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Calories In, Calories Out – What’s the Deal?

Critical Concepts Series: Energy In, Energy Out – Part Two

The pressure, drive, and desire for weight loss or a low body fat percentage is very common amongst athletes. 

These are a mixture of external and internal pressures to look a certain way, be a certain weight or body shape, or conform to certain standards or ideals within a sport. 

Many people turn to unhealthy diets and training methods because of the pressure to be thin. These methods aren’t sustainable in the long term and can actually lead to more health problems.

This is part two in a series of articles on energy intake for athletes.

  1. Part one looked at why low daily energy intake has negative effects on everything from motivation to performance in athletic women.
  2. Part two – this article – will look at why “energy in, energy out” is more nuanced than “calories in, calories out.”
  3. Part three will focus on the factors of total daily energy expenditure, including non-exercise activity thermogenesis and purposeful physical activity.
  4. And in part four, you’ll learn about how to improve your metabolism over 40.

What is Calories In, Calories Out?

Because of the pervasive influence of diet culture (1), chances are, if you were asked what is required to lose weight (body fat, more specifically), your answer would invariably describe the need for some form of energy deficit. 

As you’re so often told, losing or gaining body fat is a matter of energy balance:

  • If calories in exceed calories out, then you might expect to gain body fat. 
  • If calories out exceed calories in, then you might expect to lose body fat. 

It is highly likely, then, if you have engaged in any form of dieting to change body composition, you have applied this calories in / calories out (CICO) model to yourself.

A very common example is applying the generic concept of a 500 calories (or more!) deficit without thinking if that’s even realistic or necessary for you as an athlete.

Most likely in the common form of eat less, move more, removing or restricting food groups or meals, while simultaneously increasing some combination of exercise frequency, intensity, and duration.

There is, of course, a lot of truth to the central tenets underpinning this energy in / energy out model:

  • To gain mass (where mass can be lean body mass, such as muscle or bone, or fat mass), you need an energy surplus, usually by eating more calories. 
  • To lose mass (lean or fat) you need an energy deficit. 

Basic laws of physics (thermodynamics) have to apply. 

The Limits of CICO

However, as is often the case with biological models being stripped of all their complexity and nuance and reduced down to pithy one-liners, such simplicity can, at best, lead people astray, and, at worst, do a lot of harm to an individual’s health and wellbeing.

Building back some of the nuance and context is a fraught affair. 

You are, after all, attempting to explain a combination of thermodynamics and metabolism without adding more confusion than you’re trying to subtract. 

But with so many athletic women getting into strife as they try to match, intentionally or otherwise, low daily calorie intake (often below 1500kcal) with high energy output (often 1000kcal or more than they’re eating), a change in the discussion needs to happen somewhere.  

To get underway, we first need to unpack some key concepts around energy.

Energy In: A Calorie is a Calorie, But Energy isn’t Energy

The energy we consume or burn is most commonly measured in calories (used interchangeably with kilocalories [kcal] especially with reference to food). 

Technically, calories are a measure of heat – it is the energy needed to raise the temperature of 1 gram of water through 1 degree Celsius. 

Protein and carbohydrate are each said to contain around 4kcal per gram, while fat is more energy-dense, containing around 9kcal per gram.   

While a calorie is a calorie (1 calorie of fat and 1 calorie of protein, for example, are equal in terms of energy density), this calorie of fat and calorie of protein will have a different effect in the body.

That is, they might be equal in energy-density but not in consequence. 

Saying that a calorie is a calorie is the same as saying an inch is an inch, or a pound is a pound. It quantifies things but doesn’t qualify things.

Protein Energy vs Non-Protein Energy

This is where it is perhaps more useful to think about energy in terms of protein energy and non-protein energy (or protein: energy ratio, where the latter is predominantly carbohydrate, fat, and alcohol) (2). 

This is especially true when considering the broad consequences of under- or over- eating each form of energy. 

Let’s say you consistently eat a 500kcal surplus each day. 

If your calorie surplus is protein energy, then the extra mass (“weight”) you will gain will be more likely lean mass – muscle, connective tissue, bone. If your calorie surplus is non-protein energy (either fat or carbohydrate), then, everything else being equal, any mass gain is more likely to be fat mass. (3)

Conversely, if you’re attempting to run an energy deficit by cutting your calorie intake, the consequences will depend on the type of calories you have cut out.

