Naturally Thin? Why Some People Don’t Seem to Put on Weight

Naturally Thin? Why Some People Don't Seem to Put on WeightMost people have to work hard to avoid putting on weight as they age, but there’s a small subset of people who seem almost resistant to weight gain and are naturally thin. Despite eating a healthy diet, they struggle to keep weight on. It’s not a stretch to say that some of these people are born with genetics that helps them stay slender. Scientists have confirmed this. They’ve identified a group of 28 genes that predispose a person to be thin.

Normally, we have matching sets of chromosomes, a copy from each of our parents. Some men and women have a genetic anomaly that makes it more difficult for them to gain weight. More specifically, they have extra copies of 28 genes located on chromosome 16. Having extra copies of these genes is linked to being underweight. On the other hand, people with too few copies of these genes are at high risk for obesity. As you might expect, this group of genes plays an important role in regulating body weight.

The Drawbacks of Having Skinny Genes

How common is it to have extra copies of these genes that make it hard to gain weight? About one in 2,000 have it. Of course, there are some drawbacks to being born with “skinny genes.” Men and women with duplicate copies of these genes have no problem keeping weight off, but they can have other health problems as well. About one in one in 4 has a condition called microcephaly where their head and brain are smaller than they should be. This puts them at increased risk for neurological problems and it may shorten their lifespan. In addition, they usually grow and develop more slowly as children.

Keep in mind that people with these gene duplications are usually significantly underweight. Most have a BMI of less than 18.5 and can’t seem to gain weight no matter what they eat. What’s interesting is how important this group of genes is for controlling body weight. If you have too many copies, it’s hard to gain weight. If you have too few, you’re at 40 times increased risk for obesity.

Do Naturally Thin People Have More Brown Fat?

Ever notice how some people eat a lot but don’t seem to gain weight? Unlike people with gene duplications on chromosome 16, they aren’t necessarily underweight, they just don’t seem to gain despite eating a substantial number of calories.

How can you explain this? It’s possible naturally thin people burn more energy through NEAT (non-exercise associated thermogenesis) by fidgeting more or they’re active enough that they burn off the excess calories.

Another theory is that these people have more “brown fat.” Unlike white fat, the adipose tissue you store on your tummy and hips, brown fat is “inefficient” fat tissue. It converts food and nutrients into wasted energy in the form of heat. Babies have lots of brown fat because it helps them stay warm. Generating heat seems to be the primary purpose of this type of metabolically-inefficient fat.

At one time, experts believed people lost all of their brown fat by the time they reached adulthood. Now research shows most people retain at least some of it, usually in the neck and upper chest. This brown fat can be activated under certain circumstances. Thin people not only have more brown fat but seem to have a greater capacity to activate it. They use it to convert a portion of the food they eat into wasted energy or heat. Why is this important? When they overeat, their brown fat is turned on and they don’t gain weight. The more brown fat you have and the more you’re able to activate it, the more calories you “waste.”

Obviously, this is fat you want around – and you want it to stay active. Factors that seem to “turn on” brown fat are exercise and exposure to cold temperatures. In one small study, researchers found energy expenditure in participants rose when they lowered the temperature of the room they were in. Staying “cozy and warm” has drawbacks when it comes to controlling your weight. Brown fat seems to be activated at temperatures of 60 degrees Fahrenheit and lower.

Exercise Turns on Brown Fat

Exercise not only turns on brown fat – it may help to turn white fat into brown fat. Research in mice shows white fat, the storage form of fat you want less of, takes on characteristics of brown fat in response to exercise. When you work out, you may be creating more metabolically-active brown fat from white fat. This allows you to “waste” more calories from the food you eat as heat. At least in mice, brown fat also increases insulin sensitivity and improves metabolic health. So brown fat has other benefits as well. Turning on the brown fat you have is another motivation to work up a sweat exercising.

The Bottom Line?

Genetics do play a role in body weight. Researchers believe that around 70% of a person’s body weight may be determined by genetics. The good news? You have control over the other 30%. Eating a clean diet and exercising is a very important part of the equation. It’s nice to have good genetics and brown fat but it’s even more important to be disciplined.



Eurekalert.org. “‘Gene overdose’ causes extreme thinness”

Nature. 2011 Aug 31;478(7367):97-102.

Medscape.com. “Brown Fat and Obesity: The Next Big Thing?”

Discovery News. “Turn Down the Heat, Lose Weight?”

Medical News Today. “Exercise Can Turn Bad Fat into Good Fat”

Behav Genet. 1997 Jul;27(4):325-51.


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Safe Metabolism Boosters: Science Reveals New Ways to Burn More Fat

Types of Body Fat: White Fat, Brown Fat, and Beige Fat and What They Mean for Your Waistline

5 Scientifically-Backed Ways to Activate Brown Fat

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