Calories In Vs Calories Out | Part 1

How many calories for weight loss | Bulk Powders Core


One of the guiding principles of any nutrition program targeted at either losing body fat or gaining muscle is the concept of ‘calories in versus calories out’. Despite the fact that some people believe that calories don’t matter when looking to reach a specific physique or performance goal, it is an unequivocal truth of the laws of physics (Newton’s third law of thermodynamics in fact) that in order for us to lose or gain body mass, energy balance is the key determinant in this process.

In this two part article, we will firstly discuss the factors that can impact energy balance, how the macronutrients that contribute to our energy balance are used and stored and how this relates to muscle building and fat loss processes. In part two we will look at the impact of other factors such as meal timing, meal frequency and how the foods from which we get our macronutrients may impact on our muscle building or fat loss success!

If we want to gain energy, which translates into increased body mass, we need to consume more ‘calories in’ than we expend with our normal metabolic function with the addition of energy expended during daily activity and exercise… our ‘calories out’. Conversely, if we want to lose mass (namely body fat), we need to consume less energy than we expend, thus we tap into our energy reserves to make up the deficit. If we get our nutrition right, the reserve we tap into will be body fat… simple, right?

Now, this is where we must consider our macronutrient intake in the diet; in short, where we get our calories from. Although calories in versus calories out determines whether we will lose or gain weight, the amount of carbohydrates, fats and protein in the diet will determine the nature of weight loss or weight gain. Obviously if we lose weight we want to make sure that we preserve muscle and use stored body fat reserves, thus by weight loss we mean to say have fat loss. When we say weight gain, what we actually mean is increases in lean tissue whilst limiting excessive gains in body fat.

This is where protein plays a very important role, for two reasons. Firstly, ensuring adequate protein in the diet is obviously important for encouraging muscle growth as the amino acids required to build new tissue are derived from dietary protein. The body does not store protein efficiently anywhere except in the muscles, so when we talk about our body storing excess energy, we want that to be in the form of protein, as this directly equals more muscle! For muscle building purposes we want to be consuming around 2g per kg of body weight per day1.

Secondly, for weight loss, protein is going to play several key roles. Protein is going to help protect muscle, which is a key determinant of our metabolic rate. Protein is the most filling macronutrient, which means that it helps to regulate appetite2.

Finally, protein has the highest ‘Thermic Effect of Food’ (TEF) of all the macronutrients. TEF is the amount of energy it takes to digest, absorb and store a specific food or meal and around 30% of the energy we get from protein is immediately lost to these processes, meaning our net calorie gain is effectively lower3. Due to these important factors, protein intake for weight loss should be targeted at around 2.5g per kg of body weight.


This also means that our ‘calories in’ can directly, and quite profoundly, impact on our ‘calories out’, so although calories matter, the nature of our calories means that not all calories are created equal. This means that if we took someone with a low protein diet who was maintaining their weight on let’s say 2500kcal, and then we doubled their protein, we would still be consuming the same calories in, but due to TEF we would have actually increased our calories out, so we might be able to cause fat loss without even having to reduce calorie intake. The laws of thermodynamics are still intact, they are just not as rigid as people often realise.


So, how do carbohydrates and fats fit into all this? Well, firstly fats are essential in the diet for many physiological functions, so we need to include some to stay healthy (around 1g per kg of bodyweight). However, aside from meeting these basic demands, and despite the belief that low carb dieting is more effective, recent research shows that it doesn’t matter where you get the rest of your calories from4.

If you prefer carbs, eat carbs, or if you like fats, have fats, or any combination of the two! Fats tend to have a weight loss advantage as they are more ‘filling’ so help control appetite. However, carbohydrates, generally speaking, have a higher TEF but this can be influenced by the source of both carbohydrates and fats. Ultimately, for weight loss, if our calories are set at the right level and the diet contains adequate protein, it does not matter were the rest of our calories come from in macronutrient terms.

For those who take part in regular intense exercise then including a good amount of carbohydrates in the diet is important whatever your goal. This is simply because carbohydrates are stored in the muscles and liver as glycogen and not typically as body fat (normally it is excess dietary fat that is stored as body fat, not carbohydrate, unless consumed in serious excess), which is what many people wrongly believe5. Glycogen supports performance in the gym, which helps to build muscle and by maintaining strength/performance and overall energy output may assist in supporting fat loss by allowing us to keep our ‘calories out’ side of the equation as high as possible.

What this all means is that it is possible, through manipulating the nature of our calories to build muscle and lose fat at the same time, using stored body fat (to a certain point) to support muscle building processes. But a cautionary tale, it also means that blindly decreasing calories which might result in weight loss can include losses in both muscle and fat tissue if we do not pay enough attention to ensuring we protect muscle tissue through a high protein diet and the inclusion of exercise, particularly resistance training.

Stay tuned for part 2, in which we will discuss the ‘next level’ in our nutritional influences that will impact on reaching our goals; including meal frequency, meal timing and the influence of the types of foods we get our macronutrients from.


  1. Helms et al., (2014) Evidence-based recommendations for natural bodybuilding contest preparation: nutrition and supplementation. Journal of the International Society of Sports Nutrition.
  2. Weigle et al., (2005) A high-protein diet induces sustained reductions in appetite, ad libitum caloric intake, and body weight despite compensatory changes in diurnal plasma leptin and ghrelin concentrations. American Journal of Clinical Nutrition.
  3. Antonio et al., (2014) The effects of consuming a high protein diet (4.4 g/kg/d) on body composition in resistance-trained individuals. Journal of the International Society of Sports Nutrition.
  4. Hall et al. (2015) Calorie for Calorie, Dietary Fat Restriction Results in More Body Fat Loss than Carbohydrate Restriction in People with Obesity. Cell Metabolism.
  5. Hellerstein (1999) De novo lipogenesis in humans: metabolic and regulatory aspects. European journal of clinical nutrition.


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