Metabolism and weight

You might think a medical condition is a cause of slow metabolism and weight gain. But medical conditions rarely slow metabolism enough to cause a lot of weight gain. Conditions that can cause weight gain include Cushing syndrome or having an underactive thyroid gland, also called hypothyroidism. These conditions are uncommon.

Many things affect weight gain. These likely include genes, hormones, diet and lifestyle, such as sleep, physical activity and stress. You gain weight when you eat more calories than you burn — or burn fewer calories than you eat.

Some people seem to lose weight more quickly and more easily than others. But everyone loses weight by burning more calories than they eat. The bottom line is calories count. To lose weight, you need to eat fewer calories or burn more calories through physical activity. Or you can do both.