Obesity is a Disease: Why it Happens, and Why it Matters

Scientists now understand weight gain is not simply a problem of willpower.


There has been a pervasive idea in society that obesity is a sign of personal failure—that it is caused exclusively by lack of willpower, laziness, and “gluttony.” The science does not agree. In 1998, the National Institutes of Health declared obesity a disease and the American Medical Association followed suit in 2013.

Let’s look at three key reasons obesity is more a disease then a personal failing, and how this knowledge can help you control your own weight—or be more understanding of others’ struggles.

Human Biology. The prevalence of obesity (a body mass index over 30) among U.S. adults is 42 percent and rising. Although lifestyle choices (like what we eat and how much we move) are important, our natural biology plays a key role in the current obesity epidemic. “Human biology is designed to encourage us to eat when food is there,” says Susan B. Roberts, PhD, a professor at the Friedman School and senior scientist on the Energy Metabolism Team at the Human Nutrtion Research Center on Aging. “This normal biology combines with an unhealthy food environment and food culture to cause weight gain. So-called ‘low willpower’ is normal human behavior, not some defect.”

What to do: Take control of your food environment. Stocking the house with healthy (whole and minimally processed) foods, watching portion sizes, preparing more food at home, and choosing wisely when eating out or ordering in can all help curb the natural tendency to overeat.

Metabolic Adaptation. In addition to storing energy (calories) for future use, fat (adipose tissue) acts as a powerful endocrine organ, secreting hormones and other molecules into the blood. “Many of the hormones released by adipose tissue are involved in regulating appetite, energy expenditure, and fat storage,” says Leon I. Igel, MD, a Tufts alum who is an endocrinologist and chief medical officer for Intellihealth. For example, adipose tissue releases the hormone leptin to signal we’ve had enough to eat. Individuals with obesity can develop a lack of sensitivity to leptin, or leptin resistance. With this condition, one doesn’t get the normal feelings of fullness and satiety and the body starts to burn less calories at rest. Leptin resistance thus contributes to a vicious cycle of obesity.

Many other hormones are released by fat cells, including those involved in glucose tolerance, insulin sensitivity, cell growth, inflammation, and the formation of blood clots in veins and arteries. “As levels of adipose tissue increase,” Igel explains, “multiple metabolic pathways stop working as they should. This leads to the development of a number of medical conditions associated with obesity.” Metabolic changes are also responsible for making it difficult to maintain weight loss.

What to do: Science suggests specific foods have different effects on weight gain. “Eat more minimally processed fruits, vegetables, nuts, beans, whole grains, fish, and yogurt,” says Dariush Mozaffarian, MD, DrPH, dean of policy for the Friedman School and editor-in-chief of this newsletter. “Foods with live probiotics and fermented foods may also help protect against weigh gain. Eat less refined grains, starches, sugars, and red and processed meat. Avoid soda and other highly sweetened drinks. Alcohol is also associated with weight gain—if you drink, do so moderately. Other animal foods, like milk, cheese, poultry, and eggs, appear, in observational studies, to be relatively neutral for long-term weight gain.”

Getting adequate sleep, increasing physical activity, and finding ways to reduce or deal with stress can also help address some of the physiological reasons we gain weight.

Even metabolic adaptations that slow metabolism and make weight loss maintenance difficult may be responsive to dietary intake. “Recent controlled trials show metabolic adaptation is influenced by diet quality and composition,” says Mozaffarian. “This research found that after weight loss resting metabolic rate was best maintained on a high fat, low carb diet (60 percent energy from fat, 20 percent from carbs) compared to a low fat, high carb diet (20 percent energy from fat, 60 percent from carbs), leading to about 300 more calories of energy expenditure a day.” Effects were in between on a moderate fat, moderate carb diet (40 percent energy from each). All the carbohydrates in these diets emphasized fruits, non-starchy vegetables, beans, and whole and minimally processed grains, rather than refined grains and added sugars.

Health Impacts. Obesity is the root cause for many medical diseases, including heart disease, stroke, type 2 diabetes, and certain types of cancer. It is also a risk factor for poor self-esteem, depression, obstructive sleep apnea, osteoarthritis, gout, female infertility, gallstones, pancreatitis, and non-alcoholic fatty liver disease. “This makes obesity its own complex medical disease,” says Igel. “We treat each of these other weight-associated conditions individually, but if we are able to treat obesity itself, we reduce all of these other medical issues at the same time.”

What to do: As with the treatment of other conditions—like high blood pressure, high cholesterol, and diabetes—medication may be necessary if lifestyle modifications are not enough. New medications, in combination with lifestyle changes, can help overcome the body’s metabolic adaptations and assist with weight loss. However, these tend to be expensive and are not yet widely covered by health insurance programs.

It is important to recognize that people with obesity who struggle to lose weight are fighting a real battle with their own bodies. It’s equally important to understand that we are not entirely helpless in the face of our genes, hormones, and metabolism. Lifestyle modification, including diet and physical activity, is still the cornerstone of obesity treatment. Accepting that obesity is a disease will help reduce stigma and discrimination, increase research dollars and insurance coverage, and move us all toward a healthier future.


Please enter your comment!
Please enter your name here