How Your Body Uses Energy and What It Means for Your Metabolism

Your body uses energy every minute of the day. Each time you eat, your body breaks food down into usable fuel. That fuel powers everything you do, from breathing and thinking to walking and regulating body temperature. The total amount of energy your body uses in a full day is called Total Daily Energy Expenditure (TDEE).

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What Makes Up TDEE

TDEE has four main parts, and each one contributes differently to your total daily energy use.

1. Basal Metabolic Rate (BMR): The energy your body uses to keep you alive, even when you’re resting or sleeping. It covers things like breathing, your heart beating, and keeping your body temperature steady. For most people, BMR makes up about 60 to 75% of daily energy use.
2. Thermic Effect of Food (TEF): This is the energy your body uses to digest, absorb, and process the food you eat. TEF is usually about 10% of TDEE.
3. Non-Exercise Activity Thermogenesis (NEAT): This is the energy you use from all the small movements you don’t think about, like walking to the kitchen, standing, shifting your posture, cleaning, or fidgeting. NEAT can range from 15 to 30%, depending on how much you move throughout the day.
4. Exercise Activity Thermogenesis (EAT): This is the energy you use during planned workouts like strength training, Pilates, or running. For most people, EAT is 5 to 15%, unless they are training like a competitive athlete.

You can see that most of your energy use comes from just being alive (BMR). But the smaller parts, like NEAT and EAT, are the ones you can influence to some extent.

Knowing how these parts work helps you understand why two people can seem to live the same way and eat the same things, yet still see different changes in their weight. Everyone has their own mix of BMR, TEF, NEAT, and EAT, and that mix affects their results.

When people in fitness or healthcare talk about metabolism, they are usually talking about this whole system.

You Can Estimate Your TDEE

TDEE calculators are helpful tools you can use to estimate your TDEE, but they’re not perfect. They often give lower estimates for people with more muscle and may not be accurate for people living with obesity. That’s why it’s best to use them as a general guide, not a definite answer.

Why Knowing Your TDEE Matters

Having an idea of your TDEE helps you understand how eating and moving work together to change your body. For example:

• If you eat less than your TDEE, your body uses stored energy (fat) to make up the difference, and you lose weight over time.
• If you eat more than your TDEE, your body stores the extra energy as fat, and you may gain weight over time.
• If you eat about the same as your TDEE, your weight stays about the same.
• If you move more than usual, your TDEE goes up a little because your body is using more energy.
• If you move less than usual, your TDEE goes down a little because your body is using less energy.
• Eating more and moving more can balance each other out. You might stay the same weight, lose weight, or gain weight depending on how big the changes are.
• Eating less and moving less can also balance out. If your total energy use drops, you may not lose as much weight as you expect.

This is not something that you need to track daily, but knowing what it means helps you understand how food and exercise impact your body and why everyone’s energy needs are different.

Basal Metabolic Rate (BMR) Is Different for Everyone

There are a few key reasons why some people naturally use more energy at rest while others use less.

Factors that Influence Your BMR:

• Body size: Bigger and taller bodies use more energy because they have more cells and tissue to take care of.
• Muscle mass: Muscles use more energy than fat, so people with more muscle use more energy even at rest.
• Age: As people get older, they often lose muscle, and that makes BMR go down a little (strength training helps keep more muscle).
• Sex: Males often have a slightly higher BMR because they biologically have more muscle due to higher testosterone levels.
• Genetics: The genes you are born with help decide how your body uses energy. They affect how active your cells are and how well your mitochondria (the parts that make energy) work. Because of this, some people naturally use energy faster, and some use it more slowly.
• Hormones: Hormones are chemical messengers that tell your body what to do. Ones like thyroid and leptin help control how quickly you use energy and when you feel hungry or full.

Most of these differences come from biology and how your body works, so you can’t directly control your BMR. But you can influence it in helpful ways. Building and keeping muscle, especially during weight loss, gives your body a slight boost.

What Can Change Your BMR for a Short Time

Sometimes your BMR can shift for a short time. A few things that can cause that include:

• Eating much less than your BMR: Your body is smart and wants to protect you from losing too much weight. If you eat too little for too long, your body may slow down your BMR to save energy.
• Temperature: When it is very hot or very cold, your body has to work harder to keep your temperature normal. Shivering or sweating takes energy, so your BMR can go up. Brown fat, a special type of fat that generates heat, can also make your body use more energy in colder conditions.
• Illness or injury: When you are sick or hurt, your body uses more energy to fight germs and repair tissues. This makes your BMR rise until you are better.
• Stimulants: Substances like caffeine, nicotine, and certain medicines can make your body work faster for a short time. This can increase your BMR.

Life Stages That Can Change Your BMR

Your BMR can change during life stages like:

• Pregnancy: BMR goes up because the body has more mass and needs extra energy to help the baby grow.
• Lactation: After giving birth, the body uses more energy to make breast milk. This can increase energy use by about 15% to 25%.
• Growth: Babies, children, and teens need more energy because their bodies are building new tissues and growing fast.
• Menopause: Changes in hormones can cause a loss of muscle. Since muscle uses more energy than fat, BMR often goes down during this stage.

How Food Plays a Role

The food you eat gives your body energy in the form of calories. Your body also uses some of that energy to digest and process your meals. This is called the thermic effect of food (TEF). It means that different types of foods take different amounts of energy for your body to break down.

