Story highlights The less you weigh and the more fit you become, the fewer calories you burn doing the same amount of exercise

Irregular eating patterns can decrease the number of calories burned during digestion

(CNN) Metabolism is a term that is thrown around a lot in the nutrition world, but most people don't fully understand what it really is, what aspects of your metabolism are under your control and how you can optimize it to help you lose weight and keep the weight off.

The term metabolism actually refers to all the chemical processes that take place in your body to keep you alive. When it comes to weight loss, most people are referring to the total energy expenditure (TEE), or the number of calories that you burn on a daily basis.

Resting metabolic rate, the calories your body burns at rest, makes up approximately two-thirds of TEE and depends on your age, sex, body size, its composition, hormone levels and recent energy intake. The more body mass you have, the more you burn, and muscles and other lean tissue, especially major organs, burn significantly more than fat.

As we age, changes in hormones and protein metabolism, in conjunction with decreased physical activity, lead to a muscle loss of approximately 3% per decade beginning in your 30s, often accompanied by a slow gain in fat. Regular strength training can help offset these changes in body composition, just one factor you can control.

Genetics plays a role. And restricting calories can also decrease resting metabolism. A recent study showed a decrease of 72 calories per day after just three weeks of cutting calories in half. This may not seem like much but it could be a major factor in weight regain over time.

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