What do calories do in the body?

What do calories do in the body? They give your cells the energy they need to keep you alive and moving. Every heartbeat, every breath, every thought running through your head right now burns calories. Without them, your organs shut down and your body stops working.

A calorie is a unit of energy. When you eat food, your body breaks it down and pulls out the energy stored inside the carbs, protein, and fat. That energy powers everything from your brain to your muscles to the repair work your body does while you sleep.

Most people hear the word “calories” and think about weight loss or dieting. But calories do a lot more than make you gain or lose weight. They run your entire body, and understanding how they work changes the way you think about food.

What is a calorie and where does it come from?

A calorie measures how much energy a food or drink gives your body. The technical definition says one calorie is the amount of energy needed to raise the temperature of one kilogram of water by one degree Celsius.

Your body gets calories from three main sources in food.

1. Carbohydrates give you 4 calories per gram
2. Protein gives you 4 calories per gram
3. Fat gives you 9 calories per gram

Alcohol also contains calories at 7 per gram, but your body can’t use them for anything useful. These are “empty calories” because they add energy without any real nutrition.

When you eat, enzymes in your digestive system break carbs down into glucose and other sugars, fats into glycerol and fatty acids, and protein into amino acids. These molecules travel through your bloodstream to your cells, where they get used right away or stored for later.

According to the National Institutes of Health, more than 95% of the energy in food gets digested and absorbed from the gastrointestinal tract under normal conditions. Your body is very efficient at extracting energy from what you eat.

How does your body actually use calories?

Your body uses calories in four main ways, and most of them have nothing to do with exercise.

1. Basal Metabolic Rate (BMR) takes up 60% to 70% of your total daily calorie burn. This is the energy your body needs just to stay alive. Your heart beating, your lungs breathing, your kidneys filtering blood, your liver processing nutrients, and your body keeping itself at 37 degrees Celsius all require calories. Even if you stayed in bed all day, your body would still burn the majority of its daily calories on these basic functions.
2. The thermic effect of food (TEF) uses about 5% to 10% of your daily energy. It takes energy to digest, absorb, and process the food you eat. Not all foods cost the same amount of energy to process either. Fat has a TEF of 0% to 3%, meaning if you eat 100 calories of fat, you net about 97 to 100 of those calories. Carbs have a TEF of 5% to 10%. Protein stands out with a TEF of 20% to 30%, which means eating 100 calories of protein only nets you 70 to 80 usable calories. Your body burns the rest just processing it.
3. Non-exercise activity thermogenesis (NEAT) covers all the movement you do that isn’t planned exercise. Fidgeting, walking to the kitchen, tapping your foot, standing up from your desk, and even waving your hands while talking all burn calories. NEAT varies wildly between people. One landmark study from 1995 overfed subjects by 1,000 calories per day for six weeks and found that some people barely gained any weight because their NEAT went up. One person gained just over half a kilogram when they should have gained 3 to 4 kilograms because their body spontaneously increased physical activity without them even realising it. Research also shows that even a 10% reduction in body weight can decrease NEAT by almost 500 calories per day, which makes weight regain more likely after dieting.
4. Exercise activity thermogenesis (EAT) covers planned workouts and structured exercise. For most people, this makes up the smallest portion of daily calorie burn. And here’s something most people don’t know. For every 100 calories you burn through cardio, your body compensates by reducing your NEAT so you only actually increase your total daily burn by about 72 calories on average. Your body subconsciously moves less after exercise to partially offset the extra energy you spent.

How many calories does your brain burn?

Your brain burns about 20% to 25% of your total daily calories. That works out to roughly 320 to 500 calories every day, depending on your size and metabolic rate.

This is remarkable because the brain only makes up about 2% of your total body weight. Despite being small, it is the most energy expensive organ in the body. Research published in the Proceedings of the National Academy of Sciences confirms the brain accounts for about 20% of the body’s oxygen consumption, and this rate stays fairly constant whether you’re watching TV or solving a math problem.

