Study examines the early effects of strength training

Published in Life Sciences, the research examined what happens to adipose tissue during the early stages of strength training, before weight loss can influence the results. Supported by FAPESP, the study builds on nearly 20 years of research into how exercise affects metabolic health.

Since 2007, research groups have investigated the biological mechanisms behind the effects of physical activity on conditions such as obesity and diabetes. As molecular analysis techniques have improved, scientists have expanded their focus beyond visible outcomes such as weight loss and muscle gain to examine changes taking place inside cells.

“We’ve always heard that exercise burns fat and helps us lose weight. But we rarely stop and think about how it happens, how much exercise we need, and what that fat burning actually means,” says Leandro Moura, a physician, physical education expert, UNICAMP professor, study author, and coordinator of the Exercise Cell Biology Institute (ECeBiL). The group studies the physiological, metabolic, and biomolecular adaptations caused by physical exercise and sport.

How the strength training experiment worked

Male mice participated in the experiment. One group received a standard diet, while another received a high-fat diet designed to induce obesity. The obese mice were then divided into two groups: one remained sedentary, while the other completed a strength training program.

Both groups of obese mice continued eating the same high-fat diet throughout the experiment. Keeping their diets identical allowed the researchers to isolate the effects of exercise more clearly.

To mimic strength training, the mice climbed a 70-centimeter flight of stairs while carrying a weight attached to their tails. Each workout was performed at 70% of the animals’ maximum capacity and included 20 climbs, with 60 to 90 seconds of rest between climbs. The mice completed seven training sessions in total.

“We calculated how much weight each mouse could withstand while climbing stairs. Of course, it’s impossible to exactly replicate the strength training that humans do, but we were able to induce a similar effort in the body with more intense contractions, effectively mimicking a human strength training session,” Moura explains.

Researchers focus on visceral fat

The researchers analyzed mesenteric adipose tissue, which surrounds the intestines. Moura said this tissue is primarily representative of human visceral fat—the fat that accumulates deep in the abdomen between vital organs.

Visceral fat is generally considered more harmful to health than subcutaneous fat, which lies just beneath the skin. Excess visceral fat releases inflammatory substances into the bloodstream that can affect metabolism throughout the body.

The seven-day training period was chosen deliberately. The researchers wanted to measure how exercise affected adipose tissue before weight loss began. Weight loss can trigger additional physiological changes, making it more difficult to distinguish the direct effects of strength training.

“If you can maintain the same weight and see what happens inside your body, you can directly determine what effect your physical exercise had before you lost weight,” Moura explains. To avoid introducing another variable, both groups of obese mice received the same diet throughout the experiment.

Fat cells became smaller before body weight changed

Although the mice did not lose total body weight, those that completed resistance training had less mesenteric adipose tissue than sedentary obese mice. Their adipocytes, or fat-storing cells, also became smaller.

Fat cell size can affect metabolic health. During obesity, fat cells may become chronically stressed and swell to the point that they release inflammatory substances into the bloodstream. The reduction in cell size after exercise suggests that the tissue was moving toward a healthier metabolic state before visible weight changes occurred.

Strength training reactivated fat breakdown

The researchers also examined molecular activity inside the fat cells. They found that obesity reduced proteins involved in lipolysis, the process that breaks down stored fat and makes it available for energy.

After seven strength training sessions, part of this fat-breakdown system began working again.

The proteins studied included PLIN1, or perilipin-1, ABHD5, and ATGL. PLIN1 helps regulate how fat is stored inside fat cells. When stimulated by exercise, PLIN1 changes and releases ABHD5, which helps initiate fat breakdown.

ABHD5 then binds to ATGL, an enzyme involved in the first step of breaking down triglycerides stored inside cells.

The researchers also examined Scd1, a gene involved in producing fat molecules. This finding suggests that the body may become less inclined to store calories from food as fat.

In sedentary obese mice, ABHD5 levels were low, indicating that the lipolytic system was not strongly activated. Strength training increased the interaction between ABHD5 and ATGL and activated additional markers associated with lipolysis.

“People tend to think that if you’ve been strength training for two months and haven’t lost weight, you haven’t done anything. But what we found in this study is that after just seven days of strength training, even if you hadn’t lost weight yet, the molecular pathways that promote fat storage were less active, and the molecular pathways that stimulate fat breakdown were more active,” Moura said.

Strength training may offer benefits beyond muscle growth

Moura said the findings challenge the common belief that aerobic exercise primarily supports weight loss, while strength training mainly builds muscle.

“Both types of exercise not only activate similar pathways at different intensities, but they also activate different pathways, so combining the two remains the best recommendation and best strategy for people looking to lose weight,” he said.

What researchers plan to study next

The researchers caution that the results are experimental. Seven days of strength training in mice will not necessarily produce the same effects in people.

The study included only male mice and examined a single type of fat deposit, focusing on genes, proteins, and molecular signals associated with fat breakdown.

Future studies will follow the animals over a longer training period to determine how these early changes develop after weight loss begins. The researchers are also investigating how exercise affects other fat deposits, including fat around the heart.

Moura said the findings could encourage people who have recently started exercising but have not yet noticed a change on the scale.

“Even if we can’t see anything yet with the naked eye, metabolically there are many signals happening in our bodies that are beneficial for our health, including weight loss. That’s why we shouldn’t get discouraged if we don’t see visible results after short-term strength training,” the researchers concluded.