Not all belly fat is made the same. A small new study has found that people who are overweight or obese but who have exercised regularly for the past two years or more have healthier-looking fat in their bellies.
Compared to non-exercisers, those in the study who consistently partook in endurance exercise possessed fat deposits between the skin and muscle of their abdomens that showed a higher density of blood vessels, a lower abundance of collagen linked to metabolic issues, and fewer pro-inflammatory white blood cells.
All of these factors indicate that the body is depositing fat in the abdomen in a healthier way, even if that doesn't change the amount of fat tissue present.
"Our findings indicate that in addition to being a means to expend calories, exercising regularly for several months to years seems to modify your fat tissue in ways that allows you to store your body fat more healthfully if or when you do experience some weight gain – as nearly everyone does as we get older," explains kinesiologist and principal investigator Jeffrey Horowitz from the University of Michigan.
The study included 52 adults who were considered overweight or obese at the time of research. They were divided into two groups: 28 people who said they did not exercise regularly, and 24 people who reported they did exercise.
Those in the exercise group reported partaking in endurance-type exercises at least four times a week for a minimum of two years. The average participant reported regularly exercising for the past 11 years.
The researchers compared samples of abdominal fat tissue from just beneath the skin in 32 of the participants (half male, half female) with similar body fat percentages – 16 from each group.
People who exercised regularly had fat tissue better suited for storing fat. As well as more blood vessels, less metabolism-interfering collagen, and fewer inflammation-causing cells, exercisers' belly fat tissue had a higher density of mitochondria and proteins involved in metabolism.
In further experiments in the lab, Horowitz and his colleagues analyzed cultured samples of abdominal fat from a different sub-cohort from each of the groups.
Comparing these cohorts, researchers found regular exercisers showed a greater growth of blood vessels in their isolated belly fat tissue, as well as a higher lipid storage capacity, which could indicate improved insulin sensitivity.
Previous studies in mice have found those that are overweight or obese have enlarged fat cells in their abdomen, which can lead to fewer blood vessels distributing oxygen to tissue.
This recent research, however, found that people who regularly exercise show enhanced capacity for oxygen and nutrient delivery in the blood vessels of their fat tissue, even though the cells themselves are enlarged.
"Compared with our previous study in which we examined the effects of three months of training on fat tissue, we generally see these differences are more robust in people who exercise regularly for years versus those who don't exercise," explains Horowitz.
The findings suggest that regular exercise can remodel the content and configuration of body fat cells in a way that is better for metabolic function and that can somewhat alleviate inflammation.
Although some studies on rodents suggest that exercise can increase the percentage of brown fat, which breaks down white fat for energy, the new findings suggest this probably does not occur in humans. The proteins necessary for this process were not elevated in exercisers.
All subjects in the current study self-reported their exercise routines, and the type of endurance exercise was not considered. Longer-term studies with larger cohorts are needed to explore these results further.
That said, the initial findings indicate that adults who are overweight or obese but who exercise regularly are profoundly influencing the function and metabolism of their abdominal adipose tissue and possibly their cardiometabolic health.
The study was published in Nature Metabolism.
