If you thought spending hours in a gym every day would help you slim down, think again! A new study suggests that exercise may not prevent weight gain.

"Our study results indicate that physical activity may not protect you from gaining weight," said lead study author Lara Dugas, Assistant Professor at Loyola University Chicago Stritch School of Medicine in the US. The findings of the study involving young adults from five countries were published in the journal PeerJ.

Physical activity has many proven health benefits, ranging from reducing the risk of heart disease, diabetes, and cancer to improving mental health and mood. People who are physically active tend to be healthier and live longer.

But while physical activity burns calories, it also increases appetite, and people may compensate by eating more or by being less active the rest of the day.

The researchers followed adults aged 25 to 40 living in five countries -- the US, Ghana, South Africa, Jamaica and Seychelles. Participants wore tracking devices called accelerometers on their waists for a week.

The devices measured the wearers' energy expenditure and step count. Researchers also measured participants' weight, height and body fat. After an initial examination, participants were asked to return one year and two years later.

Surprisingly, total weight gain in every country was greater among participants who met the physical activity guidelines. For example, American men who met the guidelines gained a half pound per year, while American men who did not meet the guideline lost 0.6 pounds. The researchers did not find any significant relationships between sedentary time at the initial visit and subsequent weight gain or weight loss. The only factors that were significantly associated with weight gain were weight at the initial visit, age and gender, the study said.