Everything You Need To Know About Your Metabolism

The Internet is littered with advice for anyone looking to "boost" or "supercharge" their metabolism in order to shed unwanted pounds. But your metabolism isn't a dial you can crank up to 100 simply by chugging caffeine, taking special supplements, or eating numerous small meals throughout the day. And it does way more than control the number you see on the scale. Your metabolism is a complex collection of thousands of processes that take place simultaneously throughout your body every moment you're alive. As WebMD explained, "Metabolism refers to all the chemical reactions taking place in the body to convert or use energy."

Your metabolism allows you to break down the food you eat into energy your cells can use. It drives the formation of compounds like enzymes and hormones from simple building blocks like amino acids and minerals. It gives your cells the energy they need to grow and divide. Getting rid of the waste products created form all these chemical reactions is also the job of your metabolism.

There's a lot of confusion, misinformation, and hype surrounding your metabolism — both how it influences you and how you can influence it. Learning more about what metabolism really is won't provide the quick fixes you might have been hoping for, but it's an important step to understanding and optimizing your long-term health.

Anabolism versus catabolism

According to the Khan Academy, "Cells are constantly carrying out thousands of chemical reactions needed to keep the cell, and your body as a whole, alive and healthy. These chemical reactions are often linked together in chains, or pathways." These metabolic pathways can be divided into two broad types: anabolic and catabolic.

Anabolic pathways take simple building blocks and turn them into more complex molecules. Creating proteins from amino acids, DNA from nucleic acids, or neurotransmitters from fatty acids and minerals are all examples of anabolic processes. These processes usually require energy to complete. Anabolism is the "building up" side of metabolism. On the other hand, catabolism is the "breaking down" side of metabolism (via Khan Academy). Catabolic pathways take complex molecules and turn them into their constituent building blocks. By breaking the chemical bonds of complex molecules, these pathways release energy that the body can use for anabolic processes. Digesting food is an excellent example of catabolism.

These pathways support one another. For example, digesting a cheeseburger (catabolism) releases raw materials and energy that the cells of your muscles can then use to repair themselves (anabolism) after working hard at the gym (catabolism).

Basal metabolic rate and resting metabolic rate

Most people don't think about metabolism in terms of anabolic and catabolic pathways; they think in terms of how much energy the body needs on a daily basis. The food we eat provides calories (energy) that our bodies then use for all the various things we do. According to a 2018 paper published in the Journal of Exercise Nutrition & Biochemistry, these energy needs fall into four categories: basal or resting metabolic rate (BMR/RMR), diet-induced thermogenesis (DIT), exercise activity–related thermogenesis (EAT), and non-exercise activity thermogenesis (NEAT).

As WebMD explained, BMR refers to the calories needed just to keep your body alive. This includes things like breathing, pumping blood, maintaining a stable body temperature, repairing cells, and balancing electrolytes in the body. BMR is based largely on gender, age, height, body composition, and genes. Your BMR can be estimated with a mathematical formula that takes these factors into account, or it can be more precisely measured in a lab using a device called a calorimeter. The vast majority of the calories you burn every day (about 70%) go toward your BMR.

Resting metabolic rate (RMR) is another term that's often used interchangeably with BMR but is actually slightly more inclusive than BMR. RMR is the amount of energy your body needs to function at rest. In addition to the functions covered under BMR, RMR also includes the energy needed for things like eating and drinking, sweating or shivering, and using the bathroom.

Diet-induced thermogenesis

Diet-induced thermogenesis is the second of the four broad ways in which we expend energy. As a 2014 paper published in Nutrition & Metabolism explained, "The thermic effect of food, also called diet-induced thermogenesis (DIT), is a metabolic response to food. Food intake results in a transient increase in energy expenditure attributable to the various steps of nutrient processing." In other words, you've got to spend energy to make energy.

The authors noted that DIT is measured as a percentage increase in energy expenditure above basal metabolic rate (BMR). Protein produces the largest increase (15 to30%), while carbs produce a more modest spike (5 to 10%) and fats produce very little (0 to 3%). In other words, protein takes the most energy to break down, while fat takes the least. A 2018 paper published in the Journal of Exercise Nutrition & Biochemistry noted that DIT accounts for about 10 to 15% of our total energy expenditure.

Even water may have some impact on DIT. A 2003 paper published in The Journal of Endocrinology and Metabolism found that "drinking 500 ml of water increased metabolic rate by 30%. The increase occurred within 10 min and reached a maximum after 30-40 min." The authors concluded that drinking two liters of water daily would increase energy expenditure by approximately 400 kilojoules (95.6 calories). But a 2006 study published in the same journal found that room-temperature water had no effect on energy expenditure, while drinking water cooled to 3 degrees Celsius (37.4 Fahrenheit) only increased energy expenditure by a modest 4.5% over the course of an hour.

