JAMA Study Finds Fault with using BMI as Measure of Health Risk

A new study from the University of Iowa finds that some women considered to be a normal weight could unknowingly be at high risk for obesity-related health issues. It comes down to body shape – apple or pear – and suggests that physicians should measure risk of heart disease, cancer and metabolic disorders based on waist-hip-ratio (WHR) rather than BMI.

The study, published in the Journal of the American Medical Association’s JAMA Network Open, found that a subgroup of people who are considered to be normal weight as measured by body mass index (BMI) could actually be at high risk for death because of their waist size.

Wei Bao, PhD, professor of epidemiology in the University of Iowa College of Public Health and the study’s corresponding author, says that according to current clinical guidelines, physicians need not rely only on BMI to determine obesity-related health risk. This single modal mentality leaves women who are actually in a high-risk group because of other risk factors, such as percentage of body fat and body shape, thinking they’re healthy.

“The results suggest we should encourage physicians to look not only at body weight but also body shape when assessing a patient’s health risks,” says Dr. Bao.

The study used data from the Women’s Health Initiative, which tracked the health of more than 156,000 women between the ages of 50 and 79 from 1993 to 2017. Bao and his team linked mortality rates to the respondents’ BMI as well as their central obesity, which is the excess accumulation of fat around a person’s midsection that has been linked to an array of health problems and is measured by waist circumference.

Menopausal and post-menopausal women who were considered normal weight on the BMI scale but had a high waist circumference were found to be 31 percent more likely to die within the two-decade observation period. That’s comparable to the 30 percent increased likelihood that an obese person with central obesity – considered to be in the highest risk group – will within 20 years of observation.

The JAMA study found that the two primary causes of death in people who had normal BMI but high waist size were cardiovascular disease and obesity-related cancer.

This study is so far the largest to identify people with normal weight central obesity as a high-risk subgroup for death. The findings about cardiovascular death and death from any cause largely confirm a similar study published in 2015 by researchers at the Mayo Clinic that was based on a much smaller population.

“The Nurses Health Study (for which my mother was an original participant) starting in 1976 began collecting waist measurements in the 1980’s. My mother remembers this request very well. The findings (Joann Manson et al) have repeatedly shown an increase in cardiovascular disease and cancer risk among nurses with larger waist sizes,” says Marie Savard MD, a physician not part of the study and author of the book, Apples & Pears: The Body Shape Solution for Weight Loss and Wellness. 

“This study is large, the findings are compelling and add to the already well-recognized risk of excess visceral fat even in normal weight individuals,” Dr. Savard says. “For that reason, I think the data on waist-hip-ratio is even more compelling as it identifies those with greater waist size (visceral fat) relative to hip size (benign/protective gluteal fat).”

Dr. Savard believes that this study and many more before it are so compelling that all cardiovascular, cancer and metabolic studies with women as the subjects should divide participants into body-shape categories. Physicians should also take note of a woman’s body shape when assessing health risks and let female patients know of their risk factors before the risk is great or has manifested into disease. See below for how to measure WHR.

Instead BMI is used as the determinant for establishing a woman’s risk for health problems. “While it’s a simple number to understand and easy to determine – it involves only height and weight – it isn’t always accurate because it doesn’t include other important numbers, such as the percentage of body fat or where that fat has accumulated on the body,” says Dr. Bao.

This study was specific to women between the ages of 50 and 79, however Dr. Bao believes the findings are relevant to younger women and men.  “It’s not unusual for people with a high BMI score to be in excellent health because the bulk of their weight is muscle – football linemen, for instance. At the same time, as this study indicates, people who are in the normal range on BMI could still have a high percentage of body fat, putting them in a high-risk group.”

Despite these limitations, BMI has become the standard for health-care providers and health-policy makers when measuring obesity-related health risks. Dr. Bao says this focus on BMI leaves many people in the dark about the risks from central obesity.

“People with normal weight based on BMI, regardless of their central obesity, were generally considered normal in clinical practice according to current guidelines,” Dr. Bao says. “This could lead to a missed opportunity for risk evaluation and intervention programs in this high-risk subgroup.”

According to a study from the World Health Organization, Waist Circumference and Waist-Hip Ratio Report of a WHO Expert Consultation, here are the recommended guidelines for measuring waist-hip ratio:

  1. Most protocols measure at the midpoint(36%),umbilical level(28%)and minimal waist level(25%). Waist circumference should be measured at the midpoint between the lower margin of the least palpable rib and the top of the iliac crest, using a stretch‐resistant tape that provides a constant 100g tension.
  2. Hip circumference should be measured around the widest portion of the buttocks, with the tape parallel to the floor.
  3. Subject should stand with arms at the sides, feet positioned close together, and weight evenly distributed across the feet.
  4. Waist circumference should be measured at the end of a normal expiration,when the lungs are at their functional residual capacity
  5. Subject should be advised to relax and take a few deep, natural breaths before the actual measurement is made, to minimize the inward pull of the abdominal contents during the waist measurement.
  6. Waist measurement be made after the subject has fasted overnight or is in a fasted state, to reduce the effect of water, food or gas in the gastrointestinal tract.

This story was adapted from materials provided by University of Iowa. Original written by Tom Snee.
Source: Yangbo Sun, Buyun Liu, Linda G. Snetselaar, Robert B. Wallace, Bette J. Caan, Thomas E. Rohan, Marian L. Neuhouser, Aladdin H. Shadyab, Rowan T. Chlebowski, JoAnn E. Manson, Wei Bao. Association of Normal-Weight Central Obesity With All-Cause and Cause-Specific Mortality Among Postmenopausal Women. JAMA Network Open, 2019; 2 (7): e197337 doi: 10.1001/jamanetworkopen.2019.7337
Dr. Marie Savard is a physician board-certified in Internal Medicine, author, former ABC News Medical Contributor and founder of the Savard System for managing and controlling one’s healthcare. In addition to ABC News, her work has been featured by dozens of major media outlets including Oprah, The View, CNN, NBC and CBS, Good Morning America and ABC World News. Dr. Savard’s lifelong mission is to help people take charge of their health and health decisions. Click here to learn more about Dr. Savard.