How Weight Loss Works: Calories In Versus Calories Out: Metabolism Basics

What Does Calories In Versus Calories Out Mean?

Weight loss works on a simple principle: you need to burn more calories than you consume. This is often called “calories in versus calories out.” Your body burns calories through your basal metabolic rate (BMR) (energy used for basic functions like breathing and digestion), daily activity, and exercise. When you consistently eat fewer calories than your body needs, it uses stored fat for energy, leading to weight loss.

“Calories out” is the number of calories you burn, and “calories in” is the number of calories you get from the foods you eat.

Calorie burning occurs primarily through three bodily processes:

• Basic metabolism: The majority of the calories you consume are used by your body to support essential processes like your heartbeat. Your basal metabolic rate (BMR) is the term used to describe this.
• Digestion: Your digestive system uses between 10 and 15 percent of the calories you consume. It is known as the thermic effect of food (TEF) and varies based on what you eat.
• Physical activity: The remaining calories from your diet are intended to support your daily activities, such as walking, reading, and dishwashing, as well as your exercise regimen.

As long as you consume the same amount of calories from food as you need to maintain your metabolism, digestion, and physical activity, your weight will remain constant.

The “calories in versus calories out” model is therefore absolutely accurate. Losing weight requires a calorie deficit.

Understanding The Basics of Calories in vs. Calories Out

Energy balance is the foundation of the CICO model. Energy spent through digestion, physical activity, and basal metabolic rate (BMR) is referred to as “calories out,” whereas “calories in” refers to energy obtained through food and drink. Weight gain happens when calorie intake surpasses expenditure, and weight loss happens when expenditure surpasses intake.

The body uses calories to sustain essential processes like breathing, circulation, and cellular activity, as well as through the thermic effect of food (the amount of energy utilized in digestion) and physical activity, which includes both daily movements and exercise. A number of variables, including hormones, body composition, and metabolism, affect this dynamic balance.

There is more to health than “calories in vs. calories out.”

When it comes to your health, not all calories are created equal, even though the “calories in versus calories out” model is important for weight loss. This is due to the fact that, independent of calorie content, different foods have distinct effects on different bodily functions.

Calorie sources have varying effects on your hormones and overall health. Your hormone levels can be impacted by various foods in different ways.

An excellent example is the different effects of fructose and glucose. Though your body metabolizes these two simple sugars in quite different ways, they have the same number of calories per gram.

Insulin resistance, elevated blood sugar, and elevated triglyceride and LDL (bad) cholesterol levels are associated with a diet that is excessively high in added fructose compared to one that provides the same number of calories from glucose.

Fruit, which also contains water, fiber, and natural fructose, does not have the same adverse effects. Furthermore, the kind of fat you eat can affect your levels of reproductive hormones in different ways. For example, in healthy women, diets high in polyunsaturated fats seem to increase fertility.

Furthermore, even though unsaturated and saturated fats have the same number of calories per gram, switching to unsaturated fats in your diet may further reduce your risk of heart disease.

Your level of fullness is influenced by the foods you eat

Your hunger and feelings of fullness are influenced by the nutrients you consume.

A 100-calorie serving of beans, for example, will satisfy your hunger far more efficiently than a 100-calorie serving of candy. This is due to the fact that foods high in fiber or protein are more satisfying than those with lower concentrations of these nutrients.

Because of its low protein and fiber content, the candy is much more likely to cause you to overeat later in the day, which lowers the chance that your “calories in” and “calories out” will equal one another. In a similar vein, fructose tends to raise ghrelin levels more than glucose does.

You won’t feel as full after eating fructose as you would after eating glucose because it doesn’t activate the same parts of your brain that are responsible for fullness. Because of this, it is typically more difficult to maintain an energy balance when consuming processed foods that are high in fructose but low in protein or fiber. Your metabolism is affected differently by the source of calories

Different foods have different effects on your metabolism. For example, some are harder to metabolize, absorb, or digest than others. The thermic effect of food (TEF) is the metric used to quantify this work.

