A Romulan with blood on his face. Nanoprobes assimilating blood cells. Blood is the circulatory fluid that runs through most corporeal lifeforms in the universe. Its primary function is to carry nutrients and respiratory gases to cells and to remove waste products.
Muscle is more metabolically active than fat, and building muscle can help increase your metabolism (36, 37, 38, 39). This means you will burn more calories each day, even at rest (40). Lifting weights will also help you retain muscle and combat the drop in metabolism that can occur during weight loss (41, 42, 43, 44).
In that case I don't think their metabolism is significantly higher. I always try to keep in mind that my muscles ARE my metabolism. Yes, an increase in muscle mass usually does mean that your metabolism will be higher--if you remain lean without starving yourself. Consider Sumo Wrestlers who have quite a bit of lean body weight but they also have a lot of adipose tissue as well (fat).