We see unique body sizes and shapes, faces and personalities every day. So, it should not come as a surprise that no two people are exactly alike—not even identical twins. Aside from the obvious differences in people’s outward appearance, there are many differences in people’s physiology as well. Mary-Ellen Harper, PhD, from the University of Ottawa in Canada, has discovered that variations in muscle physiology may explain why some people seem to be able to lose weight faster than others. She presented her work last month at Experimental Biology 2021.

When Harper and her research team took a close look at the muscles of overweight people who lost very little weight while dieting versus those who lost a lot of weight, they found big differences in the way their muscles produced energy. Muscles produce energy by converting the food you eat to a form of energy called adenosine triphosphate, or ATP. Compared to people who didn’t lose much weight after dieting, the people who lost the most were not as efficient at making ATP. In other words, their muscle cells burned more calories to make the same amount of ATP.

Interestingly, the researchers found that those who didn’t lose a lot of weight got the most benefit from exercise, such as walking on a treadmill and strength training, rather than dieting. After six weeks of exercise training, this group lost a higher percentage of fat mass than those who lost a lot of weight. In addition, exercise boosted ATP-making proteins in the people who lost more fat mass.

It is too early to know if differences in muscle physiology are the reason some people have better results with dieting and exercising. But the take-home message when it comes to finding ways to lose weight and be healthier is one size does not fit all.