A food needs more energy to be metabolized if its TEF is higher. TEF is lowest for fat and highest for protein. Accordingly, it takes more calories to metabolize a high-protein diet than a low-protein one. For this reason, it’s commonly believed that eating protein will increase your metabolism more than eating fat or carbohydrates. Nevertheless, the TEF of foods seems to have little bearing on your calorie balance when it comes to weight loss.

The Metabolic Complexity Beyond Simple Math

The biological complexity of weight regulation is oversimplified by the CICO equation, despite its mathematical accuracy. Calorie restriction causes changes in the body that can change energy expenditure and make weight loss more difficult.

The body slows metabolism more than would be predicted from the loss of muscle and fat alone when calorie intake is decreased. Due in part to hormonal changes like decreased thyroid hormone secretion, this metabolic adaptation can lower resting metabolic rate by up to 15%. Because the body burns fewer calories at rest, this slowdown makes it difficult to lose weight over the long term.

The metabolism might not fully recover following weight loss, which could lead to weight cycling and make it harder to sustain fat loss.

The Function of Macronutrients and Hormones

Insulin and leptin are two hormones that affect how calories are processed and control appetite, fat storage, and energy expenditure. According to the carbohydrate-insulin hypothesis, eating a lot of carbohydrates causes insulin to rise, which encourages fat storage and decreases fat burning. Strong evidence, however, contradicts this theory, demonstrating that overall caloric intake—regardless of the makeup of macronutrients—is the main factor influencing the risk of obesity.

Thermogenesis, Protein, and Satiety

The impact of calories from various macronutrients on metabolism varies. Compared to fats and carbohydrates, protein has a higher thermic effect, which means the body needs more energy to digest and metabolize it. For instance, compared to 2% for fat, roughly 25% of protein calories are used in processing. High-protein diets can raise daily energy expenditure by 80–100 calories due to this metabolic benefit.

Additionally, protein encourages fullness, which naturally lowers appetite and calorie intake. Even when calories are equal, low-carb diets can sometimes result in greater weight loss than low-fat diets, in part because they tend to contain more protein.

The Importance Of Nutrient Density

A food’s nutrient content per calorie can vary substantially. Compared to less nutrient-dense foods, nutrient-dense foods have higher concentrations of vitamins, minerals, and health-promoting compounds per gram. Fruits, for example, are far higher in nutrients than donuts. Fruit will give you a far higher dose of vitamins, minerals, and healthy plant compounds per calorie.

Nutrient-dense foods also include dairy products, meat, fish, poultry, vegetables, whole grains, legumes, and unsalted nuts and seeds. White pasta, soda, cookies, chips, ice cream, and alcohol are examples of processed foods that are thought to have a low nutrient density.

Nutrient-dense diets have been repeatedly associated with a decreased risk of heart disease, diabetes, and other chronic illnesses. They may even extend your life.

Metabolism: Converting Food Into Energy

The process by which the body converts food and liquids into energy is called metabolism. Calories from food and beverages combine with oxygen during this process to provide the body with the energy it needs.

A body requires energy for everything it does, even when it is at rest. This covers breathing, blood circulation, hormone balance, and cell growth and repair. Basal metabolic rate, also referred to as basal metabolism, is the quantity of calories used by the body while it is at rest to accomplish these tasks.

The primary determinant of basal metabolic rate is muscle mass. Additionally, basal metabolic rate is influenced by:

• Body type and size: Even when at rest, people with greater muscle mass or size burn more calories.
• Gender: Men usually have greater muscle and less body fat than women their age and weight. Men, therefore, burn more calories.
• Age: People typically lose muscle as they age. Fat makes up a larger portion of the body’s weight, which slows the burning of calories.

