Background. The prevalence of obesity has increased in the last two decades. To maintain body weight energy expenditure (EE) should be equal to energy intake (EI). A low EE predisposes individuals to weight gain and to obesity that can also results from low resting metabolic rate (RMR). Caffeine (Caf) is an active food ingredient and is widely consumed globally, and has an important impact on energy balance. Caf reduces appetite (EI) and increases EE, thus, Caf has a potential role in body weight reduction. Caf causes higher total daily energy expenditure (TDEE) in normal weight (NW) people compared to obese (OW). Moreover Caf is linked to decreased fat oxidation in OW. There are differences between OW and NW in Caf pharmacokinetics, but no differences reported between NW and OW females in its effects on RMR. There is a gender difference in the influence of Caf on metabolism. The investigators are aware of no previous studies which compared the effect of Caf on the RMR of NW and OW men.