A person can have a body weight considered normal and still carry a pattern of fat that is linked to a higher risk of heart disease.
That is the message from a major new study showing that the size and shape of the midsection can reveal health information that body mass index, or BMI, may miss.
Researchers say routine waist measurements could help doctors identify people whose cardiovascular risk is hidden by a seemingly healthy BMI.
The study examined more than 260,000 adults and followed them for an average of around two decades. It compared BMI with waist circumference and waist-to-hip ratio to see which measurements provided useful information about future heart and blood vessel problems. The findings were published in JACC.
BMI has been used in medicine for generations because it is easy to calculate. A person’s weight in kilograms is divided by the square of their height in meters, producing a number that is then placed into categories such as normal weight, overweight or obesity.
These categories are often used when discussing the risk of diabetes, heart disease and other chronic conditions.
But BMI has a basic weakness: it describes body size, not body-fat location. It cannot show whether extra weight comes mainly from muscle or fat. It also cannot tell whether a person’s fat is concentrated around the waist or stored elsewhere.
Researchers have become increasingly interested in abdominal fat because not all body fat behaves in the same way. Visceral fat lies deeper in the abdomen and surrounds organs such as the liver and intestines.
This type of fat has been strongly linked with metabolic problems, including abnormal blood sugar, unhealthy blood fats and cardiovascular disease.
Fat stored directly beneath the skin is called subcutaneous fat. Although excess amounts can still contribute to obesity, it is generally not as strongly associated with metabolic disease as visceral fat. This difference helps explain why the shape of the body may provide clues that total body weight cannot.
The new research asked a practical question: could measuring the waist improve the risk information provided by BMI?
The team used data from the Cross Cohort Collaboration and included people who had a recorded waist circumference or waist-to-hip ratio. Researchers then examined whether these measurements were associated with later cardiovascular events and death.
They studied nine major outcomes, including heart attack, stroke, heart failure and atrial fibrillation. They also looked at coronary heart disease, cardiovascular disease more broadly, cardiovascular and coronary deaths, and death from any cause.
This wide range allowed the researchers to test whether abdominal fat was consistently linked with health risks.
The first striking result was how often BMI and waist measurements disagreed. Among people whose BMI placed them in the normal-weight category, 5% still had a high waist circumference. An even larger group, 18%, had a high waist-to-hip ratio.
The same mismatch appeared among people classified as overweight. About 39% had a high waist circumference and 40% had a high waist-to-hip ratio. At the other end of the spectrum, some people whose BMI placed them in the obesity category had comparatively low waist measurements.
These differences mattered because cardiovascular risk changed according to waist size. People classified as normal weight or overweight who had a high waist circumference or waist-to-hip ratio showed a 15% to 50% higher risk for most of the health outcomes studied.
In other words, BMI alone could make some higher-risk people appear healthier than they actually were.
Michael J. Blaha, senior author of the study and director of clinical research at the Johns Hopkins Ciccarone Center for the Prevention of Cardiovascular Disease, said waist measurements can reclassify the risk suggested by traditional BMI categories.
The researchers found people with clinically normal body weight who nevertheless carried excess fat around the abdomen. Those people tended to face higher risks across many of the outcomes.
The opposite pattern was also revealing. People classified as having obesity by BMI but who had a low waist circumference generally did not show significantly higher risks than normal-weight people with a low waist circumference for most outcomes.
This finding suggests that body-fat distribution can help explain why people within the same BMI category sometimes have very different health profiles.
Waist-to-hip ratio is calculated by dividing waist circumference by hip circumference. A larger value means that more of the body’s size is concentrated around the waist compared with the hips. It is one simple way of identifying what is sometimes called central obesity or abdominal obesity.
Why might belly fat be particularly important? Visceral fat is not simply an inactive storage area for extra calories. It releases hormones and other substances that can affect inflammation, insulin sensitivity, cholesterol and other processes involved in cardiovascular disease.
Over many years, these metabolic changes can contribute to plaque buildup in arteries and make conditions such as type 2 diabetes more likely. This does not mean everyone with a larger waist will develop heart disease. Cardiovascular risk depends on many factors, including blood pressure, cholesterol, smoking, age, family history, exercise and diet.
The value of waist measurements is that they can add another piece to that larger risk picture. They are inexpensive, quick and require no blood test or medical scanner.
Harlan M. Krumholz, editor in chief of JACC and a professor at Yale School of Medicine, argued that waist circumference and waist-to-hip ratio should become part of routine cardiovascular risk assessment.
The researchers also stressed that BMI should not simply be discarded. It remains useful for quickly describing weight relative to height and for studying populations. The problem comes when it is treated as a complete measure of a person’s fat distribution or cardiovascular health.
Like all observational research, the study has limits. Researchers did not directly account for every participant’s diet, physical activity or genetic tendency toward obesity. Those factors could influence both where people store fat and whether they eventually develop cardiovascular disease.
Another limitation is that waist measurements were taken at only one point in time. People’s bodies can change substantially over 20 years, and gaining or losing abdominal fat could alter health risk. Future studies that repeatedly measure waist size could provide a clearer picture of how changes in fat distribution relate to later disease.
The study also cannot establish cause and effect. It shows that high abdominal fat measurements were associated with higher risk, not that waist size itself directly produced every heart attack, stroke or other event.
Still, the size of the study, the long follow-up and the pattern across multiple cardiovascular outcomes make the results difficult to ignore.
The broader lesson is that the number on a bathroom scale does not tell the whole story about health. Two people can have the same BMI but very different amounts of abdominal fat, muscle and visceral fat. Adding a simple waist measurement may help reveal those differences and give doctors a more complete view of cardiovascular risk.
For people concerned about heart health, waist size should also not become another number to obsess over in isolation. Blood pressure, cholesterol, blood sugar, smoking, exercise, sleep and family history remain important. The new findings simply suggest that where the body stores fat deserves a place alongside these established risk factors.
Overall, the research provides strong evidence that BMI alone can misclassify cardiovascular risk. The clearest concern is people with normal or overweight BMI who carry substantial fat around the middle, because their risk may be underestimated. Routine waist measurements could offer a simple way to identify these people earlier and guide more complete prevention efforts.
The research involved the Cross Cohort Collaboration and investigators including scientists at the Johns Hopkins Ciccarone Center for the Prevention of Cardiovascular Disease. The study was published in JACC.
Source: Johns Hopkins University.
