In the United States, over 60% of adults are overweight, and over 30% are obese, with the rates of obesity consistently and rapidly increasing [1].

The reason for this issue is simple in nature, with modern populations continuing to eat more and move less. This drives the central dictator of weight gain – energy imbalance [2].

Since 1824, nutritionists have used the calorie – a unit of energy – to measure the energy content of a given food. After 1000’s of well-designed controlled clinical trials, obesity is now attributed to excessive calorie consumption throughout the day in relation to the bodies energy expenditure.

This finding has led to the popularly expressed diet known as “calories in calories out”. Such a phrase is in reference to the fact that creating an energy deficit (eating less calories than expended) causes weight loss, whereas an energy surplus (eating more calories than expended) leads to weight gain [3].

However, such a concept is consistently criticized to this day, as this train of thought does not necessary account for macronutrient or micronutrient type or quality.

This leads to various controversies for 2 primary reasons:

• Some nutrition experts still believe that macronutrient type plays a significant factor in weight gain or loss
• All nutrition experts agree that nutrient type has a large impact on general health

The Only Principle To “Calories In Calories Out”

To fully understand this concept, lets quickly break down what is meant by “calories in” and “calories out”.

Calories “in” is pretty self-explanatory. It is the number of calories ingested through food and drinks in a 24 hour period.

Calories “out” is where it gets a bit more complex. Calories “out” consists largely of :

• Resting energy expenditure: the daily energy requirement of the body at rest, in the absence of external work, to maintain internal functioning of the organs and cells
• Physical activity: the energy expenditure via exercise and non-exercise induced activity (daily activities)
• Thermic effect of food: the energy required to digest and absorb the nutrients contained in food

By ensuring that calories “in” is, on average, less than calories “out” over a certain period of time, weight loss is inevitable as the body will be forced to utilize stored bodyfat for energy needs.

It should be noted that such an approach requires individuals to consistently track their daily calorie intake by way of a calorie counting application, and ideally track their energy expenditure with some type of activity watch.

So Why is There Confusion About “Calories in Calories Out”?

The main reason for the confusion about this concept is because of the third factor of energy expenditure, as previously mentioned – the thermic effect of food.

This is because the 3 energy-containing macronutrients have different thermic effects, meaning they all require a different amount of energy to be processed once they have been ingested.

For example, dietary protein requires 20-30% of its energy content to process it within metabolic processes, carbohydrates 5-10%, and fat 0-3% [4].

Therefore, although 1 calorie from any nutrient is only 1 calorie, when accounting for its metabolic processing within the body, the net calories as an end result is going to be different.

This is the one of the main reasons why higher protein intakes have an advantage in weight loss and maintenance [5]. It simply has a larger effect on “calories out” compared to dietary carbohydrate and fat.

So to be truly correct about “calories in calories out”, the more appropriate term to use is “net calories in net calories out” – but hey, that just doesn’t sound as simple does it!

What About Insulin?

As a further criticism of the calorie-counting diet, many fat-based diet advocates bring up the argument of the carbohydrate-insulin hypothesis.

In general, the argument is that fat-based diets have a metabolic edge over carbohydrate-based diets due to the fact that carbohydrates cause a significant production of the insulin hormone which promotes fat storage.

So by eating less dietary carbohydrate this will significantly lower insulin production, lead to a greater release of fat from adipocytes (fat cells), and promote a better fat loss environment.

On paper this mechanistic advantage of a fat-based appears sound and would negate the “calories in calories out” model of fat loss. However, in reality, different variations between carbohydrate and fat within a calorically identical diet do not appear to induce contrasting results.

For example, 2 meta-analyses of highly rigorous metabolic ward studies have revealed that weight gain or loss is not primarily determined by varying proportions of carbohydrate and fat in the diet, but instead by the number of calories ingested [7].

WARNING – There is More to Food Than Just Calories

As much as it is tempting to simply view foods as just calories, and as much as that may be the only real determinant of weight loss, food has a much greater role to play within health and wellbeing that should not be ignored.

A key example of this when it comes to weight loss is the satiety value of food – meaning how it reduces your hunger levels throughout the day.

Although you may be able to lose weight by eating a limited enough amount of “junk food” per day, this is not a sustainable or appropriate weight loss method if it is just going to cause extreme hunger (which impacts overall wellbeing).

In an ideal scenario, eating large volumes of food that are very satiating will lead to more sustainable diets and better health outcomes [8].

So if you are going to use up 500 calories of your daily energy intake, you are probably best off opting for the lean meat with a pile of veggies as opposed to a small cheeseburger.

It is no surprise that short-term feeding studies report that lower energy-density food choices result in lower daily energy intakes and improved weight loss.

Factors such as palatability, satiety, glycemic index and fiber content play huge factors which “calories in calories out” does not account for [9].

The satiety value of foods may even be the primary mechanism which is driving obesity, and has ever been recently labelled as “hidden hunger” [10]. This relates to the psychological “need” of overweight individuals to consume more food despite an abundance of energy already being consumed.

If dietary needs such as protein, fiber, vitamins and minerals are lacking in the foods that are being eaten, the body will still constantly crave additional food until these physiological needs have been met.

This has brought about a host of recent studies on the crucial link and signaling between the gut and the brain, driven by the release of various satiety hormones and peptides following food consumption.

For example, dietary protein is the most satiating nutrient to consume. The presence of amino acids in the gastrointestinal tract induce the release of the satiating hormone CCK. In addition, the gastrointestinal tract responds to the presence of amino acids by releasing other anorexigenic substances such as GLP-1 and PYY [11].

On the other hand, refined sugar has little, if any impact on satiety hormones, and may even increase hunger once blood sugar levels seemingly crash from the initial spike.

The Positive Side to “Calories in Calories Out”

An upside to this dietary approach is that it promotes flexibility in the diet, which is often beneficial for long-term sustainability and avoids potential eating disorders from very strict mentalities.

After all, the number 1 determinant of dietary success is the level of adherence to that diet, regardless of the type of diet [12].

In fact, many successful obesity prevention programs promote a flexible dietary approach over a rigid dietary approach for this exact reason [13].

Educating people that weight loss can still be achieved with a small inclusion of “bad” foods – provided a calorie deficit is maintained – is a somewhat better approach than adopting the mindset that certain foods are always to be avoided at all costs. This mentality may lead to unhealthy relationships with food and potential binge eating if the “bad” food is ever eaten.

1 cookie every day is better than 10 cookies on a weekly binge.

Conclusion

“Calories in calories out” is the concept that all you need to do to lose weight is consume less calories than you expend per day.

This is true. However, it fails to account for the different amounts of calories needed to metabolize various nutrients, which can significantly impact the net calories in (or out) per day.

In addition, it is a fairly narrow-minded picture of weight loss. In reality, other factors such as meal satiety, micronutrient density, and the health impact of various foods should be accounted for.

So despite the general concept of calorie control being correct on paper, it is only one piece of a much greater puzzle.