The Calories-in, Calories Out Myth

If you’ve ever tried to change your body composition, you’ve probably heard the same old advice: eat less and move more to lose weight, eat more to gain weight. Calories in, Calories out (CICO). It’s on every nutrition label, every fitness app, repeated by many fitness coaches and health care practitioners. It feels like settled science: something so obvious it doesn’t even need explaining. If you intake more energy than you use, you gain weight. If you use more energy than you intake, you lose weight. Simple math.

What if I were to tell you there was more to the story? What if I were to argue that this model is not just an oversimplification, but both an unscientific explanation and not a useful heuristic for changing one’s body composition? Well before I sound outright crazy for challenging this well accepted idea, I first need to paint this argument in its strongest light because if we’re going to challenge an idea this deeply embedded, we owe it the courtesy of properly understanding it first.

The Case for CICO

On the surface this logic seems airtight. Food is comprised of macromolecules: fats, proteins, and carbohydrates. When burned in a bomb-calorimeter, each of these produce a measurable amount of heat energy defined as calories. This reflects the “Calories in” side of the equation. Your body meanwhile requires energy to breath, digest, think and exercise. This is the “calories out” side of the equation. If you consume more energy than your body requires, the excess gets stored as body fat. Consume less, and your body makes up the difference by using some of your fat reserves. 

This logic seems to line up with how we evolved. For most of human history, food was scarce. A body that could efficiently store energy during brief periods of excess would be more likely to survive the extended periods of scarcity. The problem, according to this framework, is that we now live in environment where food access is abundant, and sedentary living is the normal. Too many calories in, not enough calories out. If only we we balanced out this equation, we would solve the obesity epidemic and associated ailments.

Key Problems with CICO

1. Humans are not Bomb Calorimeters

The human body does not burn food for fuel, nor does it run on heat energy. Our cells breakdown some of the macronutrients in food to make ATP, which is then used by cells. ATP is the energy currency of the body, not calories, and the amount of ATP generated is heavily dependent on various other inputs. To describe the amount of “calories” that are theoretically contained in a given food as a substitute for the amount of ATP that the body generates as part of a tightly regulated metabolic process is fundamentally misleading.

Furthermore, a certain proportion of the macronutrients never get used to make ATP at all. Protein is primarily used to rebuild body tissues, fats can be used for our cell membranes, and fibre is indigestible by humans and doesn’t contribute to a meaningful amount of ATP besides for a small portion of fibre that is fermented by gut bacteria. Yet all of these are counted as “Calories in”.

Even the macronutrients that do get used to make ATP don’t follow the simple path that CICO implies. Excess fats and carbohydrates beyond what the body can metabolize are not converted into ATP before being stored. Instead they are converted into triglycerides, the body’s fat storage molecule. They are only converted to ATP when the body is ready to use it. So the notion of excessive “Energy in” promoting fat storage is inaccurate, as the molecules that comprise adipose tissue are never energy to begin with. This process is rather the interchange of different forms of mass.

In a nutshell, the human body doesn’t consume or use calories. Using calories as a metric is an incorrect unit that doesn’t convert in any useful way to the correct one.

2. The Calories Out Side of the Equation is not Fixed

In order to assess this side of the equation, CICO operates under the assumption that each person has a somewhat fixed basal metabolic rate, meaning the rate at which we utilize fuel to function on a regular basis, which in conjunction with that day’s exercise demands would add up to form the total energy expenditure of the day. The problem? Well the foods we eat have significant effects on our body’s metabolism, heavily influencing the so called “Calories out” side of the equation. Here are some examples:

Nutrient density- B vitamins, magnesium, CoQ10 are all required nutrients for proper functioning of the electron transport chain, a key step in generating ATP. More nutrient dense foods lead to more efficient energy production and a higher metabolic rate. This means that two diets with identical calorie counts can produce meaningfully different outcomes without any change in exercise.^[1][2]

Deuterium- Foods higher in deuterium slow down the rate at which your mitochondrial motor functions. Therefore foods equal in so-called “calories” with varying levels of deuterium produce energy at different rates and have different results.^[3]

The Randle Cycle- Eating a diet rich in both fats and carbohydrates results in substrate competition that cross-inhibits the opposing substrate’s energy pathway slowing overall energy production. The mix of what you eat changes how efficiently you can utilize it for fuel, and how likely you are to store excess.^[4]

Mitochondrial Uncoupling- In certain metabolic states, such as ketosis and cold-exposure, mitochondria dissipate energy as heat, rather than produce more ATP. More energy has been consumed without any additional storage, meaning the “calories out” number has just shifted with no way of properly accounting for it.^[5]

Thyroid down-regulation- Chronically under-eating can suppress T3 production lowering the body’s basal metabolic rate. The body senses that you have been chronically under-eating and adapts to the deficit as an evolved protective mechanism to the perceived scarcity. This is why suggesting that individuals just simply “eat less and move more” often results in stalling and rebounding.^[6]