Calories In, Calories Out Myth: Fact or Fiction?

One of the foundational principles of fitness is Calories In, Calories Out – or at least it was. Over the last decade or so, there has been an increase in outspoken critics who claim it's a myth.

They claim it's outdated and doesn't apply to our modern society. This begs the question, does it? Calories in and calories out is a highly nuanced subject that can't be solved with absolute statements. This article will attempt to make a little more sense of this subject.

Quick History Of Calories In, Calories Out

It's important to realize that calories in, calories out (CICO) is not a new concept. And we don't mean as in a few decades old. As we understand it, calories in and out can be traced back more than 100 years.

Wilbur O. Atwater, the "father of nutrition science," developed the Atwater system in the late 1800s. This system measured the calories of food that Atwater produced, as he felt the population was "influenced too much by taste" and should "regulate appetite by reason." ¹

This novel idea turned out to be extremely successful.

Ironically, we find evidence of companies back then manipulating this concept to take advantage of consumers.

So much emphasis was being placed on calories that companies began placing value on them rather than the nutritional value. For example, beer companies claimed their beer was more valuable than milk as it contained more calories. The same can be seen in comparing flour to beef – 1 pound of flour had 1,600 calories at 6 cents while 1 pound of beef had 1,000 calories at 25 cents. Obviously, the flour was better value!¹

Regardless, this trend grew and evolved until we see another crucial supporter of the calorie, the brilliant doctor Lulu Hunt Peters.

Peters was one of the first female doctors and was especially interested in the type of energy found in food, known as the calorie at the time. In fact, she's one of the first examples of a person who measured her calories successfully in a bid to lose fat—and she was successful in losing 70 pounds!

She's also one of the first authors to write a book detailing how to count calories in her book Diet and Health With Key to the Calories.² 

From here, the idea of the calorie grew and changed as we learned more about nutrition and our body's physiological systems. We obviously can't go over every bit, but the main takeaway is three-fold;

1. The concept of calorie counting and "calorie in, calorie out" has been around for more than a hundred years.
2. Following this concept proved to be very effective
3. The concept has evolved

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What Is "Calories In, Calories Out?"

Starting at the beginning, let's go a little deeper in explaining "calories in, calories out" (CICO).

A calorie is a unit of measurement used to measure how much energy a food source provides.³ Our bodies require this energy to perform all of their various functions – this is one of the major reasons we need to eat!

However, the amount of energy we eat isn't always the same amount as we consume. Depending on our diet, different people will consume various amounts of calories.  At the same time, these same people will expend different amounts of energy throughout the day.

Sometimes, we burn the same amount of calories we consume. Other times, we may eat more than we expand, and vice versa.

This relationship between calories consumed and calories expended is the basis for calories in, calories out.

How Is Energy Consumed Equated?

As mentioned above, energy consumed is derived from our diets, specifically from our macronutrients.

Our macronutrients consist of different amounts of calories and look like this;

• 1 gram of protein - 4 calories
• 1 gram of fat - 9 calories
• 1 gram of carbohydrate - 4 calories

So, let's use the following example of a diet;

• 150g of protein X 4 calories = 600 calories
• 100g of fat X 9 calories = 900 calories
• 200g of carb X 4 calories = 800 calories
• Total calories = (600 + 900 + 800) = 2,300 calories

How Is Energy Expended Calculated? 

Your total energy expended throughout the day, known as your Total Daily Energy Expenditure (TDEE), is the total sum of 4 parts;

• Basal Metabolic Rate (BMR)- Your BMR is the calories you burn to support basic life functions. This includes things such as respiration and thermoregulation. 
• Thermic Effect of Food (TEF)—TEF is the amount of calories you burn to digest a food, absorb its nutrients, and expel its waste. Not all foods have the same TEF. In general, protein has the highest TEF, 20-30%, while fat has the lowest, 1-3%, and carbs have 5-10%. Most people are equated at 10%, but this can vary depending on your diet. This will be important later.
• Exercise Aerobic Thermogenesis (EAT) - EAT is the amount of calories you burn through exercise.
• Non-Exercise Aerobic Thermogenesis (NEAT)- NEAT is the total amount of calories you burn through an activity that's not exercise, such as walking

When you combine all the energy consumed from these parts, you get the total amount of energy used. Just like your diet, this can change daily.

