Science 101

Metabolism 101: What Is a Human Metabolism?

The chemical processes that keep your body functioning are widely misunderstood. We spoke to the experts to get to the bottom of it.


There are many misconceptions about the chemical phenomenon we call metabolism. Most people have a loose concept of their body’s metabolism as the amount of calories their body burns each day, but it’s so much more than that. Metabolism is often touted as a fat-burning machine, something that slows down and is almost always linked to weight loss. But your metabolism is doing far more than just controlling your weight. In fact, the definition of your body’s metabolism is much broader than you might have even imagined.

We tend to think of our metabolism as something that is fixed, one “thing” separate from the rest of our body systems, but according to Paolo Sassone-Corsi, PhD, director of the Center for Epigenetics and Metabolism at the University of California, Irvine, that’s a common misconception. “It is extremely complex and is intertwined with other systems in the body.”

Metabolism is every biochemical process that occurs in your body and keeps you alive. It includes the production of energy, hormones, neurotransmitters, digestive enzymes, the maintenance of sleep cycles and rhythms, and the utilization of food to produce all the raw materials for the body’s processes. It encompasses both anabolism (the buildup or creation of substances) and catabolism (the breakdown of substances). These chemical reactions are organized into metabolic pathways whose steps are the transformation of one chemical into another, facilitated by enzymes.

We tend to think of our metabolism as something that is fixed, one 'thing' separate from the rest of our body systems, but it is extremely complex and is intertwined with other systems in the body.

Joseph A. Baur, PhD, associate professor of physiology at the University of Pennsylvania Perelman School of Medicine Institute for Diabetes, Obesity and Metabolism, says that typically glucose metabolism (turning carbohydrates and sugars into energy) is the first thing people learn about as science students, so it makes sense that many people automatically think of weight when they think of metabolism.

But metabolism is much broader than that, Baur says. For example, his lab spends a lot of time talking about NAD+ metabolism, a set of reactions that creates the coenzyme nicotinamide adenine dinucleotide.

“That’s a whole different set of reactions than glucose metabolism,” Baur says. “Other metabolic pathways that often get overlooked are amino acids and proteins. But these metabolic processes are complicated and there’s not a consistent, strong enough motivation for people to pay attention to.”

Meanwhile, weight and energy are very much things that people can see and feel, making those aspects of our metabolisms crystal clear and, for better or for worse, the focal point. Still, our metabolisms change with our circadian rhythms, the 24-hour internal body clock that runs our sleep/wake cycles, our exercise patterns, and more.

The Meaning of a Metabolic Rate

People often talk of having a “fast” metabolism—able to eat a lot and “burn it all off”— or a “slow” metabolism. But what does that really mean? When we talk about this, we’re really talking about something called “metabolic rate,” also known as the speed of your metabolism. Metabolic rate is how many calories you burn in a certain span of time. Our basal metabolic rate is the number of calories needed to keep us alive, while our total daily energy expenditure is the actual number of calories we burn in a day.

Anyone who has tried to diet has learned about the basic idea of calories in, calories out: you take in a certain number of calories and you burn a certain number of calories each day. If you take in more calories than you burn, you gain weight or fat. If you take in fewer calories than you burn, you lose weight or fat.

But metabolism isn’t quite as simple as calories in, calories out, says Sassone-Corsi. Not all foods are metabolized at the same rate. The first metabolic step that occurs when you eat is glycolysis, a cellular pathway that breaks down glucose and generates energy very quickly. Our cells have evolved to run this pathway first, so our bodies burn and utilize the simple sugars we take in first, transforming them to the energy we need via glycolysis. But if sugar isn’t available because we aren’t taking any in, such as on a ketogenic diet, our bodies will run a different metabolic pathway — a state of ketosis — that breaks down fat for energy instead. If we take in a meal that contains protein, fats, and sugars and has more calories than we can use, our bodies will burn the sugar first and store the fat and protein because they’re more complicated for the body to utilize, says Sassone-Corsi. 

And perhaps more important is the difference in every human’s cellular makeup and metabolic rate. Some people have a faster or slower metabolic rate naturally, but external factors like diet, exercise, and sleep can alter it. 

“Every time a cell responds to anything, whether it's food, or exercise, or whatever, those metabolic cycles are modified,” Sassone-Corsi says. 

Exercise affects your metabolic processes, not only by burning more calories during exercise but also by keeping your metabolic rate increased for hours afterward. Plus, building lean muscle mass with exercise will increase your basal metabolic rate — if you have more muscle, you need more calories to feed it.

Exercising at the optimal time for your circadian rhythm can also maximize its beneficial effects on your metabolism. Sassone-Corsi says mid-morning is ideal in terms of our natural circadian clocks, based on research he and his colleagues have done. While exercise is beneficial no matter when you do it, the research in mice has shown that mid-morning exercise resulted in higher utilization of energy, burning more carbs and ketone bodies than exercise in the early evening.

Our Circadian Rhythm and Its Effects on Metabolism

Our metabolism runs in sync with our body’s daily clock. Through millions of years of evolution, our bodies evolved to expect food during the daytime and to run regenerative and repair processes at night, while we sleep. Now, we have access to calories all the time, and often consume calories when our bodies don’t expect them, such as having a midnight snack. Feeding our bodies calories constantly destroys our metabolism. “It’s like a stick of dynamite,” says Sassone-Corsi.

If you eat a cheeseburger in the middle of the day, your body is able to process the calories fairly efficiently, using them for energy needs immediately—you’re moving around, you burn off the calories you just took in. But if you eat a cheeseburger at midnight, when your body is running restorative metabolic cycles, “those cycles go crazy,” says Sassone-Corsi. “What I mean by that is you activate a whole new program, which is related to inflammation or stress responses in the cell.” 

So meal timing is important to maintaining metabolism, and tweaking your meal timing can actually have beneficial effects on your health and metabolism. For example, time-restricted feeding, where you extend the amount of time you normally fast at night while sleeping—say by delaying your first meal until 11:30 or noon, and then stopping eating at around 8pm—can yield similar health benefits as restricting how many calories you eat. That said, Sassone-Corsi emphasizes that not all people are the same. Some are night owls who like to go to bed late and get up late, and some are early risers. (He advises that you work with your own sleep cycle to devise the best timing for your meals so that you have a few hours before sleep to digest, and fast for some hours after you wake.)

Every time a cell responds to anything, whether it's food, or exercise, or whatever, those metabolic cycles are modified.

As we age, our metabolic processes begin to slow as we tend be less active, and we also lose muscle mass. 

“We can’t stop getting older,” says Sassone-Corsi, “but we can make sure that not only do we give ourselves a chance at the longest life, but increase our quality of life. And at least some of our life quality is based on these very simple principles of following our circadian rhythm.”

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