Well, a calorie (note small 'c' ) is the amount of energy needed to raise 1 gram of water by 1°C. This is a tiny amount of energy, so dieticians use big 'C' calories, which is the amount of energy needed to raise 1 kilogram of water by 1°C.
As Robin said Calories are measured in a Bomb Calorimeter. This consists of a sealed vessel of known weight and material, within which the foodstuff is burned in an excess of Oxygen. The before and after ignition temperatures allow the energy released by combustion to be measured accurately.
Obviously this is not how animal digestive systems work, but the calorimeter gives the maximum number of Calories available if the animal could recover all of them. Species have adapted to recover energy from different foodstuffs. Cow stomachs have four compartments which allow them to digest grass, mostly cellulose, which humans cannot digest at all – it passes straight through. Cows can't fully digest herbage either, which is why cow poo can be dried and burnt.
Although human digestive conversion efficiency is difficult to measure accurately, it's well known that we can extract almost all the energy available from certain foodstuffs, especially Sugars and Fats. As we evolved from hunter gatherers who needed a lot of energy to find food, and often went hungry, we are configured to put on weight in times of plenty, and burn it off later. This causes severe problems to modern man who mostly sits about scoffing an unlimited supply of crisps, Mars Bars, Coca Cola, Alcohol, and Steak and Chips. These are high Calorie foods in a Calorimeter and the human stomach, just the job before the winter famine starts, but we all get fat when there is no famine. Vegetables contain fewer Calories and are digested less efficiently, so we're unlikely to get fat on them. They contain trace minerals, vitamins, and roughage essential to our Biology – digestion isn't just about Calories.
Humans are unique in that we're the only species that can convert inedible food into something we can eat. Cooking, fermentation, dissolving, distillation etc. These processes are also good at removing toxins and killing bacteria. On the downside, processing often removes wanted minerals, roughage and vitamins as well, and it's not unusual for them to be chemically restored.
Digestion is complicated and still not fully understood. To avoid becoming a diabetic slob, it's best to cut back on high-Calorie foods, but Calories are only one indicator. More recently, British ready meals have started carrying a colour code. Trouble is I like food where Energy, Saturates (Fat), Sugar, and Salt are all code RED, and don't enjoy anything that's good for me.
Obesity has become a serious problem in the UK; the NHS is burdened with people who have made themselves ill due to poor lifestyle choices.
When I was younger, Brits used to laugh at Americans because they were all so outrageously fat – not now. Modern Brits are also packed full of sugary goodness…
Dave
Edited By SillyOldDuffer on 13/07/2023 10:23:27