My understanding is that any carbs that aren’t used for energy are converted and stored as fat. Hypothetically, could a human just survive off of protein and a lot of carbs?
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Fat has two basic uses for the body: energy storage and ingredient for stuff, like hormones. The second use can’t be fulfilled by stored fat, it must be dietary (eaten) fat.
And no, you can’t live without fat, there’s even a thing caused “rabbit starvation”, which is literally people starving to death because they only ate rabbit meat, which is VERY lean, for a long period of time.
The thing is, there are tons of different kinds of potential molecular arrangements for fat. Fatty acids can be small and short, with only 4 carbons (butaric acid) and as many as 18 carbons long (oleic acid). There can also be double bonds between any of these carbons, making it a monounsaturated fat, or multiple double bonds making it a polyunsaturated fat. While you can make a few types of fats, your body can’t actually produce every single kind of fat that you need. Fats are an essential cellular building block, with all cellular membranes being made from fats. This is what allows our cells to have some give, and not be stiff and rigin like plants. Basically, fats allow you to walk!!
Body fat is not the same as dietary fat even though they are both called fat.
If you consume more calories than you burn, the excess consumption is converted and stored as body fat. No matter what you eat, if it is excess calories it will get converted and stored as body fat although the conversion process is slightly different based on the macro.
Dietary fats are essential to give your body energy, support body functions, protect your organs and produce important hormones.
Fat isn’t just used for energy, it’s important in a range of functions e.g. hormonal regulation, vitamin absorption. But these require essential fatty acids, which the body can’t make itself. So, we need to consume a minimum amount of fat from external resources (i.e. dietary fat) to supply these.
There are multiple kinds of fat. We need many of them. Our bodies can convert carbohydrates into some fats, but not all. We also cannot convert every type of fat into every other type of fat. So, we need to consume fats to get the ones we can’t make ourselves. And it can be more efficient to eat a particular kind of fat than making it from other fats or carbohydrates.
You [need cholesterol](https://www.youtube.com/watch?v=b7zWNabebxs) so you absolutely need fat
also saturated fatty-acids are good for building cell membranes… I wouldn’t worry much about cholesteral and saturated fatty-acids (I know the latter are not the topic but I wanted to mention them because food with a lot of cholesterol often has saturated fatty acids).
I think you could survive on carbs and proteins but it wouldn’t be healthy.
(also people who have too high cholesterol in their blood get that mostly from sugar. The book grain brain is really interesting and has a good chapter about Cholesterol. I didn’t read it in english so it’s hard for me to explain)
The short answer is that there are a lot of different fatty acids.
When the body converts glucose to fat, it creates palmitic acid. It can be converted to another 5 fatty acids, but there are a lot of other fatty acids that the body can’t create, so they need to come from the diet.
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