Two additional factors determine how many calories a body burns each day, in addition to the basal metabolic rate:

• How the body uses food: Calories are burned during food digestion, absorption, movement, and storage. Nutrient absorption and food digestion account for about 10% of total caloric intake. Not much can be done to change this.
• How much a body moves: The remaining calories a body burns each day are made up of any movement, like playing tennis, going to the store, or chasing a dog. Both increasing physical activity and simply moving more throughout the day can significantly alter this.
• Non-exercise activity thermogenesis (NEAT) refers to daily activity that is not considered exercise. Walking around the house is part of this. It also covers housework, gardening, and even fidgeting. Every day, between 100 and 800 calories are used for NEAT.

Weight and metabolism

You may want to attribute weight gain and a sluggish metabolism to a medical condition. Rarely, however, does a medical condition slow metabolism to the point where significant weight gain results. Cushing syndrome or hypothyroidism, or an underactive thyroid gland, are two conditions that can lead to weight gain. Such conditions are rare.

Genes, hormones, diet, and lifestyle factors like stress, physical activity, and sleep are probably some of the many factors that influence weight gain. Eating more calories than you burn, or burning fewer calories than you consume, causes weight gain.

It appears that some people lose weight more quickly and effortlessly than others. However, everyone loses weight when they burn more calories than they consume. Calorie count is the most important factor. You must consume fewer calories or increase your caloric expenditure through exercise if you want to lose weight. You can also do both.

How To Establish a Deficit In Calories

Figuring out how many calories you consume is the first step in creating a calorie deficit.

The quantity of calories you burn each day is known as your total daily energy expenditure, or TDEE. This figure is composed of:

Your resting caloric expenditure is known as your basal metabolic rate, or BMR.
The thermal effect of food, or TEF, is the number of calories burned during food processing.
The “thermic effect of activity,” or TEA, is the term used to describe the calories you burn when exercising.

After determining your TDEE, you can choose a calorie deficit (e.g., 500 kcal per day) and use that to calculate your calorie target. This is how many calories you can consume each day while still sticking to your selected calorie deficit. For instance, a 500 kcal deficit would result in a daily calorie target of 1500 kcal if your TDEE was 2000 kcal.

The My Vital Metrics TDEE calculator allows you to precisely determine your BMR and TDEE. In contrast to other calorie calculators available online, this one offers choices to:

• Input data from a DEXA scan
• Input data from an RMR test
• Give specifics about your daily routine, exercise, and sleep patterns

It’s important to ensure your TDEE is accurate. Internet BMR calculators may provide erroneous results, which could result in unsuccessful weight loss attempts and the misconception that CICO “doesn’t work.” The greatest way to position yourself for weight loss success is to have the appropriate information at your fingertips.

Your body composition—more especially, your muscle mass—is closely related to your metabolic rate. Accordingly, a DEXA scan for body composition can be a very useful tool for obtaining an accurate RMR figure and making sure you’re dealing with the correct number.

Physical Activity and Metabolism

You can’t easily control the speed of your basal metabolic rate, but you can control how many calories you burn through physical activity. You burn more calories when you’re more active. In actuality, some people who appear to have a quick metabolism are most likely simply more active than others, and they may even fidget more.

The Physical Activity Guidelines for Americans advise doing the following to increase caloric expenditure:

Aerobic activity: Try to get in at least 30 minutes of moderate exercise each day as a general rule. You may need to increase your exercise if you want to reach certain fitness goals, lose weight, or keep it off.

Moderate aerobic exercise includes things like bicycle riding, walking slowly, and lawn mowing. Vigorous aerobic exercise includes activities like aerobic dancing, running, and hard work.

Strength training: At least twice a week, perform strength training exercises for each of the major muscle groups. Strength training can include the use of weight machines, your own body weight, heavy bags, resistance tubing, or resistance paddles in the water, or activities such as rock climbing.

Factors that affect CICO

It’s worthwhile to think about whether any of the following factors might be limiting the effects of your hard work if you’re not seeing results, or even if you are.

