The calorie is a unit of energy, in particular heat.
The small calorie, gram calorie, or calorie (symbol: cal) is the amount of heat (energy) required to raise the temperature of one gram of water by 1 °C.
The large calorie, kilogram calorie, kilocalorie (symbol: kcal), or Calorie (capital C) is the amount of heat (energy) needed to increase the temperature of one kg of water by 1 °C, exactly 1000 small calories, or about 4.184 kJ.
The second form is the one commonly used to express food energy. Its most common name is calorie; kilocalorie is sometimes used, more often in the symbol “kcal” than in the spelled out word.
Calorie counts for foods are done in two ways:
1) Using the established approximation of 4-4-9, i.e. 4 calories per gram of carbohydrates or protein and 9 calories per gram for fats.
2) Using the USDA database that has calorie values for a large variety of foods based on actual measurements, or the so called Atwater Conversion Factors. (Most online calorie calculators use the USDA database.)
The two methods give different results, and the food manufacturer doesn’t have to state which one they used. How much would the difference be? I don’t know. Perhaps as much as 10% depending on what the ingredients are?
Calories are typically measured with a bomb calorimeter. This method and the foundation for the USDA database were developed by
Atwater used a bomb calorimeter. A bomb calorimeter is measuring the energy (heat) created by a food item when it is burned. We then assume that this amount of energy is representative for how our digestive system extracts energy out of the same food item.
A simple bomb calorimeter by Sciencebuddies
One question about energy/metabolism I have not been able to find an answer to is how much of what we eat is actually used by the body. There have been studies made that show that the body is a very efficient machine and that little goes to waste. However, these studies have been made in an “underfed” situation where the food supply was below the body’s energy needs. What would happen in an “overfed” situation? If I had a meal with 5000 cals? Would I instantly gain 1 pound of fat?
The theory is that for people that do not gain weight, a 5000 calorie meal would be compensated by increased metabolism so that they burn all these calories. You would think the energy produced in this process would make their skin hot to the touch! Could it be that the body instead just extracts the energy it thinks it needs from the food and discards the rest? To me, this seems a much more plausible explanation but I can find no source that entertains the idea. There are “set point” theories, i.e. that the body tends to find a comfortable weight and stays there regardless of how much or how little we eat. The set point theory is still assuming increased/decreased metabolism though.
To lose weight, the available energy from food needs to be less that what the body needs. We are told that to lose 1 pound of fat, we need to eat 3,500 calories less that what we burn. However, all the weight loss studies I have seen fail to confirm this stated “truth”. Long term weight loss is always less than what the theoretical value dictates. Why is that? Calorie count wrong? Food composition plays a role? Metabolic rate incorrect? Why are they never interested to find out the reason for this discrepancy in the studies?
Weight loss continues to be pure guesswork. Recommendations based on 100 year old science. Trial and error. Individual adjustments required. Is it surprising that so many dieters fail? Just give up due to frustration due to lack of understanding of why our efforts are not producing results?
If the medical community would provide us with tools to lose weight in a safe and healthy manner there would be no need for drastic measures like Kimkins. Kimkins appeal is that starvation works. For a short period of time. They just don’t tell you that you are bound to regain the weight and may have developed medical problems from it. Slow and steady is the way to go with weight loss. We didn’t gain weight overnight and shouldn’t expect to lose it overnight either.