Eli5 So we need calories to survive. If there are 100 calories in a spoon of oil, how come we can’t survive on oil for emergencies?
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Fun fact, arctic explorers bring lots of butter with them to eat because they have one of the highest calories to weight ratio.
https://abcnews.go.com/Technology/GlobalWarming/story?id=4853093
A small correction.
We don’t need calories to survive. Calories are just an (outdated) measurement for consumed energy. We don’t consume food (just) for energy. We consume it for nutrients, such as vitamins, minerals, fibres, proteins and carbs. These things aren’t just responsible for providing us with energy. They are needed for our bodies to function, develop and maintain itself properly.
You can google what each of the nutrient categories and subcategories are needed for, but long story short, the nutrients maintain your health
Everyone is talking about nutrition, but I am pretty sure drinking 2,000 calories of oil, or even just 1,500, would act as a pretty extreme laxative, resulting in you becoming dangerously dehydrated long before malnutrition became a concern. Remember the rule of 3, you can survive 3 minutes without air, 3 days without water, and 3 weeks without food. Water is a far more pressing concern, don’t mess with your ability to absorb it.
Your body also needs oxygen to survive, but you cannot breathe water (H2O). Its about your body’s ability to access that oxygen, or in your case the calories.
Calories are a measure of energy. Think of it more of a physics term than a biology one. If we burn a substance, it gives off heat. For most things it gives off more heat than we put into it. 100 calories worth of oil will give off the same amount of heat as 100 calories of biscuits*.
The problem is, your body doesn’t literally set fire to its fuel. It uses biological processes to break down the molecules into smaller parts, which also happens to give off energy.
*the unit commonly used for food is kilo calorie (kCal), which is actually 1,000 calories.
Others have mentioned micronutrients, amino acids etc. I just want to add one more important point about sugar:
Sugar is fairly important for the body’s functioning. It is possible to survive without it, but this can cause problems.
The body can get sugar in several ways. It can be eaten. Starches can be broken down for it. A reserve of sugar in the liver can be broken down. If there are no sugar or starches in the diet, then the body can use the *energy* from oils combined with the *carbon* from proteins to make sugar. If the body is only getting oils, it uses muscle mass to get the carbon. When the body runs of of muscle mass that it can use, then it can no longer make sugar even with lots of oils.
The not ELI5 explanation for this is that fatty acids are oxidized into 2-carbon acetyl thioesters which cannot be turned into pyruvate without first going into the Krebs cycle. Here they must be converted to malate first which loses 2 carbons for every 2 carbons that goes into the cycle. Gluconeogenesis from only fatty acid substrates depletes Krebs intermediates.
Now the body can still survive without the sugar, but it must make toxic acids and ketones in order to keep the brain alive. There is some debate and controversy about how harmful this is.
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