Muscle mass –The amount of muscle tissue you have can be a factor that affects metabolism. Muscle requires more energy to function than fat because fat is not metabolically active. As your percentage of lean muscle increases, your metabolism also increases.
Gender – Metabolism is different between men and women. Generally, due to their greater muscle mass, most men have a higher metabolism rate than women, and women lose muscle mass faster than men.
Age –There is a natural decline in our metabolism as we age, starting as early as 30. After 40, our metabolism slows down about 5% every 10 years. Also, weight gain is usually stored in the middle section of the body, where there is often a loss of muscle mass.
Body size –Bigger bodies usually have larger internal organs and more fluid to maintain. Taller people have a larger skin surface, causing their bodies to have to work harder to maintain a constant temperature, which generally causes a faster metabolism. People who weigh less, need less energy to fuel their bodies.
Genetics – Can also play a role in whether you have a slower or faster metabolism, and some genetic disorders can also affect your metabolism.
Hormonal factors - Imbalances caused by certain conditions, including hypo- and hyperthyroidism, can affect your metabolism. Hypo- may decrease your energy needs and hyper- may increase them.
Environmental factors – The weather can affect your metabolism. For example, if it is very cold or very hot, your body has to work harder to maintain its normal core temperature