Diet Principles #1: Calories In VS Calories Out

Welcome to the first article in what is to become the Diet Principles series – a series of posts that are intended to break down the fundamentals of nutrition. Through this series we want to educate the masses on the basics of calorie intake and take a look into macro-nutrients and metabolic adaptation. Some of this may sound complicated but don’t worry it is not as hard as it sounds and will be explained very clearly. This information will be relevant for those wanting to lose weight, gain weight and maintain weight as these principles apply to all weight related goals.

So let’s get into it.

Diet Principles #1: Calories In VS Calories Out

No matter what you have heard or been told about dieting, whether that be to lose weight or gain weight, the single and only factor that determines this is calories – or more specifically your total daily calorie intake. It is changes in your daily calorie intake that will determine if you will lose weight, gain weight or remain the same weight (maintenance). The simple formula of Calories In VS Calories Out (Also known as CICO) is the simple and only formula that determines weight loss or gain.

Calories In

Diet Principles #1: Calories In VS Calories Out

‘Calories In’ refers to how many calories you are consuming each day. Quite simply the food and drinks that you are eating in a day. So when you hear Calories In, I want you to think about the food that you have ate.

Calories Out

The human body uses calories as energy to function. Just like a car burns through petrol to operate, your body is burning through calories to do everything from basic human functions such as breathing to intense exercise such as running or weight lifting.

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Just how much calories your body burns each day varies from person to person. Variables such as your height, your body weight, metabolism and muscle mass all effect how much calories your body burns in each day. For example someone with more muscle will burn more calories sitting on the couch than someone with less muscle would. This leads us to the topic of Basal Metabolic Rate (BMR).

What is BMR?

Basal Metabolic Rate or BMR is quite simply how many calories your body burns per day at rest. Simply put, it is the amount of calories your body burns when physically doing absolutely nothing. Of course most of us are busy and burn more calories than this even if not actively exercising, simply through activities such as walking to work etc. This brings us on to Total Daily Energy Expenditure (TDEE).

What is TDEE?

Your Total Daily Energy Expenditure or TDEE is your total estimated daily calorie expenditure (how many calories you are burning) when exercise is taken into account. Put simply, this is your BMR with the addition of calories burned through exercise. This is partly why exercise is an important and helpful factor in weight loss although not entirely necessary. Your TDEE is what we will refer to as Calories Out. 

Calories In VS Calories Out

Now that we have a definition of Calories In and Calories Out, we can now look at the very basic law of weight loss or gain. The formula is as follows:

Calories In – Calories Out = Outcome

For weight loss the ‘Outcome’ must be negative. This means your Calories Out is greater than your Calories In. Put simply, you are burning more calories than you are eating therefore your body needs to burn its own stored energy ie fat. This is known as a Calorie Deficit. The larger the calorie deficit the more aggressive the weight loss will be, I would not recommend going aggressive however as it will leave you feeling very sluggish and drained. Instead choose a more moderate deficit of 200 – 300 kcals to begin with.

For weight gain the ‘Outcome’ must be positive. This means that your Calories In are greater than your Calories Out, or you are eating more calories than you are burning each day. This results in your body storing this extra energy in the form of muscle or fat. This is known as a Calorie Surplus.

For weight maintenance (staying the same weight) the ‘Outcome’ must break even or equal zero.

Let’s Look at an Example

Meet John. He is a 30 year old male office worker and weighs 185lbs at 5ft 10in tall. He currently does not exercise at all. Using a TDEE calculator we workout that John’s Total Daily Energy Expenditure is 2,243 calories per day.

Great, now that we know John’s ‘Calories Out’ we can work out how many calories John needs to eat in order to Lose Weight, Gain Weight and Maintain his current weight.

Lose Weight

To lose weight, John must eat less than 2,243 kcals each day.

Example: Calories In = 2,000 kcals (2,000 – 2,243 = -243 kcals)

This puts him in a Calorie Deficit of 243 kcals.

Gain Weight

To gain weight, John must eat more than 2,243 kcals each day.

Example: Calories In = 2,450 kcals (2,450 – 2,243 = 207 kcals)

This puts him in a Calorie Surplus of 207 kcals.

Maintain Weight: 

To maintain his current weight, John must consume 2,243 kcals each day.

Example: Calories In = 2,243 kcals (2,243 – 2,243 = 0 kcals)

As you can see the formula leaves John at 0 kcals therefor breaking even and maintaining the same body weight.

Diet Principles #1: Calories In VS Calories Out

But what if John started to take a run every evening after his work to clear his head? On his 20 minute jog, he burns around 300 kcals. This now puts John’s TDEE at 2,543 kcals each day. This changes the outcome of the CICO formula.

If John was still wanting to lose weight, this would put him at a deficit of 543 kcals. Now he has two options here, he can keep the deficit at this level and lose weight more aggressively or eat 300 calories more worth of food to remain at the original deficit of 243 kcals.

The same principles apply if John wanted to gain weight or maintain. He would need to consume more calories to adjust to his new TDEE and allow him to step into a calorie surplus or break even. This shows how exercise can manipulate your TDEE and either allow you to lose weight more aggressively or to eat some more food to make up for the increase in Calories Out.


As you can see the basic concept of weight loss or gain is very simple. All these fad diets that exist all work on this concept, but take it to unnecessary extremes that are not sustainable and can cause more harm than good.

The secret to losing or gaining weight successfully and maintaining it is to follow this method in a paced and consistent manner. Do not expect to change overnight – trust the process and if you have a hiccup (we all do – it’s human nature) just let it be and get back to it. Do not be one of these people who will ‘start again next Monday’ just because you had a few too many calories one day.

In the next article of this series, we will take a look at Metabolic Adaptation. This is an important follow up to this article to be sure to follow our social media platforms to keep an eye out for it being published.

As always let us know what you think in the comments below, and if you have any questions or would like to discuss further don’t hesitate to ask.

Get Smart, Train Smart, Achieve Goals.


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