Gillian Goddard

7 minute read Gillian Goddard

Gillian Goddard

Why BMI Doesn’t Dictate Health

A new study shows the number on the scale is less important than previously thought

Gillian Goddard

7 minute read

When I first consult with patients struggling with overweight or obesity, they often ask what they should weigh. They sometimes want to know what a healthy weight is for someone of their height, thinking that to be healthy they need to reach a “healthy” body mass index (BMI). I always try to steer them away from setting a specific number on the scale as their goal. 

Longtime readers know that I am wary of applying BMI to an individual person. BMI, which is just a calculation that relates your weight to your height, was developed as a tool to study populations and risk for chronic disease, not as a vital sign like blood pressure.

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Instead, I prefer to take a more holistic approach. Common goals my patients and I set together include establishing healthy diet and exercise habits; improving their blood pressure, cholesterol, and blood sugar; and, most importantly, feeling good and being able to do the physical activities they want to do with ease, regardless of their weight.

But the data supporting this idea has been surprisingly hard to come by. For one thing, the data we have is all observational. You cannot randomize people to different BMIs, so you have to come at the problem in another way. As a result, I was excited to read this study that does just that — so excited that I have already shared it with several of my patients. I think you will find it helpful as well. 

What does the data say?

The study linked above is a meta-analysis that includes data from 20 published articles and nearly 400,000 participant observations. It uses the data from those papers to divide the participants into six groups based on BMI and physical fitness: normal BMI (less than 25 kilograms per square meter, or kg/m2) and fit; overweight BMI (25 to 29 kg/m2) and fit; obese BMI (30 or more kg/m2) and fit; normal BMI and unfit; overweight BMI and unfit; and obese BMI and unfit. The participants in all the studies were followed for several years and any deaths were recorded, including cause of death. 

There was some variation in how fitness was measured, but all the studies included measures of cardiorespiratory fitness — that is, a participant’s ability to perform aerobic exercise. At least some of the data came from treadmill testing, where subjects were asked to walk on a treadmill using a standard treadmill protocol. If participants are able to continue the test for longer, they are more fit. This data is reported in units called metabolic equivalents of task (METs). The ability to perform 8 METs — the equivalent of running 5 miles per hour — for about 3 minutes was considered fit. 

The authors found that fit participants with BMIs in the overweight and obese range did not have increased mortality compared with fit participants with BMIs of less than 25. Additionally, they found that unfit participants with BMIs of less than 25 were two to three times as likely to die of any cause as fit participants with comparable BMIs. 

The authors conclude that fitness, defined as the ability to perform high-intensity exercise, is a better predictor of mortality than BMI. 

Why is this data important?

Part of the reason I am so excited about this study is that this data and more studies like it are forming the basis of a new conversation among experts about reconsidering BMI as a metric and its role in managing patients. In fact, a panel of 58 experts just published a new proposal for defining clinical obesity. Currently obesity is diagnosed based on BMI. In the new proposal, the authors argue for using BMI as a surrogate measure of health in epidemiological studies or as a screening measure among individuals but not as the sole criterion for diagnosing an individual with clinically significant obesity. BMI should be seen as just part of the equation, not the only way obesity should be diagnosed. 

This data also helps to define concrete steps patients can take regardless of BMI to improve their health and reduce their risk of dying prematurely. For years, we have recommended that patients lose weight. But anyone interested in goal setting will tell you the goal of losing weight is one likely to lead to frustration. I have many patients who follow a healthy diet and are physically fit but have been unable to lose weight, often due to insulin resistance. Still, the message they have historically heard from their providers is that their efforts are worthwhile only if they result in weight loss. This new data suggests that healthy habits are beneficial regardless of whether they result in weight loss.

We cannot control whether we lose weight. But we can control whether or not we practice healthy habits such as exercise. Setting a goal to move our body for 30 minutes five days per week will, over time, result in improved fitness, especially if we increase the intensity of that movement with time. Now I can tell my patients that the data suggests that doing so will reduce their risk of premature death. 

This is not just about those with high BMIs, though. This data should also be a wake-up call to those who are thin but not fit, and for those who are focused solely on weight loss — with or without anti-obesity medications. The data suggest that being unfit at any size substantially increases the risk of dying. Put another way, being fit at any size is better for long-term health than being thin and unfit.

How am I using this data now?

I have already shared this study with a number of patients. I still think weight loss is important for some patients. Many of my patients with obesity find that exercise can be painful, and weight loss can unload the back and knees and make exercise more comfortable. Weight loss allows many of my patients to engage more often in physical activities that they enjoy.  

But now more than ever, I am using this data to support setting goals that are not related to a number on a scale. I am also using it to reassure my patients who have overweight and obesity and are physically fit that their exercise efforts are not in vain. Rather than looking at exercise as a tool for losing weight, the goal of exercise is that it improves physical fitness. 

Lastly, I am using this data to talk about physical fitness with patients who do not have overweight or obesity. Because we have long equated thinness with health, the thin unfit group is often ignored and may not realize they are at risk. When people are thin and unhealthy, we have assumed it is because they are doing unhealthy things to stay thin. However, those who are genetically thin may also be at risk if they aren’t practicing healthy habits. It is critical to educate them as well. 

Ultimately, I think publications like these are important first steps to a more holistic view of body size, health, and disease risk.   

The bottom line

  • New data suggests that aerobic fitness is a better predictor of a person’s risk for premature death than BMI or weight alone.
  • As a result, health goals should likely be focused on improving physical fitness rather than on achieving a certain goal weight.
  • Experts are using data like this to redefine clinically significant overweight and obesity.
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Mary Kate
5 days ago

This is so reassuring. I’m overweight according to BMI and have been consistently weight lifting and doing HIIT for over a year and have gained weight. Sometimes I want to give up because of the scale.

MitochondriaFan
MitochondriaFan
12 days ago

I’m curious what kind of healthy diet doesn’t help improve patients’ metabolic symptoms. Totally agree that weight isn’t the primary causal problem but is downstream (and correlated!) with the actual issue of metabolic syndrome.

Anonmom
17 days ago

I’m genetically thin and have always joked that I’m “skinny fat” because there isn’t really a way to say that a person in our culture is thin but still unfit. I’m glad this study is shining a light on folks like me who may feel they don’t need to “work” at being healthy. We’re often ignored in the discussion. (Now to go hit the gym!)

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