Is Your Metabolism Fast or Slow? How to Tell the Difference

You and your friend eat and drink the same amount of the same foods and beverages, and you gain weight and your friend never does. It's possible that you and your friend have different metabolisms.

"People with fast metabolism are able to eat large quantities of food and not gain any weight," says Rajsree Nambudripad, MD, an integrative medicine specialist with St. Jude Medical Center in Fullerton, California. People with a slower metabolism may have to eat less to maintain their body weight, according to Harvard Health.

Metabolism is your body's process for converting the food you eat and the beverages you drink into energy for all you do, explains the Mayo Clinic. What determines whether you have a fast, a slow or a normal, steady metabolism?

One factor is your thyroid gland, according to the Nemours Foundation.

"The thyroid is one of the most important endocrine glands in our body and serves as a 'thermostat' for our metabolism," Dr. Nambudripad says. Disorders such as an overactive thyroid (hyperthyroidism) can cause a fast metabolism, while having an underactive thyroid (hypothyroidism) will cause a slow metabolism, she says.

Your metabolism is also strongly influenced by your diet and exercise, Dr. Nambudripad says. "If you are mostly sedentary, sitting at a desk all day, you may have a lower metabolism." she says. "If you are very active at your job, like a waitress or nurse, constantly walking around, you will have a higher metabolism."

Your genes and your body composition also play a role in your metabolism speed, according to the Nemours Foundation. Generally, the more muscle and less fat you have, the higher your metabolic rate, even when resting.

If you take medications, they may also affect your metabolism. Medications that may decrease your metabolism and result in weight gain include corticosteroids, antidepressants, diabetes medications such as insulin, some heartburn drugs, hormone therapy or contraceptives and anti-seizure drugs, according to the Obesity Action Coalition.

At the same time, caffeine, a stimulant, may slightly increase your metabolism and help curb your appetite, according to Mayo Clinic. (Talk to your doctor about these side effects before stopping any medication.)

Your age is yet another influencer. As you get older your muscle mass declines, and unless you exercise more, your metabolism could slow, according to the Academy of Nutrition and Dietetics.

Signs of a fast metabolism due to an overactive thyroid, according to the American Thyroid Association, include:

• Weight loss
• Irritability and nervousness
• Increased energy (at first)
• Fatigue
• Increased sweating
• Difficulty sleeping

And, according to the American Thyroid Association, signs of a slow metabolism due to an underactive thyroid can include:

• Weight gain
• Fatigue
• Feeling cold when others are warm
• Dry, itchy skin
• New or worsening constipation

You should talk to your doctor if you suspect you have a thyroid imbalance, which can be treated medically.

"For high thyroid, we investigate for Graves' disease (autoimmune high thyroid or an overactive thyroid nodule) and we sometimes treat with medications like methimazole to reduce thyroid hormone levels," Dr. Nambudripad says.

"For low thyroid, we investigate for Hashimoto's thyroid disease (autoimmune low thyroid), and we supplement with thyroid replacement hormone if needed. Sometimes subtle cases of hypothyroidism can be corrected with iodine and selenium supplementation."

Read more: How to Eat Right for Your Metabolism Type

What else can you do to correct your metabolism? Start with a healthy diet.

"If you eat a whole-foods diet with a lot of fiber from vegetables, your metabolism will improve as well," Dr. Nambudripad says. "This is because the vegetables improve the health of your gut microbiome (bacterial ecosystem in the gut), which has a strong influence on your hunger hormones: leptin, ghrelin and insulin. In contrast, eating a lot of processed foods with chemicals will have the opposite effect on the gut microbiome, and you may find your metabolism slows down."

Eating regular meals and snacks, though limited in size, will help prevent you from overeating mindlessly because you're famished, according to the Academy of Nutrition and Dietetics.

Also, get physically active. Exercise can help you build muscle mass, which burns more calories, according to Mayo Clinic. Plan a regular program of aerobic and strength training. Start slow and build to a program you enjoy and can keep up.

Diet and exercise also play a role if your metabolism is faster than most.

A fast metabolism isn't a license to eat junk, notes the Center for Young Women's Health. Eat well-balanced meals and nutrient-dense snacks, such as avocados, nuts, seeds and peanut butter. The center also advises doing some strength training and resistance exercises to help build up your frame.

Read more: 13 Day Metabolism Diet