Calories In, Calories Out Equation

It's important to realize that calories must go somewhere. When we eat the same amount we burn, it evens out, and there are no changes in the body.

But what happens when we consume more calories than we expend? Our bodies must store this extra energy (as it must go somewhere) and do this in the form of fat; in other words, we gain fat.

And what happens if we expend more calories than consumed? In this case, we're consuming enough calories to fuel our energy so the body taps into its stored energy which is released; thus, we lose fat.

As you can see, the concept of CICO is pretty simple—it's a basic concept that describes how weight loss or gain works.

• Calories consumed - Calories expended = Change In Weight

With that said, there are ultimately 3 possibilities;

• Calories consumed - Calories expended = Surplus (Weight Gain)
• Calories consumed - Calories expended = Deficit (Weight Loss)
• Calories consumed - Calories expended = Same (Maintain)

Arguments Against Calories In, Calories Out

We went over the general idea of CICO above. It seems pretty simple, so why are people now saying it doesn't work?

This section will look at some of the most common arguments against it.

The Quality Of Food Affects How Our Body Processes It

One common claim against CICO is that the quality of foods can affect the way our body processes them.

For example, Harvard Health published an article titled "Stop Counting Calories" and states that "calories in, calories out" is outdated.⁴ They then give 3 reasons why CICO and calorie counting don't work.

One of those is that the type of food you eat can have different effects on your eating habits—specifically, they note how different foods can affect how much you eat.

They point to a 2019 study published in Cell Metabolism to do this. In this study, two groups were given meals and told they could eat what they wanted.⁵ However, one meal consisted of ultra-processed foods and one meal consisted of unprocessed foods. 

They followed their original diet for 2 weeks and then switched diets.

What happened was that when either group ate the ultra-processed food, they consumed an extra 500 calories. 

Their conclusion is that not all calories are equal because some foods have higher satiety, and people naturally eat less.

Yes. But if "calories in, calories out" doesn't matter, why does it matter if they eat 500 more calories? Of course, calories matter.

Calories In, Calories Out DOES NOT Say Quality Doesn't Matter

Nowhere does "calories in, calories out" state that the quality of food doesn't matter. In fact, it makes no comments about the diet except how the sum of calories affects our weight.

This would be like saying, "Investing isn't a good way to increase wealth" because some investments consistently provide good returns while others don't.

This "proof" comes from conflating CICO with other principles that CICO does not claim to solve. In fact, there are numerous people who have lost weight only by eating fast food to prove controlling calories is what determines weight gain.⁶⁻⁷

Now, we are 100% not suggesting the only thing that matters is calories. However, poor quality ultimately causes more weight gain as you tend to eat more of it. If you can control the calories, this won't be an issue.

Different Food Sources Are Metabolized Differently

Similar to food quality, people will say that not all calories are equal because your body will metabolize them differently.

This is 100% true. As discussed above, TEF is the energy your body takes to process food. For example, let's look at the breakdown of two people's diets.

• 35% Protein/ 30% Fat/ 35% Carbs
• 25% Protein/ 50% Fat/ 25% Carbs

Even if both people ate the same total calories, the first person will burn more calories as their diet produces a higher TEF.

However, this doesn't prove anything. 

• TEF is already calculated within your TDEE. This isn't a forgotten variable. While it can be different, a person just needs to adjust.
• TEF existed 100 years ago when people lost weight counting calories when TEF wasn't known.
• At the end of the day, TEF is still burning calories, which adds to the "calories out" of the equation.

Different Foods Affect Our Hormones Differently

Similar to how our body metabolizes different foods differently, they can also affect our hormones differently.

Here are some of the major interactions noted;

• Carbs can cause spikes in glucose, which triggers insulin release. When this happens frequently, it can lead to insulin resistance.
• Carbs can cause crashes after sugar spikes, which may lead to cravings for more sweets.
• Carbs can increase levels of ghrelin, which increases hunger.

As you see, most are directed towards carbs. Regardless, the last two still note the problem is that it can lead to eating more. So again, calories do matter, as do food sources.

Concerning insulin resistance, while this is obviously true, the amount of sugar spikes required to cause insulin resistance indicates a person is eating a surplus of calories, specifically from processed foods.

