9 Medical Causes of a Slow Metabolism

What's really causing that weight gain and sluggish feeling? These hidden factors may surprise you.

You have too much cortisol


Your metabolism is how your body turns calories into energy, so when you say you have a "slow metabolism," you really mean your body is hanging onto calories, causing unwanted weight gain. But what causes this to happen? For some people, it could be too much cortisol, known as the "stress hormone." Normal amounts of cortisol can help you burn fat if it's working in tandem with other chemicals in your body. But if you have too much cortisol—like if you're really stressed out for a long time—your body may think you're under duress and could need extra energy. which is why it clings to calories. This also happens if you have a medical condition called Cushing's Syndrome, a disorder of the adrenal glands that releases too much cortisol into the bloodstream. "The excess cortisol can promote a significant amount of weight gain," Michael West, MD, an endocrinologist at the Washington Endocrine Clinic in Washington, DC, told everydayhealth.com. These are other signs stress is making you sick.

Your insulin levels are too high


This is a bit of a chicken-or-egg problem: Being overweight is a cause of insulin resistance and type 2 diabetes; and having type 2 diabetes is linked with problems losing weight. According to the National Institute of Diabetes and Digestive and Kidney Diseases, insulin helps the body use glucose for energy; if your body is "insulin resistant," your body's cells can't absorb the glucose, which results in high blood sugar. This may also trick your body into thinking your fully stocked with energy, so your metabolism slows down. Even if you're not diabetic, other factors, like stress, can raise your insulin levels. A study from Ohio State University's Wexner Medical Center found that among women who were asked to eat the same high-calorie meal, those who had reported being stressed out had higher levels of insulin. They burned 104 fewer calories—which could add up to 11 pounds a year! "This means that, over time, stressors could lead to weight gain," lead author Jan Kiecolt-Glaser, MD, a professor of psychiatry and psychology at Ohio State, said in a press release. Here are some simple steps to cut down on sugar cravings.

Your thyroid is out of whack


Your thyroid gland, located at the base of your neck, helps regulate thyroid hormones, which greatly affect your body's metabolism. So if you don't make enough of the hormone (a condition known as "underactive thyroid" or "hypothyroidism") your metabolism will slow down. According to the Office of Women's Health, a part of the United States Department of Health and Human Services, the most common cause of hypothyroidism in this country is Hashimoto's disease. "Hashimoto's disease is an autoimmune thyroid disease that affects the thyroid gland," explains Shane LeBeau, MD, endocrinologist at University of Pittsburg Medical Center (UPMC). "Patients who have this disease produce antibodies that attack the thyroid gland—and if their immune system damages their thyroid glands sufficiently, they may become hypothyroid and require [synthetic] thyroid hormone." As for how exactly the thyroid controls metabolism, Dr. LeBeau says that's less clear. "What we do know is that thyroid hormone increases the body's consumption of oxygen, which is used to determine what one's 'basal metabolic rate' is," he says, referring to the body's baseline metabolism. "Therefore, patients who are hypothyroid consume less oxygen and in turn have a lower basal metabolic rate. I personally tell patients I see in my clinic that thyroid hormone is necessary for all of our organ systems to operate smoothly." These are other silent signs you could have a thyroid problem.

Your estrogen is low


Women entering menopause may start asking, "Why is my metabolism so slow?" "Low estrogen is one of the manifestations of menopause, which takes place at age 51.4 years on average in the United States," says Ghasak Amer Mahmood, MD, an endocrinologist and metabolism internal medicine expert at PIH Health in Whittier, California. "Lack of estrogen typically increases fat mass and decreases lean mass." Studies have shown that weight gain can be, unfortunately, a result of this lack of estrogen. According to the National Heart, Lung and Blood Institute, women gain an average of five pounds during menopause and add more fat around their middle—and fat is less efficient at burning calories than muscle, even at rest. These natural menopause remedies can help you deal with menopause symptoms.

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Your testosterone is low


Men aren't off the hook for hormone changes. "With testosterone deficiency, as men age their muscle mass declines and fat mass increases," Dr. Mahmood says, slowing metabolism. A study of the effects of a testosterone-reducing drug for prostate cancer patients showed that low testosterone levels were associated with higher body fat mass. Testosterone replacement—though controversial—tends to reverse these changes, Dr. Mahmood says, as a study from Germany shows. Increasing vitamin D levels may also help to raise testosterone.

You're just getting older

iStock/Christopher Futcher

We often attribute our slow metabolism to advancing age, and in fact, this might be spot on. In addition to the hormonal changes that occur as both men and women get older, our body loses muscle tissue with every passing birthday. Because lean body mass (i.e., muscle) burns more calories than fat even at rest, less muscle means fewer calories burned, and therefore, more stored as fat. According to the National Institute on Aging, you can combat the weight gain that comes with an older metabolism by exercising more and eating less. The Mayo Clinic says that women need about 200 fewer calories in their 50s than during their 30s and 40s. Here's how to get the metabolism of a 25-year-old.

You don't have enough muscle tissue relative to fat


If you are overweight or obese, you have more fat than muscle—and in a vicious cycle, less fat means fewer calories burned while at rest, which in turn means more fat. If you want to burn more calories while at rest, you need more lean muscle. Which means you need to exercise. Pick up weight training, which builds lean muscle, and try interval training, which the American College of Sports Medicine says smokes more calories than traditional workouts. Also focus on eating a healthy diet and getting enough sleep. If you're tired you'll have less energy for exercise—and your lack of energy can make you crave energizing (read: high-calorie) foods. A study from the New York Obesity Research Center showed that people who were sleep-deprived consumed a whopping 300 calories more a day, likely because the body is searching for fuel.  Here are ways to lose weight while you sleep.

You're taking medications


Certain drugs can slow your metabolism. Long-term use of anti-inflammatory steroids, including prednisone, can increase appetite and lead to overconsumption of calories. They're also associated with insulin resistance, higher blood glucose, and fat storage. Some studies have shown antidepressants to be linked with weight gain due to a decrease in the body's basal metabolic rate and an increase in appetite. Beta blockers to treat high blood pressure slow heart rate and give you less energy for exercise, according to the American Heart Association, so those could play a role in weight gain. To offset the effects of your meds on your metabolism, eat a healthy diet and exercise regularly (after you get the OK from your doctor).

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You're not taking in enough calories


It seems to make sense that if you eat fewer calories you'll lose weight, right? Not always: You need enough fuel to keep your body going, otherwise it will slow down your metabolism to conserve energy. "Your body will start to store fat because it thinks it's not going to get more fuel," Kimberly Lummus, MS, RD, of the Austin Dietetic Association told everydayhealth.com. "You will be at a point where your body is kind of at a standstill." Focusing on getting enough protein, especially early in the day, can give your metabolism a boost. Here are some ideas for high-protein breakfasts.

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