No, “Skinny Genes” Aren’t Really a Thing

The concept of "skinny genes" doesn't refer to tightly fitting pants, but instead to the notion that there are set of genes — units of DNA — that make a person skinny throughout his or her life.

It's not just genetics that determine someone's size and shape. (Image: Westend61/Westend61/GettyImages)

But is there really a standard pattern of genes and/or genetic variations that make people skinny? On the flip side, does that mean there's also a pattern of genes that makes you overweight or obese?

A 2019 study published in PLOS Genetics claimed to identify regions of the genome that could reflect healthy skinniness. Lots of media coverage spun this as saying that thinness is determined by what genes you have.

But as a medical doctor and genetics expert, I strongly disagree with this notion that skinniness is inherently genetic. At best, I would argue that the absence of certain gene variations that contribute to obesity can make it easier to be slimmer, but — and this is important — only if you control your environment (i.e. eat a healthy, balanced diet and get regular physical activity).

However, like most questions in genetics, the answer to this is complex. So let's dig into this a little more to answer the question: What makes someone skinny?

The Truth Behind the "Skinny Genes" Myth

People like simple answers. So it makes sense that with the more we've learned about how genetics determine the way we look — hair color, eye color, skin color, etc. — that we'd start to infer that our DNA also determines our body type. But it's not a simple one-to-one correlation.

It's possible that the presence of absence of particular gene variations (keep reading for more specific examples) give an individual a predisposition to be thinner or heavier, but I don't believe that there are variations that directly make you skinny — apart from those that will lead you to be malnourished and/or deficient from micronutrients. If you eat 10,000 calories a day and don't exercise, guess what? You're going to gain body fat regardless of your genes!

However, understanding your genetics does leave you with clues to help you achieve body composition-related goals. And if you're interested in finding out more about your metabolic efficiency or predispositions to certain eating patterns and behaviors, you can head over to FitnessGenes.com and purchase a DNA test or upload your 23andMe or AncestryDNA data.

There isn't a direct relationship between one gene and how skinny you are (or aren't). (Image: Peter Cade/Photodisc/GettyImages)

So, What Does Determine How Thin You Are?

First and foremost, I'm not making any judgements or statements about whether or not being "skinny" is desirable. Nevertheless, it is a physical trait that's been studied, so it's worth looking into the science behind it. Here are three of the factors that can help predict how thin someone will be:

  • Energy storage is less than energy expenditure* Consistency and never dieting* Not overcompensating for indulging

1. Energy Storage Is Less Than Energy Expenditure

This is not the same as "calories in versus calories out." In order for someone to have a low body fat percentage, there's one fundamental concept that's important to understand. For whatever reason, someone who is 'skinny' is burning more or equivalent energy compared to the energy they're storing. This may of course mean that they're consuming fewer calories than they're burning with their daily activity, but not always.

Some people have a genetic predisposition to be metabolically inefficient. It's become popular to describe such people as having a fast metabolism, but they don't actually have a fast metabolism. Rather, such people lose more energy from food as heat energy as opposed to storing it in tissues.

This doesn't make them skinny per se, but it does make it more difficult for them to gain weight. When their calorie intake is very high, these are the types of people who may start to become "skinny-fat." Far from being healthy, being skinny-fat actually puts you at risk of obesity later in life due to increased inflammatory markers and other negative metabolic changes that lead to fat deposition.

By contrast, being metabolically efficient means you store energy from food very easily. From an evolutionary perspective, this would have been very useful when food may have been scarce for prolonged periods. At a time when energy dense foods are so readily available, however, this can make it difficult to keep body weight down.

2. Consistency and Never Dieting

People who are thin tend to be quite consistent with their eating habits, and often never actually go on a diet. This is an important point because dieting can have some serious consequences on your metabolic health, body composition and risk factors for future disease. In other words, yo-yo dieting rarely leads to long-term weight control.

Nowadays, we can actually predict who might be at risk of yo-yo dieting based on understanding genes like the FTO gene and how these genetic variations could influence psychological factors related to diet, including emotional eating, disinhibition and cognitive restraint. So again, genes can play a role in whether you have a stronger potential to be thin, but your genes can't make you thin.

3. Not Overcompensating for Indulging

Another factor to consider with skinny people is the possibility that they tend to obsess less about food. For example, if they have a night out and consume too much one day, they don't correct for it the following day.

They simply revert back to their regular eating pattern, which is again a characteristic that can be reflected in variations of the FTO gene. This behavior can also be impacted by genetic variations seen in genes (ex. your PPARA genes) that can make people feel fulfilled from eating, especially carbohydrates.

About FitnessGenes

FitnessGenes is the first DNA-testing platform of its kind to eliminate the guessing game from fitness and nutrition. With a quick-and-easy DNA test and an analysis of 681,000 SNP variations (or you can upload you 23andMe or AncestryDNA data), its team of genetic scientists reveals specific traits, including metabolic tendencies, dietary sensitivities, fat-burning capacity, muscle type, recovery time and more. FitnessGenes then prescribes personalized, week-by-week exercise programs and nutritional guides based on your genetic profile.

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