Calories in = Calories out. Simple, elegant and utterly incorrect.
Bold Statement I know… Let me explain.
The calories in calories out diet philosophy (or CICO as it’s known) has its roots in 1878 when German physiologist and hygienist Max Rubner published his theoretical work on the law of conservation of energy in organisms, demonstrating that energy intake equals energy output plus storage.
This is pure First Law of Thermodynamics: “The total energy of an isolated system remains constant.”
In 1891 he started a 3 years mission, built sophisticated calorimeters (completely sealed “chambers” that could measure heat production in living animals) and published detailed experimental results that proved his earlier hypothesis.
Max Rubner’s Calorimeter Study: The Experiments From Which CICO Are Built
Rubner’s work was seen as brilliant – yet I see it as physics applied to biology.
After all, a calorie is a unit of heat measurement, not a unit of chemical energy measurement. (And yes, the body is powered by chemistry… there’s a whole ‘nother article to be written about ATP, which is the body’s basic energy chemical – mammals do not use protein, fat or glucose directly, it must be converted to ATP in the mitochondria to power us)
He put dogs in big tanks and created an environment where there were no real-world variables to contend with.
As an aside, I can’t see how the dogs didn’t get stressed out by it all. Especially given that one of those poor doggies was in there for 45 days.
Specifically:
“In a famous experiment of Rubner’s (6) in which a dog lived in a calorimeter for 45 days, heat production based on gas exchange and nitrogen excretion totalled 17,406 kcal and the heat loss measured by direct calorimetry was 17,349 kcal. The extraordinary agreement between these two values provided convincing evidence for Rubner’s “law of maintenance of energy in the animal body” as well as demonstrating the rigor of his experimetal(sic) technique. His results proved that under steady-state conditions, the energy derived from the oxidation of food, measured by indirect calorimetry, was equal to the heat generated and lost to the environment, measured by direct calorimetry.”
No exercise. No social contact. Probably no light variation to mark day from night, as a candle would introduce an uncontrollable heat variable and so in reality, the tank would probably have been pitch black.
Just breathing through tubes while someone measures how much heat you’re producing.
I shudder to think of its mental suffering yet it didn’t even get a name in the papers. Listed as “a dog” under lab equipment.
And back then, that was considered perfectly acceptable experimental practice.
Anyhoo…
Why Eating Less and Moving More Doesn’t Work: What Rubner Couldn’t Measure
When Rubner’s work got simplified and applied to human weight loss advice 60 years later, when people like Jean Mayer broke away from previous finding and popularised the “eat less, move more” mantra that ignored everything Rubner’s experiments couldn’t measure: insulin response, cortisol, leptin, thyroid function, gut microbiome, sleep, stress, and metabolic adaptation.
Rubner’s science was sound and so we then built our entire modern approach to weight management on data collected this way.
Every diet book that tells you to count calories, every “3,500 calories equals one pound” rule, every fitness app calculating your deficit… they all trace back to lonely dogs in dark sealed boxes.
Rubner also posited that the then newly discovered macronutrients (protein, fat, carbohydrate) could simply substitute for each other based on their caloric values.
He calculated that:
- 1g protein = 4.1 calories
- 1g fat = 9.3 calories
- 1g carbohydrate = 4.1 calories
These values are still used today (rounded for simplicity to 4, 9, and 4) and yet biologically we do not use all the food we consume as pure energy.
The Calories In Calories Out Myth: Why A Calorie Is NOT a Calorie
Eating and digesting takes energy in and of itself, not to mention the slice off the top that the gut microbiome takes for itself before the liver gets a sniff of what we eat.
We do not use protein as energy unless it is an emergency. Protein is exclusively used by the body in building its structures, as are around 20% of the fats we consume, which again are processes that use energy.
We have to convert 100% of the carbohydrate eaten into energy as the body isn’t made of bread or sugar, although our fat stores certainly are!
Our very biological existence creates spontaneity.
The dogmatic calories in calories out myth became the dominant paradigm in weight management advice by the 1960s-1970s.
If you believe it (and the 57 calorie difference in Rubner’s numbers are for me literal proof that CICO doesn’t hold true for living animals) you are caught in what I call the First Law trap.
The First Law Trap: Why Physics Alone Cannot Explain “Weight Loss”
We worship at the altar of the First Law while completely ignoring the Second Law – “In any spontaneous system, entropy increases”
Whilst Rubner’s experiment (almost) proved that the First Law holds for dogs in a controlled calorimeter, the very fact that we live life “in the wild” means that we are not “an isolated system”.
So, biology falls squarely under the dominion of the Second Law, and by golly, our entropy is all over the shop, depending on stress, sleep, hormones, gut health and inflammation!
And we’ve spent a century making women feel like failures when reality didn’t match the CICO myth.
Metabolic Adaptation and Entropy: What Actually Controls “Weight Loss”
Entropy is:
- Hormones that change metabolic rate.
- Stress responses that tank thyroid function.
- Inflammatory cascades that alter how we partition nutrients.
- Gut bacteria that extract different amounts of energy from the same food.
- Circadian rhythms that affect insulin sensitivity.
- Hormones deciding whether to store or burn.
- Enzymes determining how efficiently you extract energy.
- Metabolic processes shifting between pathways.
- Mitochondrial efficiency changing.
- Thermogenesis ramping up or down.
And most critically of all, we have bodies that ADAPT to input.
Why Your Metabolic Rate Slows Down When You Eat Less
Reduce intake of food containing the building blocks of the body and your metabolism slows.
And your body gets more efficient. It acts sharpish to protect you from what it perceives as famine.
It starts reducing what energy it expends (which is why you feel cold and tired if you are eating low calorie and low fat) and storing as much as possible (which is also why “weight loss is slow” because the body is fighting to hang onto everything it has!)
The elegance of “eat less, move more” completely ignores that your body has an active vote in this process.
CICO is not junk science because Rubner got his measurements wrong. And by the gods, he was utterly meticulous with his measurements… and they “prove” the first law for sure.
You’ll get no argument from me on that, even if I do think the margin of error shown gives pause for thought…
No, it’s junk science because we took his carefully controlled laboratory conditions and pretended they applied to real life.
Calories In Calories Out Myth: Why Hormones Matter More Than Heat
A calorie isn’t a calorie because the PROCESS of converting food into ATP is never the same twice.
Your body’s response is completely different at the bacterial, hormonal, enzymatic and even chronological level to 100 calories of carbs vs 100 calories of protein vs 100 calories of fat.
Same “energy” in. Wildly different entropy increase. Completely different outcome.
Rubner measured energy balance in a CONTROLLED system. Constant temperature. Constant food. No variables. Simply… (grim) existence.
But humans living our lives? We’re spontaneous systems. This amazing biological system we all have is adapting constantly.
Not being able to sustain ‘eat less, move more’ is not your problem. The model is.
Download my guide ‘When Change Feels Too Much‘ to understand what your body was actually asking for all along.
About the author: Nikola specialises in metabolic health and the biological realities that conventional weight loss advice ignores. With 26 years of experience watching the CICO model fail, she helps people understand what their bodies are actually asking for instead of forcing them into broken paradigms.
