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Why Is My Metabolism So Slow?

By Lindsay Boyers ; Updated August 14, 2017

When you’re struggling with weight loss, you often blame your metabolism. You may be wondering if your metabolism is slow, and if it is, what you can do to speed it up. While some factors do affect your metabolic rate, little evidence exists to demonstrate that a slow metabolism is the primary cause of most people's weight loss woes. Some causes of a slow metabolism are under your control, while others you may not be able to do much about.

What Is Metabolism?

Metabolism refers to every physical and chemical process in the body that converts or uses energy, or calories, including breathing, body temperature regulation, muscle contraction, digestion, blood circulation and brain function.

When people refer to slow metabolism as it applies to weight loss, they often mean basal metabolic rate, or BMR. Your basal metabolic rate is the number of calories your body burns at rest – or when you’re doing absolutely nothing other than being still. Your BMR is the biggest factor in determining your metabolic rate or whether you have a “slow” or “fast” metabolism.

Genetics and Metabolism

Your BMR is partly determined by genetics. Some people are born with a BMR that's slower than others; they just burn fewer calories at rest. Although the reason is still under investigation, some researchers believe it’s because your genes play a role in both your muscle size and your ability to build new muscles, two factors that also affect metabolism. If you naturally have a lower muscle mass, then your BMR will be lower.

Your age and gender also play a role in metabolism. As you age, your metabolism starts to slow because you typically start to lose muscle mass and gain more body fat. Women also tend to have a slower metabolism than men, because women have more body fat, less muscle mass and lighter bones.

Body Composition and Metabolism

Although genetics plays some role in your body composition, you have the ability to change your muscle-to-fat ratio through exercise, specifically strength-training exercises. Research shows that muscle is more active and uses up more energy than fat, so the more muscle you have, the more calories your body will burn at rest. On the other hand, if your body comprises mostly fat, your BMR will likely be lower.

Erratic Eating Affects Metabolism

A study done by the Hebrew University in 2012 found that the time you eat may also play a role in metabolism. Researchers fed different groups of mice on different schedules -- two on a fixed schedule, and two on an erratic one. All groups received the same amount of calories. The study authors concluded that the mice fed on a regular schedule experienced a boost in metabolism that burned fats instead of storing them, which ultimately resulted in a loss of body weight. Although more research needs to be done on humans, this study suggests an erratic eating schedule may be partly to blame for a slower metabolism.

Weight Loss Affects Metabolism

One unwelcome side effect of weight loss is that the more weight you lose, the harder it becomes to lose weight. When you initially cut calories, your body responds with what is called "adaptive thermogenesis." This means that as your body -- and all your organs and tissues -- decreases in size, you need fewer calories to live, so your basal metabolic rate slows down. As you become smaller, your metabolism continues to slow down, which means that your body now requires fewer calories to function. When this happens, you'll have to cut calories even further to continue to lose weight.

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