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?

You could for a short time. But we need more than calories to survive. We need nutrients. Things like vitamins and small amounts of minerals. Our digestive systems also need fiber.

If we don’t get the right nutrients in the right amounts, we can get serious medical conditions, and even die.

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You can survive on oil in short term emergencies.

A healthy diet is defined by the state of New Zealand (for an assumption of a 2000 calorie/day base metabolic rate) as 50 grams of protein, 70 grams of fat, 24 grams of saturated fatty acids, 310 grams of carbohydrates, 90 grams of sugar, 2.3 grams of sodium, and 30 grams of dietary fiber

You also need a lot of different types of vitamins that your body just doesn’t produce on its own

All of that is stuff thar you will just get eating normal food in regular portion sizes, and at least most of that stuff you’re not going to go through negative consequences immediately if you don’t have them.

But, if you just start eating 2000 calories of something without all of that stuff, you will start to experience malnutrition as your body will start to cannibalize less essential parts of itself in order to get the resources that vital systems need to keep you alive

A good example you might be familiar with is Scurvy

Your body doesn’t produce Vitamin C, Vitamin C is a building block in collagen among other things, and is necessary for your intestines to absorb iron.

So, in order to continue getting the iron you need to make blood, when your body is running on pure empty, it starts to break down your hair, your gums, and your scars and skin, all of which are made out of substances containing Vitamin C

Because your skin is much thinner you get cuts more easily, because you don’t have the ability to make new scar tissue (on account of that needing Vitamin C) you just keep bleeding

Because your gums are thinner those start bleeding and the stuff holding your teeth in are a lot less good at their jobs so your teeth fall out

Eventually you just get so many wounds or infections from the open wounds that you die

Directly caused by malnutrition

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

You can easily survive on just oil or simply not eating in emergencies.

The most imminent need is water. If that’s covered you can last for several weeks without trouble, as long as you weigh enough.

This time really cannot be extended much by solely consuming oil, because it‘s the lack of vitamins and minerals that is actually causing you to die.

Our bodies of healthy, and of average weight store quite a large amount of fat that would already be used if you are starving. Doesn‘t really matter whether you fat storage gets used or the oil you drink.

But there‘s vitamins that we only have stores for a couple of weeks, same with minerals. You are constantly losing salt through sweat and urine. So eventually your electrolytes won‘t work out anymore if you just have tap/well water to drink.

But on the other hand rhere have been experiments done with obese people, they weren’t given any calories. Just plain solutions of vitamins and minerals in water. And they weren‘t at all harmed after several months of not eating any calories.

They simply continuously lost weight by their fat storages being burned.

However this cannot be extended undefinetely without adding protein, because our bodies aren‘t perfect at recycling protein, so eventually you need to also supply a minimum amount of protein.

Usually calories are by far your least concern in an emergency. It‘s lack of minerals/electrolytes and vitamins that kills you first, before lack of calories; unless you are already starving weight before.