How Many Calories Are in a Human Body?

An average adult man of about 66 kg holds somewhere around 125,000 to 145,000 calories, roughly two months of food for an average person.1NPR. Bison Or Brian? From A Calorie Perspective, Cannibalism Didn’t Pay For Paleo Humans Most of it sits in fat rather than muscle, and it rises with every pound of stored fat.

Where the Calories Are Stored in Your Body

Body fat is the main store. One pound of it holds about 3,750 calories, because about 87% of fatty tissue is pure fat, and fat carries more than twice the calories of protein or carbohydrate per gram.2PubMed Central. Time to Correctly Predict the Amount of Weight Loss with Dieting Even a lean adult carrying 15 pounds of fat stores more than 50,000 calories in it.

Fat is also the most compact fuel the body has. It holds very little water, so nearly all of its weight is fuel. That is why the body keeps its long-term energy as fat.

Muscle is bulky but low in calories. All the skeletal muscle in an average man adds up to only about 32,000 calories.3NPR. Bison Or Brian? From A Calorie Perspective, Cannibalism Didn’t Pay For Paleo Humans Muscle is 65% to 80% water, about 18.5% protein and only about 3% fat.4Texas Tech University. Composition of Muscle A pound of human muscle therefore yields roughly 650 calories, less than a fifth of what a pound of fat holds.5Science. Cannibalism Study Finds People Are Not That Nutritious

Muscle adds weight without adding many calories, so two people of the same weight can differ by tens of thousands of calories. A bodybuilder may store fewer calories than a softer person of the same weight.

The organs and stored carbohydrate add comparatively little:

About 400 g of the glycogen sits in the muscles and about 100 g in the liver, so muscle is by far the larger store of carbohydrate.12Cathe Friedrich. How Much Glycogen Can Your Body Store?13PubMed. Carbohydrate storage in man: speculations and some quantitative considerations Small as it is, stored carbohydrate is the fuel the body reaches for first between meals. Eating carbohydrate refills it, and without food the liver’s share can be used up within a day or so.

Why a Human Body Is Low in Calories for Its Size

An adult body is about 68% water, 14% protein, 12.5% fat and 5% mineral, mostly bone.14Journal of Biological Chemistry. The Chemical Composition of the Adult Human Body and Its Bearing on the Biochemistry of Growth Water and bone mineral carry no calories, so only the protein and fat count. The whole body averages about 1.9 calories per gram, under 900 calories per pound.15Journal of Biological Chemistry. The Chemical Composition of the Adult Human Body and Its Bearing on the Biochemistry of Growth

That average moves with fat. Fat is the densest part, so a person with more of it holds more than 1.9 calories per gram and a leaner person holds fewer.

A pound of muscle from a wild boar or beaver holds about 1,800 calories, nearly three times a human’s 650.16Science. Cannibalism Study Finds People Are Not That Nutritious A whole horse provides about 200,000 calories and a bison close to a million.17PBS NewsHour. If you had to eat a human, which body part should you pick first? By calories alone, a human is a poor meal compared with hunted animals.18Science. Cannibalism Study Finds People Are Not That Nutritious

Burning human tissue releases relatively little heat, about 17 kilojoules per gram, enough for a fire that is unlikely to spread.19Journal of Forensic Sciences. Experiments in the Combustibility of the Human Body But how well tissue burns is a separate question from how much energy the body can use. Wet tissue spends much of its heat boiling off water, and bone mineral adds weight but no fuel.

How the Number Changes From Person to Person

The roughly 143,000-calorie figure describes an average adult man, not everyone.20NPR. Bison Or Brian? From A Calorie Perspective, Cannibalism Didn’t Pay For Paleo Humans The figures come from a small number of adult men, so they are averages. A man carrying 38 kg of fat holds more than six times the fat of a lean man with 6 kg.21PubMed Central. A mathematical model of weight loss under total starvation: evidence against the thrifty-gene hypothesis

Women generally carry a higher share of body fat than men of the same weight, so at the same weight a woman usually stores more calories in fat. Daily burn is a different matter. The calories a person burns each day to stay the same weight depend on sex, age, weight and how active they are.22PubMed Central. The effect of bigger human bodies on the future global calorie requirements A larger body both stores more and burns more each day, so a bigger reserve doesn’t automatically last longer per pound.