Go to sleep

Sleep is essential for losing weight. Consistently sleeping fewer than six hours a night has been linked in studies to weight gain, a higher BMI, and poorer health outcomes. Likewise, inadequate sleep is associated with increased intake of high-calorie foods that are high in fat and/or sugar. It is easier to maintain your calorie deficit and accomplish your weight loss goal when you get enough good sleep, which helps control hunger and satiety.

Controlling stress

Stress is a natural part of life for everyone, but chronic stress, which lasts for a long time, can significantly affect our weight and efforts to control it.

Insulin resistance, metabolic alterations, and trouble losing weight are all associated with high levels of stress. Our ability to lose weight can be further hampered by the time and energy constraints imposed by various sources of high stress, including work, family, and illness. It has been demonstrated that stress reduction strategies help people lose weight, especially those who suffer from binge eating disorder (BED).

You can reduce your stress levels by incorporating techniques like regular exercise, yoga, meditation, and doing things you enjoy.

Balance of macronutrients

Another hotly contested issue is the ideal macronutrient ratio for weight loss.

The quantity of calories you consume is far more significant than the macronutrients from which those calories are derived. However, the way your diet is broken down into macronutrients can affect:

• How satisfied are you
• Your rate of metabolism
• Your athletic ability
• Recovery of muscles following exercise

In light of this, a lot of experts advise eating a well-balanced diet that is well-balanced and contains a good mix of healthy fats, proteins, and carbs. For most people, a macronutrient balance of 40% carbohydrates, 30% protein, and 30% fat can be a great goal.

It will be more crucial to hit your calorie target than to worry about a detailed macronutrient breakdown if you’re just starting with weight loss. Tracking your macros, however, can provide more information about your diet once you become used to doing so.

Note which foods make you feel full and which make you wish you had more calories. Increasing your intake of protein and healthy fats can help fight off persistent hunger. In other cases, you might need to increase your total caloric intake because you are in too big a deficit.

Nutritional deficiencies

Being hungry and exhausted are not good ways to lose weight.

Making any necessary corrections can boost your energy levels and enhance your training results. You can make sure you get a range of micronutrients by eating a lot of different foods, such as a mix of fruits and vegetables. Testing can provide additional information if you suspect a deficiency that isn’t being addressed by diet alone.

Common deficiencies like iron deficiency anemia and vitamin D deficiency can be readily treated with prescription drugs or over-the-counter supplements. As always, before taking any new supplements or medications, make sure to speak with your doctor.

Levels of hormones

Hormones are often held responsible for unsuccessful weight loss attempts, but they play a complex role in weight management.Your hormonal balance may be influencing your weight loss efforts in a few common ways, such as:

• Thyroid hormones: Weight gain and metabolic abnormalities may result from low levels of the thyroid hormones T3 and T4 in hypothyroidism.
• Sex hormones: Weight and body fat can be impacted by both elevated and decreased levels of sex hormones, such as oestrogen and testosterone.
• Insulin: Weight gain may be caused by insulin resistance in both diabetics and non-diabetics.

It’s worthwhile to look into each of these hormonal imbalances with your healthcare provider. Hormonal imbalances can impact and hinder weight loss, but it’s important to remember that they only change the numbers in the CICO equation.

Exercise and non-exercise daily activity

Increasing your overall level of activity is one of the best strategies to lose weight. Both may be included in this:

Targeted exercise: What we typically think of when we hear the word “exercise” – lifting weights, running, cycling, engaging in sports, or going to the gym.
Non-exercise daily activity: All the other movements in our day, such as standing, walking, climbing stairs, housework, or commuting.

It’s crucial to keep in mind that movement need not be a workout to “count.” Both forms of exercise raise your calorie burn and add to the calories-in-calorie-out equation. Exercise also has the added benefits of lowering stress, enhancing sleep, and boosting self-efficacy.

