When you consider metabolism, you likely think about digestion and energy expenditure. Although it's a bit more complicated than that, metabolism is often simplified to this. For example, the Mayo Clinic says that metabolism is a conversion process where the body turns the calories you ingest into energy.
But the conversion of calories into energy is actually a complex process that includes more than just digestion. According to the U.K. National Health Service, metabolism refers to all of the chemical processes that occur within the human body. These processes work to repair damage and keep your body functioning continuously, regardless of whether you're asleep or awake.
Metabolism occurs in organisms of all sizes, from whales and elephants to tardigrades and bacteria. The only exceptions are viruses, which do not have their own metabolism. However, a 2020 study published in the journal Nature Communications reported that despite their own lack of metabolism, some viruses can influence their host's metabolism.
Related: Can you speed up your metabolism?
What are the different types of metabolism?
Metabolism is a process that occurs on a cellular level. It can be broken down into two forms: anabolism and catabolism. Anabolism is generally considered to be constructive metabolism, meaning it works to build complex molecules, while catabolism is seen as destructive metabolism, meaning it breaks down complex molecules for fuel. According to the Cleveland Clinic, both of these processes are regulated by hormones such as adrenaline, estrogen, testosterone and insulin.
Catabolism is the mechanism that's used during digestion. This process is what allows the body to break down the food and drink you consume and turn them into usable nutrients. If you don't consume enough calories, catabolism will use the body itself for fuel, breaking down fat and eventually muscle.
Anabolism is the opposite process to catabolism. It occurs whenever the body forms tissue — from wound-healing after an injury to muscle-building through exercise.
High metabolism vs. low metabolism
Metabolism is most often associated with a certain speed, as in high, or fast metabolism and low, or slow metabolism. People who talk about having a naturally high or low metabolism are usually referring to their metabolic rate at rest (their basal or resting metabolic rate).
"Your basal or resting metabolic rate refers to the amount of energy your body must generate to stay alive and function well," said Russell Jones, a professor and principal investigator at the Van Andel Institute, a nonprofit biomedical research and science education organization in Grand Rapids, Michigan. This includes physiological processes such as breathing or your heart pumping blood throughout your body, but it also refers to processes at the cellular level," he said. The human body needs a minimum amount of calories in order to carry out these basic functions while at rest.
Humans aren't constantly at rest, though. The body also burns calories when performing any activity, such as walking, working and exercising. A person's total calorie expenditure comes from all the activities a person performs in a day in addition to their basal metabolic rate, according to research published by the Centers for Disease Control and Prevention. In general, consuming more calories than the body's total daily energy expenditure is what causes weight gain, and exerting more energy than you consume causes weight loss.
A number of factors influence how low or high your basal metabolic rate is, including age, height, weight and body composition.
Does metabolism slow down with age?
Metabolism slows down as people get older because the basal metabolic rate decreases. This is a normal process that occurs in everyone, regardless of other factors like genetics or body composition. "Humans have distinct phases of metabolism that change over your lifetime," Jones told Live Science, "with newborns up to toddlers requiring more energy than adults."
Until recently, researchers believed that metabolism fluctuated around key points in life, such as puberty, pregnancy and menopause. It was assumed that some of these changes were the cause of weight gain that often occurs in people ages 40 to 60. However, a 2021 study published in the journal Science suggested otherwise.
When an infant is born, it shares the same basal metabolic rate as its mother for about the first month of life. After these first few weeks, the infant's metabolism starts to increase and continues to do so for the first year of life, until their metabolism is 50% higher than the average adult rate.
The first major decline in metabolism occurs between ages 1 and 20, when metabolism gradually slows down by about 3% each year. After this period, metabolism levels out. "A person's overall metabolic rate is pretty steady from age 20 to 60, with little evidence of a slowing metabolism in middle age," Jones said.
Around age 60 is when metabolism starts to gradually slow down again. At this point, metabolism slows down by about 0.7% a year, causing a gradual decrease in the caloric needs a person's body requires.
Are there ways to boost metabolism?
"Your genetics control many facets of body composition such as your height, how much muscle you have and how much fat you have," Jones said, which all play a role in determining metabolic rate. However, there are ways to boost and maintain a healthy metabolism. "You exert a large amount of control over your overall metabolic rate simply through your level of physical activity and how much you eat," he said.
One popular diet, known as intermittent fasting, restricts calorie consumption to a set period of hours each day. This diet can positively influence metabolism in several ways, Jones said. Intermittent fasting is able to "regulate the production of insulin — a key metabolic hormone that regulates your blood sugar levels and tells your body to store excess calories as fat. By tightly regulating insulin production, your body can better partition the nutrients you consume from food and lead to less fat storage." It can also help reduce total calorie consumption, which also supports weight loss.
"You can also influence metabolism by changing your body composition through exercise," Jones added. "Muscle accounts for a large amount of the calories you burn in a day — even at rest — so the more muscle you have, the higher your metabolic rate." If you're looking for a surefire way to improve your metabolism, he recommends building muscle through aerobic exercise and resistance training.
- Find out more about metabolism myths and facts on the U.S. National Library of Medicine's website, MedlinePlus.gov.
- Learn more about intermittent fasting and its benefits from Harvard Health Publishing.
- Estimate your caloric needs using this chart from the Food and Drug Administration.
This article is for informational purposes only and is not meant to offer medical advice.
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Siddhi Camila Lama is an independent science, health and gastronomy writer who is also the managing editor of HairScience.org. She's written for Orb Media, Atlas Obscura, BrainFacts, Medium's science and tech publication, One Zero, and more. Siddhi is a certified nutritionist with a bachelor's in Human Development, a master's in Organ, Tissue, and Cellular Transplantation, and a Ph.D. in Bioengineering.