Quick heads up—physics and chemistry were my favourite subjects in high school. I was absolutely obsessed with physics (which, to be honest, still hasn’t worn off!).
So yes, CICO—Calories In vs Calories Out—is rooted in one of the most fundamental laws of physics: the First Law of Thermodynamics, aka the Law of Energy Conservation.
The Science Behind It
The First Law of Thermodynamics states that energy cannot be created or destroyed—only converted from one form to another.
In the context of the human body:
- Calories in = energy you consume through food and drink
- Calories out = energy your body burns through metabolism, movement, digestion, heat, etc.
If you eat more energy than you burn, that surplus is stored—mainly as body fat (chemical energy).
If you eat less than you burn, your body taps into stored energy—usually body fat (and sometimes muscle) to make up the shortfall.
This is why you cannot gain fat in a true calorie deficit, and you cannot lose fat without one. It doesn’t matter if your diet is keto, vegan, paleo, carnivore, or low GI—if you’re not in an energy deficit, fat loss won’t happen. Period.
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Why the Confusion?
Most confusion around fat loss stems from these common beliefs:
- “But hormones like insulin control fat storage!”
- “But stress, sleep, and gut health affect weight!”
- “But I was in a deficit and didn’t lose weight!”
Now, all of these things influence either the ‘calories in’ or ‘calories out’ side of the equation. They impact your hunger, cravings, digestion, metabolic rate, water retention, and more. But they don’t override the law of thermodynamics.
You can’t “break” the First Law of Thermodynamics with hormones, carbs, or cortisol. Your body is a biological machine, but it still runs within the framework of physics.
What If You Think You’re in a Deficit But You’re Not?
This is a big one. Many people believe they’re in a calorie deficit but aren’t losing fat. In the case of my online fitness clients, sometimes they’ll come to me and say ‘Brad, I’m in a calorie deficit and I’m not shredding!’ My response over the past 8+ years of coaching is always… ‘no, you are not in a calorie deficit. You think you are!’
I’ve worked with thousands of people over the years through my Ultimate Body Transformation program and my online fitness coaching services. Here are some of the biggest reasons I’ve learned that stops people from shredding…
- Tracking errors – most people underreport what they eat by 20–50% (most common reason!)
- Hidden calories – sauces, oils, bites, sips, and weekend blowouts add up (when you are tracking, you must track everything inside MyNetDiary or MyFitnessPal)
- Overestimating activity – fitness trackers often overstate calorie burn (don’t eat back the calories you burn, except under very specific scenarios. One of the biggest points I give my clients! Don’t eat them back!)
- Water retention – stress, sodium, hormonal shifts can hide fat loss on the scale (this is why I tell my clients that I care about their average weekly weight change… not the day-to-day change!)
It’s entirely possible to be in a perceived deficit, but not an actual one.
CICO is not a trend. It’s thermodynamics. You can’t gain fat unless you’re in a calorie surplus, and you can’t lose fat unless you’re in a calorie deficit. That’s non-negotiable. Every single metabolic ward study confirms this. Even when macronutrients, food timing, and insulin levels are manipulated, the law of energy balance still holds true.
Related: Still stuck in the skinny-fat trap? You train hard, eat “healthy,” but still look soft instead of strong—sound familiar? Stop spinning your wheels. I’ll show you exactly how to build muscle, drop fat, and finally get the lean, athletic physique you’ve been chasing 👉 Click here to apply for online coaching today!
CICO Explained with an Aviation Analogy
This might sound a little out there, but I hold a Commercial Pilot’s Licence and fly privately. There are parallels between how a plane flies and how your body works. Think of your body like an aircraft, and fuel as your calories.
- Calories In = Fuel Loaded into the aircraft (your food and drink)
- Calories Out = Fuel Burned during flight (your metabolism, movement, training, digestion, heat production)
Now…
🔺 If you load more fuel than you burn, the excess gets stored—just like adding extra weight to the aircraft. In the body, this “extra weight” is stored as body fat.
