Calories – A Guide for Weight Manipulation



During my years of resistance training I always trusted others on how much I should eat to achieve my fitness goals. As I research more and more, tapping into different sources of information, it became clear that too much information is based on opinions and myths. Somehow it became common practice not to reference scientific studies in order to support a statement. I came to a point where I’m super critic of all the information that is not grounded in the scientific method.

So I decided to do the research myself.  On this post we discuss calories. All backed by science.

The Calories In, Calories Out Theory

The joule is the international systems’s unit of measurement of energy. When calculating the energy necessary to lift a rocket or anything along the same line, joule is used to quantify this amount of energy. But like many different physical quantities, different units can be used to quantity the same thing like the degrees Celsius and Fahrenheit to quantify temperature. The same happens for energy, like the joule there also exists the calorie. For some reason the calories is the unit of energy most used in the nutrition and fitness context. But it is still just the same, energy.

A calorie is defined by the amount of energy needed to raise the temperature of one gram of water by one degree Celsius at a pressure of one atmosphere.

Because this amount of energy is quite small compared to the amount of energy packed in the foods we eat, a sub-unit of the calorie is used, the kilocalories (1000 calories). You don’t refer to your weight in grams but in kilograms. It is more convenient. Everytime you read some nutritional chart and look at the calories, mind the units. Here in Europe, the calories are always in kilocalories (kcal) and kilojoules (kJ), but I have seen that some places give the calories in Calories. Note the capital C. One Calorie is equal to 1 kilocalories.

NOTE: Unless you are a scientist and work with calories in the lab as a measure of unit, everytime you see the unit “calorie” it is actually 1000 calories. If you eat a cake and you check the package and says 600 kcal and then you go to the treadmill to burn all those calories, please don’t run until the screen says 600.000 calories. The treadmill must definitely is displaying the amount of energy in kcal even though the units in the screen say calories.

The calorie unit is typically used to indicated the amount of energy a food contains but also to quantity how much energy is spent at doing a certain activity. This is done for convenience, making it easy to compared energy intake and energy output. An example.

An average apple has 95 kcal, meaning that when you eat an apple, our body will use that energy to power the body and all its systems. Hopefully you eat more then an apple but your get the idea. The same idea applies to physical exercise. A person who weighs 75 kg and runs 5 km in 30 minutes will burn 388 kcal (energy wise the equivalent of about 4 apples).


Figure 1 – Visualization of the Calories in, Calories out model. Adapted from [6]

And here enters the First Law of Thermodynamics, which states:

In a thermodynamic process involving a closed system, the increment in the internal energy is equal to the difference between the heat accumulated by the system and the work done by it.

Our body (a closed system) will change (internal energy) depending of the balance between energy gained/retained heat (heat accumulated) and the level of activity (work). Resuming, we can play with 2 factors:

  • How much we move (lifestyle and workouts)
  • How much energy we consume (food) or retain (heat/cool environments)

Our first step is to assess how many calories our body needs to maintain a energetic equilibrium, aka our maintenance calories.

The Calorie Conspiracy Myth

If you search “calories in calories out myth” you will find loads of different people arguing against this “formula”. Like taxes, the rules of thermodynamics are always present, so I was quite intrigued by this myth. After searching through a bunch of bad science and esoteric opinions, I found a fantastic article and book called “The Calorie Myth” by Jonathan Bailor. I recommend you to read both by yourself but the take away message is this:

The quality of the food you eat is also a major factor in manipulating your weight. Our body is a complex biochemical machine composed by different components and mechanisms. Even though the Calories In, Calories Out Model can be applied to the human body, it is not enough to fully describe what is happening inside us.

This post only strives to define the amount of energy necessary to achieve a certain fitness goal. Energy, no macro nutrients, no vitamins, no healthy fats, only energy.

