Most of us know that metabolism plays a role in our weight. But what is that role? For that matter, what is metabolism? Metabolism is the process your body uses to turn food and drink into energy. Knowing exactly how it works is the first step in reaching your body weight goals, according to UNC REX Healthcare dietitian Diane Danchi, RD, LDN.

"Metabolism is the amount of calories your body needs every day to sustain its weight,” Danchi says. “It’s determined by how quickly your body burns calories.”

Things That Can Affect Your Metabolism

Metabolism is affected by your:
– Height and weight: The bigger your body is, the more calories it needs to sustain itself.
– Age: Your metabolism slows as you get older.
– Sex: Men have more lean muscle mass, so they often have a higher metabolism.
– Genetics: What is your family’s metabolism like? That will affect yours.
– Lean muscle mass: More lean muscle means higher metabolism.
– Hormones: Your thyroid can slow down or speed up calorie burning; decreased estrogen in menopause also slows metabolism.
– Cardiovascular exercise: Regular exercise consumes calories in the moment and boosts metabolism for hours afterward.

Measuring Metabolism

Metabolism is measured by determining how much oxygen your body consumes over a specific amount of time. Basal metabolic rate (BMR) is a measure of the calories needed to sustain basic body functions at rest, such as breathing, circulation, and kidney function. Resting metabolic rate (RMR) is BMR plus the calories needed for basic daily activities. It does not include calories for heavy work or exercise. RMR can be determined by breathing into a device called an indirect calorimeter, which measures the oxygen you consume over eight to 10 minutes and tells you how many calories you need per day without heavy exercise or heavy work – such as 1,500 or 2,000 calories, for example. The numbers from the indirect calorimeters are used in nutrition counseling with clients, Danchi says. “Once we know their RMR, we can figure out what they need to maintain their current weight, and then, if they’re trying to gain or lose weight, we can determine how many calories a day will help them meet their goals.”

Metabolism, Food, and Eating

Eating can actually increase your metabolism because of the thermal effect of food. It takes calories to process food, which involves eating and chewing, digesting, and eliminating waste. “It’s estimated that the amount of calories you need to eat, digest, and absorb food is about 10 percent of your total calorie needs for the day,” Danchi says. So, if you consume 2,000 calories in one day, your body will burn about 200 calories just processing that food.

“Chewing the food takes energy, as does the working of your stomach to break it down,” Danchi says. “Your stomach actually contracts when you eat. After that, digestive enzymes and acids work, and then your colon works.”

Scientific studies suggest that some foods might slightly increase metabolism or increase fat-burning signals to the brain, Danchi says, including hot peppers and caffeine. Also, because it takes more physical energy to eat them, very chewy foods slightly increase your daily caloric needs, as do foods very high in fiber, which require more energy to chew and digest.

Exercise and Metabolism

“If you want to increase your metabolism, do at least 30 minutes of cardiovascular exercise on most days and do a full-body strength-training workout two to three times a week to keep up your lean muscle mass,” Danchi says. Aerobic exercise can burn calories at the time you do it, and it can trigger your body to burn calories at a higher rate for hours afterward. Lifting weights to increase lean muscle mass will also increase the amount of calories you need.

This becomes even more important as people get older. “One of the reasons people gain weight and need fewer calories as they age is they’re doing nothing to keep their lean muscle mass at least stable,” Danchi says. “Lean muscle naturally atrophies starting in your mid-20s, and when your lean muscle mass is shrinking or atrophying, your daily calorie needs are gradually declining as well.”

Metabolism and Hormones

If you start gaining or losing weight unexpectedly and for no apparent reason, you should get your thyroid checked with an “easy blood test,” Danchi says. Hypothyroidism is a condition in which your thyroid functions at a lower level than normal, causing your body to burn calories at a lower rate. The American Association of Clinical Endocrinologists changed the norms it used for diagnosing hypothyroidism several years ago. She says the change in thyroid norms resulted in more people being diagnosed with hypothyroid disease and being treated a little more aggressively. “For a lot of people, that [diagnosis] has been very welcome news because they’ve struggled with all the symptoms of hypothyroidism, including weight gain, and were getting no help,” Danchi says.

Another way hormones can affect metabolism is that women after menopause lose about 30 percent of their calorie needs with the loss of estrogen. That’s why postmenopausal women tend to gain weight and need to be careful, Danchi says. “But if they pay attention, they will notice that their appetite has fallen. Unfortunately, a lot of women just keep eating,” she says. “I had one female client who tracked her metabolism with an indirect calorimeter all the way through menopause. It fell from about 1,750 calories that she needed per day down to around 1,300 over about five years.” Because the client’s metabolic rate was tracked consistently, she was able to take steps to avoid significant weight gain.

Losing Weight with a Slow Metabolism

If someone has a lower metabolic rate, they can still lose weight; it’s just going to take longer, Danchi says. You have to burn 3,500 calories of body fat to lose one pound, so that takes time if your daily calorie needs are already low. If you have a 500-calorie deficit every day for seven days, that’s going to give you about a pound of body fat loss per week. That’s not as difficult for people who need 1,700 calories or more to maintain their weight. They can cut out 500 calories with diet and exercise. But if you can only eat 1,500 calories or less to maintain your weight, you won’t be able to cut 500 calories per day and stay healthy, Danchi says. “We don’t really like to go below the threshold of 1,200 calories a day because it’s almost impossible to get basic nutrition if you drop lower than that,” she says. However, “If you eat a 1,200-calorie diet of all healthy foods, you should be able to meet your nutrition needs.”