Written by: Sarah Jayawardene, MS
Reviewed by: Emily J., MSc RD
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Metabolism is the breakdown of food for energy, growth, and development. Each macronutrient — carbs, fats, and proteins — are metabolized differently and can impact your metabolic health in many ways.
Metabolism is necessary for the survival of all living organisms. It allows you to breathe, circulate blood, fight infections, repair cells, and make new proteins, just to name a few among countless other roles. But what exactly is metabolism, and how does it work?
Your metabolism is a tightly regulated and highly organized system of biochemical reactions that converts food into energy and keeps your body in homeostasis, or biological balance . It constantly runs to maintain your body temperature, move your muscles, and keep you alive and functioning.
The fuel for your metabolism is the food you consume.
When you eat, your digestive system breaks down macronutrients (i.e., carbohydrates, proteins, and fats) into smaller components that are absorbed into your bloodstream and circulated throughout the body. Then they’re converted into the energy that powers your brain, heart, lungs, and muscles, as well as all other organ systems.
But these macronutrients aren’t all the same — your body metabolizes them in different ways. Understanding these distinctions is key to understanding your metabolic health, and is a great starting point for developing better eating habits.
So what exactly happens in your body after you take a bite of food? Throughout the article, we’ll use the example of eating a cheeseburger — which has carbs, protein, and fat — to explain how each component of food is metabolized.
Carbohydrates are broken down into glucose (your body’s preferred energy source) through a process aptly named carbohydrate metabolism . This allows your body to choose between using glucose as energy immediately or storing it for later use.
When you eat a cheeseburger, the carbs from the bun and vegetables will be digested and broken down into smaller molecules, called monosaccharides and disaccharides, which are chains of sugar. These are then absorbed into the bloodstream through transporter molecules in your small intestine — which results in your blood glucose (or blood sugar) rising.
Once in your bloodstream, these molecules of glucose make their way to cells in your body that need them for energy, and wait to be let into the cells by the hormone insulin.
You can think of your cells as event spaces where guests (glucose) are attending a party. When you eat, you’re sending big groups of guests to the front doors of your cells. To control the flow of traffic, your pancreas sends insulin, which acts like a doorman, to unlock your cells and let glucose in .
Insulin unlocks your cells so that glucose can enter and be turned into energy.
But what happens once glucose is inside of a cell?
After insulin lets it pass through the cell membrane, glucose is broken down into smaller molecules (called pyruvate) through a process called glycolysis and shuttled into the mitochondria, the energy factory of your cell, where it’s used to produce adenosine triphosphate (ATP) .
ATP comes up a lot in cell metabolism — it’s a molecule that can carry energy throughout cells and provide energy for reactions to keep our cells (and us) alive.
In other words:
After you eat and metabolize the carbohydrates from the bun and vegetables, there may still be a surplus of glucose. If there’s already enough glucose in your body, and adequate levels of ATP, glucose will instead be sent to your liver and muscles for storage in the form of glycogen — energy your body can use later on.
After digesting the proteins from the meat in your cheeseburger, amino acids (the building blocks of proteins) can be circulated in the body and used by your cells.
The main role of amino acids is to make new proteins that your cells need to function. These include, but are not limited to:
Some types of amino acids (13 out of the 20 total amino acids) can also be converted into glucose through a process called gluconeogenesis, which takes place mostly in the liver and is an important part of protein metabolism [5, 6, 7].
The beef patty in our cheeseburger example contains isoleucine, valine, threonine, and methionine — four of the amino acids that can be converted into glucose.
Your body will digest the patty and use some of the amino acid building blocks to make new proteins, while others may undergo gluconeogenesis and become glucose that your body can use directly as fuel or further transform into fat via a process known as lipogenesis.
Lipid metabolism is the process of breaking down and using fats and fatty acids for energy . This breakdown of lipids is necessary to provide energy for the body, and it also serves as a source of building blocks for new cells and hormones.
The beef and cheese of your burger contain fat, and when you eat these, your digestive system first breaks down the lipids, composed of molecules called triglycerides into their individual components of glycerol and free fatty acids. Your liver will use the fatty acids and convert them into glucose for metabolism.
This is done via a process called beta-oxidation, which results in a molecule called acetyl-CoA that can be used to aid the production of ATP (the energy-carrying molecule).
We know now what happens to carbohydrates in an absorptive state (when food is actively being digested and nutrients absorbed), but what about in between meals, while you sleep, or if you skip meals?
Remember — your metabolism keeps your body running, and has a number of intricate systems in place to ensure there is always a source of glucose/ATP to power your organs, particularly your brain and heart.
Once you’ve eaten, you typically don’t eat again until your next meal. In other words, you enter a fasting state.
About 90 minutes after eating, your blood glucose peaks. Around the 2-hour mark, it stabilizes and reaches pre-meal levels . After several hours of not eating, when your body needs more glucose to create ATP, it tells the liver to release stored glycogen, which is turned back into glucose by the hormone glucagon.
Once glycogen stores begin to get depleted, your body uses gluconeogenesis to supply the glucose it needs to function when glycogen stores in the liver are running low.
This can happen after about 8 hours of fasting, when you’re on a low-carb diet, and while you’re sleeping. (Since gluconeogenesis takes time, the newly formed glucose is released into the blood slowly and doesn’t raise blood sugar.)
When there aren’t enough glucose or fatty acids readily available for energy, insulin can help regulate blood sugar levels by increasing circulating fatty acids when necessary . It signals adipose tissue (fat tissue) to liberate and release stored lipids, which can then be transported to the liver for eventual ATP production.
In an ideal world, your body is able to supply glucose to your cells from all three macronutrients. That said, conditions such as insulin resistance or metabolic dysfunction can disrupt your body’s ability to make glycogen — meaning that your body can’t properly maintain a supply of “back-up” glucose to create energy .
Understanding your metabolic health — which often begins with a tool like a CGM that allows you to track your blood glucose levels in real-time — is a critical part of ensuring that your body’s intricate metabolic balancing act is functioning properly — and thus allowing you to continuously have the energy to survive .
Many different processes contribute to metabolism, including digestion and absorption of food, as well as chemical reactions that occur throughout the body. Metabolism is an ongoing, dynamic process that helps your body maintain a state of energy homeostasis.