Why Do Certain People Eat A Lot, But Never Gain Weight?

This is one of the greatest injustices in life: some people have to pay careful attention to whatever they eat to control their weight; others, however, can eat donuts at will, and their weight would rarely change. What’s the secret? Why do some people hardly ever gain weight?

Kathleen Melanson, professor of nutrition and food science at University of Rhode Island, said that there’s no simple answer to this question. “It involves genetic, nutritional and even behavioral factors,” Melanson told Live Science, “and the extent of impact may vary with individual differences.”

One of the most important factors is perception. It has nothing to do with body size or metabolism, neither with casting a spell on a full moon night. Melanson said that many people seem to eat whatever they want without gaining weight. In fact, what they really eat is no more than ours. For example, your friend who eats ice cream every day may naturally eat less meals or snacks to offset the heat of ice cream. Or when eating pizza, they eat small pieces very slowly, and they would stop when they’re full.

“If you measure the calorie intake of these people, you will find that they don’t eat as much as you think,” said Dr. Frank Greenway, chief medical officer of the Pennington Biomedical Research Center, “They will eat high-calorie foods, but it is difficult for other people to control themselves when eating this kind of food.”

Exercise also makes a difference, but it is not necessary to sweat in gym. Melanson said: “Some people just move more, even if they are not athletes.” For example, they may be more active, having a manual job, or having to follow their children and run around all day long. Melanson adds that evidence suggests that some people’s genes determine their more willingness to move. Such extra exercise can speed up their bodies’ metabolism. “The more you move”, she said, “the more the number and activity of mitochondria in muscle cells will be. These are the ‘power plants’ that produce energy. And people use this energy for exercise.” The more mitochondria, the more calories will be burnt.

Dr. Ines Barroso, a researcher on genetics of obesity at the University of Cambridge, said there is little evidence to prove that certain people could consume more calories than others without exercising. However, Melanson said that there may be some physical differences that allow people to naturally consume the calories they ate without too much strict exercise. A large number of nervous system signals and hormones circulating in our blood tell us when we are hungry and when we are full. Melanson said this is called the appetite regulation system, and some people may have a more sensitive one.

An important hormone in this system is leptin. It can help us regulate our appetite for a long time, not the appetite for the next meal only. So people with a more sensitive system might eat at will at parties, and then feel not hungry for the next few days and eat little. “They can adjust their energy balance automatically because their appetite regulation system will say: ‘All right, we’ve eaten enough calories’,” Melanson said. 

Heredity could affect a person’s tendency to gain or lose weight. According to an article published in PLOS Genetics in 2019, researchers have identified more than 250 different DNA regions related to obesity. For this study, researchers compared healthy group of 1622 subjects with low BMI, severely obese group of 1985 subjects and control group of 10433 subjects with normal weight. They found that lean participants had fewer genes related to obesity. But Barroso, a co-author of the study, said that genes only cannot affect your weight. “We didn’t find certain gene that specifically prevents or induces obesity. It seems to be a continuum,” Barroso said, “There are also people who have genetic determinants of obesity, but they are just not obese.”

To sum up, the answer to this question is complicated: the tendency to gain or maintain weight is not destined, but it is not completely under our control. Melanson said that there is no genetic switch that allows some people to eat at will without gaining weight; and vice versa, gaining weight is not necessarily due to lack of self-control.

“All men are born different.”