For decades, body mass index, or BMI, has been one of the most common ways to classify people as having normal weight, overweight or obesity.
But a large new study suggests that BMI can miss an important part of the picture. Where body fat is stored, especially around the waist, may provide valuable information about a person’s future risk of heart and blood vessel disease.
The study included more than 260,000 people and followed them for an average of about 20 years.
Researchers found that waist circumference and waist-to-hip ratio could identify higher cardiovascular risk even among people whose BMI placed them in the normal-weight range. The research was published in JACC.
BMI is calculated using a person’s weight and height. Because it is simple and inexpensive, doctors and researchers have used it for many years as a rough measure of whether someone is carrying excess weight. However, BMI cannot tell the difference between fat and muscle, and it cannot show where fat is located in the body.
That location may matter greatly for health. Fat stored deep inside the abdomen around organs is known as visceral fat. It is more strongly linked with problems such as insulin resistance, type 2 diabetes and cardiovascular disease than fat stored just beneath the skin.
This is why two people with the same BMI can have very different health risks. One person may carry more fat around the abdomen, while another may carry more around the hips and legs or have greater muscle mass. BMI alone may place them in the same category even though their metabolic health is different.
The researchers wanted to see whether simple measurements of the midsection could improve cardiovascular risk assessment. They examined data collected through the Cross Cohort Collaboration and looked at people who had measurements of either waist circumference or waist-to-hip ratio. They then tracked a wide range of major health outcomes.
These outcomes included heart attack, stroke, heart failure and atrial fibrillation, a common abnormal heart rhythm. The researchers also studied coronary heart disease, overall cardiovascular disease, deaths from coronary disease, deaths from cardiovascular disease and deaths from any cause.
This allowed them to examine whether body-fat distribution was linked with many different forms of poor health.
The results showed that BMI categories did not always match abdominal fat levels. Among people classified as normal weight by BMI, about 5% had a high waist circumference and 18% had a high waist-to-hip ratio. This means a notable group of apparently normal-weight people still carried relatively high levels of fat around the middle.
Among people classified as overweight, 39% had a high waist circumference and 40% had a high waist-to-hip ratio. The study also found that some people classified as having obesity by BMI had relatively low waist measurements. These differences show why weight and height alone may not fully describe body composition.
The most important finding involved people whose BMI looked reassuring but whose waist measurements did not. People with normal weight or overweight who also had a clinically high waist circumference or waist-to-hip ratio had about a 15% to 50% higher risk across most of the cardiovascular outcomes studied.
Their abdominal fat measurements identified risk that BMI alone could have overlooked.
Senior author Michael J. Blaha of the Johns Hopkins Ciccarone Center for the Prevention of Cardiovascular Disease said waist measurements appeared to change the risk picture created by traditional BMI cutoffs.
In particular, some people considered normal weight by BMI had high levels of central body fat and higher health risks. This supports looking beyond the number on the scale.
The findings involving people with obesity were also interesting. Those classified as having obesity by BMI but with a low waist circumference did not have significantly different risks for most outcomes compared with people who had normal weight and a low waist circumference. For death from any cause, their risk was actually lower in the analysis.
This does not mean that obesity is harmless or that BMI has no value. BMI remains a quick and useful screening measure for large populations and can help doctors start conversations about weight-related health. The study instead suggests that BMI becomes more informative when it is combined with information about where fat is stored.
Waist circumference is easy to measure with a tape, while waist-to-hip ratio compares the size of the waist with the size of the hips. A higher ratio generally indicates that more body mass is concentrated around the abdomen. These measurements cost almost nothing and could be added to routine health checks.
Lead author Zeina A. Dardari said the results emphasize the importance of recognizing excess abdominal fat across the entire BMI range.
Relying only on BMI could lead doctors to underestimate cardiovascular risk in some people. The researchers encourage clinicians to consider fat distribution when assessing people who have not yet developed cardiovascular disease.
There are several possible reasons abdominal fat may be especially harmful. Visceral fat is biologically active and can release substances that influence inflammation, blood sugar control and fat metabolism. These changes may contribute over time to damaged blood vessels, diabetes and heart disease.
The study has important strengths, including its very large number of participants and long follow-up period.
Looking at more than a quarter of a million people over roughly two decades gave researchers an opportunity to study major cardiovascular events that can take many years to develop. The results were also seen across several different health outcomes.
However, the research has limitations. The investigators did not have complete information about physical activity, diet or genetic factors related to obesity, all of which could influence cardiovascular health.
Waist circumference and waist-to-hip ratio were also measured only once, so the study could not show how changes in abdominal fat over the years affected risk.
Because the study was observational, it also cannot prove that a large waist directly caused the cardiovascular problems that followed.
Waist measurements may partly reflect other biological or lifestyle factors that contribute to disease. Even so, the long follow-up and consistent associations make the findings important for future research and clinical practice.
Overall, the study makes a strong case for moving beyond BMI as the only measure of weight-related cardiovascular risk. BMI remains useful, but it can hide important differences between people with similar height and weight.
Measuring the waist may reveal risks that would otherwise go unnoticed, especially in people whose BMI is considered normal or only moderately elevated.
The research was conducted by investigators from the Cross Cohort Collaboration, including researchers at the Johns Hopkins Ciccarone Center for the Prevention of Cardiovascular Disease. The study was published in JACC.
Source: Johns Hopkins University.
