What are Calories?

In this Article (Index)

Calories: All the facts

• Definition of Calorie
• Calories in Food
• Ideal Daily Caloric Intake
• Calories and Weight Loss
• Not All Calories Are The Same
• Thermic Effect of Food
• Food Satiety Effect
• Food, Calories and Your Gut's Microbiome
• Closing Comments

Definition of Calorie

The term Calorie comes from physics. Back in the late 1700s, steam power became more common and engineers needed to design and build steam engines. This required careful calculations of the energy provided by heating water, and converting it into steam. In Britain, the engineers adopted the British Thermal Unit or BTU to define the heat provided by burning different substances (coal, wood, peat). In other parts of the world, using the metric system, a similar unit was formulated. French scientist Nicolas Clement designated it as the calorie.

A calorie is the amount of heat needed to increase the temperature of 1 gram of water by 1 degree Celsius. In American or British units this is roughly equivalent to heating half a thimblefull of water by 2°F. Soon it was found to be too "small" to use in steam engines, so it was named "small calorie," and another unit was creawted, "large calorie."

The large calorie was the equivalent of heating 1,000 grams of water by 1°C, it was named with a capital "C" as "Calorie", it was also known as a Kilocalorie. In the metric system, "kilo" stands for 1,000, so one Kilocalorie = 1,000 calories.

This was quite confusing as the large and small calories had the same name, differing only in a lower or uppercase letter, so a different energy equivalent was adopted, the Joule with he Kilojoule being 1,000 Joules.
In the European Union, food nutritional value on labels is indicated both in Kcalories and Kjoules.

In the U.S. large calories are used but, surprisingly, they are written with a lowercase "c" as "calories." Even the Food and Drug Administration or FDA uses this unit with the lowercase "c". (2)

In our website we use the uppercase name "Calorie" which is equivalent to 1,000 calories, and 4.184 KJoules.

Calories in Food

Food provides the energy that the body needs to sustain life. We eat to stay alive. Food intake means incorporating energy and this energy is measured in Calories.

The food is digested and broken down into smaller molecular components and during this process, energy is released. The body uses this energy immediately or stores it for future use. It can be stored as fat deposits or as glycogen.

Excessive Caloric Intake

Eating too much food, and ingesting too many Calories leads to an accumulation of the unused energy as fat. This leads to obesity. Being obese entails health risks such as an increased risk of cardiovascular disease (stroke, heart attack), diabetes type-2, gallstones, non-alcoholic fatty liver disease, joint diseases (osteoarthritis and gout), sleep apnea and cancer.

Obesity also causes mental health issues such as depression, low self-esteem, anxiety. (1)

Reduced Calorie Intake to manage weight

People who are overweight can ingest fewer calories, eat healthy food, with better nutritional value, and this way reduce their body's energy intake. This will prompt the body to consume stored energy ("burning" fat). Exercise and increased physical activity helps to consume more energy, and reduces the health risks associated with obesity.

Not Eating Enough Calories

The opposite is also true, not eating enough calories as in certain diets or eating disorders (anorexia, bulimia) also has a negativ impact on health. It causes disruption in the body's cells, extreme weight loss, mental health issues. Malnutrition deprives the body of essential elements and vitamins, electrolyte imbalances that can cause dangerous heart conditions. Bone and muscle mass are lost, the kidneys are also affected. These disorders can lead to death.

Ideal Daily Caloric Intake

The daily needs in terms of energy required by the body to function normally, are influenced by many factors such as gender, age, height, weight, and level of activity.

Furthermore, it isn't only a matter of energy, but of the quality of the food you eat. The same amount of Calories doesn't mean the same effect on the body.

Quality of the Calories

Ingesting your daily caloric needs consuming a balanced diet made up of whole grain, fruit, vegetables, lean meat or fish and healthy oils, nuts and legumes does not have the same effect on your body as getting that same amount of energy by only eating sugar or only eating lard.

The vitamins and minerals contained in a balance diet are lost if you eat only refined sugars and carbohydrates and ultraprocessed foods. The impact on your gut's microbiome is also different, and this in turn affects your overall health. Furthermore, refined carbs and sugars cause spikes in blood sugar levels leading to insulin resistance and type-2 diabetes.

Healthy fats like those found in olive oil, avocados, and nuts are Calorie-dense meaning that a small helping has many Calories, but this type of food is healthy and unlike Calorie-dense sugars, have a beneficial effect on body health.

An average adult woman should eat around 2,000 Calories per day to maintain her weight, and the average man should eat 2,500 Calories daily to keep a stable weight. Eating less than these levels say, 1,500 for women and 2,000 for men, will create a caloric deficit leading to weight loss.

Calories and Weight Loss

The rule of the thumb is that ingesting 500 fewer Calories per day or 3,500 Calories per week will cause a weight loss of 1 pound per week.