Maintenance Calories

Our body need energy all the time. Even if you don’t move during the whole day, the body will require energy to keep you alive. This is called the Resting Energy Expenditure (REE) or Resting metabolism according to Figure 1. REE can be calculated using the formula developed by Mark Mifflin and Sachiko T. St. Jeor in their publication [1]. There are different formulas but the Mifflin-s formula is the most commonly used.

REE=(9.99 x weight(kg) )+(6.25*height(cm))-(4.92*age)+(166*sex(male=1,female=0))-161

This value is then multiplied by a coefficient named Physical Activity Level (PAL). It reflect how active the person is. As expected, this value is difficult to estimate but multiple sources created categories of activities with the respective PAL [2] [3].

Table 1 – PAL coefficients for different levels of activity












The above PAL are only estimations. For the best results try the following:

  1. Pick a PAL value that reflects your lifestyle.
  2. Use the “Maintenance Calories Calculator” in this page.
  3. Weigh yourself.
  4. During a 2 or 3 weeks, eat the amount of caloric indicated by the calculator.
  5. After that period, check your weight. If you lost weight increase the PAL, if you gain decrease it.

By the multiplication of PAL with REE we then obtain the Total Energy Expenditure (TEE). It represents the number of calories you need to maintain your current weight.

Now that we have a baseline to work around, let’s define an objective. Do you want to lose fat, maintain your current weight or gain muscle?


Calculator based on [1] [2] [3]


Losing Body Fat

Following the thermodynamic model from before, to lose weight we need either to consume less then the maintenance calories or to practice a physical activity that burns enough calories to put us into a caloric deficit or a combination of both.

The next rational question to ask is: “How fast is possible to loss weight?”

The bigger the difference between the maintenance calories and the consumed calories, the faster is the weight loss. But if this process is too fast, health issues might occur. Research indicates that a healthy rate for weight loss fall between 0.5 and 1 kg per week [4] [5].

We know the rate of weight loss and now it’s necessary to know how many calories are in what we want to lose: body fat. It turns out that to lose 1 kilogram of body fat is necessary to burn 7716 kcal. The calculator below works using this two factors.  Select how many kilograms you want to lose per week and the calculator gives you how much you need to subtract from your previous calculated maintenance calories. Remember: play with food restriction and/or exercise.


The Numbers Can Lie

Although we are calculating things with some kind of precision, don’t get to attached to the results. They are just ballpark number to guide you in the right direction. As I mentioned previously, our body is a complex machine making it difficult to reflect its complex inner works with 2 or 3 simple equations. For example, the resting energy expenditure (REE) reduces with caloric restriction [7]. This metabolic change negatively affects the weight loss process and a bigger deficit between maintenance calories and daily intake needs to happen. Also remember that the maintenance calories are dependent of the (PAL) physical activity level coefficient. As we saw previously, the PAL value is “guesstimated” so keep that in mind when adding or subtracting calories to your baseline. For example:

Jimmy wants to lose body fat. He has 75 kg, 173 cm and is 23 years old. By using the calculator, he determines that he needs 2,843 calories to maintain his weight (using a PAL of 1.65). To lose 0.5 kg per week he needs to consume 2290 calories per day. But if he uses a PAL of 1.78, putting him at 2520 calories per day to lose the same body fat.

This goes to show that you need to keep your weight under a close watch every week. If you are losing body fat to fast, change something in your diet. If you are not losing enough, eat less or exercise more. Take away message: Measure, track and adjust.

Gaining muscle

Two things are necessary for muscle building: surplus of energy and a stimulus. In this post only the surplus of energy is discussed and defined but eventually I want to discuss the stimulus component in a future post. To gain lean tissue (aka muscle), the body needs a surplus of energy provided by proper nutrition. Although the macro nutrients and micro nutrients are important in this process, here we are looking at the big picture, the overall energy necessary to consume to gain muscle.

To define this overall energy we need to define how fast to body can produce lean tissue and how much energy is needed for this process. Let’s look into the first variable, how fast muscle can be produced.