Exercise

It should come as no surprise that exercise (that is, structured physical activity for the sake of health and fitness) is one of the four general categories of energy expenditure. Obviously running, lifting weights, dancing, or any other form of exercise burns calories — that's kind of the whole point. But there's a lot of confusion about just how big a role exercise plays.

According to a 2018 paper published in the Journal of Exercise Nutrition & Biochemistry, exercise activity-related thermogenesis (EAT) accounts for only about 15 to 30% of total daily energy expenditure among people who meet the government's guidelines for physical activity. It makes up a much smaller percentage for those who aren't getting the recommended amount of exercise.

Unfortunately, the belief that exercise burns calories long after you stop is wishful thinking. Energy expenditure may stay elevated for an hour or so after exercising, but after that, it returns to baseline (via MedlinePlus). The idea that maintaining a pound of muscle requires significantly more calories than maintaining a pound of fat is also a myth. The oft-cited claim that adding a pound of muscle mass burns an extra 50 calories a day is a wild exaggeration. In reality, it only takes about six calories a day to maintain a pound of muscle (via Verywell Fit). There are, of course, other important benefits to exercise and building lean muscle — revving up your metabolism just isn't exactly one of them.

Non-exercise activity thermogenesis

The fourth and final way in which we burn calories is non-exercise activity thermogenesis (NEAT). As a 2018 paper published in the Journal of Exercise Nutrition & Biochemistry explained, NEAT includes anything that requires movement but isn't part of structured exercise. Getting up from a seated position, walking through a store, cleaning the house, and typing are all examples of NEAT. NEAT also includes unconscious movements like fidgeting or tapping your foot. Basically, any time your body is moving and it's not part of intentional exercise, it's NEAT. NEAT actions may last only a minute or two (like walking up a flight of stairs) or hours (for instance, if you walk around as part of your job).

Some parts of your metabolism, such as basal metabolic rate (BMR), are pretty fixed and don't vary much from day to day. Levels of NEAT, however, can vary widely between individuals. For instance, one person may have a desk job and then relax on the couch at night, while another person may work in a warehouse and spend their leisure time doing home renovation projects. NEAT also fluctuates within each person. You might lay in bed reading for most of one day and then replant your garden the next. In individuals with a mostly sedentary lifestyle, NEAT accounts for only about 6 to 10% of total energy expenditure, according to an article in Endotext. For active individuals, it may account for 50% or more of total calories burned in a day.

Getting rid of metabolic waste

While most people focus on the "burning calories" part of metabolism, getting rid of waste products is also a critical (if less sexy) part of your metabolism. Waste products can be divided into two categories: metabolic and nonmetabolic (via Britannica). Metabolic waste products are created as a result of chemical processes taking place in cells. Nonmetabolic waste products are things that pass through the body (usually via the digestive system) without ever being involved in a metabolic pathway, either because they can't be broken down for use or because the body doesn't need them right now. For instance, fiber can't be digested and becomes a nonmetabolic waste product. Similarly, if you take a supplement with way more vitamin B12 than your body can absorb, the rest will pass through you as nonmetabolic waste.

Metabolic waste products can come in gas, liquid, or solid form. Heat is also considered a metabolic waste product, since it results from chemical reactions and needs to be eliminated. In humans, carbon dioxide is the gaseous metabolic waste product, while water is the liquid metabolic waste product. There are a variety of solid metabolic wastes, including nitrogen-containing compounds created as a result of breaking down proteins and inorganic salts. Just as these various waste products were created by the action of your metabolism, other metabolic pathways help move this waste out of the body. While this isn't a side of metabolism we often think about, if there are problems with the metabolic processes that help flush waste from the body, individuals can get very sick very quickly (per Britannica).

Controllers of metabolism

While metabolic processes take place in cells throughout your body, a few organs are largely in charge of calling the shots. The thyroid is a butterfly-shaped gland located in the neck that regulates how quickly or slowly cells use energy. It does this by producing two hormones: T3 (triiodothyronine) and T4 (thyroxine). The thyroid is in turn controlled by the pituitary gland and hypothalamus in the brain, which use chemical feedback loops to tell the thyroid how much hormone to produce (via Endocrineweb).

As a 2016 paper published in Experimental & Molecular Medicine explained, the pancreas produces a variety of substances that impact metabolism. It secretes enzymes that power the catabolic pathways of digestion, such as amylase (which breaks down carbs into simple sugars) and pancreatic lipase (which breaks fats into smaller fatty acids). It also produces the hormones insulin and glucagon, which control the movement of glucose into cells, where it can be turned into energy.