You can get a ballpark for your own body. Start with your body fat in pounds, which is your weight times your body fat percentage; a 180-pound person at 25% fat carries about 45 pounds of fat. Multiply by about 3,750 calories, which gives roughly 169,000 calories in that example. Then add about 30,000 for muscle and a few thousand more for organs and stored carbohydrate. Treat the total as rough, because body fat readings can be off by several percentage points and muscle size varies.

Bathroom scales with body fat readings give rough estimates, and clinics can measure more accurately with body scans. Readings can shift with how much water you have drunk.

How Long Stored Calories Can Keep You Alive

In the hours after a meal, the liver releases its stored carbohydrate and fat starts being released from fatty tissue.23PubMed. Energy metabolism in feasting and fasting As fasting continues, muscle protein is broken down for a short time to help the liver make sugar.24PubMed. Energy metabolism in feasting and fasting After about three days, the body switches to protecting protein and runs mainly on fat and ketones, a fuel the liver makes from fat.25PubMed. Energy metabolism in feasting and fasting

The first days are the hardest. Hunger and low energy are often strongest in the first day or two, while the body still leans on stored carbohydrate and hasn’t shifted toward fat. Much of the early weight drop is water, because stored carbohydrate is held with water and releases it when used. After a few days, as ketones build up, hunger often eases.

Fat sets survival time. The more fat a person stores, the longer they could survive with no food, and women survive longer than men carrying the same amount of fat.26PubMed Central. A mathematical model of weight loss under total starvation: evidence against the thrifty-gene hypothesis

The rough maths is stored calories divided by daily burn. An average person burning about 2,000 a day stores weeks of fuel. The real figure is shorter, because not all tissue can be used safely, though the body also slows its energy use when starved.

Muscle pays a price over time. A short period of eating too little doesn’t measurably shrink muscle, though its stored carbohydrate drops by half or more.27PubMed. Biochemical composition of muscle in normal and semistarved human subjects: relevance to anthropometric measurements Long-term starvation, however, can shrink muscle by more than half.28PubMed. Biochemical composition of muscle in normal and semistarved human subjects: relevance to anthropometric measurements What remains holds relatively more water and fewer calories.29PubMed. Biochemical composition of muscle in normal and semistarved human subjects: relevance to anthropometric measurements

A large fat reserve still doesn’t make a long fast safe. Fat supplies calories but not vitamins, minerals or salts, and shortages of these can cause serious problems however much fat remains. The heart is a muscle, and the protein loss of prolonged starvation can weaken it and disturb its rhythm. Survival estimates describe outer limits, not a plan. Anyone thinking about a long fast, especially with a medical condition or on medication, should talk to a doctor first.

What a Pound of Body Fat Means for Weight Loss

The familiar rule of about 3,500 calories per pound comes from the energy in a pound of fatty tissue, closer to 3,750.30PubMed Central. Time to Correctly Predict the Amount of Weight Loss with Dieting A 500-calorie daily deficit adds up to about 3,500 calories a week, so in theory it removes about a pound of fat a week. The deficit can come from eating less, moving more, or a mix, and the body doesn’t care which, though a very large cut is harder to sustain.

The rule misleads because the weight people gain and lose isn’t pure fat; it also contains muscle and water.31PubMed Central. Obesity Tissue: Composition, Energy Expenditure, and Energy Content in Adult Humans In men, each kilogram of that mixed weight holds about 4,560 calories, and in women about 5,525.32PubMed Central. Obesity Tissue: Composition, Energy Expenditure, and Energy Content in Adult Humans A pound of lost weight is therefore worth roughly 2,100 calories in men and 2,500 in women, less than a pound of pure fat. Actual weekly loss can run somewhat faster than the rule predicts at first.

Then it slows. As you get lighter you burn fewer calories each day, so the same diet produces slower loss over time. Fat loss itself is slower and steadier, and several weeks give a truer picture than a few days.

Very steep calorie cuts take more of the lost weight from muscle. Eating enough protein helps hold on to it, and strength exercise signals the body to keep it. Keeping muscle helps keep daily calorie burn from dropping too far.