What is Metabolism?

Most often, when we hear about metabolism it relates to weight loss. But the speed of a person’s metabolism doesn’t usually relate to weight gain or loss. Here’s what metabolism is and what you can do to affect your weight.

Metabolism refers to all of the processes and chemical reactions that turn what we eat into the stuff our bodies need. Food, water, and oxygen enter the body, and hundreds of reactions magically transform them into energy and components to build and repair cells plus waste products, like carbon dioxide.

For instance, if you eat a peach, the digestive system breaks it down into molecules: sugars, fats, amino acids, vitamins, minerals, and phytonutrients. The molecules are absorbed by the intestines, enter the blood stream, and are distributed to cells.  Some of these components are used for energy, while others go on to support cellular processes. All of the foods you eat—carbohydrates, protein, and fat—serve critical functions. Chemical reactions transform them into cellular parts and energy. That’s metabolism.

Cells are growing and being repaired constantly. For instance, your skin cells are replaced every 48 days. Bones are renewed every 10 years. And the energy molecules go on to make muscles contract and allow the brain to form thoughts. So you can imagine how important it is to eat a healthy diet. When you don’t, your body is missing something it needs to function. Damage persists and poor health results.

So how does your body use the energy it makes?

• Depending on your body size, sex, and age, you need a certain amount of energy to carry out all of your basic functions, like breathing. A large young man will need more energy than a smaller older woman. This is called your basal metabolic rate.
• A second set of energy—about 10 percent of the calories you get from carbohydrates and proteins—is needed just to digest and absorb the nutrients. This is called thermogenesis.
• A third set of energy is for non-exercise activity thermogenesis (NEAT), or everything that you do that is not automatic and not exercise. Typing, walking from room to room, brushing your teeth… People use between 100 – 800 calories for NEAT daily. (This might be what people mean when they say that they have a fast or slow metabolism, meaning that they are just either generally a high energy user or not.)
• The top three uses of energy, the ones that meet your body's basic functions, stay fairly consistent and aren't easily changed. The biggest source of variability is physical activity.

Genetics and hormones play a small role, but how much you eat and how much physical activity you do largely determine your weight. Weight gain happens when you eat more calories than you burn. Note that if you’re having a hard time losing weight, ask your doctor to be checked for hypothyroidism. It is a very common condition that, if left untreated, may impact your metabolism.

Some companies produce supplements that claim to change your metabolism. But they are not required to prove that their products work. And some can come with dangerous side effects. Do not take any supplement without discussing it with your healthcare provider first.

Habits that support a healthy diet, good sleep, stress management, and increased physical activity are most beneficial in affecting weight. Tackling one small change at a time and mastering it before moving on to the next beneficial change is a great way to make weight loss happen in a healthy and sustainable way.

Amy Freeth, MD, is an endocrinologist at SVMC Endocrinology in Bennington, Vermont.