BODY SIZE AND COMPOSITION If you way more or have more muscle mass, you will burn more calories even at rest. So people who were more are more likely to have a faster metabolic rate, not a slower one because a portion of excess weight is muscle tissue.
AGE As you get older, muscle mass decreases, which slows down the rate at which you burn calories.
GENETICS Genetics also play a role in whether you have a slower or faster metabolism, and some genetic disorders can also affect your metabolism.
DIET set an aspects of your diet can also affect metabolism. For instance, if you don't have enough iodine for optimal thyroid function, it can slow down your metabolism.
MUSCLE MASS muscle requires more energy to function, pat show the more muscle tissue you carry, the more energy for body needs just to exist.
ENVIRONMENTAL FACTORS The weather can also have an effect on your metabolism. If it is very cold or very hot, your body has to work harder to maintain its normal temperature and that increases the metabolic rate.
DRUGS caffeine and nicotine can increase your metabolic rate, while medication, including some antidepressants and anabolic steroids, can contribute to weight gain regardless of what you eat.
PHYSICAL ACTIVITY Regular exercise increase muscle mass and encourages your body to burn kilojoules at a faster rate, even when at rest.