When it comes to health, does body mass index (BMI) matter? That question has been a source of debate for decades. A mathematical formula originally developed to track trends in population health, BMI is used by many health-care providers to diagnose individual patients — something it was never intended to do.
A different metric may tell us more about disease risk.
New research out of the Women’s Health Initiative focusing on 156,624 women with an average age of 63.2 found that those who have a waist measurement of 34 inches or more had a significantly increased risk of early death from cardiovascular disease, cancer and all causes combined. This was true even if their BMI was in the “normal” range.
In fact, the women with “normal” BMIs (18.5-24.9) and larger waist circumferences — officially known as “normal-weight central obesity,” but often referred to as “skinny fat” — had a risk of early death similar to that of women with an “obese” BMI and a larger waist. Women with “overweight” BMIs and larger waists also had an elevated risk, but it wasn’t as high. As for women who had “overweight” or “obese” BMIs but waists less than 34 inches around? Their risk was similar to women with “normal” BMIs and smaller waists — it was even slightly lower in some cases.
I asked one of the study’s authors, Marian Neuhouser, Ph.D., R.D., head of the cancer-prevention program at Fred Hutchinson Cancer Research Center, if these results were expected. “It was really surprising that someone with a so-called normal BMI but with a high waist circumference has an equal risk to someone with an obese BMI and a high waist circumference,” she said. “This suggests that those groups of people may have a similar metabolic profile.”
Why is a larger waist a problem?
Many factors influence health — genetics, socioeconomic status, access to health care, nutrition, chronic stress, physical activity, alcohol use, smoking status. Adipose tissue (aka body fat) can affect health too, but it’s not a slam dunk to say that the amount of fat someone has on their body determines their health risks. The truth is far more nuanced, and takes into account factors like how fat cells are functioning and where they are.
Excess visceral fat — the fat that collects around your abdominal organs — has been linked to poor metabolic health. The problems start with unhealthy cholesterol and triglyceride levels, chronic inflammation and insulin resistance, when the insulin the body produces has trouble getting blood sugar into cells. These conditions increase the risk of heart disease and other chronic health problems.
The women in the Women’s Health Initiative study who had larger waists and more visceral fat were also more likely to smoke, get less physical activity and come from a lower socioeconomic background. All of these things can harm health on their own; having a lower socioeconomic status in particular contributes to chronic stress, which encourages the body to store fat in the abdomen. However, the association between a larger waist and increased risk of early death remained even after accounting for those other factors. Neuhouser said this suggests that visceral fat independently contributed to the increased health risks.
What are health-care providers doing about this? Not as much as they could. Very few people have their waists measured in a doctor’s office. But based on the findings of this and previous studies showing excess fat around the midsection to be a health risk, that oversight may be a disservice to women of all BMI levels. Why the disconnect? Perhaps it’s because guidelines from the American Heart Association, the American College of Cardiology and other groups only recommend waist measurement if someone’s BMI is above 30.
Why are we hung up on BMI?
Neuhouser said there’s a growing awareness in the research community that tracking BMI on its own isn’t a sufficient way to assess health risks. But this knowledge isn’t necessarily trickling down to the health-care setting. “I would suggest that BMI is a tool, but it’s not a perfect tool,” she said. “It’s just one point of information. Just because someone has a normal BMI doesn’t mean that they’re metabolically healthy.”
She emphasizes looking at the whole patient, taking waist measurements and doing blood work, and adds that even if a patient with a normal BMI doesn’t smoke and reports exercising and healthful eating, that doesn’t mean they’re not at risk — not all of the women in the study with normal-weight central obesity had unhealthy lifestyle habits. “Measuring waist circumference is relatively easy to do, and it’s easier to do on someone when they’re relatively slim,” she said, acknowledging that while taking a waist measurement may not seem like a burden, adding one more thing to all of the information health-care providers need to collect may feel like one.
The study’s authors noted that the health risks associated with excess visceral fat may be partly due to having less muscle and less subcutaneous fat around the hips and thighs. Both muscle mass and hip-and-thigh fat are associated with better metabolic health. So why isn’t this getting more attention from doctors?
“I don’t think it’s getting enough attention clinically, but it certainly is in the research world,” Neuhouser said, adding that the health risks of sarcopenia (age-related muscle loss) are well established. While it’s more challenging to measure lean tissue, she did point out that when women get a DEXA scan to measure bone density — typically postmenopause — it also measures lean mass.
What can you do?
If you have excess fat around your midsection and want to improve your health odds, start with exercise. A recent meta-analysis of 17 previous randomized controlled trials found that physical activity was effective in reducing visceral fat. Interestingly, the authors speculated that the reason physical activity reduces the risk of cardiovascular disease may be that it reduces visceral fat and increases muscle mass.