Q: My friend complains that she keeps gaining weight no matter how little she eats because she has a slow metabolism. How much does a person's metabolism really affect body weight?
A: There is a lot of misunderstanding about the impact metabolism has on our health, especially in terms of weight loss.
In simple terms, metabolism is the internal process by which our body expends energy and burns calories. It runs 24/7, even when we're resting or sleeping, by converting the food and nutrients we consume into the energy our body needs in order to breathe, circulate blood, grow and repair cells, and everything else it does to survive.
This process works at different intensities in different people. How fast your friend's metabolism works is determined mostly by her genes. People might have fast, slow, or average metabolism, regardless of their body size and composition. Age also affects metabolism, as it can slow over the years, even if a person starts out with a fast metabolism.
Differences in metabolism speed are evident in how easy or hard it is for people to gain or lose weight. A slow metabolism burns fewer calories, which means more get stored as fat in the body. That's why some people have difficulty losing weight by just cutting calories. A fast metabolism burns calories at a quicker rate, which explains why some people can eat a lot and not gain extra pounds.
But you can't entirely blame a sluggish metabolism for gaining weight. The reality is that metabolism often plays a minor role. The factors that always will matter the most are diet quality, total daily calorie intake and level of activity.
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While it may only be a small change, a person can speed up a naturally slow metabolism, or rev up one that has become sluggish over time. Here's how:
Add some high-intensity intervals and weight training to your regular exercise routine. After a period of interval training, your metabolism can stay revved up for as much as a full day. Weight training builds muscle mass, and that extra muscle will require more calories, turning up your metabolic rate.
Your metabolism increases whenever you eat, digest, and store food, a process called thermic effect of food. Protein has a higher thermic effect than fats and carbohydrates, so eating more healthy protein-rich food sources might also help speed metabolism a bit.
Some studies suggest that drinking green tea can also rev up metabolism. Green tea contains a compound called epigallocatechin gallate, which may slightly increase the calories and fat you burn.
(Howard LeWine, M.D., is an internist at Brigham and Women's Hospital in Boston and assistant professor at Harvard Medical School. For additional consumer health information, please visit www.health.harvard.edu.)