Just one week of strength training was enough to increase visceral fat breakdown in obese mice, even though their body weight had not yet changed. Researchers at the State University of Campinas (UNICAMP) found that seven days of exercise activated biological processes that break down stored fat, reduced the size of fat cells, and lowered the amount of fat surrounding the animals’ intestines.
The study, published in the journal Life Sciences, was designed to examine what happens to adipose tissue during the earliest stages of strength training, before weight loss could influence the results. Supported by FAPESP, the research builds on nearly two decades of work investigating how exercise affects metabolic health.
Since 2007, the research group has explored the biological mechanisms behind the effects of physical activity on conditions including obesity and diabetes. As molecular analysis techniques have improved, the scientists have expanded their focus beyond visible outcomes such as weight loss and muscle gain to investigate changes taking place inside cells.
“We’ve always heard that physical exercise burns fat and helps you lose weight. But we rarely stop to think about how that happens, how much exercise is needed, and what that fat burning actually means,” says Leandro Moura, a physician, physical education professional, professor at UNICAMP, author of the study, and coordinator of the Exercise Cell Biology Lab (ECeBiL). This group is dedicated to studying the physiological, metabolic, and biomolecular adaptations induced by physical exercise and sports.
How the Strength Training Experiment Worked
The experiments involved male mice. One group received a standard diet, while another was given a high-fat diet to induce obesity. The obese mice were then separated into two groups. One remained sedentary, while the other completed a strength training program.
Both obese groups continued eating the same high-fat diet during the experiment. By keeping their diets identical, the researchers could more clearly isolate the effects of exercise.
To mimic strength training, the mice climbed a 70-centimeter staircase while carrying weights attached to their tails. Each workout was performed at 70% maximum capacity and consisted of 20 climbs, with 60 to 90 seconds of rest between climbs. The animals completed seven training sessions in total.
“We calculated how much weight each mouse could carry while climbing a staircase. Obviously, it isn’t possible to replicate the exact strength training exercises performed by a person, but we were able to induce a similar effort in the body, with more intense contractions, effectively mimicking a strength training session for humans,” Moura explains.
Researchers Focus on Visceral Fat
The researchers concentrated their analysis on mesenteric adipose tissue, which surrounds the intestines. According to Moura, this tissue is the primary representative of “visceral fat” in humans – the fat that accumulates deep within the abdomen, among the vital organs.
Visceral fat is considered more harmful to health than subcutaneous fat, which lies directly beneath the skin. When visceral fat becomes excessive, it can release inflammatory substances into the bloodstream that affect metabolism throughout the body.
The short seven-day training period was chosen deliberately. Researchers wanted to examine what exercise itself was doing to fat tissue before weight loss began, since losing weight can trigger additional physiological changes that might make it harder to separate the direct effects of strength training.
“When we’re able to maintain the same body weight and observe what happened in the body, we can directly isolate what physical exercise contributed to before weight loss,” Moura explains. The two groups of obese animals were fed the same diet throughout the experiment to avoid introducing another variable.
Fat Cells Shrink Before Body Weight Changes
Despite showing no reduction in total body weight, the mice that performed strength training had less mesenteric adipose tissue than the sedentary obese mice. Their adipocytes (cells responsible for storing fat) were also smaller.
Fat cell size can have important consequences for metabolic health. In obesity, adipocytes can expand until they become chronically stressed and begin releasing inflammatory substances into the bloodstream. The shrinking of these cells following exercise indicates that the tissue is moving toward a healthier metabolic state even before changes in body weight become visible.
Strength Training Reactivates Fat Breakdown
Researchers then examined the molecular activity inside the fat cells. They found that obesity had disrupted proteins involved in lipolysis, the process through which stored fat is broken down and made available for energy.
After seven strength training sessions, part of this fat breakdown system had begun functioning again.
Among the proteins studied were PLIN1 (perilipin-1), ABHD5, and ATGL. PLIN1 helps control how fat is stored inside fat cells. When exercise stimulates PLIN1, it changes and releases ABHD5, which helps initiate fat breakdown.
ABHD5 then binds to ATGL, an enzyme that participates in the first stage of breaking down triglycerides stored inside the cell.
Researchers also detected lower expression of the Scd1 gene, which plays a role in producing fat molecules. That finding suggests that the body may become less inclined to store calories from food as fat.
In sedentary, obese mice, ABHD5 levels were lower, meaning the fat breakdown system was not strongly activated. Strength training increased the interaction between ABHD5 and ATGL and also activated additional markers associated with lipolysis.
“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 comments.
Strength Training Does More Than Build Muscle
Moura says the findings challenge the common idea that aerobic exercise is mainly responsible for weight loss while strength training is useful primarily for building muscle.
“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,” says Moura.
What Researchers Want to Study Next
The researchers caution that the results remain experimental. The findings do not mean that seven days of strength training will necessarily produce the same effects in people.
The study included only male mice, examined a single type of fat deposit, and focused specifically on genes, proteins, and molecular signals associated with fat breakdown.
Future studies will follow animals through longer training periods to see how these early changes develop once weight loss begins. The team is also examining how exercise affects other fat deposits, including fat located around the heart.
Moura says the findings could still offer encouragement to people who have recently started exercising but have not yet noticed changes on the scale.
“Even if we aren’t seeing anything yet with the naked eye, metabolically, inside our bodies, various signals that are beneficial to health – including weight loss – are taking place. That’s why we shouldn’t get discouraged by the lack of visible results after a short period of strength training,” the researcher concludes.
