9 Medical Causes of a Slow Metabolism

Updated: Mar. 24, 2021

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

Man looking stressed out at work

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 and they release too much cortisol into the bloodstream. “Increases in cortisol causes an increased availably of all fuel substrates (carbs, fats, and proteins) by mobilization of glucose, free fatty acids, and amino acids from the body’s reserves,” explains Roger Adams, PhD, personal trainer, doctor of nutrition, and owner of eatrightfitness.com.  “While this may lead to a loss in lean tissue (muscle), it also seems to increase appetite and fat mass, which can be a dangerous combination.” These are other signs stress is making you sick.

doctor pulling liquid into a syringe

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 Dr. Adams, insulin helps the body use glucose for energy. “If a person is pre-diabetic, or insulin resistant, the cells fail to respond to inulin,” he says. “This then results in higher than normal amounts of glucose in the blood, which in turn may signal the pancreas to produce even more insulin.” The result? Elevated insulin and blood sugar levels, which may progress to type 2 diabetes. 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! Here are some simple steps to cut down on sugar cravings.

Female doctor checking a woman's neck lymph nodes

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.

dejected woman staring out a window

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. Estrogen plummets during menopause, which is one of the reasons why women experience a slower metabolism during and after this point. These natural menopause remedies can help you deal with menopause symptoms.

yellow translucent capsules spilling out of a bottle

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.

man in a spin class smiling
iStock/Christopher Futcher

You’re just getting older

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. Strength training is key as we age to slow down muscle loss, according to Dr. Adams. “As we strength train, our muscles can absorb more protein, so the diet must also have a corresponding increase in protein to thwart his age-related muscle loss,” he says. “A well-planned exercise program, including strength training, coupled with a slight reduction in calories but an increase in protein, will help slow down the natural process of lean tissue loss.” Here’s how to get the metabolism of a 25-year-old.

heavy man lifting dumbbells out to his sides

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.

pill bottles on a pharmacy shelf

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).

man cooking eggs in a skillet on a stove

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. “When we continually eat below our basal metabolic rate (BMR), our body may adapt by adjusting the amount of calories we need to function,” says Dr. Adams. “If we eat just a bit less than the BMR, we can lose weight without drastically affecting our metabolism; however, if we go too far below our BMR for too long, the body may compensate by going into a lower metabolic state to conserve mass.” Focusing on getting enough protein, especially early in the day, can give your metabolism a boost. Here are some ideas for high-protein breakfasts.

Reader's Digest
Originally Published in Reader's Digest