What Is Metabolism?

You’ve probably heard the term “metabolism” thrown around. Some people may blame their body size on their metabolism, describing it as fast or slow. Or maybe you’ve tried to boost your metabolism through a particular diet or exercise regimen.

But what exactly is your metabolism, and is there anything you can do to change it? This article will define metabolism, its types, how it’s regulated, and the factors that impact it.

“Metabolism” is a term that refers to all chemical processes or changes in your body at the cellular level. At any given moment, thousands of complex chemical processes are happening in your cells to keep you healthy and thriving. Such processes help with breathing, circulating blood, controlling your body temperature, and ensuring your brain and nerves function.

You have to consume food to create energy for these chemical processes. However, your body cannot use the food directly. The energy in the food must be converted into a form your cells can use for normal functioning, including growth, development, reproduction, repair, and elimination (getting rid of waste from the body).

Types of Metabolism

There are two types of metabolism: anabolism and catabolism. Anabolism processes require energy, while catabolism processes create or release energy. Both co-occur in the body.

Anabolism

Anabolism is any chemical process involved with synthesis, or building, of complex molecules from simpler molecules. Your body is constantly repairing and building new structures necessary for life.

Sometimes anabolism is visible, such as in building muscle for sports, a healing wound, a growth spurt, or pregnancy. Other times, it happens without our noticing, such as when producing new blood cells, repairing DNA (deoxyribonucleic acid; the hereditary material in humans and other organisms), mineralizing bone, or synthesizing hormones like insulin, estrogen, or testosterone.

Catabolism

Catabolism is any chemical process involved with the degradation, or the breakdown, of complex molecules into simpler molecules. The process is highly regulated by enzymes in the body, which control chemical reactions.

When you eat food, your body needs to digest it and catabolize or break it down into a form usable by the body. Another example of a catabolic process is when your body breaks down muscle and fat to release stored energy during strenuous exercise or starvation.

What Regulates Metabolism?

Metabolism is regulated by hormones, chemicals acting as messengers in the body.

While many hormones are involved in various chemical processes in the body, the thyroid hormone is largely responsible for regulating your metabolism.

Hormones are made in the endocrine glands, including the pituitary, pineal, thymus, thyroid, adrenal glands, and pancreas, and send chemical messages to all parts of the body, including organs, organ systems, and tissues like bone, skin, and muscle.

Hyperthyroidism, or excess thyroid hormone, increases your basal metabolic rate (resting energy expenditure) and encourages weight loss, the breakdown of fats and protein for energy, and reduced cholesterol levels. Hypothyroidism, or reduced thyroid hormone, has the opposite effect, lowering overall metabolism.

Factors Impacting Metabolism

Your metabolism, or the rate at which your body’s chemical reactions use energy, can be affected by many factors, such as:

• Age: A large-scale study showed that metabolism has these four distinct phases:
• From birth to 1 year old, metabolism is very high (approximately 50% above normal adult metabolism).
• From age 1 to 20, metabolism will slowly decline.
• From age 20 to 60, metabolism will remain steady.
• From age 60 onward, metabolism will decline.
• Sex: Overall, women’s metabolic rate is about 5% to 10% lower than men’s. This is generally explained by differences in body composition since fat has less metabolic activity than muscle tissue.

Women tend to carry approximately 10% more body fat than men who have a similar age and body mass index (BMI) value.

BMI is a dated, flawed measure. It does not take into account factors such as body composition, ethnicity, sex, race, and age. Even though it is a biased measure, BMI is still widely used in the medical community because it’s an inexpensive and quick way to analyze a person’s potential health status and outcomes.

Other factors that can affect your metabolic rate include:

• Diet: Resting metabolic rate is regulated by the number of calories consumed in your diet relative to energy used. Research suggests that when excess calories are consumed, resting metabolic rate increases, whereas fasting and low-calorie consumption (under 1,000 calories per day) causes basal metabolic rate to drop.
• Exercise: The impact of exercise on metabolic rate is controversial, despite being widely studied. However, research suggests that exercise may regulate metabolism because bed rest in sedentary individuals leads to lower metabolic rates. Additionally, in highly trained runners who suddenly stop running, their resting metabolic rate can decrease by 7% to 10%.
• Sleep: A study in which healthy participants were sleep deprived (getting just four hours per night) showed that their resting metabolic rate decreased compared with participants who had a full night of rest (10 hours per night). However, when the sleep-deprived participants returned to regular sleep, their metabolic rate increased, returning to their normal rate.
• Hyper- or hypothyroidism: Since thyroid hormone is largely responsible for regulating your resting metabolic rate, excess or reduced thyroid hormone can impact your metabolism.
• Injury or disease: When a person is undergoing illness or injury, such as cancer, sepsis, trauma, or burns, their body’s usage of energy increases by 20% to 25%. Metabolism also increases during the healing process after an operation by 15% to 30%. This is why proper nutrition is critical to healing and success after operation and treatment.

Can You Increase Your Metabolism?

Several factors contribute to your metabolism or energy usage. Unfortunately, much of your metabolism is out of your control. That’s because your basal metabolic rate, or resting energy expenditure, makes up about 70% of your metabolism. This includes all normal cellular functions occurring without conscious participation, such as breathing, pumping blood, and a functioning brain and nervous system.

Another 10% of energy is used to digest your food. Physical activity accounts for the remaining 20%. This includes not just voluntary exercise but maintaining your posture and fidgeting.

While you may not be able to change your metabolism drastically, some things can help:

• Build more muscle: While the direct effect of physical activity itself may not increase your daily energy expenditure by a lot, the impact of building more lean muscle mass may, as it is more metabolically active tissue.
• Get enough rest: Sleep deprivation can lower your metabolism, so getting enough sleep may give you a boost if you are regularly sleep deprived.
• Eat enough food: Similarly to getting enough rest, you may also want to examine if you get enough calories each day. While most people may choose calorie-restricted diets to lose weight, this is counterproductive, as fasting or extremely low-calorie diets actually decrease your basal metabolism.

Summary

Metabolism is the sum of all chemical reactions in the body required to sustain life. These chemical processes involve energy and the breakdown and buildup of molecules and are regulated by hormones.

Your metabolism can be impacted by age, sex, diet, exercise, sleep, and injury or disease. While some lifestyle factors can affect your metabolism, no magic bullet or pill will drastically increase your metabolism.

However, a balanced diet with sufficient calories, regular exercise, and enough sleep can help regulate your metabolism.

By Rebecca Valdez, MS, RDN
Valdez is a registered dietitian nutritionist, health writer, and nutrition consultant. She received her MS degree in nutrition from Columbia University.