I recently saw a discussion online about lab-grown meat (cultured muscle cells in a growth medium), in which someone was describing the potential problems with it—which boil down to it probably has a different nutrient profile and the assumption that it is equivalent to meat from a living animal is completely unjustified. Animals are phenomenally, astonishingly complex and respond in a myriad of ways to their environment. Meat grown from cells cultured in a lab would be grown under radically different conditions from meat on an actual animal, so we know that it will be different. There will be different quantities of micro-nutrients and probably of macro-nutrients, too. (Protein is a catch-all term; there are actually nine essential amino acids which we have to get from our diet, and different proteins have very different quantities of the 26 amino acids which make up proteins.) He complained that people (scientists) often regard the human body as if it was something simple, like a car, with food being a simple fuel, like gasoline, where you just put in gasoline into the car and it goes, instead of the astonishingly complex thing that it is with significant second- and third-order effects from the complexities of the food that we eat.
It struck me as funny that this overly simplistic model of cars doesn’t even work for cars. You will not get the same gas mileage from low, medium, and high octane fuels, despite them having the same energy. (The octane rating refers to the average length of the hydrocarbon chains; high octane gasoline has longer average hydrocarbon chains.) You also will not get the same acceleration from the different fuels; an engine designed for high octane gasoline will produce less power with low-octane gasoline; the reverse can be true as well. (The reason for high-octane gasoline is that it can tolerate higher compression ratios than low-octane gasoline can and so more powerful engines can take advantage of this to produce more power. In an engine with a lower compression ratio, it doesn’t matter that the higher octane gasoline can tolerate more compression because it won’t get it.)
And heaven help you if you put diesel fuel into your gasoline engine, despite it also being a long-chain liquid hydrocarbon with only slightly more energy per gallon than gasoline.
In other words, “a calorie is a calorie” doesn’t even work with cars and the fuels you can buy at the gas station.
It should be noted that there is a domain where “a calorie is a calorie,” assuming that one limits this to the sensible macronutrients such as glucose, protein, and plant and animal fats. In very minor cases does it work to consume large quantities of ethanol, despite it technically containing calories that we can extract. In no cases does it work to drink gasoline or diesel fuel despite them having plenty of calories in them. Those (I hope, obvious) caveats aside, a calorie is a calorie when you are trying to fuel hard labor.
Suppose you have a person who you want to swim for eight hours a day. Swimming can consume around 900 Calories per hours (depending very much on body size, etc), so that will take 7,200 Calories per day in addition to base metabolic needs. If we assume that the base caloric requirement is 2,000 Calories per day, then the swimmer will be consuming 9,200 Calories per day. That is, if we feed the swimmer fewer than 9,200 Calories, the swimmer will be in a Caloric deficit and will eventually starve to death. (More realistically, their ability to do work will go down and we won’t get the full 8 hours a day of swimming out of them.) When it comes to “will this person have enough energy to do the work we want them to do,” it is indeed true that “a calorie is a calorie.” They may or may not be productive of overall health, but when it comes to the question of energy balance for work performed, this is pretty accurate. This doesn’t come up much in life (unless you’re running a sports team or prison chain gang), but this is the context where it’s true.