How Dieting And Age Affect Your Metabolism
If you have ever dieted without exercising, you have most likely lost some of your muscle tissue as you lost weight. That's because when you diet, especially at low-calorie levels, you break down muscle mass along with fat to meet your daily calorie needs. With muscle loss comes a reduction in calorie requirements because muscle is an active calorie burner. Since "metabolism" refers to the calories your body burns to maintain itself, it seems like dieting created a change in your metabolism--when really it didn't.
The Yo-Yo Diet Effect
While you are restricting calories and losing weight - or "dieting" - your BMR rate slows down. That's the rate at which your body burns calories to perform normal functions like breathing and staying warm. Very low calorie diets (around 800 calories a day) can reduce your metabolic rate by as much as 30 percent. Your body perceives itself to be "starving", so it reduces the calories it burns. It was once thought that this reduction in calorie burning would affect your resting metabolic rate indefinitely, but research does not support that idea. A more recent study showed that women who diet frequently do not have significantly lower metabolic rates than women who have not been chronic dieters. You will sometimes hear dieters say "I've ruined my metabolism", but this is not true or possible. It may seem like you are burning fewer calories than you did at one time and that is true. But the reason is because you have lost muscle through past weight loss efforts and you are not as active as you should be, especially with strength training.
Aging and Metabolism
As you age, your muscle mass diminishes and your resting metabolic rate slows so you lose strength. With this loss of strength, it's natural to reduce your activity because you feel tired and old. Over time, you continue to lose more muscle due to inactivity. All the while, your weight is creeping up, which makes you feel more sluggish and the cycle continues. This is why your calorie requirements decrease as you age, making weight gain easier and weight loss more difficult than in the past.
Thankfully, the effect of muscle loss through the aging process can be prevented and reversed through physical activity. One study looked at the effect of regular exercise on muscle mass, body fat, and resting metabolic rate in women. After six months of regular exercise, the women's' bodies had changed. They had more muscle, much less fat and their resting metabolic rates were significantly higher when compared with inactive women. This study shows that reviving old muscle tissue and speeding up your metabolism is definitely achievable.
Though you can't control your age, you can control how well your body ages. If you need to lose weight, focus on it so you can preserve your muscles. Keep as active as possible and periodically alter your exercise routines to maximize your metabolism's potential.
- McCargar LJ, Sale J, Crawford SM. Chronic Dieting Does Not Result in a Sustained Reduction in Resting Metabolic Rate in Overweight Women. J Am Diet Assoc.96; (1996):1175-1177.
- Gilliat-Wimberly M, Manore MM, et al. Effects of Habitual Physical Activity on the Resting Metabolic Rates and Body Compositions of Women Aged 35 to 50 Years. J Am Diet Assoc 101;(2001):1181-1188.
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