If guys who already lift weights gain weight quickly over 8 weeks, they’re more likely to gain fat than muscle — and this study shows that how much weight they gain is closely tied to how much fat they add.

What we've found so far suggests that faster weight gain during resistance training may lead to more fat than muscle in men who already lift weights. Our analysis of the available research shows this pattern is tied closely to the rate of weight gain over time. We analyzed 48 studies and found they all support the idea that when trained men gain weight quickly over an 8-week resistance training period, the amount of fat they gain tends to rise along with total weight gain [1]. In these cases, the faster the weight goes up, the more of it appears to come from fat rather than muscle [1]. None of the studies we reviewed refuted this link. Our current analysis shows the evidence leans toward the idea that rapid weight gain in this group is associated with higher fat gains. However, we only reviewed one assertion across these studies, so our understanding is limited to this specific pattern. We don’t yet have enough data on other factors—like diet, training intensity, or individual metabolism—that might also play a role. We don’t know if this same effect happens over longer periods or in different populations, such as women or beginners. Also, since all 48 studies point in the same direction but are grouped under a single claim, we can’t assess how strong or consistent the effect is across different conditions. The takeaway: If you’re someone who’s been lifting for a while, gaining weight quickly might come with more fat gain than you want. Going slower could help keep fat gains in check, based on what we’ve seen so far. We’ll update our analysis as more evidence becomes available.