The adrenal glands produce cortisol, which, in addition to being one of the "fight-or-flight" hormones, assists with the breakdown of fats and carbohydrates (via MedicineNet). The liver is another metabolism powerhouse. According to InformedHealth, "It converts the nutrients in our diets into substances that the body can use, stores these substances, and supplies cells with them when needed." It also helps neutralize metabolic waste products so they don't damage the body.

Metabolism and weight management

Despite how important and multifaceted metabolism is, most of us only think of it in terms of weight loss and gain. Based on the energy balance model, it all comes down to energy in versus energy out. If you consume more calories than all your metabolic processes need, the excess will be stored for later use and you'll gain weight. Alternatively, if your metabolic processes require more energy than what you're taking in from food, you'll break down stored fats, carbohydrates, or proteins for energy and you'll lose weight (via WebMD). But calories in versus calories out is tricky in practice because metabolism is so complicated.

In addition to calorie counts on nutrition labels often being inaccurate and not taking into account the effects of processing or cooking techniques, these totals ignore the reality that everyone's metabolism runs at a different pace. Some people are more efficient at breaking down and extracting energy from food than others because of differences in digestive enzymes and gut bacteria (via Scientific American).

In terms of calories out, it's hard to know exactly how much energy your metabolism is using unless you're in a lab. True, there are ways to estimate components such as resting metabolic rate (RMR) and how many calories are burned during exercise, but these are just best guesses. For instance, there are multiple formulas that can be used to estimate RMR, but the results they give can vary significantly and certain formulas may be better or worse for particular groups of people, such as athletes (via Medical News Today). Even something as seemingly straightforward as measuring the calories burned during a sweat session is complicated, since calorie trackers on equipment are wildly inaccurate and can't take into account personal differences in metabolism (via Business Insider).

The role of genes

People often attribute their slow or fast metabolism (and, by extension, their weight) to genetics. But how big of a role genes have on your metabolism is up for debate. Yes, there are hundreds of genes that control various aspects of your metabolism, from levels of certain enzymes and hormones to how efficiently your cells use energy. But metabolism is only part of the picture — lifestyle and environmental factors also matter. For some individuals, as much as 70 to 80% of the predisposition of having overweight can be attributed to genetics. For these people, their metabolism may indeed play a big part in their weight. For others, the role of genetics may be as low as 25%, suggesting that their metabolism isn't a major factor (via Harvard Medical School).

One metabolism-related gene that's gotten a lot of attention is melanocortin-4 receptor (MC4R). As a 2010 paper in Endocrine Reviews explained, this gene plays an important role in preserving energy homeostasis (balance). It helps regulate both how much energy can be extracted from food and how efficiently cells use that energy to carry out their functions. Mutations in the MC4R gene are the most common cause of monogenic obesity (that is, obesity caused by a single gene). There are over 150 known mutations of the gene.

Sleep and metabolism

Getting the recommended seven to nine hours of sleep a night is important for your health, but don't expect a solid night's sleep to somehow supercharge your metabolism. In fact, your metabolism slows by about 15% when you're asleep (not really a surprise, since you're not moving and many of your bodily functions, like breathing, have slowed). Your energy expenditure actually reaches its lowest level in the morning before you wake up (via National Sleep Foundation). A 2002 study published in the International Journal of Obesity and Related Metabolic Disorders found that metabolic rate during sleep decreased faster among subjects with a higher BMI.

Even if getting more sleep isn't a viable way to "hack" your metabolism, being chronically sleep-deprived is a surefire way to screw it up. As a 2007 paper published in Sleep Medicine Review noted, poor sleep quality has frequently been linked to obesity and metabolic disorders such as diabetes. There are at least three ways in which sleep deprivation can negatively affect our metabolisms. First, it can interfere with the body's systems for regulating glucose, which in turn has a major impact on cells' ability to produce energy. Second, sleep deprivation can increase the hunger hormone ghrelin, which causes us to take in more energy (calories) than usual. Third, sleep deprivation likely lowers total energy expenditure through reductions in exercise and non-exercise activity thermogenesis (NEAT). In other words, tired people are less likely to work out or do other physical activities.

Can what you eat or drink affect your metabolism?

Contrary to what you may have heard, eating or drinking particular foods and beverages won't speed up your metabolism. At least not in any long-term, meaningful way. As MedlinePlus noted, "Some may provide a small boost in your metabolism, but not enough to make a difference in your weight."