When calories are controlled, and assuming you're not getting 90% of your diet from table sugar, carbs won't magically give you insulin resistance.

Calories In, Calories Out Doesn't Work Because Labels Can Be Wrong

Another claim many protestors will make is that "calories in, calories out" doesn't work because nutrition labels don't always label the correct nutritional information on products.

For example, an article from MedicalExpress states that a 20% discrepancy is allowed when labeling nutrients.⁸⁻⁹ This is true, and all countries have similar guidelines.

However, using this logic, we would have to conclude that protein doesn't support muscle mass because those labels are off. And what about all the vitamins and minerals? Does that mean our body doesn't process them normally because the labels are off?

Calories In, Calories Out Is Not Calorie Counting

Remember, "calories in, calories out" simply describes the general mechanism by which your body processes energy. The policies of governing nutritional agencies don't change this. 

And why is this a problem? 

Because people don't know how many calories they're consuming because calories in, calories out is the primary mechanism by which we lose weight. 

All this proves is that calorie counting isn't always accurate if you take the nutritional labels as 100% accurate and don't monitor your weight.

This is why you always need to monitor your weight loss and make adjustments, especially in the beginning.

Adaptive Thermogenesis 

Yet another claim suggests that the concept of CICO isn't realistic because your body's metabolism will slow when in a caloric deficit over time.

This is a process known as "adaptive thermogenesis."

Specifically, adaptive thermogenesis occurs when a person's resting energy expenditure (REE) lowers. Ultimately, this actually proves CICO as a decrease in REE is a decrease in calories out.

When an excessive amount of calories needs to be lost, successful dieters combat this by cycling their caloric deficit. This might look like;

• A person gets into a 300-500 caloric deficit for 4 weeks
• In the fifth week, they will bring their calorie backup to maintenance levels
• This allows their REE to return
• They then repeat

Studies have shown that this is a very effective method for long-term fat loss.¹⁰ And notice how this is done? By counting calories and entering a deficit.

What this proves is that weight loss is not a straight drop. It's a process that can sometimes require us to manipulate the body. However, ultimately, this is all done so we can create a caloric deficit.

We can compare this to weight lifting when we take recovery days or deload weeks. It's also common to use periodization to see consistent increases in mass and strength. However, no one says that lifting weights doesn't work because it's not consistent.

Weight Rebound

It's not uncommon for people to regain weight after losing it. While this happens, I challenge you to explain how it happens.

Almost certainly, it's because they begin eating more calories.

You can probably see the pattern by now. None of these disprove CICO; they merely show that controlling calories can be difficult in modern times.

The Problem With Calories In, Calories Out

As we saw above, all common arguments against CICO ultimately argue against other mechanisms. Or, they just show that weight loss is not a straight trajectory.

Introduction Of Processed And Ultra-Processed Foods

The biggest factor that has made this more complicated is the introduction of ultra-processed foods.

When calorie counting was born in the late 1800s and early 1900s, our diets consisted of healthy, whole foods. Of course, there were some "sweets," but nothing compared to what we eat today,

Back then, people didn't need to worry as much about many of the issues raised in the arguments. Their foods were naturally filling and nutritious compared to our diet today. 

What To Consider In Addition To Calories In, Calories Out

So, considering that the arguments definitely have validity. We now have more to consider than calories.

1. Macronutrients Of A Food. One way this has evolved is that we now pay closer attention to the macros (protein, fat, carbs) of a meal. In fact, this is how most people count their calories now—they have a predetermined amount of each macro they want to eat and design their meals around that.

2. Quality Of Food. As we went over above, food quality makes a huge difference. It can affect your satiety but also improve energy levels (more movement equals more calories burned), sleep, and overall health. We recommend following the 80/20 diet, which is a basic form of flexible dieting that suggests you get 80% of your diet from whole foods

3. Personal Factors. We're all different and have various issues that can affect losing weight. This can be health issues such as diabetes or even mental health issues such as high levels of anxiety. At the same time, you may live somewhere that lacks sufficient healthy food options.

4. Following a Proper Weight Loss Plan. Another important factor is following a proper program. This includes using a proper deficit but should also include resistance training and cardio. Collectively, these can help optimize your weight loss while mitigating many of the issues we spoke about above. 

