When it comes to losing weight, it isn’t as simple as finding more willpower. Just ask anyone who has ever busted their butt at the gym five times a week, spent their entire Sunday evening prepping healthy meals for the week, and said “no thanks” to the dessert menu—while the needle on the scale still won’t budge.
Why exactly are some people able to maintain a healthy weight while others can try and try with no luck? The process of weight gain and weight loss involves a complicated combination of genetics, complex body systems and the environment.
Your brain on weight loss
Eating, like reproduction, is a motivated behavior that is part of our biologic drive to survive. We need to actually take action and do it, unlike breathing, which happens automatically, explains Ellen Schur, M.D., M.S., an internal medicine physician who practices at the Weight Loss Management Center at University of Washington Medical Center-Roosevelt.
Without this, we might forget to eat and starve or never have babies and risk our population dying off.
When you’re restricting calories and losing weight, your body can perceive this as a threat to survival, says Schur. In other words, your body can’t really tell the difference between losing weight on purpose and losing weight because you’re starving.
At the same time this is happening, the rational part of your brain is being suppressed and overridden by hormones and the drive to eat to restore the perceived energy imbalance.
“You’ll experience increased appetite and a reduced ability to be rational about it,” she says.
Obesity is an energy homeostasis disorder
People who are overweight or obese are often made to feel as if their weight and their inability to keep it off are moral failings, says Michael Schwartz, M.D., director of the UW Medicine Diabetes Institute and the Nutrition Obesity Research Center.
In reality, obesity is a disorder of the body’s energy homeostasis system—where the body works to reach its old, higher weight to maintain the status quo—rather than a passive accumulation of weight, Schwartz and colleagues write in an Endocrine Society scientific statement on the cause of obesity.
Let’s say that someone weighed 200 pounds and put in tons of work to lose 100 pounds. Now they weigh the same as their friend who maintains a weight of 100 pounds without much effort. In order to maintain the weight loss, they would have to eat 25 or more percent fewer calories than the friend who normally maintains the same body weight, says Schwartz.
That means one person might eat 2,000 calories a day to maintain their weight, while the person who lost all that weight would need to eat just 1,500 calories.
“The fact that they lost weight has activated their energy homeostasis system. At the same time that energy efficiency is increased, appetite is also much greater, so the system is pushed from both sides,” he says. “It happens to anyone who tries to lose weight, it’s just not as dramatic.”
Your genes and environment are to blame, too
While it is starting to become clear why some people have higher body weights no matter how hard they try, you may be wondering what determines who this will happen to.
It turns out, at least half of the population has an inherited predisposition to defend an increased body weight, says Schwartz. In fact, researchers have identified about 100 gene variants that are associated with obesity—and genetic predisposition is almost always from multiple genes inherited together that raise risk, he says.
For the rest of people, genetic predisposition and environmental factors are a potent combination that leads to weight gain, says Schwartz.
Imagine that someone has a genetic predisposition to have a higher body weight, but they work a physically demanding job and have access to healthy, nutrient-rich foods—and the time and energy to cook. That person probably wouldn’t become obese just because of their genes. But we all know how rare that scenario is.
More often in the U.S., the genetic risk factor becomes an issue when environmental factors—including poverty, a sedentary lifestyle and stress—come into play. Americans who live in the poorest counties are most prone to obesity, research shows.
Solving the obesity problem
Nearly 40 percent of American adults have obesity, according to the Centers for Disease Control and Prevention. Physicians and researchers who treat and study the disease hope that changing the perception of what causes obesity will help with the stigma people face—and help find treatments that actually work long-term, says Schur.
“The longer it stays about people’s personal failings, the less likely we are going to make progress in the kinds of treatments that really make a dent in the problem or that cure it,” she says.
Don’t throw in the towel
Despite how difficult it is to lose weight and keep it off, both Schur and Schwartz stress that eating well and exercising are important steps to take for your health, and that it is possible to be healthy while overweight.
“There are benefits of having a healthy lifestyle independent of how much weight you lose,” says Schwartz, citing lowered blood pressure, improved blood sugar levels, a lower dementia risk and a lower risk of heart disease.
Even if you don’t lose all the weight you had hoped, losing just 5 percent to 10 percent of your body weight (10 to 20 pounds for a 200 pound person) significantly improves your health if you have obesity, says Schur. In one study, people who lost 7 percent of their bodyweight through lifestyle changes lowered their risk of diabetes by 50 percent. The effect of the lifestyle changes to lower diabetes risk lasted over 10 years, even when the participants regained some of the weight.
“Obesity risk is determined by both genetic and environmental factors, so if you modify the environment you can reduce the risk,” he says. “Most adults continue to gain weight and increase body fat mass until at least age 60, so it’s kind of a life-long challenge—or opportunity, depending on how you look at it.”