Diets which cut either fat (low-fat diets) or carbohydrate (low-carb diets) broadly tend to have roughly the same effect on body fat over the longer term – 6 months or more, especially if protein energy is equivalent between the different diet types. (4) 

Run a calorie deficit which cuts a lot of protein energy from your diet, however, and you will tend to lose a lot of muscle mass in the mix. (5)  Body weight might reduce, but only because muscle mass has been lost (with detrimental metabolic health consequences as a result).

Example: Keeping Protein Energy Higher

Let’s illustrate this concept with some numbers. 

A common approach is to start with a total number of calories in mind, then divide this into protein, carbohydrate, and fat, using percentages. 

So let’s say you’re currently eating 2000kcal and 15% total protein. 

This is 300kcal of protein, or 75g per day (there are 4kcal per gram of protein). 

You then might decide you’re going to cut your calorie intake to 1200kcal. Sadly, this is a very common target amongst women going on diets. This is less than what children need. 

You believe your protein energy is fine because you’re still eating 15% of these calories as protein.  But 15% of 1200kcal is now 180kcal of protein, or 45g per day – a very low intake, insufficient to meet your basic needs if you’re training or exercising. (6) 

If you consider, in the example above, that the original 75g of protein would unlikely be optimal for a female athlete (perhaps 120-150g per day might be more appropriate), and now you have cut this even further. 

With your non-protein energy having also been cut quite drastically, more of the protein you’re eating is needed for energy production, (7) leaving even less protein available for repair and recovery, let alone more basic and essential functions the body requires protein energy for. 

To make up the shortfall, your body needs to pull protein out of ‘storage’.  Where do we store protein? In our large lean tissue pools, specifically our musculoskeletal system. 

You force your body into a strategy of robbing Peter to pay Paul.


Part 3 of this series will cover more about metabolism and resting metabolic rate.

And you might be wondering, “How do I figure out my total daily energy expenditure (TDEE)?”

There are several methods of estimating this, including the online TDEE calculators based on equations such as Katch-McArdle or the Harris-Benedict equation.

We’ll cover this in later articles.

To sum it up:

  • Energy balance is a more nuanced concept than simply energy in / energy out or calories in / calories out
  • You’re better served to consider the balance between protein energy and non-protein energy than total calories
  • If you cut our energy intake without considering, at least in the first instance, the impact this will have on our protein energy, you run a very real risk of losing a significant amount of lean muscle mass, with detrimental consequences to your health and performance.


  1. https://recreation.ucsd.edu/2021/01/diet-culture-social-media/
  2. Millward, D. (2013). The use of protein:energy ratios for defining protein requirements, allowances and dietary protein contents. Public Health Nutrition, 16(5), 763-768. doi:10.1017/S1368980013000396
  3. Leaf A, Antonio J. The Effects of Overfeeding on Body Composition: The Role of Macronutrient Composition – A Narrative Review. Int J Exerc Sci. 2017 Dec 1;10(8):1275-1296. PMID: 29399253; PMCID: PMC5786199.
  4. Gardner CD, Trepanowski JF, Del Gobbo LC, et al. Effect of Low-Fat vs Low-Carbohydrate Diet on 12-Month Weight Loss in Overweight Adults and the Association With Genotype Pattern or Insulin Secretion: The DIETFITS Randomized Clinical Trial. JAMA. 2018;319(7):667–679. doi:10.1001/jama.2018.0245
  5. Willoughby D, Hewlings S, Kalman D. Body Composition Changes in Weight Loss: Strategies and Supplementation for Maintaining Lean Body Mass, a Brief Review. Nutrients. 2018 Dec 3;10(12):1876. doi: 10.3390/nu10121876. PMID: 30513859; PMCID: PMC6315740.
  6. Jäger, R., Kerksick, C.M., Campbell, B.I. et al. International Society of Sports Nutrition Position Stand: protein and exercise. J Int Soc Sports Nutr 14, 20 (2017). https://doi.org/10.1186/s12970-017-0177-8
  7. Helms ER, Zinn C, Rowlands DS, Brown SR. A systematic review of dietary protein during caloric restriction in resistance trained lean athletes: a case for higher intakes. Int J Sport Nutr Exerc Metab. 2014 Apr;24(2):127-38. doi: 10.1123/ijsnem.2013-0054. Epub 2013 Oct 2. PMID: 24092765.

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Steph Gaudreau

Hi, I'm Steph Gaudreau (CISSN, NASM-CPT)!

Nutrition and fitness coach for women, Lord of the Rings nerd, and depending on who you ask, crazy cat lady. My mission is to help you fuel for more: bigger muscles, strength, energy, and possibilities. We’ll do it with my signature blend of science, strategy…and a little bit of sass.


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