One Last Thing To Consider

Here's one last thing to consider when someone says that "calories in, calories out" is a myth. 

The next time you hear this, ask them, "Then how do I lose weight? Should I eat more calories?"

More importantly, how have people lost weight by counting calories for the past century?

Remember that CICO has been successful for over a hundred years.

When we look at interviews of old-school bodybuilders like Arnold Schwarzenegger, we see they didn't count calories. However, during the off-season, they'd eat more, and during the in-season, they'd eat less.

Even though they didn't necessarily follow calories in, calories out precisely, they still used the same general idea.

Calories In, Calories Out: Final Verdict 

Calories in, calories out works.

Much of the confusion seems to be derived from semantics and what they mean when they say calories in, calories out. The problem is that people have misinterpreted this principle as if it's supposed to explain every facet of weight loss. 

It's not; it's simply the overarching mechanism that controls weight loss.

Of course, there are individual circumstances that one needs to account for.

Put it like this. Imagine if someone speaks to a group of people and says they need to get up early if they want to see the sunrise on the beach. However, different people chime in;

• That's impossible because I didn't get home until 3am after the night shift!
• That won't work because my car just broke down!
• I don't have an alarm clock!
• I have night blindness and can't drive!
• Awesome, I live on the beach and love waking up at 5am!

As you see, everyone has different circumstances that make seeing a beach sunrise more or less easy. Some need to drive farther. Some need help with transportation. For some, it's extremely easy.

While all of those are valid, some more than others, none negate the fact that if they want to see a sunrise on the beach, they must wake up early to drive there.

The issue is we live in a society that wants a simple answer without discussing the nuance and this doesn't always work. Calories in, calories out is not meant to be the only rule in nutrition but rather the basis by which our understanding of diets are built from. 

As we learn more about the body, we will likely need to make even more considerations. However, these likely won't disprove calories in, calories out but rather just build upon it as our understanding evolves.

References

1. Stacey M. This doctor pioneered counting calories a century ago, and we're still dealing with the consequences. Smithsonian Magazine. https://www.smithsonianmag.com/science-nature/doctor-pioneered-counting-calories-century-ago-were-still-dealing-with-consequences-180984282/. Published May 14, 2024.
2. Diet and health; with key to the calories. Published March 6, 2025. https://www.gutenberg.org/cache/epub/15069/pg15069-images.html
3. Hargrove JL. History of the calorie in nutrition. J Nutr. 2006;136(12):2957-2961. doi:10.1093/jn/136.12.2957 https://pubmed.ncbi.nlm.nih.gov/17116702/
4. Harvard Health. Stop counting calories. Harvard Health. Published October 1, 2020. https://www.health.harvard.edu/staying-healthy/stop-counting-calories
5. Hall KD, Ayuketah A, Brychta R, et al. Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomized Controlled Trial of Ad Libitum Food Intake [published correction appears in Cell Metab. 2019 Jul 2;30(1):226. doi: 10.1016/j.cmet.2019.05.020.] https://pubmed.ncbi.nlm.nih.gov/31105044/
6. Quinn D. Man Who Ate Only McDonald's for 100 Days Completes Challenge — and Drops 58½ Lbs.: 'I Feel Amazing' People.com. https://people.com/kevin-maginnis-mcdonalds-100-days-results-7506887. Published June 1, 2023.
7. MSN.  https://www.msn.com/en-us/health/other/man-loses-132-pounds-by-eating-this-1-chick-fil-a-menu-item-every-day/ar-AA1C4s2v?ocid=BingNewsSerp
8. Fuller N. It's time to bust the "calories in, calories out" weight-loss myth. MedicalXpress. https://medicalxpress.com/news/2023-06-calories-weight-loss-myth.html. Published June 6, 2023.
9. Fabiansson SU. Precision in nutritional information declarations on food labels in Australia. Asia Pac J Clin Nutr. 2006;15(4):451-458. https://pubmed.ncbi.nlm.nih.gov/17077059/
10. Varady KA. Intermittent versus daily calorie restriction: which diet regimen is more effective for weight loss?. Obes Rev. 2011;12(7):e593-e601. doi:10.1111/j.1467-789X.2011.00873.x https://pubmed.ncbi.nlm.nih.gov/21410865/