomes examined.”The data showed how often BMI and measures of abdominal fat pointed in different directions. Among people classified as normal weight by BMI, 5% had a high waist circumference, while 18% had an elevated waist-to-hip ratio.Among those classified as overweight, the gap was even wider: about 39% had a high waist circumference and roughly 40% had a high waist-to-hip ratio, indicating abdominal obesity that BMI did not capture.The mismatch also worked in the opposite direction. Among those classified as obese by BMI, 9% had a low waist circumference and nearly half, 45%, had a low waist-to-hip ratio, suggesting lower risk than BMI alone would imply. In other words, for a substantial share of participants, BMI and abdominal-fat measures painted very different pictures of cardiovascular risk.BMI has long been used as an independent predictor of cardiovascular disease(Photo: Shutterstock)And those differences were not merely theoretical. Among people with normal weight or overweight BMI, a high waist circumference or waist-to-hip ratio was associated with a greater risk for most of the clinical outcomes examined.One particularly striking finding involved participants classified as having obesity by BMI but with a low waist circumference. They did not show an increased risk for most cardiovascular conditions compared with people who had both normal BMI and a low waist circumference, and they had a lower risk of death from any cause.The study does not establish why that pattern occurred. “It appeared that waist circumference was what distinguished the groups,” Dr. Berkovich said. “Their risk of adverse clinical outcomes was very similar.”Visceral fat surrounds internal organs and has repeatedly been linked to higher disease risk(Photo: Shutterstock)The waist-to-hip analysis also revealed differences between men and women. Among men with obesity, those with a lower waist-to-hip ratio had a lower risk across nearly all the outcomes examined.Among women, however, the picture was different. Women with obesity remained at higher risk than normal-weight women even when their waist-to-hip ratio was relatively low, although their risk was lower than that of women whose fat was more concentrated around the abdomen.“In women, it did not appear to make the same degree of difference,” Berkovich said. “In men, it reduced the risk across almost all the outcomes.”Taken together, the findings suggest that a person’s BMI category does not necessarily capture their full cardiovascular risk. Two people with the same BMI may differ substantially in body composition and fat distribution, and therefore in risk.“Fat distribution is no less important than the amount of fat,” Dr. Berkovich said. “Fat concentrated around the abdomen, an ‘apple’ body shape, is associated with higher cardiometabolic risk than fat concentrated around the hips, a ‘pear’ shape.”The clinical message is not that BMI should be discarded and replaced with one other measurement. Instead, Dr. Berkovich said, doctors may need to combine several measures to build a more accurate picture.“We need to think beyond a single measure,” she said. “We will probably need to combine several measurements to assess cardiometabolic risk more accurately. What is clear is that in some populations, we cannot rely on BMI alone.”Body shape, ‘apple’ or ‘pear,’ can affect risk(Photo: Shutterstock)This is not the first study to point in this direction, but it adds substantial evidence across nine cardiovascular outcomes and two decades of follow-up. “The overall message is not new,” Dr. Berkovich said. “But this study strengthens the evidence with an exceptionally large sample and a very long follow-up period.”Despite the clear pattern in the data, the study does not fully explain why BMI was associated with different levels of risk depending on waist circumference and waist-to-hip ratio. “There are other factors we still don’t fully understand,” Dr. Berkovich said. “Some of these differences may be influenced by factors the researchers could not fully measure or isolate.”However, the findings show an association and do not prove that fat distribution caused the differences in disease risk. “This was an observational study, so it can identify associations but cannot establish causation,” Dr. Berkovich said. The researchers also lacked data on participants’ physical activity, diet and genetic predisposition to obesity, while waist circumference and waist-to-hip ratio were measured only once.Still, the size of the study and the length of follow-up give the findings considerable weight. “This is a very large study,” Dr. Berkovich said. “Twenty years of median follow-up is extremely difficult to achieve. A randomized trial designed to isolate the effects of fat distribution over several decades would be almost impossible to conduct, both practically and ethically.”