What body-mass index tell you?
Excessive body fat predicts increased risk of chronic diseases including cardiovascular disease, type 2 diabetes, and cancer. But it’s not just the amount of fat that matters—it’s the location that really matters. The common standard for measuring human body fat is called the body-mass index (BMI). It’s calculated as a person’s weight in kilograms divided by the square of the person’s height in meters. Many websites can make this calculation for you, provided you know your height in feet or inches and your weight in pounds.
In spite of its wide use, BMI had two critical drawbacks. First, it does not distinguish between lean mass (muscles, ligaments, tendons, bones) and fat mass. Two, BMI doesn’t account for body fat location. Body fat concentrated around your waist in much more detrimental to your health that body fat on your hips or thighs. Body fat around your middle strongly predicts the amount of fat in your internal organs, such as your liver, pancreas, and heart. Excess liver and pancreas fat predict higher risks of type 2 diabetes and premature death.
What about other measures of body fatness?
Would direct measures of body fatness, rather than BMI, be better predictors of health risks? Researchers in Iran and Canada conducted a systematic review and meta-analysis of 72 cohort studies that evaluated 8 measures of body fatness. The measures included waist circumference, waist-to-hip ratio, waste-to-height ratio, waist-to-thigh ratio, body adiposity (fat) index, A body shape, hip circumference, and thigh circumference. Higher levels of each of the measures of significantly and independently predicted greater risk of all-cause mortality, except for larger hip and thigh circumference, which predicted lower risk of mortality. The best predictor of risk of all-cause mortality during follow-up appeared was the waist-to-height ratio (WHtR), which is your waist circumference in inches divided by your standing height in inches.
Waist circumference-to-height ratio: Easy to measure, simple to understand
While BMI is the most widely used measure of body fatness in the US, the waist-to-height ratio (WHtR) appears to be a better measure of health problems associated with overweight and obesity. British researchers used cross-sectional data from 7,414 participants in the nationally representative Health Survey for England to determine the prevalence of obesity by age and gender. Specifically, researchers also determined the incidence of mortality and years of life lost in relation to increasing values of BMI and WHtR. The number of life years lost rose more steeply in relation to increased WHtR than BMI. Thus, WHtR is a more sensitive measure of mortality risk. Analyses showed that WHtR values lower than 0.5 for men and 0.45 for women predicted the fewest number of life years lost. The number of life years lost increased strongly for WHtR values greater than 0.6 for both sexes but more so for men than women and for younger compared to older persons. For example, a 50-year old man with a WHtR value of 0.66 would likely live 3.8 fewer years than a 50 year old man with a WHtR of 0.5. Here’s an easy way to remember the message of this study: Limit your waist circumference to half of your height. More on this later.
Waist-to-height ratio as a simple and sensitive screening tool
When you go to the doctor, you typically fill out forms that inquire about various aspects of your physical and mental health. A nurse likely measures your height, pulse, blood pressure, and body temperature. British researchers proposed that the nurse should also measure your waist circumference. Why? Because the ratio of your height (in inches) to your waist circumference (in inches) seems to be the best proxy for the amount of fat stored in your internal organs, especially your liver, pancreas, and heart. Research shows that a waist-to-height ratio of more than 0.5 exceeds the healthy range for people regardless of sex, race, and ethnicity. Relying on body-mass index to identify people with excess body fat will likely misclassify about 10 percent of people as normal weight but who actually have excess body fat. Medical providers can use the waist-to-height ratio as a simple, easily obtained, and sensitive screening tool to identify people who are at increased risk for health problems. The simple message of, “Keep your waist circumference to less than half of your height,” can help you keep on track for better health and well-being.
Does the waist-to-height ratio work for diabetic persons?
Diabetics who develop hypertension have greater risk of additional adverse outcomes including stroke, coronary artery disease, and eye retina deterioration. A recent study investigated the ability of body-mass index, waist-to-hip ratio, and waist-to-height ratio to predict the risk of hypertension in persons with type 2 diabetes. Researchers used data from 1,685 Iranians who were followed over an average of 5 years. After accounting for confounding factors, body-mass index and waist-to-height ratio significantly predicted 5 and 230 percent higher risks of developing hypertension, respectively. Thus, waist-to-height ratio proved to be a much more useful predictor with an optimal cutoff point of 0.59. That is, Iranians with type 2 diabetes who keep their waist circumference to less than 59 percent of their height will greatly reduce their risk of hypertension and its complications.
Here’s a pro tip for Americans. Find a piece of inelastic cord or line that’s longer than your height. Measure your height and cut the cord or line to half of your height. On the first day of every month, wrap the piece of cord or line around your waist. If the ends don’t meet, you’ve got too much belly fat and it’s time to reduce your waist circumference. The photo shows that the ends of my measured piece of line (its length is half my height) exactly meet. Thus, I'm right on the edge to too much belly fat. Any waist expansion would mean that I have too much belly fat and need to trim down.