But Calorie counting alone is not enough, the body will react to the lower energy intake and reduce its metabolism to consume less energy.

Adding physical activity will increase energy expenditure and promote the development of muscle tissue. Muscles consume more energy than fatty tissue: 13 Calories ⁄ kg per day for muscles vs. 4.5 Calories ⁄ kg per dayfor adipose tissue.

Building muscle increases the energy output in comparison to a person with the same weight but with more fatty tissue. An average male will have roughly 2 kg of muscle for every 1 kg of fat. In women the ratio is 1 to 1. (3)

Calorie Content

The energy content in food macronutrients is the following:

• 9 Cal per gram. Fats
• 4 Cal per gram. Proteins
• 4 Cal per gram. Carbohydrates
• 7 Cal per gram. Alcohol (it has no nutritional value whatsoever)

Two different people could consume the same amount of Calories over a period of years and eat a very different diet, and a different nutrient balance.

A person eating 200 grams of protein and 200 grams of Carbohydrates per day will intake 1,600 Calories. Another person eating 100 grams of fats, 100 grams of Carbs and 75 grams of proteins will also consume 1,600 Calories. But the impact on their bodies will be different. Especially if the Carbs are whole in one case and refined in the other. (4)

Weight in America and Caloric Intake

The average American man eats between 2,360 and 2,640 Calories per day and the women eat from 1,640 to 1,800 Cal per day, and this is roughly 10% lower than it was back in the 1970s. So, Americans are consuming less calories than 50 years ago.

However, Americans are gaining weight! Why? The answer is simple. To maintain a stable weight, the following equation must balance:

If more calories are ingested, the excess energy is stored as fat, and weight is put on. If more calories are burned the body will use it stores and weight is lost.

Americans have reduced their intake of Calories, but have also become less active, and burn less energy. Some groups are overeating, and underexercising leading to weight gain.

The data also shows that 60 to 90 percent of the weight lost by people following caloric restricted diets, is regained. People diet cyclically, they gain weight, go on a diet, lose it, regain it, diet again, and so on. These cycles seem to have a negative impact on heart health and also force the body to use energy more efficiently, making it more difficult to lose weight. (5)

Not All Calories Are The Same

Thermic Effect of Food

Not all Calories are equal.

This concept sounds difficult, but it is simple: The body consumes energy to process, digest, absorb and metabolize food. This is expressed as a percentage of the energy content in a macronutrient that is consumed to digest it.

Fats have a 2 to 3% thermic effect, carbohydrates 6 to 8%, while proteins have a 25-30% value.

For every 100 Calories of fats you eat, the body consumes 2 to 3 Cal. and ends up with 97-98 Cal. But for every 100 Cal of protien, it only keeps 70-75 Cal and consumes the other 25-30 to process the protein.

The body also favors using the energy contained in carbohydrates and protein over fats. So it oxidizes these to obtain energy and stores the fats in adipose tissue. (6)

Satiety Index

Feeling satiated depends on the food you eat. Different foods produce different satiety.

It is easy to eat 300 Calories of chocolate, but trying to do the same with lettuce is far more difficult.

So you can overeat foods with lower satiety index.

Studies have shown that protein, fibre, and water have higher satiety index scores than fat content, which has a lower score. (7)

Food, Calories and Your Gut's Microbiome

Finally, different foods have a different impact on the absorption of calories. A study published in 2024 (8) followed two groups of subjects eating different diets. One group consumed a high-fiber, whole-food diet; the other a typical Western diet, consisting of processed foods. Both calories were identical in the number of calories and had similar proportions of protein, carbohydrates and fat. The outcome was very interesting.

• Those eating a high-fiber diet absorbed fewer calories: 116 Cal per day were not absorbed because they were lost in the feces. The high-fiber diet nourished the gut microbiome so the bacteria consumed the energy instead of it being absorbed by the body.
• The high-fiber group lost slightly more weight than the processed foods group (despite eating the same number of calories with the same proportion of macronutrients).
• The high-fiber group showed increased levels of the hormone GLP-1, that induces a feeling of fullness.

Closing Comments

Calories are a useful way to mesasure the energy content of foods. They can provide guidance on your body's energy requirements so that you consume the correct ammount for your age, sex, height, weight and level of activity.

Reducing the intake of Calories is a tool used to manage body weight and shed pounds. However the quality of the food you eat, and its content of fiber and protein has an impact on the actual number of calories absorbed by your body.

Eating the same number of Calories but with a different mix of macronutrients (fats, protein and carbohydrates) will have a different effect on your body, health and weight.

Satiety, fullness, and gut microbiota health also affect the number of Calories absorbed.

Consult your physician before starting any weight loss program or beginning any exercise program.