“Every now and then a man’s mind is stretched by a new idea or sensation, and it never shrinks back to its former dimensions.” Oliver Wendall Holmes
Can an overweight or obese person who is fit also be healthy? Even without losing weight, is regular exercise enough to reduce the risk of heart disease in obese people? Or does being overweight, by itself, put a person at a higher risk of inflammation and cardiovascular disease, regardless of physical activity and exercise habits? These questions are part of the “fat but fit” controversy and have been debated in the scientific community for decades. There are several large-scale studies confirming that fitness is more important than fatness, and just as many that show the opposite to be true. And with every new study comes a flurry of news reports that frequently misrepresent and sensationalize the research. But, regardless of the hype, most doctors remain firmly suspicious of the claim, with most believing there is no such thing as “healthy” obesity.
That a scientific controversy exists about whether an obese person can be metabolically healthy— “fat but fit”—comes as a surprise to some people. Most of us assume that extra pounds, and especially obesity, are unhealthy; it is common to grow up hearing stories about the dangers of being overweight. The belief that poor health and weight go hand in hand is deeply rooted in our narratives and is reinforced by the media as well as in those in health care. Many physicians have a weight bias, so an unfavorable number on the scale during a doctor’s visit will most likely include a directive to lose weight along with a recommendation to eat less and exercise more.
There is plenty of evidence that being overweight and obese can be problematic. Carrying too much weight is often associated with low-grade inflammation, which we now know contributes to cardiovascular disease, and carrying excess pounds puts strain on the joints. But weight is just one of many symptoms associated with chronic disease; by itself, it does not predict a doomsday scenario of disability and disease. Proponents of healthy obesity argue that focusing only on weight loss is problematic. We now know that some factors related to weight are within our control, such as lifestyle behaviors, while others are influenced by genetics, medications, gut microbiome, and environmental factors. Experts on both sides of the “fit but fat” controversy agree that focusing only on weight is ineffectual at best and stigmatizing at worst, especially as a growing body of research points to the critical role exercise plays in keeping us healthy, regardless of our weight.
It is common in our culture to conflate thinness with fitness, a misconception that is deeply rooted in cultural beliefs. Generally, we tend to judge thin people as energetic, motivated, and successful, while those who are obese are seen as lazy and unproductive. Who hasn’t looked at an obese person with the thought that he or she should simply “stop eating and start moving”, without knowing anything about that person’s lifestyle? And we can be our own worst critics, deriding ourselves for being sluggish and unmotivated with every workout we skip. But the association between exercise and weight is a complicated one that is not accurately measured by the “move more” standard. We now know that adipose (fat) tissue is not an energy piggy bank in which the calories we eat are stored while physical activity and exercise makes a withdrawal. Rather, it is a highly sensitive and sophisticated organ that responds to movement in very specific ways that can keep us healthy, even though we may not shed a pound.
Given our cultural bias against fatness, many find it hard to believe that body fat can be healthy, especially when most people spend a lifetime trying to get rid of it. As we continue to learn about adipose tissue, we find that not all body fat is created equal. What matters most is the size of our fat cells, not the total amount. Each adipocyte—fat cell—has a specific storage capacity, expanding as fat enters the cell for storage and shrinking when its contents are released for energy. When the adipocyte is close to being full, it starts to leak its contents into the bloodstream. Whereas lipids stored in a fat cell are an important source of energy, they become problematic in the bloodstream where they are converted into fatty acids and lodge in other organs. Unlike adipose tissue, the heart, muscle, and liver are not equipped to handle fatty acids; once they attach to these organs, they wreak havoc and initiate inflammation. In contrast, small fat cells have more available storage space, enabling them to expand and extract the fatty acids from the bloodstream, keeping them safely tucked away within the cell wall where they belong. The goal is not to get rid of fat, but to be sure our cells have the capacity to store fat properly, keeping it out of the bloodstream.
Compared to dysfunctional fat cells, healthy fat cells have lots of active mitochondria, the power center of a cell that converts oxygen and food into energy. With more mitochondria, the fat cell becomes resilient, adaptable, and responsive. Essentially, mitochondria help the fat cell do a better job at providing energy. Healthy fat cells are also rich in blood vessels, which bathe them in a rich supply of oxygen, nutrients, and cells that help reduce inflammation. Dysfunctional fat tissue, with large, leaky cells, fewer mitochondria, and a poor blood supply, has less immunity, making it more likely to become inflamed and initiate inflammation elsewhere in the body.
Although we don’t fully understand all the components of metabolically healthy fat, studies show that exercise plays an important part in improving the functioning of our fat tissue. Exercise training at moderate and intense levels has been shown to be particularly effective at reducing the size of fat cells, increasing the number and functioning of the mitochondria, and supplying the cell with more capillaries that help to reduce inflammation. All these benefits seem to improve with the dose—the more movement, physical activity, and exercise we engage in, the greater the change in the cells.
Although all movement seems to keep our fat tissue healthy, some studies suggest that exercise training at moderate and intense levels may offer additional benefits. Fat tissue biopsied from people who exercised regularly, for an extended period of time, at moderate to hard levels of intensity had the healthiest fat tissue, compared to other, less active or sedentary subjects, regardless of their age and weight. The tissue of the exercisers had more mitochondria in the fat cells, which had a much richer blood supply and showed less signs of inflammation than less active subjects.
Although extremes in weight are never healthy—most would agree that being too thin or morbidly obese is cause for concern—decades of public health policies that focus only on weight loss have been an abysmal failure for most people. Exercise, physical activity, and physical fitness may result in only small changes in body weight but results in significant improvements in overall health. When it comes to coronary heart disease, studies that take into account physical activity have concluded that fitness is more important than fatness, at least for those who are overweight or moderately obese.
We will all be better off when the scientific community drops the “fat” controversy and focuses exclusively on the “fit”. Such a shift in perspective leads to treatment options that many will find easier to follow—rather than “lose weight”, starting an exercise regimen is easier to stick with than restrictive and ineffective diets. Importantly, shifting our focus on who is fit rather than fat creates a less myopic approach to medicine. Doctors who associate thinness with health tend to overlook risk factors in normal weight patients. Studies have found that normal weight subjects who were sedentary had a much higher risk of coronary heart disease than healthy obese people. A focus on fitness rather than a number on the scale helps to level the medical playing field and provides better health care for all, regardless of size.
Given the complexity of health care, it is doubtful any of us will receive exercise prescriptions from our doctor anytime soon. In the meantime, what would happen if you shifted your attention away from your weight to focus more on your fitness? What would change in how you feel about your health? How would you view others who are overweight or obese? Keep in mind that our narratives about weight are deeply ingrained and not easy to change, and most people struggle with the idea that you can be fit and fat. It’s hard to change beliefs that are held so deeply. Even if it is only for a moment, I invite you to try on a different attitude about your body, dropping the focus on fat to celebrate your fitness.
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