What if making one part of the body’s energy machinery work harder did not prevent weight gain—but actually came with more body fat? In male mice, that is exactly the puzzle this study found. Obesity and its associated metabolic complications, including type two diabetes and cardiovascular disease, represent a growing global health burden.

Much of that burden is linked to disrupted control of body energy and expansion of fat tissue. Mitochondria act as key regulators of energy metabolism, and mitochondrial dysfunction is strongly implicated in the development of diet-induced obesity and insulin resistance.

The study focuses on a gateway that helps protect mitochondria. If that gateway stays open abnormally, the mitochondria lose their electrical drive and their energy-making process is disrupted. So the researchers removed the gene that makes cyclophilin D, a protein that helps regulate this mitochondrial gateway, in male mice.

They followed the mice on either a standard diet or a high-fat diet. The central question was whether losing cyclophilin D would affect the whole body in the same way, or whether its effects would depend on the diet and the tissue. The findings point to a tissue- and diet-specific role in controlling body energy balance.

Even before the high-fat diet, mice lacking cyclophilin D had higher body weight and more fat, while their lean mass was comparable to that of the other mice. They ate the same amount, but they moved more during the dark phase. Their energy expenditure was comparable, and their overall handling of glucose was also comparable, although insulin response showed a slight late difference.

The timeline makes the comparison easy to follow: measurements began before diet exposure, while body weight and body composition were tracked continuously for thirty-four weeks. After high-fat feeding, the mice lacking cyclophilin D developed greater body weight and fat mass than the comparison mice, while lean mass remained comparable.

They still ate similar amounts and showed similar movement. Here is the surprising part: after accounting for lean mass, their total energy expenditure was higher. They also showed signs of using more fat as fuel during the dark phase.

Higher energy expenditure did not prevent mice fed a high-fat diet from developing greater obesity, while food intake and locomotor activity remained comparable. The researchers compared mitochondrial respiration in quadriceps muscle, brown fat, and liver in mice fed standard or high-fat diets.

In quadriceps mitochondria, ADP-stimulated State three respiration was significantly higher in knockout mice than wild-type mice under both dietary conditions. The increase did not appear in brown fat: oxygen consumption profiles and respiratory control ratios were comparable between knockout and wild-type mice under both dietary conditions.

The direct measurements supported the same pattern: obese mice lacking cyclophilin D had selectively increased energy production in leg muscle, but not in brown fat or liver, without changes in mitochondrial abundance or efficiency. In other words, losing cyclophilin D enhanced the muscle’s ability to respond to an energy demand, while other metabolic tissues remained comparatively unaffected.

Its influence on whole-body metabolism depended on both diet and tissue. Only the quadriceps showed increased respiration; brown fat and liver did not. That muscle-restricted increase may help explain why energy expenditure rose after adjustment for lean mass, even while body fat also increased.

But the study cannot establish that the stronger muscle energy production caused either the altered energy expenditure or the higher body fat. The connection remains unresolved. Future studies will need to test how fuels are used and how energy moves through the body, including measurements earlier during high-fat feeding.

Removing cyclophilin D made diet-related obesity worse, while increasing energy production specifically in leg muscle. The result suggests that metabolism cannot be understood as one whole-body switch: different tissues can respond in opposite ways.