Body Mass Index (BMI) can not distinguish between fat and muscle, which tends to be heavier and can tip more toned individuals into overweight status, even if their fat levels are low. In the journal Science, the latest data from University of Pennsylvania shows that BMI also does not tear apart different types of fat, each of which can have different metabolic effects on health. Belly fat, which is known as visceral fat, is more harmful than fat that simply sitting under the skin. Visceral fat develops deep among muscles and around organs like the liver and by releasing certain hormones and other agents, it disrupts the body’s ability to balance its energy needs. Even relatively thin people can have high levels of visceral fat, which means they might be considered healthy by BMI standards, but internally they may actually be at higher risk of developing health problems related to weigh gain.
A person’s Body Mass Index (BMI= weight in kg divided by height in m2) is a flawed measure of health. The study, led by A. Janet Tomiyama, an assistant professor of psychology at the University of California-Los Angeles (UCLA), analyzed the link between BMI and cardiometabolic health using data from the most recent National Health and Nutrition Examination Survey (NHANES).
The cardiometabolic health data available in the NHANES gives measures of blood pressure, triglycerides, cholesterol, glucose, insulin resistance and C-reactive protein (a marker of inflammation).The analysis shows nearly half of Americans whose BMI puts them in the overweight category (34.4 million people) are actually healthy according to cardiometabolic measures, as are 19.8 million whose BMI classes them as obese.
The researchers also found that 15% of Americans (2 million people) whose BMI calculation is 35 or higher – thus classing them as very obese – are also healthy. The study also finds over 30% of Americans whose BMI puts them in the normal range (nearly 21 million people) are actually unhealthy according to cardiometabolic measures.
CT scans and MRIs can provide a clearer glimpse at the body’s make-up by separating out fat from muscle, for example, but these are expensive, BMI is relatively easy measurement for the doctors to take.