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Why do some people never gain weight?

woman eating pizza
(Image: © Shutterstock)

It's one of life's great injustices: Some people must carefully pay attention to everything they put in their mouths in order to maintain their weight, while others can eat doughnuts to their heart's content and achieve the same result. So what's the secret? How do some people manage to never gain weight?

There isn't one simple answer to this question, said Kathleen Melanson, a professor of nutrition and food sciences at the University of Rhode Island. "There's genetic, nutritional, and even behavioral factors involved," Melanson told Live Science, "and the extent to which each of those factors come into play in any given individual is going to vary."

One of the most important factors has nothing to do with body type, metabolism, or performing a spell during the full moon: It's perception. Many people who appear to eat whatever they like without gaining weight aren't actually eating more than the rest of us, Melanson said. For example, your friend who eats ice cream on a daily basis might naturally compensate for those extra calories by eating less at another meal, or snacking less throughout the rest of the day. Or perhaps, when they eat pizza, they're eating slowly, getting full, then stopping after just a few slices. 

Related: Can you turn fat into muscle?

"If you measured these people's calories, they may not eat as much as you think," said Dr. Frank Greenway, the Chief Medical Officer at Pennington Biomedical Research Center. "They're just eating calorically dense foods when they do eat; things other people might have a difficult time not overeating."

Physical activity can also make a difference, but it doesn't have to be a gym workout. "Some people just move more, even if they're not necessarily athletes," Melanson said. For instance, they might fidget or pace, have an active job, or spend all day chasing their kids around. There’s even evidence that some people are genetically predisposed to want to move their body, Melanson said. That extra movement can also rev up the body's metabolism, or how much energy your body spends throughout the day, not including exercise. The more you move, the more  "the mitochondria within the cells of the muscle will increase in number and in their activity. And those are the power plants that are creating energy, using energy for movement," Melanson said. More mitochondria, means more calories burned. 

There's little evidence to suggest that — without exercise — some people are born burning significantly more calories than others, said 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. Cascades of nervous system signals and hormones that circulate in our blood interact to tell us when we're hungry or full. This is called the appetite regulatory system, and it may be more sensitive in some people than others, Melanson said. 

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

Genetics can play a role in a person's tendency to gain or lose weight. Researchers have identified over 250 different regions of DNA that are associated with obesity, according to a 2019 study published in PLOS Genetics. For this study, researchers compared 1,622 healthy people with low body mass index (BMI) against 1,985 people with severe obesity and 10,433 control people of normal weight. They found that the thin participants had fewer genes associated with obesity. But according to Barroso, who was a co-author on the study, genes alone don't determine your weight. "We didn't find genes that were exclusively either protecting from obesity or predisposing someone to obesity. It seemed like a continuum," Barroso said, "You also have people who have the genetic determinants for obesity yet they're not [obese]." Barroso said.

In the end, the answer is complex: our tendency to gain weight or maintain our weight isn't pre-determined, but it's also not entirely under our control. There's no genetic on-off switch that allows some people to eat all they want without gaining weight; at the same time, a tendency to gain weight isn't necessarily due to a lack of self control, Melanson said.

"It's not the same from one person to another."

Originally published on Live Science.

  • Debunk the Junk
    Genetics will play the largest factor. Scientists don't like saying this too often because it carries cultural implications but it is what it is. We are biological beings, there's no getting around it.

    Also I find the first 3-4 paragraphs of this article not really science at all, rather anecdotal opinion based off junk science that produces inconsistent evidence. I.e. the calories in:calories out arguement. The hypothesis that you can consume a bowl of ice cream ridden in tons of sugar, high-fructose genetically modified corn syrup, and preservatives as oppsed to organic chicken breast within the same caloric intake value and the result is negligible is not reputable science you should give any mind too. As soon as you introduce the pancreas and liver into the equation this in-out argument becomes extremely silly and misinformed.

    It's important to remember that in the scientific method, hypothesis is not evidence, and evidence is not proof. When researching studies, aways look at which body is conducting the study, what is the sample size and what are the controls. Was it peer-reviewed independently by a reputable source. You can pretty much toss out studies conducted internally by manufacturers and always get a second opinion on anything FDA regulators pass. The current American nutrition pyramid is basically upside down. A real boon to those industries that rank low on the pyramid.

