Metabolism
This is the process that extracts and utilizes energy (measured in calories) from food. Even at rest, energy is needed for many functions, such as breathing, the beating of your heart, and cell growth and repair.
The amount of energy used for these basic functions while you are at rest is known as your “resting metabolic rate,” which accounts for approximately 70% of your body’s use of energy each day. The rest is taken up with the physical exercise you do each day to expend calories. The more activity, the more likely you will be to lose weight, but your metabolism can be a trick beast to deal with.
Your resting metabolic rate is affected by your weight, age, level of physical activity, and the amount of muscle in your body. Having more muscle increases your metabolic rate, since muscle utilizes more energy than fat, even at rest.
This is why people who used to be athletes often pack on the pounds when they stop competing, and why many of us start to suffer from ‘middle aged spread.’
Your resting metabolic rate is also in part genetically determined.
The act of eating also uses up energy, because energy is needed to digest food, absorb nutrients, and store excess calories as body fat. This process—called thermogenesis—accounts for 10–15% of the body’s total daily energy expenditure.
Some research suggests that obese people require slightly less energy for thermogenesis than normal-weight people, and thus more of the calories they eat are stored as fat rather than used to process food.