In the case of caffeine, one landmark study published in The American Journal of Clinical Nutrition examined the effects of caffeine on the metabolism of lean and obese individuals. The researchers found that one 100-milligram dose of caffeine (equivalent to roughly one cup of coffee) increased the resting metabolic rate of all participants, but only by 3 to 4% and only for about two and a half hours. According to a 2012 study published in Appetite, capsaicinoids (the compounds that give hot peppers their kick) may increase total energy expenditure, but only by about 50 calories a day — not much in the grand scheme of things. A 2013 study published in Metabolism concluded that although ginger didn't increase resting metabolic rate, it did increase diet-induced thermogenesis, but only by about 43 calories per day.

There's no dietary shortcut to boosting your metabolism. It all comes down to burning more calories through being more active in your daily life.

Metabolism changes

If it feels like you can't even look at a slice of cake without gaining a pound, whereas you used to be able to eat an entire cake without seeing a change on the scale, you're not alone. Many people blame "the middle-age spread" on a slowdown in their metabolism. But according to research conducted by the Pennington Biomedical Research Center and reported on by Science Daily in 2021, metabolic changes over our lifetime don't happen when you might think and aren't as drastic as you might expect.

According to the researchers, infants less than a year old have the highest metabolisms, which makes sense considering how quickly they're growing and developing. Adjusting for body size, infants burn calories 50% faster than adults. Metabolism then drops by about 3% a year until it levels out in a person's 20s. So even though puberty is also a time of rapid growth, it doesn't lead to an upswing in metabolism.

Contrary to what you might expect, a person's metabolism stays relatively stable until about age 60. After that, it gradually decreases by about 0.7% a year. So a person in their 90s needs about a quarter fewer calories than someone in middle age. Part of this decline in metabolism may be due to decreased muscle mass as we age, but the researchers concluded that the metabolic slowdown has more to do with the fact that our cells simply become less active (and therefore need fewer calories for energy) as we age.

Metabolic damage

At this point, we all know how bad yo-yo dieting is for your physical and mental health. You've probably heard that perpetually alternating between overeating and extreme calorie restriction can wreck your metabolism and put your body into "starvation mode." This is true to some extent, but the full story is a bit more complicated.

In an interview with HuffPost Australia, dietitian Robbie Clark explained, "'Metabolic damage,' or better known as 'starvation mode,' is the phenomenon referring to the body's physiological adaptation or natural response to long term calorie restriction or deficit." This deficit can be the result of not eating enough calories, overexercising, or both. Also referred to as adaptive thermogenesis or metabolic adaptivity, this is your body's attempt to maintain energy balance. Your metabolism isn't physically damaged. If you don't have enough energy available to meet your body's needs, then it will work to drop those energy needs to meet the supply. For example, if you regularly only consume 1,000 calories because you're trying to lose weight, eventually your metabolism will adapt and drop your energy needs, and then that meager 1,000 calories will only maintain your weight. And if you suddenly start eating 4,000 calories a day (or even a more healthy 1,800 to 2,500), you may see the weight pile on quickly because it takes a while for your metabolism to adapt again.

But before you go blaming metabolic damage for your weight-loss plateaus, Clark cautioned, "This phenomenon is very real, but whether this response is so powerful that it can prevent you from losing weight, or even start gaining weight despite continued calorie restriction, is still not clear."

Metabolic disorders

Metabolism is a collection of complex processes, and problems with any one of those processes leads to a metabolic disorder. Metabolic disorders include issues with breaking down food into energy, transporting raw materials needed for metabolic processes, and getting rid of metabolic waste products. Enzymes and hormones carry out and regulate most metabolic processes, so metabolic disorders usually involve defective or absent enzymes or problems with hormone production (via Intermountain Healthcare).

Many metabolic disorders are genetic, while others come about because an organ involved in metabolism (such as the thyroid or pancreas) is damaged or diseased. Others, such as type 2 diabetes (the most common metabolic disorder), appear to be the result of a combination of genetic and lifestyle factors. Each metabolic disorder has its own unique characteristics, but common symptoms include extreme weight loss or gain, abnormally high or low appetite, and a lack of energy, according to Intermountain Healthcare.

WebMD noted that "in most inherited metabolic disorders, a single enzyme is either not produced by the body at all or is produced in a form that doesn't work." Inherited metabolic disorders affect 1 in every 1,000 to 2,000 people, although rates for certain conditions may be much higher among specific demographic groups. Lysosomal storage disorders (in which toxic metabolic waste products build up within cells), mitochondrial disorders (in which mitochondria, the energy-producing powerhouse of the cell, don't function properly), and glycogen storage disorders (affecting how sugars are stored in the body) are just a few of the many types of inherited metabolic disorders.