Good walkthrough of the history of calorie in the context of the insulin model.
summerizer
Origins: heat, respiration, and measurement
- Lavasier linked respiration with slow combustion and focused on measurable heat output.
- Ice-melt calorimetry produced repeatable heat numbers; the calorimeter measured heat, not hunger.
- 1800s calorimetry improvements enabled fuel comparison and efficiency accounting.
Late-1800s shift: apply combustion measurement to food
- Population growth and industrial scaling increased demand for standardization and measurement.
- Atwater burned food in a bomb calorimeter; released heat became the food “energy” number.
- A key assumption followed: the body handles food energy roughly like combustion heat.
Where the calorie model omits biology
- Digestion differs from sealed-chamber burning; humans are open systems with variable absorption.
- Digestion costs energy, and hormones route energy to different fates; the bomb calorimeter omits this.
- A single number can reduce attention to context once it exists.
Why the calorie spread
- The USDA used calories for efficiency: feeding large groups cheaply and reliably, optimizing output per dollar/pound/shipment.
- Calories enabled food swapping on paper and scaled ration planning, institutional feeding, and policy metrics.
- Once embedded in reports and education, calories felt official; labels and regulation made them ubiquitous.
Incentives and downstream behavior
- One number enabled food arithmetic: eat less, burn more, track the count.
- Low-calorie options can gain default preference when taste is high; this rewards “tastier with fewer calories” products.
Equal calories, different outcomes example
- 100 calories of steak and 100 calories of chocolate match on paper but differ in-body.
- Steak: higher digestive cost, stronger satiety signaling, minimal insulin response, hunger quieter for hours.
- Chocolate: rapid absorption, blood sugar and insulin spike, brief satiety, hunger returns quickly.
Thermodynamics and system condition
- Energy conservation constrains what can happen, while complex systems vary within those constraints.
- “Calories-only” fat loss is like “speed-only” flight: speed matters, but lift, wind, and control determine takeoff and stability.
- System condition changes outcomes; chronic inflammation narrows margins, reduces efficiency, and alters handling of identical inputs.
Bottom line
- The calorie persists because it standardizes the unstandardizable and supports administration and marketing.
- A calorie measures heat energy in food; it does not determine what the body will do with that food.
I thought I’d write my thoughts about this down someplace: on various other communities the first law of thermodynamics is often implicitly cited as the reason why calories are relevant to a discussion about what food we eat, in the form of calories in calories out. Here’s the first law of thermodynamics:
Stated alternately, the change in internal energy of a system equals the net heat transfer into the system minus the net work done by the system. There are several reasons this doesn’t apply to human beings when we think about loss of mass:
The most obvious one is that we aren’t isolated systems. The human body is an open system that constantly interacts with its environment, such as when we breathe. So measuring calories is unlikely to be accurate or even useful at all. In ketosis, the body exhales some ketones as waste, which again aren’t measured in any dietary consideration of caloric intake/usage.
The calorie is a unit of heat energy, and if net heat transfer mattered at all in terms of us gaining/losing mass even if we were to consider ourselves as a closed system, then sitting in a sauna would result in a tremendous amount of net transfer of heat energy into the body. If calories mattered, then we should then gain mass. But going in a sauna doesn’t make us put on any weight at all, rather it’s quite the opposite. Yes, this was a sauna thought.
Related to 2: heat energy has no mass, so we can’t eat it. A calorie is a way of thinking about things and how their temperature is affected by other things. Nutrition is all about about mass and biochemical processes, and less so about temperature, although ambient temperature does affect the human body and metabolism!
When measuring the caloric energy in foods, there is a margin of error of 20-25%. In other words, it’s not possible to determine accurately how much you’re eating from the measurement, which additionally is done by the food manufacturers. So it’s pretty pointless anyway.
Consuming an iso-caloric amount of food results in different biological outcomes that are affected by, among other things, time of day, the the type of the food consumed (caloric “quality”), and even order of macronutrients consumed (e.g. eating carbs/sugars first results in a higher blood glucose spike than if they were consumed last). If calories in calories out was true, none of this would matter.
Well said!