Is “Calories In, Calories Out” Too Simple?

The way our body uses calories is incredibly complex, let alone how that translates to weight loss or weight gain. Our nutrition team explains why we should think beyond "calories in, calories out".

As a very general statement, if you eat less calories you will likely lose weight, and if you are eating more calories than you are using you will likely gain weight. But there are many factors that complicate the relationship between energy intake and weight change that it’s important to take consider. Some people online might shout about it being as simple as “calories in, calories out”, but it really isn’t. Let’s go a bit deeper.

What is a calorie?

A calorie is a unit of energy. It can be difficult to calculate the amount of calories you are consuming accurately. In countries like Australia or the USA, there can be up to a 20% margin of error for the calories reported on the label. So the calories “in” you would count may not be entirely accurate.

How does our body use calories?

Our body's total energy expenditure – how we use energy – can be broken down into three main categories: basal metabolic rate, thermal effect of food, and activity level.

Resting energy expenditure

You may also see this referred to as resting metabolic rate or basal metabolic rate. It is the amount of energy your body needs to simply function. It is the energy needed to support normal body functions and homeostasis. It accounts for about 60-70% of our total energy expenditure.

Thermal effect of food

This is the energy involved with digesting, metabolizing, and storing energy and nutrients from our food. It typically accounts for about 10% of our total energy expenditure.

Physical activity

Physical activity energy expenditure is the most variable of the three, and is the energy burned due to moving the body. This includes movement through exercise and as well as the movement built into our lives like walking to work or even typing. This can be calculated by adding up the resting energy expenditure and thermal effect of food, and subtracting it from total energy expenditure. So if REE is 70% and TEF is 10%, physical activity makes up 20% of our total energy expenditure.

What impacts the way we use calories?

Not eating enough

Eating less than your basal metabolic rate can lead to metabolic adaptation. This occurs when we are eating less calories than our body needs to perform basic functions.

When this happens, our body thinks we are experiencing food scarcity, and in an effort to keep us alive it will lower our basal metabolic rate (the amount of energy we need in order to perform basic functions). When this rate is lower, this means your body is using less energy at rest, and therefore not “burning” as many calories as it was before.


Our genetics play a role in dictating the way our body metabolizes calories. Some research suggests our bodies have a set point weight, which is a predetermined weight range that our body tries to maintain.


Chronically elevated or chronically low hormones can impact the way we metabolize energy, our appetite, and body fat distribution. This is why people with certain thyroid conditions may struggle with weight loss or weight gain.

What can impact weight changes beyond calories consumed?

In addition to all the points mentioned above, there are a plethora of things that can impact our weight changes beyond the amount of calories you consume/burn. These factors include (but are not limited to) medications, medical conditions, sleep, stress, age, gut bacteria, and diet quality.

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