What is the "calories in, calories out" model?

The "calories in versus calories out" model is based on the idea that in order to maintain a stable weight, the number of calories you eat needs to match the number you burn.

"Calorie input" refers to the calories you get from the food you eat, while "calorie output" is the number of calories you burn.

Three main body processes that burn calories:

  • Basal metabolism. Your body uses most of the calories it gets from food to maintain basic functions, such as your heartbeat. This is often called your basal metabolic rate (BMR).
  • Digestion. About 10-15% of the calories you eat go toward digestion. This is called the thermic effect of food (TEF) and varies depending on the food you eat.
  • physical activities. The remaining calories you get from your diet are intended to fuel your physical activities, including exercise and daily tasks such as walking, reading, and washing dishes.

Your weight will remain stable when the number of calories you consume from food matches the number of calories you burn to maintain metabolism, digestion, and physical activity.

Therefore, the “calories in vs. calories out” model is absolutely correct. You need a caloric deficit to lose weight.

Losing weight requires a calorie deficit

From a biological perspective, you need to eat fewer calories than you burn to lose weight. There is no way around it.

Once your body's energy needs are met, the extra calories are stored for future use—some in the form of glycogen in your muscles, but most in the form of fat. Therefore, eating more calories than you burn can lead to weight gain, while eating fewer calories than you need can lead to weight loss.

Some studies show that what you eat is more important than how much you eat, meaning the calorie content of your diet has nothing to do with weight loss. However, these studies are based on some incorrect assumptions.

For example, those who adhere to a low-carb diet that helps people lose weight despite eating the same number of calories (or even more) often rely on diet magazines to estimate calorie intake.

The problem is that food diaries are notoriously inaccurate, even when filled out by nutrition experts.

What's more, some studies only report total weight loss without mentioning whether the weight loss comes from muscle, fat or water loss.

Different diets affect muscle and water loss differently, which may make them appear more effective for weight loss, but they are not.

Studies that control for these factors consistently show that weight loss is always caused by a calorie deficit. This is true whether your calories come from carbohydrates, fat, or protein.

Health is more than just “calories in versus calories out”

While the "calories in versus calories out" model is important for weight loss, not all calories are created equal when it comes to your health.

This is because different foods have different effects on various processes in your body, regardless of calorie content.

The source of calories affects your hormones and health in different ways

Different foods can affect your hormone levels in different ways.

The different effects of glucose and fructose are a good example. These two simple sugars provide the same number of calories per gram, but your body metabolizes them in completely different ways.

Compared to a diet that provides the same number of calories from glucose, a diet that adds too much fructose is associated with insulin resistance, increased blood sugar levels, and levels of triglycerides and LDL (bad) cholesterol.

That said, fruits that contain naturally occurring fructose along with fiber and water do not have the same negative effects.

What's more, the types of fat present in your diet can have different effects on your reproductive hormone levels. For example, a diet rich in polyunsaturated fat appears to improve fertility in healthy women.

What's more, replacing saturated fat with unsaturated fat in your diet may further reduce your risk of heart disease, even though both types provide the same number of calories per gram.

The types of food you eat affect how full you feel

Your nutrient intake affects your feelings of hunger and fullness.

For example, eating a 100-calorie serving of beans will reduce your hunger more effectively than eating a 100-calorie serving of candy.

This is because foods high in protein or fiber are more filling than foods containing less of these nutrients.

Candies that are low in fiber and protein are more likely to cause you to overeat later in the day, reducing the likelihood that your "calories in" will match your "calories out."

Likewise, fructose tends to increase levels of the hunger hormone ghrelin more than glucose.

It also doesn’t stimulate the satiety centers in the brain the way glucose does, so you don’t feel full after eating fructose the way you feel after eating glucose.

That's why most processed foods, which are high in fructose but lacking in protein or fiber, often make it harder for you to maintain energy balance.

Sources of calories have different effects on your metabolism

Foods affect metabolism differently. For example, some people need more work to digest, absorb, or metabolize than others. The metric used to quantify this work is called the thermic effect of food (TEF).

The higher the TEF, the more energy required to metabolize food. Protein has the highest TEF, while fat has the lowest TEF. This means that a high-protein diet requires more calories to metabolize than a low-protein diet.

This is why people often say that eating protein boosts your metabolism better than eating carbs or fat. That said, when it comes to weight loss, a food's TEF appears to have only a small impact on your calorie balance.

Why nutrient density matters

The amount of nutrients per calorie in foods varies widely.

Nutrient-dense foods provide more vitamins, minerals, and beneficial compounds per gram than less nutrient-dense foods.

For example, fruit is more nutritious than donuts. Calorie for calorie, fruit will provide a greater dose of vitamins, minerals and beneficial plant compounds.

Other nutrient-dense foods include vegetables, whole grains, legumes, meat, fish, poultry, dairy products, and unsalted nuts and seeds.

On the other hand, processed foods, including white pasta, soda, cookies, potato chips, ice cream, and alcohol, are considered low in nutrient density.

A diet rich in nutrient-dense foods has been consistently linked to a lower risk of chronic diseases like diabetes and heart disease, and may even help you live longer.

The "calories in versus calories out" model fails to take nutrient density into account, which is a good reason to doubt its relevance to your health.


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