Body-mass index (BMI) is the most widely used measure of body fatness
Sometimes ways of doing things don’t change in the face of compelling evidence that a better way exists. Such seems to be the case with respect to measuring human body fat. Aside from aesthetics, 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 current standard for measuring human body fat is called 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 will make that calculation for you if you know your height in feet and inches and your weight in pounds.
In spite of its popularity, BMI doesn’t account for body fat location plus it doesn’t work well for exceptionally muscular people. For example, during last year’s NBA season, LeBron James’ height (6’, 8.5”) and weight (242 pounds) equated to a BMI of 24.9, just barely under cut-off of 25 for overweight. It defies imagination that LeBron James could be even close to overweight.
Waist circumference and waist-to-hip ratio are better measures
Visceral fat (within the abdominal cavity) supports metabolic derangement including increased risk of insulin resistance, excessive blood fats, high levels of small LDL-cholesterol particles, and low levels of HDL-cholesterol. Alas, measuring visceral belly fat requires expensive and time-consuming equipment. Waist circumference and waist-to-hip ratio both reflect belly fat and are quick and easy for anyone to measure. Both measures reflect greater central obesity (belly fat), which is highly correlated with visceral fat and increased risk of cardiovascular disease and other chronic diseases. Researchers typically evaluate obesity according to BMI values. But BMI does not adequately distinguish between body fat and lean muscle mass, especially in normal weight people.
A team of international researchers evaluated BMI, waist circumference, and waist-to-hip circumference ratio as predictors of mortality of people with cardiovascular disease using data from four published studies plus unpublished data from the Mayo Clinic. Waist circumference and waist-to-hip ratio were better predictors of mortality risk than was BMI for people with cardiovascular disease. The former measures reflect central obesity, which is highly correlated with visceral obesity, which predicts increased risk of insulin resistance, excessive blood fats, high levels of small LDL-cholesterol particles, and low HDL-cholesterol. All these factrors predict increased risk of cardiovascular events.
Waist circumference-to-height ratio could be even better measure
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 the Health Survey for England to determine the prevalence of obesity by age and gender. The researchers also used data from 7,414 participants from England, Wales, and Scotland in the nationally representative Health and Lifestyle Survey to determine 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 of 0.5 for men and 0.45 for women predicted the lowest number of life years lost. The number of life years lost increased strongly for WHtR values greater than 0.6 for both sexes, but was greater 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.
SADHtR could be better still
Researchers at the Center for Disease Control in Atlanta studied the ability of two other measures of body fat and BMI to predict risk of poor metabolic health markers. The metabolic markers included triglycerides, triglycerides/HDL-cholesterol ratio, and insulin resistance/liver fat. One alternative measure was the sagittal supine abdominal diameter (SAD) divided by height (SADHtR). SAD measures belly fat as the height of a person’s belly after exhaling while lying on an examining table. The other alternative was the waist-to-height ratio (WHtR). Data for the study came from 4,398 participants in the National Health and Nutrition Examination Study during 2011-2014. Results showed that SADHtR was the best predictor of unhealthy levels of the three metabolic health factors, followed by WHtR and BMI. Measuring SAD in a health clinic is quick and easy and involves no more effort that measuring waist circumference. The study authors proposed that medical providers use screening criteria for SADHtR and WHtR values greater than 0.140 and 0.606, respectively, to flag patients with high risk of metabolic diseases. Values above these threshold numbers indicate substantial risks of high unhealthy levels of the metabolic markers.
More body fat measures
Knowing the amount of one’s body fat isn’t all that useful. What’s much more useful is knowing the degree to which one’s body fat predicts risk of bad health outcomes, mainly arising from the amount of abdominal fat. Researchers in the UK compared a set of three body weight-based and five body-shape-based measures of body fat as predictors of the risk of the metabolic syndrome or its constituents. The cross-sectional data for the study came from a convenience sample of 550 young adults (average age 21 years) living in England and enrolled in the Collaborative Investigation on Nutritional Status of Young Adults. The body-weight measures (body-mass index, new body-mass index, Clinica Universidad de Navarra - Body Adiposity Estimator) did not adequately predict cardiometabolic risk. Four of the five body-shape measures (waist-to-height ratio, waist circumference, waist-to-hip ratio, body adiposity index) adequately predicted cardiometabolic risk. However, waist-to-height ratio and waist circumference performed the best.
Ordinary people can easily understand these two measures and values above which cardiometabolic risk increases considerably. Here are two possible goals: 1) Keep your waist-to-height ratio below 0.5. In other words, limit your waist circumference to half your height. Thus, a 6-foot (72 inch) tall man (or woman) would keep his (her) waist circumference under 36 inches. 2) Men, keep your waist circumference below 94 cm (37 inches); women, keep your waist circumference below 80 cm (31.5 inches). If you reach your goal, you’ll enjoy better health and you’ll feel better, too.