🔻 If you burn more fuel than you load, the aircraft pulls from its reserve tanks—in the body, that’s your stored fat. You’re still flying, just tapping into backup energy.
No matter what, the plane follows the laws of physics (I won’t bore you with aerodynamics—one of my favourite subject areas in aviation!)
You can’t magically fly further than your fuel allows, and you can’t burn more energy than you have onboard without consequences (i.e., an emergency landing… or in human terms, fatigue, metabolic slowdown, or muscle breakdown if pushed too far for too long). Some people think hormones, carbs, or specific diets “override” this equation. But that’s like saying the type of fuel or weather conditions somehow cancel out gravity or the lift equation (L= CL x 1/2ρ x V^2 x S).
In reality, things like:
- Hormones
- Sleep
- Stress
- Food quality
…they all affect how efficiently you burn fuel, how much drag you’re creating, or how smoothly you’re flying—but they don’t change the laws of aerodynamics. The energy balance equation still governs the flight.
I hope you liked my little analogy 🙂
Whole Foods Are Great—But They Don’t Bypass Energy Balance
Whole foods are brilliant for fat loss:
- Higher in fibre and protein
- More filling
- Less processed
- Require more energy to digest (higher TEF)
But they still contain calories. Overeat on whole foods, and you’ll still gain fat. Clean eating isn’t a free pass. You can gain weight eating brown rice, chicken, avocado, and olive oil if you’re in a surplus. This was exactly one of the main reasons why I was stuck in a rut because I was eating predominantly raw vegan cheesecakes, raw cocoa smoothies, and protein balls, wondering why I wasn’t shredding.
The Role of Hormones—Partners, Not Opponents
Hormones absolutely matter—but they don’t cancel out CICO.
- Insulin helps store nutrients but doesn’t cause fat gain unless there’s excess energy
- Cortisol can increase water retention and cravings
- Thyroid hormones impact BMR (your base calorie burn)
Think of hormones like traffic lights. They influence how fast or slow your metabolism and hunger signals move—but they don’t add or remove cars (calories) from the road.
Starvation Mode Is a Myth
If you eat too little for too long, your body adapts—it burns fewer calories (adaptive thermogenesis). You feel tired, move less, and metabolism slows. But fat loss still occurs in a deficit. Even in extreme conditions (think famine victims or those with anorexia), weight is lost. The idea that your body “stops” burning fat because you’re eating too little is a myth.
It’s just that pushing too hard for too long makes the process miserable, not impossible.
CICO Is Simple, But Not Easy
This is the nuance:
- Calories in isn’t just what you eat—it’s affected by digestion, gut health, and food type
- Calories out includes your BMR, NEAT, TEF, exercise, subconscious movement—and it changes as you lose weight
So yes, hormones, food quality, and sleep all matter—but they influence calorie balance. They don’t override it.
Mindset, Habits & Environment Still Matter
You can’t out-science a bad lifestyle. Emotional eating, sleep deprivation, stress, alcohol, and poor food environments all increase your likelihood of consuming more calories without realising. That’s why sustainable fat loss isn’t just about maths—it’s about making the numbers easier to stick to.
Final Thoughts
At the end of the day, CICO is always true. But how you apply it is what separates frustration from results. If you’re constantly hungry, exhausted, or stuck—it doesn’t mean CICO is broken. It means your approach needs adjusting. Just like my approach many years ago… I had to change my mindset and approach to tackle fat loss and muscle building from a science-based point of view.
That’s why I coach people online to:
- Create sustainable calorie deficits
- Eat high-protein diets
- Lift weights to preserve muscle
- Prioritise sleep and stress management
- Build habits that make fat loss automatic
Because fat loss doesn’t need to be extreme—it needs to be consistent.
Related: Still stuck in the skinny-fat trap? You train hard, eat “healthy,” but still look soft instead of strong—sound familiar? Stop spinning your wheels. I’ll show you exactly how to build muscle, drop fat, and finally get the lean, athletic physique you’ve been chasing 👉 Click here to apply for online coaching today!
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