Carbohydrates are the human body’s main source of energy. When you eat foods like bread, rice, fruit, or pasta, your digestive system breaks them down into glucose, a simple sugar that fuels nearly every cell in your body. If you’re thinking bigger picture, oil remains the world’s main energy source at the societal level, supplying 34% of total global demand in 2024.
Why Your Body Prefers Glucose
Your body can technically run on fats, protein, or carbohydrates, but glucose is the preferred fuel. The moment you eat carbohydrates, your digestive tract starts converting them into glucose, which enters your bloodstream. Rising blood sugar signals your pancreas to release insulin, which tells your cells to absorb that glucose and either use it immediately or store it for later.
Your brain is especially dependent on glucose. Despite making up only about 2% of your body weight, the brain consumes roughly 20% of all glucose-derived energy. It burns through about 5.6 milligrams of glucose per 100 grams of brain tissue every minute, making it the single largest consumer of glucose in the body. This is why skipping meals can leave you feeling foggy or irritable before any other symptom shows up.
How Cells Turn Food Into Fuel
Glucose on its own doesn’t power your cells directly. It has to be converted into a molecule called ATP, which is the actual energy currency your cells spend. This conversion happens in two stages, and the difference between them is dramatic.
The first stage, called glycolysis, happens in the main body of the cell and extracts only a small fraction of the energy locked inside glucose. The real payoff comes in the second stage, which takes place inside mitochondria, the tiny power plants found in nearly every cell. Mitochondria take the partially processed glucose, combine it with oxygen, and generate about 15 times more ATP than the first stage alone. This is why breathing matters so much for energy: without oxygen, your cells would be stuck with that small initial trickle of ATP.
Inside mitochondria, the fuel molecule gets fed through a cycle that strips away high-energy electrons. Those electrons pass through a chain of proteins embedded in the mitochondrial membrane, and their energy is used to pump hydrogen ions across that membrane. When the ions flow back through a specialized protein machine called ATP synthase, the flow drives the assembly of ATP, roughly three to four ions per molecule. It’s an elegant system, and it produces the vast majority of the energy your body uses every day.
Where Your Body Stores Energy
You don’t use every bit of glucose the moment it enters your bloodstream. Your body stores the excess as glycogen, a compact form of glucose tucked away in two main locations. Skeletal muscles hold about 500 grams of glycogen, and the liver stores around 100 grams. Together, that’s roughly 2,400 calories of quick-access energy.
Muscle glycogen fuels physical activity directly. Liver glycogen serves a different purpose: it gets converted back into glucose and released into the bloodstream to keep your blood sugar steady between meals, which is especially important for your brain. Once those glycogen stores are full, any additional excess energy gets converted into fat and stored in adipose tissue around your abdomen and under your skin.
When Your Body Switches to Fat
Your body isn’t locked into burning glucose. It can shift to burning fat under certain conditions, a process sometimes called metabolic flexibility. During fasting or prolonged exercise, your muscles increasingly rely on fat oxidation instead of glucose. This transition serves two purposes: it preserves blood glucose for your brain and delays the depletion of muscle glycogen so you can keep moving longer.
The switch happens inside the same mitochondria that process glucose. Both fatty acids and glucose-derived molecules get converted into the same intermediate compound, which then enters the same energy-producing cycle. Mitochondria can use both fuels, and the competition between them plays out at several points along the internal membrane where energy production occurs. During fasting, measurements of the fuel mix being burned in leg muscles show a clear tilt toward fat oxidation in healthy individuals.
If the body still isn’t getting enough calories from carbohydrates or stored fat, it will start breaking down protein as a last resort, converting it into molecules that can be used for energy. This is why severe calorie restriction can lead to muscle loss.
Fat Packs More Energy Per Gram
Carbohydrates provide about 4 calories per gram. Fat provides about 9. This makes fat an incredibly efficient storage medium, which is exactly why your body preferentially stores surplus energy as fat rather than as more glycogen. Your glycogen reserves might last a day of normal activity, but even a lean person carries tens of thousands of calories in fat stores.
This efficiency is also why the body deposits fat so readily. It’s not a design flaw. It’s a survival strategy built for environments where food was unpredictable. The trade-off is that accessing fat for energy is a slower process than tapping glycogen, which is why high-intensity exercise burns through carbohydrates first and your body shifts to fat burning during longer, lower-intensity efforts.
The World’s Main Energy Source
If your question was about energy at a global scale rather than inside the body, the answer is fossil fuels, with oil leading the pack. In 2024, total global energy supply hit a record 592 exajoules. Oil accounted for 34% of that total, making it the single largest source. Natural gas supplied about 25% of global energy demand, and coal also reached a record level of demand at 165 exajoules.
Nuclear energy met just over 5% of global demand, growing 3% in 2024. Electricity demand grew at 4%, outpacing total energy demand growth, a sign that the world’s energy system is steadily electrifying. Solar, wind, and hydropower continue to grow as shares of electricity generation, but fossil fuels still dominate the overall energy mix when you account for transportation, heating, and industrial processes alongside electricity.