Metabolism: A Look at the Facts

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The following information was taken from a professional handout I came across recently. I thought it would help dispel some of the myths that are circulated in the media.

Metabolism is the amount of energy that you need per day to keep your body functioning and to complete physical activity and lifestyle needs. The energy used for these functions comes from only one source - the foods that you eat. Your resting metabolic rate (RMR) accounts for 60%-70% of the total calories that you burn by regulating your body temperature, keeping your heart pumping, your brain, liver and other organs functioning, keeps your lungs breathing, etc.

Some people say that they gain weight easily because they have a “slow metabolism.” Is this true?

Studies show that the difference in metabolism between two people of the same age, gender, height, and body composition is probably less than 3%. If two 35-year-old women who are both 5′7″ and have the same percentage of body fat would both have their metabolism tested, and woman A burned 1600 calories/day, woman B would most likely burn somewhere between 1552-1648 calories/day. Keep in mind that overweight people usually have faster metabolisms than thinner people. This is because the more you weigh, the more your body has to work.

Is it true that exercise increases your metabolism, helping you to burn calories even when at rest?

Even if you increased your muscle mass by 15%, you would only gain a 5% increase in RMR, which would mean that a person who generally eats 2000 calories/day, would need an extra 75 calories. 

Is it true that your metabolism slows down with age?

Yes, this is technically true. Beginning at age 30 and each decade thereafter, the average person’s metabolism slows down by 2%-3%. This slowing down is not inevitable though. Having an active lifestyle keeps one’s metabolism steady during the decades after age 30.

How will dieting affect my metabolism?

Crash dieting or restrictive eating can have a very strong impact on your metabolism. After 2 weeks of starvation or semi-starvation, resting metabolic rate (RMR) is likely to drop by as much as 15% and can continue to slow down even further during prolonged restriction. Once you have cut your daily calorie level to 1000-1200 calories/day, your metabolism will slow down. However, once a person begins to eat a sufficient amount again, metabolism will “bounce back.” Chronic dieters cannot permanently slow down their metabolism, contrary to popular belief. What does happen though is that with each subsequent diet of 1200 calories or below, your metabolism will slow down just as quickly. Over time, your body composition can shift to a lower proportion of muscle. Muscle is “metabolically active”. If you have a lower proportion of muscle, your metabolism will not be as fast.

Does working out increase your metabolism for several hours after you have finished?

Following a high-intensity workout, your body may burn a bit more for a few hours. If however, you are eating a low-calorie diet, your metabolism will continue to be slow as a result of your restrictive diet. High-intensity working out with a restrictive amount of food will only cause your body to lose muscle and slow down your metabolism as a means of conserving calories.

Is it true that eating spicy foods will increase my metabolism, helping me to lose weight?

If that were true, many people would be devouring chili peppers. The effect of eating spicy foods is so minimal and short-lived that it does not make a difference as far as weight loss is concerned.


References and recommended readings

Finn C. The myth about muscle and your metabolic rate. Available at: Accessed May 14, 2009.

Funderberg L. Ten truths about metabolism. Available at: Accessed May 14, 2009.

Laquatra I. Nutrition for weight management In: Mahan LK, Escott-Stump S. Krause’s Food, Nutrition, and Diet Therapy. 11th ed. Philadelphia, PA: WB Saunders; 2004:562-563.

Mayo Clinic. Metabolism and weight loss: how you burn calories. Available at: Accessed May 14, 2009. The truth about metabolism. Available at: Accessed May 14, 2009. 

Uhland V. The burning questions about metabolism. Available at: Accessed May 14, 2009.

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