As predicted, the amount of muscle an individual can gain is limited. Therefore the amount of muscle that can be gained depends of how much muscle the individual already possesses. Two fitness experts, Alan Aragon and Lyle McDonald, defined rates of muscle gain athletes should expect under a resistance training regiment. I found these rates of muscle gain in literature but because they claim athletes can expect an increase of 0.5 to 1 kg PER WEEK, I decided to stay with the Aragon and McDonald rates. It would be interesting to know why they rates in literature are so far off to what athletes and experts experience.

Below is the Alan Aragon’s table of rates for muscle gain. I refer this one to the McDonald’s rate because it takes into account the body weight. But both have similar rates.

Table 2 – Alan Aragon’s rates of muscle gain
Category Description Rate of Muscle Gain
Beginner 1 year of less 1-1.5% total body weight per month
Intermediate 2 to 3 year 0.5-1% total body weight per month
Advanced 4 to 5 years or more 0.25-0.5% total body weight per month

For the second variable we need to define how much should be the caloric surplus to achieve the desired rate of muscle gain. Finding this value in literature also wasn’t an easy task. I ended up looking at the  caloric intake recommended by institutions like the (American) National Research Council and the International Sports Sciences Association (ISSA). The first recommends a surplus of 5 kcal are required to synthesize 1 g of muscle while the ISSA recommendations are based on activity level and body weight (Table 3). These recommendations are inline with Dr. Christine Rosenbloom’s statements about muscle building diet requirements, where men should consume 55 kcal per kg and women should consume 44 kcal per kg to gain lean muscle mass [7].

Table 3 – ISSA’s daily caloric intake recommendation of gaining muscle

Resistance training Daily caloric intake
1 to 3 times per week 35 – 40 kcal per kg
3 to 4 times per week 40 – 44 kcal per kg
5 to 7 times per week 44 – 49 kcal per kg

NOTE: The ISSA recommendations account for an increase in caloric intake that support the resistance training activity. We also considered this in the maintenance calories calculations so please that that into account if you use the following table 3. If you use a PAL (Physical Activity Level) that represents both your daily activity and resistance training, you should get more accurate results. Remember, the recommended values are computed for a big sample size, meaning that they are not optimized for your specific case.

The next calculator has into account the previously mentioned recommendations.


Although the caloric intake it is just a first assessment in planning a complete nutrition plan, this post presents the logic behind different weight manipulation strategies. In next posts, I want to do the same for other topics like macro-nutrients, hypertrophy and strength oriented training, cardiovascular exercises and supplements.

What do you think about the style of this post? Do you find them informative?

I prefer doing the research myself and looking at expert in the fitness world with academic background. Below I leave some authors and books I found useful for this topic during my research for the blog post.


  • The Calorie Myth – Jonathan Bailor
  • Sports Nutrition: A Practice Manual for Professionals –  Christine A. Rosenbloom and Ellen J Coleman


[1] A new predictive equation for resting energy expenditure in healthy individuals, M. Mifflin, S.T. St. Jeor, 1990

[2] Physical activity and energy balance, Health Nutrition: 2(3a), 335–339, Marleen A. van Baak, 1999


 [4] Essentials of Strength Training and Conditioning, Thomas R. Baechle,Roger W. Earle, 2015 (pg. 224) 

[5] Energy-Yielding Macronutrients and Energy Metabolism in Sports Nutrition, Judy A. Driskell, Ira Wolinsky, 1999 (pg. 300)

[6] Essentials of exercise physiology,  Victor L. Katch, William D. McArdle, Frank I. Katch.— 4th ed.

[7] Sports Nutrition: A Practice Manual for Professionals – Christine A. Rosenbloom and Ellen J Coleman

Road Delta is where I share my interests, experiments and creativity with the world. All blog posts and videos in this website are 100% made by me, which makes it difficult to define a theme. And all over the place. Nutrition, physiology, public speaking, travelling, fitness… You can find it all here.

Your opinion is welcomed :)

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