Study Finds Strength Training Shrinks Fat Cells Before Weight Loss Begins

A week of strength training was enough to trigger fat-burning changes in obese mice, even though the animals’ body weight stayed the same, according to a study from the State University of Campinas (UNICAMP) published in Life Sciences. The study is part of a research line that has examined how exercise affects metabolic health for nearly two decades, and it looked specifically at what happens inside fat cells before weight loss can complicate the picture. 

Male mice were split into a standard-diet group and a high-fat-diet group to induce obesity. The obese animals were then divided again: one group stayed sedentary, the other began strength training, climbing a 70-centimeter staircase with weight attached to their tails, at 70% of their maximum capacity, for seven sessions total. Both obese groups kept eating the same high-fat diet throughout, isolating exercise as the only variable. 

The researchers focused on mesenteric adipose tissue, which study author Leandro Moura describes as the primary stand-in for visceral fat in humans—the deep abdominal fat linked to inflammation throughout the body. Although total body weight didn’t drop, the trained mice had less mesenteric fat mass than their sedentary counterparts, and their fat cells were smaller.

At the molecular level, the training partially restored the action of proteins involved in lipolysis, or fat breakdown, that obesity had impaired. Specifically, exercise triggered PLIN1 to release ABHD5, which then bound to ATGL to begin breaking down stored triglycerides—a chain that was blunted in sedentary obese mice due to reduced ABHD5 levels. The study also found reduced expression of the Scd1 gene, tied to fat synthesis.

“People tend to think that if they’ve been strength training for two months and haven’t lost weight, it hasn’t done any good. But what we saw in this study is that after just seven days of strength training—even when there was still no weight loss—the molecular pathways that promote fat accumulation were less active, while those that stimulate fat breakdown were more active,” Moura says.

The findings, Moura notes, also push back on the idea that only aerobic exercise supports weight loss. “Both types of exercise activate similar pathways—at different intensities—as well as different pathways. Therefore, combining the two remains the best recommendation and the best strategy for those who want to lose weight.”

The study was conducted only in male mice and examined a single fat deposit; the team next plans to track longer training periods and other fat deposits, including tissue around the heart.