Protein:

• Uses the most energy to digest, about 20–30% of its calories.
• One gram of protein has 4 calories.
• Takes more work for your body to process, which helps you feel fuller and slightly boosts energy use throughout the day.
• If you eat 100 calories of protein, your body keeps only about 70–80 because it uses energy to break it down.

Carbohydrates:

• Uses about 5–10% of their calories for digestion.
• One gram of carbs has 4 calories.
• Give your brain and muscles quick energy.
• If you eat 100 calories of carbs, your body keeps about 90 to 95.

Fat:

• Uses the least energy to digest, around 0–3%.
• One gram of fat has 9 calories.
• Important for hormone health and nutrient absorption.
• If you eat 100 calories of fat, your body keeps almost all of it, about 97 to 100 calories.

Alcohol:

• Has 7 calories per gram.
• Does not give your body nutrients, so it isn’t counted as part of a healthy nutrient breakdown.

This is why protein keeps you full, carbs give you quick energy, and fats help your body work properly.

What is a Macro Split

A macro split is just the way you divide your daily calories between protein, fats, and carbohydrates. Instead of only looking at total calories, it shows you where those calories come from so you can stay energized, recover well, and feel your best.

Most people do well with higher protein, moderate fats, and enough carbohydrates to keep their energy steady and help muscles repair. A common place to start is around 30% protein, 30% fats, and 40% carbohydrates, but your needs can shift based on your appetite, training, goals, and how your body feels. You can calculate yours using this tool.

Why NEAT Changes from Person to Person

NEAT includes everything you do that is not exercise. How much NEAT a person has is shaped by both behavior (what you do) and biology (how your body works). Your habits, environment, and daily choices affect how much you move. But your brain and hormones also play a role.

Some people naturally move more because their brains send stronger “get up and move” signals, while others are more still, and scientists have found that genes and brain chemicals like orexin also influence these signals¹. They can shift based on sleep, food, stress, and other life factors.

You can increase your NEAT through small choices, like standing more often, stretching, or walking during work breaks. Even if your body makes up for a little of that added movement by resting later, most of the extra energy still counts. That’s why NEAT remains a powerful way to use more energy throughout the day even if your natural baseline is partly set by genetics.

How Exercise Fits In

Exercise is the smallest part of TDEE, but it is one of the most important factors you can control for long-term health, strength, metabolism, and weight stability. The energy you burn during a workout is just one piece. The bigger impact comes from how exercise changes your body on the inside.

Strength training and neuromuscular work create adaptations that no amount of dieting, medication, or daily movement can replace. These adaptations raise your total daily energy use over time by increasing muscle, improving movement efficiency, and supporting the systems that control metabolism.

Strength training improves:

• muscle mass (the strongest predictor of long-term metabolic health)
• bone density (extremely important as we age)
• posture and joint alignment (helps avoid the hunchback look later in life)
• balance, coordination, and movement quality (these change very quickly with rapid weight loss)
• insulin sensitivity and blood sugar control
• confidence, self-trust, and body awareness
• sleep and stress regulation

These changes influence every part of TDEE, not just the “exercise” portion. Muscle requires energy to maintain. Better movement reduces pain and compensations. Stronger neuromuscular pathways improve how efficiently your body performs everyday tasks.

A structured plan that gradually increases challenge and focuses on quality, not just quantity, supports weight stability during rapid body changes. It helps preserve the tissues and movement patterns that protect your long-term health.

Calories In vs. Calories Out

At the simplest level, if you eat more energy than you use, your body stores the extra. If you eat less, your body turns to stored energy. That part is always true. But the way your body handles that energy is not the same for everyone, and biology plays a big role in how easy or difficult that balance feels.

• Hormones: Insulin, leptin, and thyroid hormones influence how you store or release energy. You cannot directly control these.
• Sleep and stress: Poor sleep and high stress can lower NEAT, increase hunger hormones, and change how your brain reacts to food.
• Gut bacteria: Your microbiome affects how many calories you pull out of the same meal. Some people absorb more; some absorb less.
• Genetics: Your genes help decide how quickly you use energy and how your body prefers to store extra.

So “calories in vs. out” is always the foundation, but biology decides how easily or efficiently that balance works for each person.

The Big Picture

Your body is always trying to stay balanced. If it gets less energy from food, it may lower NEAT or slightly reduce BMR to save fuel. If it gets more energy, it may raise NEAT or increase body temperature to use some of it.

That’s why long-term health and strength come from understanding how your energy systems work instead of relying on quick fixes. You can:

• Build muscle through resistance training to influence BMR over time.
• Eat enough protein to support TEF and metabolism.
• Stay more active throughout the day to boost NEAT.
• Start an exercise routine to increase EAT.
• Try to get enough sleep most nights, keep stress in check where you can, and stay consistent so your body can regulate energy more easily.

When you move often, eat balanced meals, and rest well, your body becomes better at managing and using energy. You feel it in your strength, your confidence, and how your body supports you in daily life.

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Resources

1. Scammell TE, Winrow CJ. Orexin receptors: pharmacology and therapeutic opportunities. Annu Rev Pharmacol Toxicol. 2011;51:243-66. doi: 10.1146/annurev-pharmtox-010510-100528. PMID: 21034217; PMCID: PMC3058259. https://pmc.ncbi.nlm.nih.gov/articles/PMC3058259/#:~:text=Considered%20together%2C%20these%20behavioral%20and,reducing%20arousal%20and%20improving%20insomnia. ↩︎

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