Why Do Some People Never Gain Weight?

August 28, 2020


It’s one of life’s great injustices. Some people have to watch everything they put in their mouths to keep their weight down, while others can eat as many doughnuts as they want and achieve the same results. So what’s the secret? How do some people manage to never gain weight?

Katherine Melanson, a professor of nutrition and food science at the University of Rhode Island, says there’s no simple answer to this question.” There are genetic, nutritional, and even behavioral factors involved,” Melanson told Live Science, “and the degree to which these factors play a role in any given individual will vary.”

One of the most important factors has nothing to do with body size, metabolism, or casting spells during a full moon. It’s perception. Melanson says that many people who appear to eat whatever they like without gaining weight don’t actually eat more than the rest of us. For example, your friend who eats ice cream every day may naturally compensate for those extra calories by eating less at another meal or snacking less at other times of the day. Or maybe they eat pizza very slowly, get full, and then stop after just a few slices.

“If you measure these people’s calories, they’re probably not eating as many as you think,” says Dr. Frank Greenway, chief medical officer at the Pennington Biomedical Research Center.” They’re just eating calorie-dense foods; it may be hard for other people not to overeat things.”

Physical activity can also make a difference, but not necessarily gym workouts.” Some people are just hyperactive, even if they’re not necessarily athletes,” says Melanson. For example, they may be sedentary or pacing, have an active job, or chase their kids around all day. There’s even evidence that some people are genetically predisposed to want to move their bodies, Melanson says. This extra movement can also make a difference in your body’s metabolism, or how much energy your body spends in a day that doesn’t include exercise. You move that more “mitochondria within the muscle cells will increase in number and in their activity. And these are the power plants that create energy and use it for movement,” Melanson says. More mitochondria means more calories burned.

There’s little evidence that – without exercise – some people are born to burn significantly more calories than others, says Dr. Ines Barroso, a researcher at the University of Cambridge in England who studies the genetics of obesity. But there may be physiological differences that allow some people to naturally moderate the number of calories they consume without exercising tremendous self-restraint, Melanson said. A cascade of neurological signals interacts with hormones circulating in our bloodstream to tell us when we’re hungry or full. This is known as the appetite regulation system, and it may be more sensitive in some people than others, Melanson says.

One important hormone involved in this system is leptin. It helps regulate how much food we want to eat for a longer period of time, not just for our next meal. So a person with a more sensitive system might go back to eating for a few seconds and three seconds at a party and then feel full and eat less for the next few days.” They just automatically can kind of recalibrate their energy balance because their appetite signaling system can say, ‘Okay, we have enough energy,'” says Melanson.

Genetics can play a role in a person’s tendency to gain or lose weight. According to a study published in PLOS Genetics in 2019, researchers have identified more than 250 different regions of DNA associated with obesity. In the study, researchers compared 1,622 healthy individuals with a low body mass index (BMI) to 1,985 severely obese individuals and 10,433 normal-weight controls. They found that the lean participants had fewer genes associated with obesity. But according to Barroso, who co-authored the study, “genes alone don’t determine your weight.” We didn’t find genes that specifically either protect against obesity or predispose people to obesity. It seems to be a continuum,” says Barroso, “and you also have people who have the genetic determinants of obesity, but they don’t [have obesity].” Barroso says.

In the end, the answer is complicated: our tendency to gain or maintain weight isn’t predetermined, but it’s not entirely under our control, either. Melanson says there is no genetic switch that allows some people to eat whatever they want without gaining weight; at the same time, the tendency to gain weight is not necessarily due to a lack of self-control.