What Is Metabolism? Medical Doctors Explain the Actual Definition

We often think of metabolism as a superpower: There’s that friend with a speedy metabolism who can seemingly eat whatever they want without gaining weight. Or, once we hit middle age, the common belief is that the power of our metabolism starts to wear off—so we may turn to fad diets or calorie restriction to fight the pounds.

But recent research has been emerging to reveal that our conventional thinking about metabolism is mostly mistaken. The complex biochemical process called “metabolism” is tied to a lot more in our bodies than just our weight…and, we have more influence over how well it works than you’ve probably realized.

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What is metabolism?

The human metabolism is the sum of all the chemical reactions that occur in our bodies to maintain life. That means your metabolism powers everything in your body, from your breathing to blood flow, tissue repair, hormone regulation, digestion, and even cognitive processes like memory and decision-making. In fact, the blog for Johns Hopkins Medicine highlights the relationship between metabolic problems and many chronic diseases, including diabetes, heart disease, kidney disease, liver disease, thyroid problems, asthma, and some forms of cancer.

Everyone has an individual “basal” metabolic rate, which refers to the energy (or “calories”) your body uses to keep all of its various tissues, organs, and processes functioning well. Research suggests 70% of the calories your body uses daily comes down to this resting metabolic rate—and, as Dutch neuroscientist Michael van der Kooij, PhD, suggested in a 2020 paper, 20% of daily calorie consumption occurs simply from “cerebral faculties,” or everyday brain function.

So while exercise burns calories and your diet will affect caloric intake, that’s only part of the equation. The efficiency of your metabolism is what largely guides weight management as well as the healthy functioning of nearly every other process in your body.

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What role do genes play in metabolism?

Your genes play a significant role in your metabolic rate, explains Felix Spiegel, MD, a bariatric surgeon with Memorial Hermann in Houston, TX. Researchers are still trying to understand exactly how or why genetics influence metabolic rates, but it’s well-established that body composition, how you use and store calories, and hormone function—all of which affect your metabolism—are passed down from mom and dad. Additionally, about one in 500 people have an inherited metabolic disorder, according to 2021 genetics research published in Molecular Genetics and Metabolism Reports. We also know that some people are at a genetically greater risk of chronic conditions known to impair metabolism, such as obesity, high blood pressure, and diabetes.

That said, your lifestyle is a major—and controllable—factor behind your metabolism, Dr. Spiegel says. “A healthy diet and exercising can increase your metabolism,” he explains. “The more overweight you are, the slower your metabolism becomes because fat levels suppress metabolic hormones and increase hormones that contribute to a lower metabolism.”

One 2021 endocrinology study showed that not getting enough sleep affects the hormones involved in regulating metabolism. Dr. Spiegel emphasizes that eating habits are incredibly important, too. Limiting your meals, like only eating once a day or restricting how much you eat throughout the day, may initially trigger weight loss, but it actually works to slow your body’s metabolism down. This not only makes it difficult to maintain your weight but can have far-reaching effects throughout the body.

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Why is my metabolism slow?

If you notice your metabolism is slow, there may be one simple factor. Chief among the lifestyle factors that affect your metabolism is one very common experience: Stress. (Neuroscientist Dr. van der Kooij, PhD, said in the 2020 paper cited above that chronic stress is also linked with slow metabolism and a higher risk of developing a metabolic disorder.) Lack of sleep, poor nutrition and physical inactivity contribute to biochemical alterations, or stress in the body, says Dan LeMoine, a nutrition consultant board certified by American Fitness Professionals & Associates (AFPA). “And what happens is the body says, Whoa, something’s out of balance, I need to protect myself.”

That’s why, at its core, a slowed metabolism is an evolutionary response: The body reacts to stress by slowing down its use of energy so it can conserve more in the event it should face a potential threat. “Our brains and our bodies just haven’t caught up to the reality that there is a restaurant on every corner where we can go and grab unlimited calories if we need to,” he says. When the stress comes from our lifestyle, the slowed-down metabolism effects sustain. “So when I have folks saying, ‘I’m having weight issues’ or ‘I’ve got really low energy,’ it’s usually because of these [lifestyle-induced] biochemical changes.”

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Does metabolism slow as we get older?

Recently, a team of more than 80 researchers collected data from nearly 6,500 people ranging from newborns to 95-year-olds spanning a 40-year time period. And their findings, published 2021 in the journal Science, contradict several long-accepted beliefs about metabolism.

One of the major myths the study busts: The idea that our metabolism naturally slows with age. While this is true to some extent, age is not as much of a factor as previously thought. The researchers identified four distinct metabolic rate life stages:

• Infants have a very high caloric burn.
• Then from age one to age 20, metabolism gradually slows by about three percent a year.
• But from age 20, there’s no significant age-related change to your metabolic rate until you reach 60 years old when it declines by about 0.7% per year.

So, if you feel like your metabolism isn’t what it used to be when you were younger, this likely has less to do with aging and more to do with your lifestyle.

The study also found no biological difference in metabolic rate among men and women. The key finding here again circles back to modifiable lifestyle changes: Men and those assigned male at birth tend to have more lean muscle mass—and more muscle contributes to a faster metabolism.

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What are signs of a slow metabolism?

LeMoine suggests struggling to lose or maintain weight, even if you’re eating well and exercising, is generally the main red flag that something’s interfering with your metabolism. But there are other signs of a slow metabolism, too. “Low energy is a big one,” he explains. “Feeling cold, especially if it’s warm out, can also indicate your body is not utilizing energy appropriately; constipation can sometimes be a sign that your bowel movements are slowing down because your body’s trying to conserve energy; and dry skin can be a sign that you’re not well-hydrated, which affects your metabolism.”

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