Scientists Discover Why Males and Females Store Fat in Different Body Parts

"I did not expect to see such contrasts between males and females."

Thanks to the body measurements of 362,499 people, we can officially say that the places where fat accumulates in the body in men and women is far from random.

The fate of fat cells — whether they end up in the stomach, legs, or hips — might come down to the effect of just 98 genes, regardless of how many avocados you eat, how much apple cider vinegar you chug, or whether you are male or female. The study results were published this week in Nature Communications.

It’s common knowledge that men tend to accumulate fat in the midsection and women tend to accumulate in the legs and waist, says Mathias Rask-Andersen, Ph.D., lead author of the paper and researcher at Uppsala University’s medical genetics and genomics research group. His work corroborates this observation with hard data.

"I did not expect to see such contrasts"

“I did not expect to see such contrasts between males and females,” Rask-Andersen tells Inverse. “In general, each variant contributes a small amount to the overall trait. However, it is possible to sum the effects and generate a polygenic score that is applicable to each individual and describe a meaningful amount of the variation of these traits.”

Legs, Hips, and Trunk

In the study, Rask-Andersen and his colleagues found that men carried 28 percent their body fat in the legs and hips, whereas women carried 39.7 percent of their body fat in those areas. In their trunks, men carried 62.2 percent of their body fat, whereas women carried only 50 percent. He calls those differences “striking,” chalking them up to the action of 98 genes that he identified in the genetic data of the individuals he studied.

As important as those genes are, Rask-Andersen adds that they’re part of a very complex system: It isn’t simply that women have genes that shuttle fat into their lower halves and men don’t. Rather, it’s that 37 of these genes might only be active in women. Even if a man has one of those genes that would help shuttle fat away from his middle, that gene isn’t actively working to redistribute the fat cells in his body, the way it is in a female.

"So in males, genetic variation related to these traits will have almost no effect at all.

“Our results indicate that the same genetic variant will have an effect in females, but not in males, in particular for distribution of body fat between the legs and trunk,” Rask-Andersen says. “This is indicative of a biological program that is active in females, but inactive in males. So in males, genetic variation related to these traits will have almost no effect at all.”

Manipulating Fat Genes

While we have thousands of genes in each cell in the body, those genes aren’t always expressed at the same time. Environmental factors from light exposure to diet influence when cells express certain genes, turning them “on” or “off.” One of the factors that might activate these genes in women, Rask-Andersen says, are sex-specific hormones that are more abundant in women’s bodies than men’s, though he adds that this still needs to be demonstrated in other studies.

Segmental body impedance analyses. This method uses bio-electrical impedance to estimate body composition: fat mass, muscle mass, etc.

Though the paper doesn’t say what people can actually do about their fat genes, it emphasizes that it’s worth caring where fat accumulates. Previous research suggests that accumulation of fat in the midsection, called visceral fat, has been linked to higher risk of cardiovascular conditions or insulin resistance. Fat in other areas of the body — including the legs and hips — is called subcutaneous fat and doesn’t carry the same risks.

The findings aren’t especially great news for men, but they’re an important step toward understanding all the different factors that contribute to weight-related health, from diet and exercise to genetics.

Abstract: Body mass and body fat composition are of clinical interest due to their links to cardiovascular- and metabolic diseases. Fat stored in the trunk has been suggested to be more pathogenic compared to fat stored in other compartments. In this study, we perform genome-wide association studies (GWAS) for the proportion of body fat distributed to the arms, legs and trunk estimated from segmental bio-electrical impedance analysis (sBIA) for 362,499 individuals from the UK Biobank. 98 independent associations with body fat distribution are identified, 29 that have not previously been associated with anthropometric traits. A high degree of sex-heterogeneity is observed and the effects of 37 associated variants are stronger in females compared to males. Our findings also implicate that body fat distribution in females involves mesenchyme derived tissues and cell types, female endocrine tissues as well as extracellular matrix maintenance and remodeling.

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