The brain runs almost entirely on glucose. About 75% of the brain’s energy goes toward processing information and transmitting neural signals between billions of neurons. The remaining 25% handles housekeeping tasks and maintenance of cell structures, according to research by British neuroscientist David Attwell and colleagues.

In children aged 5 to 6, the brain can use up to 60% of the body’s total energy, according to Doug Boyer, associate professor of evolutionary anthropology at Duke University. This massive energy demand supports the rapid brain development happening during those years.

Hard mental work does burn a few extra calories, but not as many as you might hope. A person doing intense cognitive work for eight hours burns about 100 more calories than someone watching TV for the same time, according to research from the University at Albany.

What happens when you eat too many calories?

Your body stores the extra energy as fat. It’s that simple.

When you take in more calories than your body needs for its daily functions and activities, the excess gets converted and packed away in fat cells. Your body does this because it evolved to survive periods without food, so storing extra energy made sense for survival.

According to the NCBI (National Center for Biotechnology Information), body fat is the primary energy store in humans. This storage system works incredibly well. One kilogram of body fat stores roughly 7,700 calories worth of energy.

Over time, consistently eating more calories than you burn leads to weight gain. And this doesn’t require eating huge amounts either. An extra 100 calories per day beyond what your body needs would add up to roughly 4.7 kilograms of fat gained per year. That’s roughly one extra tablespoon of peanut butter per day.

Weight gain from excess calories also increases the risk of serious health conditions. A review of the evidence shows that excess body fat raises the risk of type 2 diabetes, cardiovascular disease, high blood pressure, and non-alcoholic fatty liver disease. Even modest weight loss helps though. Losing just 1 kilogram of body weight has been associated with approximately a 1 millimetre of mercury drop in blood pressure.

What happens when you eat too few calories?

Your body starts shutting down non-essential functions to keep you alive.

When calories are restricted, your body prioritises energy for the most important things first. The survival functions like your heart, lungs, kidneys, brain, and liver get about 800 calories per day. After that come the “luxury” functions like temperature control, hair growth, nail growth, wound healing, and menstruation, which need about 400 calories per day. Growth and physical activity come last.

According to the NCBI, when your body doesn’t get enough calories, cortisol production increases, metabolism starts slowing down, and the body goes into a survival mode where it refuses to let go of stored fat and holds onto every calorie it can.

This is why very low calorie diets often backfire. Research shows the more aggressively you cut calories, the more your metabolic rate drops. A good target for weight loss is about 0.5% to 1% of your body weight per week. Going faster than that increases the chances your metabolism tanks and makes it harder to keep losing weight.

In the most extreme cases of starvation where no food is consumed, death typically occurs in 8 to 12 weeks according to the Merck Manual. The body starts breaking down muscle protein for energy, organs take damage, and eventually cardiac arrest becomes a real risk.

Do all calories affect your body the same way?

No. A calorie is a calorie when it comes to pure energy, but the source of those calories changes how your body processes them and how hungry you feel afterward.

The clearest example is protein versus fat. When you eat 100 calories of protein, your body burns 20 to 30 of those calories just to digest and absorb it. When you eat 100 calories of fat, your body burns only 0 to 3 calories on digestion. You net far more usable energy from fat than from protein, even though the total calories going in are identical.

Studies have shown that switching from a low protein diet to a high protein diet can raise your daily calorie burn by about 4% to 5%. That might sound small, but it equals the same calorie burn as a 10 minute jog every single day and can add up to an extra half kilogram of fat loss per month.

The type of carbs matters too. A recent study compared two groups eating 2,100 calories per day. One group ate mainly processed foods stripped of fibre and starches like chips, white bread, and juice. The other group ate mainly whole foods with lots of fibre and resistant starch like potatoes, oats, and fruit. Even at the same total calorie intake, the whole foods group saw better results because fibre and resistant starch change how your body handles the energy.

The bottom line is that while energy balance determines weight change, the source of your calories affects your hunger, your metabolic rate, your muscle retention, and your overall health.

