In the world of fitness and nutrition, the acronym CICO—Calories In, Calories Out—gets thrown around a lot. Some people swear by it, while others dismiss it as overly simplistic or even “fake.” So, what’s the real deal with CICO? Does it actually work, or is it just another dieting myth? Let’s dig into the science and unravel the complexities behind this popular concept.

1. What Exactly is CICO?

CICO stands for “Calories In, Calories Out.” It’s a simple way of describing the balance between the energy you consume through food and the energy you expend through physical activity and bodily functions. The idea is straightforward: to lose weight, you need to consume fewer calories than you burn. If you consume more than you burn, you gain weight. Sounds easy, right? Well, it is and it isn’t. While the basic principle is true, there are layers of nuance that make the topic a bit more complex.

2. Are All Calories the Same for Body Composition?

Technically, yes. From a purely mathematical standpoint, a calorie is a unit of energy, whether it comes from a doughnut or a piece of broccoli. If you’re in a caloric deficit (eating fewer calories than you expend), you will lose weight. That said, it’s not that simple when it comes to body composition.

Different foods have varying effects on hunger, metabolism, and how our body processes nutrients. For example, proteins have a higher thermic effect of food (TEF) than carbohydrates or fats, meaning they require more energy to digest. Foods rich in fiber, like vegetables and fruits, help you feel fuller longer, making it easier to maintain a caloric deficit. In contrast, sugary snacks and processed foods can lead to quick energy crashes and increased hunger. So, while all calories are technically the same, how you source those calories significantly impacts your ability to stick to a diet and achieve optimal body composition.

3. The Core Truth: A Caloric Deficit = Weight Loss

Here’s the crux: if you consume fewer calories than your body needs, you will lose body mass. Period. However, some methods of eating make this easier to accomplish than others. If your diet consists mostly of vegetables, fruits, and lean proteins, you’ll likely find it easier to maintain a caloric deficit because these foods are nutrient-dense and satiating. In contrast, a diet filled with candy and junk food will likely leave you hungry and craving more, even if you’re hitting your calorie target.

But let’s clear up a common misconception: you can lose weight eating candy as long as you’re in a caloric deficit. Of course, you wouldn’t be healthy, as your body would be starved of essential nutrients. But when it comes to the simple math of weight loss, it’s still about CICO.

4. What About Slow or Fast Metabolism?

“But my metabolism is different!” you might say. Yes, everyone’s metabolic rate varies due to factors like age, genetics, activity level, and even hormonal imbalances. However, it doesn’t change the fundamental principle of CICO. Your body, regardless of its unique metabolic quirks, still runs on a certain number of calories each day. If you consistently consume fewer calories than that number, you will lose weight.

Metabolism can fluctuate based on numerous factors, but it’s not immune to the laws of physics. If you eat less than your body expends—even if that threshold is different from someone else’s—you’ll lose weight.

5. “I Tried Tracking Calories, and It Didn’t Work!”

This is a common frustration. You meticulously track your calories, but the scale doesn’t budge. Does this mean CICO is broken? Not at all. More likely, there’s an error in estimating how many calories you’re consuming or burning. Studies have shown that people often underestimate their calorie intake and overestimate their calorie expenditure.

To get a clearer picture, try weighing yourself daily and track the trend over time. Use at least a 7-day moving average to account for normal daily weight fluctuations due to factors like water retention. Most dieting apps can do this for you. The changes in your weight will reveal whether you’re truly in a caloric deficit.

6. “Keto/Carnivore/Vegan is the Only Thing That Works for Me!”

That’s great! The reason it works for you is because it helps you consume fewer calories than you expend. Different diets work for different people due to a myriad of factors, such as individual food preferences, satiety levels, and metabolic responses. Some people find it easier to eat less on a keto diet because fat and protein are more satiating. Others prefer plant-based diets because they can eat larger volumes of food while keeping calories low.

The key is finding a diet that you can sustainably adhere to while creating a caloric deficit. There is no “one-size-fits-all” approach, but all effective diets share the same underlying principle: CICO.

7. The Complexity of the Human Body and Why CICO Still Works

Yes, your body is a complex, highly regulated machine. Science hasn’t fully mapped out the intricate workings of human metabolism. Hormones, gut bacteria, stress levels, and countless other factors can influence how our bodies use energy. However, CICO doesn’t need us to understand every tiny mechanism to work.

Think of it like a machine that can multiply large numbers correctly every single time. You might not know how the internal components work, but you know it reliably provides the right result. CICO operates similarly. Even if we discover a magical food that increases your metabolism, CICO isn’t invalidated. You’ll simply adjust your understanding of your caloric expenditure: “Oh, this food makes me burn 500 extra calories daily? Great, I can eat 500 more calories now.”

The Bottom Line

CICO isn’t fake—it’s a proven model based on the simple laws of energy balance. While the human body is complex and many factors can influence how easily you can stick to a caloric deficit, the basic principle remains unchanged. Finding the right diet for you is key, but remember: whether it’s keto, veganism, or anything else, its success hinges on creating a caloric deficit.