    No mention of thyroid conditions eh?
    Reply
  • Nemo
    Debunk the Junk said:
    Genetics will play the largest factor. Scientists don't like saying this too often because it carries cultural implications but it is what it is. We are biological beings, there's no getting around it.

    Also I find the first 3-4 paragraphs of this article not really science at all, rather anecdotal opinion based off junk science that produces inconsistent evidence. I.e. the calories in:calories out arguement. The hypothesis that you can consume a bowl of ice cream ridden in tons of sugar, high-fructose genetically modified corn syrup, and preservatives as oppsed to organic chicken breast within the same caloric intake value and the result is negligible is not reputable science you should give any mind too. As soon as you introduce the pancreas and liver into the equation this in-out argument becomes extremely silly and misinformed.

    It's important to remember that in the scientific method, hypothesis is not evidence, and evidence is not proof. When researching studies, aways look at which body is conducting the study, what is the sample size and what are the controls. Was it peer-reviewed independently by a reputable source. You can pretty much toss out studies conducted internally by manufacturers and always get a second opinion on anything FDA regulators pass. The current American nutrition pyramid is basically upside down. A real boon to those industries that rank low on the pyramid.

    No mention of thyroid conditions eh?
    Agree. Lots of magic in this "study". 85% females are obese because they intake too many calories and don't move. Cause and effect, girl scientista. Females are lazy leaches and that's why they are fat.
    Reply
  • senecarr
    Debunk the Junk said:
    Genetics will play the largest factor. Scientists don't like saying this too often because it carries cultural implications but it is what it is. We are biological beings, there's no getting around it.

    Also I find the first 3-4 paragraphs of this article not really science at all, rather anecdotal opinion based off junk science that produces inconsistent evidence. I.e. the calories in:calories out arguement. The hypothesis that you can consume a bowl of ice cream ridden in tons of sugar, high-fructose genetically modified corn syrup, and preservatives as oppsed to organic chicken breast within the same caloric intake value and the result is negligible is not reputable science you should give any mind too. As soon as you introduce the pancreas and liver into the equation this in-out argument becomes extremely silly and misinformed.

    It's important to remember that in the scientific method, hypothesis is not evidence, and evidence is not proof. When researching studies, aways look at which body is conducting the study, what is the sample size and what are the controls. Was it peer-reviewed independently by a reputable source. You can pretty much toss out studies conducted internally by manufacturers and always get a second opinion on anything FDA regulators pass. The current American nutrition pyramid is basically upside down. A real boon to those industries that rank low on the pyramid.

    No mention of thyroid conditions eh?
    All genetics are only expressible in an environment. What's more, metabolic functions are pretty nuts and bolts stuff that don't usually have huge variances. So I'm not sure what you think is so biologically determined.
    Calories in versus calories out isn't an argument. It is both an expression of one of most well observed science laws - thermodynamics, as well as a consistent observation of nutrition. You lock someone in a metabolic ward so you can truly measure all the output, and control all the dietary input, and even back into the 1800s, we don't see more than 4% disagreement between the two - smaller for any modern ones.
    I don't know what it matters about the GMO in high-fructose GMO corn syrup. Corn can be a GMO crop, but high-fructose syrup with the same fructose/glucose percentage is the same chemical, and none of it can be genetically different because saccharide molecules don't have genes. If you think the saccharide molecules of a particular type, e.g. fructose, glucose, sucrose, can be different while the same type by virtue of the plant making them, you've misunderstood somethings in chemistry.
    That said, if you keep calories the same and account for calories out (a food with more protein will have higher thermic digestion effects for output), yes, the change in weight over time between the two foods will be negligible. A step further, if you control for differences in the macros - fats, carbs, proteins - you'll have the biggest factors accounting for dietary control of body weight and body composition. That's where chicken versus ice cream can start being non-negligible.
    Your take on who conducts the research comes off as a bit much. Conflicts of interest are reasons for caution in analyzing the findings of research but they are never evidence of themselves of anything wrong with a finding or methodology.
    Thyroid conditions generally change how active someone is and perhaps a bit of their appetite. It doesn't change anything fundamental about if people can lose or gain weight.
    Reply