How many calories does the average person need per day?

Most adults need somewhere between 1,600 and 3,200 calories per day. The exact number depends on your age, sex, body size, muscle mass, and activity level.

The most recent Dietary Guidelines recommend women consume 1,600 to 2,200 calories per day and men aim for 2,200 to 2,800 calories per day. But these are just starting points.

Your resting metabolic rate makes up the biggest chunk. You can estimate it roughly by multiplying your body weight in pounds by 10. So a 75 kilogram (165 pound) person would have a resting metabolic rate of about 1,650 calories. Add 30% for basic daily activities even with a sedentary lifestyle, and you get about 2,145 calories as a rough daily total.

Metabolic rates vary enormously between people though. A massive 2022 study on daily energy expenditure published in Science looked at over 6,000 subjects. At the same body weight of 80 kilograms, the person with the slowest metabolism burned only about 1,400 calories per day while the person with the fastest burned a massive 5,700 calories per day. That is a difference of over 4,000 calories at the exact same weight.

Age affects calorie needs too. The highest calorie needs per kilogram of body weight happen during infancy and decline through childhood. After age 20, your metabolic rate drops by about 2% per decade because of declining lean body mass.

Fitness trackers and calorie calculators give you a ballpark, but they’re not perfect. A 2018 meta-analysis found that fitness watches overestimated energy expenditure by between 28% and 93% depending on the brand. The best approach is to track your weight consistently, weigh yourself first thing in the morning every day, take the weekly average, and adjust your food intake based on whether that average is going up, down, or staying flat.

Does building muscle change how many calories you burn?

Yes. Muscle burns about three times more calories at rest than fat does.

One pound of fat burns roughly 2 calories per day at rest, while one pound of muscle burns about 6 calories per day. That gap adds up fast when you carry significant amounts of each.

Here’s an example. If a new lifter puts on about 13.5 kilograms (30 pounds) of muscle over five years, and they were burning 2,500 calories per day before, they would now burn about 2,680 calories per day. That’s a 180 calorie daily difference, which equals about four extra kiwis per day. Not massive, but meaningful over time and it’s one of the largest natural metabolic boosters available.

Building muscle also improves your body’s sensitivity to insulin, helps regulate blood sugar, and supports bone density. Exercise independent of anything that happens with your body weight will make you healthier. It improves insulin sensitivity, reduces inflammation, and improves cardiovascular markers even without weight loss on the scale.

Research on long-term weight loss success shows that over 70% of people who lose weight and keep it off for multiple years engage in regular exercise. Among people who regain the weight, less than 30% exercise regularly.

What role does protein play in calorie burning?

Protein has the highest thermic effect of any macronutrient, burning 20% to 30% of its calories during digestion alone.

If you eat 100 calories of protein, you only net about 70 to 80 usable calories. Compare that to carbs at 5% to 10% TEF and fat at 0% to 3% TEF. This makes protein the most metabolically expensive food you can eat.

For daily intake, research supports aiming for about 1.6 to 1.8 grams of protein per kilogram of body weight. So a 90 kilogram person would aim for about 144 to 162 grams per day. If you’re well under that target right now, increasing protein intake should be one of your first priorities.

Beyond the thermic effect, protein also increases satiety and helps you feel full for longer. Multiple studies show that higher protein diets reduce overall calorie intake because people naturally eat less at subsequent meals. And of course, adequate protein protects your muscle mass during weight loss, which helps keep your metabolic rate from crashing.

Can you speed up your metabolism to burn more calories?

Some things work, some might work, and some probably don’t work at all. Here’s what the research actually shows.

What works

1. Building muscle increases your resting metabolic rate. Adding 13.5 kilograms of muscle can boost daily burn by about 180 calories.
2. Staying physically active throughout the day through NEAT (taking stairs, parking farther away, standing at your desk) reliably burns extra calories.
3. Avoiding very low calorie diets prevents your metabolic rate from tanking. Aim to lose 0.5% to 1% of body weight per week.

