Energy in your body comes from the food you eat. Carbohydrates, fats, and proteins are broken down through a series of chemical reactions that ultimately produce a molecule called ATP, the universal fuel your cells run on. But zoom out further and nearly all energy on Earth traces back to one source: the sun.
How Food Becomes Fuel
Your body extracts energy from three macronutrients, each at a different density. Fat is the most energy-rich at 9 calories per gram. Carbohydrates and protein each provide about 4 calories per gram. Alcohol, though not a nutrient your body needs, delivers 7 calories per gram.
These raw materials all funnel into the same basic process. Carbohydrates get broken into glucose, which is split in half through a series of reactions in your cells. Those halves are shuttled into your mitochondria, the small structures inside nearly every cell that act as power generators. Fats take a different entry point: fatty acids are clipped two carbons at a time and fed into the same mitochondrial machinery. Proteins can also be converted and routed into this system when needed, though your body prefers to use them for building and repair.
No matter which macronutrient you started with, the process converges on the same molecule: a two-carbon unit that enters a loop of chemical reactions (often called the Krebs cycle). This loop strips away electrons, which are then passed along a chain of proteins embedded in the inner wall of each mitochondrion. As electrons move down this chain, they release energy that drives the assembly of ATP. Oxygen waits at the end of the chain, accepting the spent electrons and combining with hydrogen to form water. That’s why you breathe: to supply the oxygen that keeps this entire system running.
A single molecule of glucose can generate roughly 32 molecules of ATP through this process. Fat produces even more ATP per molecule because fatty acid chains are longer and yield more electron carriers per trip through the cycle.
What ATP Actually Does
ATP works like a rechargeable battery at the molecular level. It has three phosphate groups linked together, and those links are under tension because the negatively charged phosphates repel each other. When your cells snap off the last phosphate group, that tension releases energy your body can use immediately.
Your cells spend ATP on almost everything: contracting muscles, firing nerve signals, pumping molecules in and out of cells, building DNA, and synthesizing new proteins. The leftover molecule (ADP, with only two phosphates) gets recycled back into ATP inside your mitochondria, over and over, thousands of times per day. Your body turns over roughly its own weight in ATP every 24 hours, even though you only carry a few hundred grams of it at any given moment.
Where Your Body Stores Energy
Your body keeps energy in two main reserves. The fast-access supply is glycogen, a branched chain of glucose molecules stored primarily in your liver and skeletal muscles. Liver glycogen feeds your bloodstream between meals, keeping your blood sugar stable. Muscle glycogen fuels contractions during exercise. A single glycogen particle can contain up to 55,000 glucose units, but total glycogen stores are limited, typically enough to cover roughly a day of normal activity.
The larger, slower reserve is body fat. Fat tissue (adipose tissue) stores energy as triglycerides and holds far more calories than glycogen ever could. A pound of body fat contains about 3,500 calories. This is why your body preferentially stores excess energy as fat: it’s more than twice as energy-dense as carbohydrates by weight, making it an efficient long-term reserve.
How Your Body Spends Energy at Rest
Even when you’re doing nothing, your body burns a substantial amount of energy. Your resting metabolic rate accounts for 60 to 75 percent of the total calories you burn each day. This covers the basics: keeping your heart beating, your lungs expanding, your brain running, your cells dividing, and your body temperature stable. For most adults, that comes out to roughly 25 to 30 calories per pound of body weight per day.
The remaining energy goes to two things. Digesting and absorbing food (the thermic effect of food) costs a small percentage of your daily calories. Physical activity, from structured exercise to fidgeting and walking around your house, makes up the rest and is the most variable piece of the equation.
Vitamins That Keep the System Running
Your energy-producing machinery depends on more than just macronutrients. B vitamins act as essential helpers at key steps in the process. Thiamine (B1), for instance, works alongside magnesium to drive critical reactions in the Krebs cycle and in glucose metabolism. A deficiency in any B vitamin can impair your mitochondria’s ability to process glucose, fatty acids, and amino acids into ATP. This is one reason severe B vitamin deficiencies cause profound fatigue, even when calorie intake is adequate.
Where Energy Ultimately Comes From
Almost all energy in food traces back to the sun. Plants capture sunlight using pigment molecules that absorb light and funnel that energy into a reaction center, where it drives a charge separation. This converts light energy into chemical energy, which plants use to build glucose from carbon dioxide and water. When you eat plants, or eat animals that ate plants, you’re consuming stored solar energy. Your mitochondria then extract that energy through the oxygen-dependent process described above.
Fossil fuels work on the same principle, just on a much longer timeline. Coal, oil, and natural gas are the compressed remains of ancient organisms that originally captured the sun’s energy through photosynthesis hundreds of millions of years ago. As of 2023, fossil fuels still supply about 81 percent of the world’s total energy: oil at 30 percent, coal at 28 percent, and natural gas at 23 percent. Nuclear power contributes nearly 5 percent, while solar, wind, and hydropower together account for about 6 percent. Biofuels and waste make up the remaining 9 percent.
The Chemistry Is Counterintuitive
A common shorthand says that energy is “stored in chemical bonds” and released when those bonds break. The reality is more nuanced. Breaking a bond actually requires energy. What releases energy is the formation of new, stronger bonds. When your body metabolizes glucose, the carbon-carbon bonds in glucose do break, but the energy payoff comes from forming the strong carbon-oxygen and oxygen-hydrogen bonds in carbon dioxide and water. The products are more stable than the starting material, and that difference in stability is what your body captures as usable energy. The net result is the same, though: glucose plus oxygen in, ATP plus carbon dioxide and water out.