In young, lean men, those with higher cardiovascular fitness before overeating tend to gain less fat and total body energy when consuming excess calories for 100 days, indicating that fitness level...

We analyzed one assertion on this question and found that in young, lean men, those with higher cardiorespiratory fitness before overeating tended to gain less fat and less total body energy when consuming excess calories over 100 days [1]. This suggests that a person’s fitness level before overfeeding may influence how their body handles extra calories, possibly by affecting how energy is stored or used. The evidence we’ve reviewed so far leans toward the idea that better cardiorespiratory fitness — meaning the ability of the heart and lungs to deliver oxygen to muscles during sustained activity — might help limit fat gain when calories are eaten in excess. This doesn’t mean fitness prevents weight gain entirely, but it may shift how the body responds to overeating. The study followed men for 100 days of overfeeding and measured changes in fat mass and total energy stored, finding a pattern linked to their starting fitness level. We did not find any studies that contradicted this observation. However, this is based on a single assertion, and we don’t yet know if the same pattern holds for women, older adults, or people with different body compositions. We also don’t know whether the effect is due to muscle metabolism, hormonal changes, or other factors tied to fitness. What we’ve found so far is limited but consistent: higher fitness before overfeeding may be associated with less fat gain in young lean men. If you’re young, lean, and trying to manage body composition during periods of higher calorie intake, maintaining or improving your cardiorespiratory fitness could be one factor worth considering — though it’s not a guarantee against fat gain.