What might work

1. Drinking cold water burns about 8 extra calories per glass because your body has to heat it up. It adds up slowly but the compensation effects might cancel some of it out.
2. Eating spicy food containing capsaicin showed an average increase of about 69 calories per day in a 2017 meta-analysis of nine studies, but only in people with a BMI over 25.
3. Weighted vests show promise. Early case studies suggest wearing a weighted vest for most of the day can trick your body’s gravitastat sensors into thinking you weigh more, potentially increasing calorie burn and reducing hunger.

What probably doesn’t work

1. Green tea showed no meaningful fat loss benefits in long-term studies despite popular claims of up to 8% metabolic boosts.
2. Sauna burns only about 5 extra calories per 10 minute session beyond what you’d burn sitting in a normal room. Not worth counting.
3. Cold plunges burn about 14 extra calories per 10 minute session beyond resting. Also not meaningful for weight loss.
4. Eating more meals per day doesn’t keep the “metabolic furnace” burning. A 2012 study found no difference in energy expenditure between eating 3 meals or 14 meals per day with the same total calories. A 2015 meta-analysis of 15 studies also found no significant difference in fat loss between different meal frequencies.

FAQ

How many calories should I eat to lose weight?

You need to eat fewer calories than your body burns. A deficit of about 500 calories per day below your total daily energy expenditure leads to roughly half a kilogram of fat loss per week. Track your weight daily first thing in the morning, take the weekly average, and adjust intake based on the trend rather than relying on calorie calculators alone.

Do calories from fruit make you fat?

Excess calories from any source can lead to fat gain. But fruit comes packed with fibre, vitamins, and water which all help fill you up and slow digestion. You would need to eat enormous amounts of fruit to overconsume calories from it alone. Whole fruit is one of the healthiest sources of carbohydrates you can eat.

How accurate are calorie counts on food labels?

Food labels can have up to a 20% error. Something listed as 100 calories per serving could actually contain 80 or 120 calories. The good news is that if the label is off, it tends to be consistently off. So if you track your food consistently using the same products, you can still get reliable results over time by adjusting based on your actual weight trends.

Does your metabolism slow down as you age?

Yes. Your metabolic rate declines by about 2% per decade after age 20, mainly because of declining lean body mass. This is why strength training becomes even more important as you get older. Maintaining or building muscle is the most effective way to fight age related metabolic decline.

Is it true that eating breakfast boosts your metabolism?

Skipping breakfast does not slow your metabolism. Your total daily calorie intake and expenditure matter far more than when you eat. Research on time restricted eating versus normal eating patterns shows no difference in fat loss when total calories are the same. Eat breakfast if you enjoy it, skip it if you don’t. Pick whatever pattern helps you stay consistent with your total intake.

How many calories does walking burn?

Walking burns roughly 60 to 100 calories per kilometre depending on your body weight and pace. A 75 kilogram person walking at a moderate pace burns about 80 calories per kilometre. Walking 10,000 steps covers roughly 7 to 8 kilometres and burns about 350 to 500 calories. Walking is one of the most underrated fat loss tools because it burns meaningful calories without spiking your appetite the way intense exercise can.

Can you eat whatever you want as long as you stay in a calorie deficit?

For pure weight loss, yes. A calorie deficit will cause weight loss regardless of what foods you eat. But for your health, body composition, energy levels, and hunger management, food quality matters enormously. Protein keeps your muscle and burns more calories during digestion. Fibre fills you up. Whole foods regulate your blood sugar better than processed foods. Eating well within your calorie budget makes the whole process easier and healthier.

Understanding caloric metabolism helps you avoid both nutritional deficiencies and excesses like overhydration. This knowledge becomes especially practical when you’re trying to lose 5kg quickly while maintaining energy for workouts. For expert support in managing your caloric balance and achieving sustainable weight loss, a personal trainer in South Yarra can create science-based strategies that work with your body’s metabolism.