Are “eat less, move more” and CICO interchangeable?

Although it plays a significant part, the well-known saying “eat less, move more” is not the same as CICO.

Losing weight by consuming fewer calories than you expend is always going to involve some degree of eating less and/or incorporating more movement into your routine.  A lot of the controversy we discussed above stems from a fundamental misunderstanding of CICO.

Other factors influence the calories in, calories out equation, even though eating less and exercising more are crucial for weight loss. Individual differences such as hormonal levels, appetite, stress, sleep, and body composition play a significant role in how the CICO equation works on an individual level, as do more nuanced levels of our intake and exercise routine, such as overall nutrition, macros, non-exercise activity thermogenesis, and much more.

It becomes simpler to comprehend how to successfully apply CICO to our weight loss goals once we realize that calculating calories in versus calories out involves more than just diet and exercise.

It’s worthwhile to look into each of these hormonal imbalances with your healthcare provider. Hormonal imbalances can impact and hinder weight loss, but it’s important to remember that they only change the numbers in the CICO equation.

Does CICO Mean “Eat Less, Move More”?

Though it plays a significant part, the well-known saying “eat less, move more” is not the same as CICO.

You will always need to reduce your calorie intake and/or increase your physical activity if you want to lose weight by eating fewer calories than you burn. That being said, a lot of the controversy we discussed above stems from a fundamental lack of comprehension of CICO.

Although eating less and exercising more are crucial for weight loss, the calories-in, calories-out equation is influenced by other factors as well. In addition to more subtle aspects of our diet and exercise regimen, such as overall nutrition, macros, non-exercise activity thermogenesis, and much more, individual differences like hormone levels, appetite, stress, sleep, and body composition also have a big impact on how the CICO equation functions on an individual basis.

It becomes simpler to comprehend how to effectively apply CICO to our weight loss goals once we realize that estimating calories in versus calories out involves more than just diet and exercise.

Does Burning Calories Help You Lose Weight?

Indeed. Maintaining a calorie deficit is how all weight loss occurs. Calculating a calorie deficit can be challenging due to the numerous factors that affect weight loss, but this doesn’t alter the basic mechanism by which weight loss happens.

Weight loss results from consuming fewer calories than you burn off, which is influenced by your metabolic rate, general activity level, and exercise. You will gain weight if you consume more calories than you expend.

Why is losing weight so difficult if it’s that easy?

One of the main reasons is that it can be difficult to determine how much food and exercise you need to lose weight, and there is a lot of false information available. Let’s examine how to do it correctly.

Conclusion

From a purely biological perspective, the “calories in versus calories out” model is crucial for weight loss because, regardless of the type of food you eat, if you consume fewer calories than you burn. This model ignores nutrient density, which is crucial for your health. Furthermore, the way that different foods influence your hormones, metabolism, hunger, and feelings of fullness may all have an impact on how many calories you ingest.

Practically speaking, some foods can enhance your overall health and assist you in maintaining a healthy weight. If you focus solely on calories, you run the risk of missing the bigger picture.

FAQS

References

• Can you boost your metabolism? (n.d.). Mayo Clinic. https://www.mayoclinic.org/healthy-lifestyle/weight-loss/in-depth/metabolism/art-20046508
• Rd, J. K. M. (2021, January 28). What is the CICO diet? Here’s what a dietitian thinks. Healthline. https://www.healthline.com/nutrition/calories-in-calories-out
• Infs, & Infs. (2025, June 2). Calories in vs. Calories Out – The Simple Truth Behind Fat Loss. Institute of Nutrition and Fitness Sciences. https://blog.infs.com/2025/05/29/calories-in-vs-calories-out-the-simple-truth-behind-fat-loss/
• Angier, E. (2024, October 1). What is Calories in vs Calories Out (and Does it Work for Weight Loss?). My Vital Metrics. https://myvitalmetrics.com/blog-what-is-calories-in-vs-calories-out/