When we use the word "metabolism," we are usually referring to resting metabolic rate, which is the amount of energy (calories) that the body requires per day for its basic functions at rest. Most people believe that it is desirable to raise their metabolism, because they will burn more calories and consequently lose weight. Having a slightly lower resting metabolic rate is thought to predispose some individuals to weight gain, especially in the obesogenic food environment that we live in. (1-2) However, there are unfavorable consequences to running your body at a faster-than-normal speed, and raising your metabolism is not the key to weight loss.
Toxic byproducts of metabolism and biological aging
The chemical reactions of normal, everyday physiology produces byproducts. In particular, cellular energy production produces reactive oxygen species as a byproduct, which can damage DNA, proteins and lipids. Although we have natural antioxidant defenses, oxidative damage can still occur, especially if we don't take in adequate antioxidants from our diet. (3) Oxidative damage accelerates aging.
Aging here refers to biological aging -- the progressive decline in the efficiency of the body's physiological functions over time. This leads to tissue and organ damage, and increased risk of chronic disease and death.
Why do our bodies age? It is a combination of factors. One, the accumulation of oxidative damage over time damages the body's tissues, leading to accelerated aging. (4,5) And two, just the rate of living slowly wears out our cellular machinery, so if we function at a faster rate -- i.e. a faster metabolism -- the body will "wear out" more quickly. In animals, energy expenditure is indeed inversely related to lifespan, supporting this. (6) Though both these mechanisms of aging are related, as a faster metabolic rate means faster energy turnover and greater production of free radicals, leading to increased oxidative damage.
Metabolic rate and lifespan
A study on thyroid function published last year further supported the idea that a slower metabolic rate could prolong lifespan. Now, a new study has measured resting metabolic rate directly and come to the same conclusion. Metabolic rate was measured by two different methods at the start of the study. Subjects were followed for 11 to 15 years, and deaths from natural causes were recorded. For each 100-calorie increase in 24-hour resting metabolic rate, the risk of natural mortality increased by 25 to 29 percent. These results strongly support the hypothesis that a slow metabolic rate promotes longevity. (7)
Do we have any control over our resting metabolic rate? How can we slow it down?
Resting metabolic rate is largely genetically determined, but our calorie intake has an effect as well. (8) Caloric restriction and negative energy balance have been shown to reduce resting metabolic rate, and in contrast overeating increases resting metabolic rate. (9-10) Furthermore, caloric restriction has been consistently shown to prolong maximal lifespan by up to 60 percent in animals. (11) My findings have demonstrated that an optimal micronutrient intake reduces the desire for calories and reduces body temperature and white blood cell counts. This means that if you follow a high-nutrient eating style that reduces calorie intake while meeting micronutrient demands, we can reduce our resting metabolic rate and potentially increase our longevity potential dramatically.
Keep in mind that although exercise raises total calorie expenditure, it does not raise the body's basal metabolism. Exercise is the only safe way to "raise metabolism" because it activates the peripheral tissues to utilize more calories and also increases muscle mass, which in turn increases total calorie expenditure. (12) Plus, exercise promotes longevity. (13)
The goal here is to eat so healthy that it reduces your desire to overeat and reduces your metabolism slowly, so you can comfortably desire less food, though not get too thin. Eating a high-nutrient diet actually makes you more satisfied with less food, and actually gives the ability to enjoy food more without overeating.
So, this new study suggests that having a fast metabolism does not mean that you are healthier -- in fact, it probably means that you are aging more quickly. Instead of trying to increase your metabolism with the goal of losing weight, try to slow your metabolism with a low-calorie, high-nutrient diet for a longer, healthier life.
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