No Stupid Questions

Gonna try to give a very general ELI5 sort of answer

There's basically 3 main types of carbohydrates

Simple carbs- basically sugars (mono- and di-saccharides)

Complex carbs- starches, whole grains, etc. (polysaccharides)

Fiber- arguably these are just really complex carbs that your body can't really break down

In general, sugars are the source of energy your body actually runs on, especially glucose. Everything else basically gets broken down into glucose.

Your body can pretty much use simple sugars as-is or can easily break them down into a form it can use. There's some variation just how quick and easy it is for your body to use different sugars, but in general your body will start to feel the effects of eating sugar in the space of a few minutes, and the effects will peak within about an hour or two.

Complex carbs take a little more digesting to break down into a form your body can make use of. They're basically being turned into simpler sugars, but that process takes a while. You might hear about athletes carbo-loading with a big spaghetti dinner or something the night before a big competition. The idea there is that the energy from that big, complex carb-heavy dinner won't really hit them for a few hours or even until the next day, and it will keep providing that energy for a longer period of time.

Fiber is, for the most part, indigestible, your body can't really break it down into simpler sugars that it can make use of. It goes in your mouth, through your digestive tract, and out the other end relatively unchanged. That doesn't mean it's useless though, it still plays an important role in digestion. It takes up space in your stomach helping you feel more full. It absorbs water and helps keep your stool soft and helps waste move through your intestines, and it minds to things like bile acids and cholesterol so that they can be passed as waste.

Again, this is meant to be a very general answer, there's a lot of details I'm glossing over both just to keep things simple, and because I'm not a doctor or anything of the sort and I'm not 100% sure myself.

There is no difference between carbs as sugar and carbs. They all end up in the blood stream as glucose.

Fructose is really bad and usually doesn't get converted to glucose.

Fiber is not absorbed by the body and just passes through.

Carbohydrates are not essential for human life. You can be perfectly healthy without eating them. The body does use glucose for blood cells and about 15% of brain metabolism, but that can be created by the liver on a as needed basis.

There are people who differentiate carbohydrates based on how quickly they arrive in the blood stream as glucose, in my view that's a unnecessary distinction. Elevated blood glucose is really dangerous for the body, regardless of how delayed it is. That's why the body prioritizes burning the glucose off first when it's elevated in the body.

What function does elevated glucose have? (I.e. why eat carbohydrates)

• increase blood sugar for hypoglycemia
• raise insulin levels
• some athletes believe it helps their performance

The sports term is carb loading, and for non-fat adapted athletes it does help their endurance, prevents bonking. But for fat adapter athletes who don't eat carbs, it's unnecessary as they don't bonk. The blood only carries 5g of glucose, and when you use it all up you can bonk, the body does not store glucose at all.. the body stores fat. So people who burn fat don't bonk because they don't run out of energy

What benefit does fiber have?

• reduces transit time of all food
• reduces absorption of other food
• can be converted into short chain fatty acids in the intestines

Obviously, reducing the nutrition from the food you eat is only a "good" thing if your diet is unhealthy.

The SCFA's are interesting, you can eat fiber and have some of it turn into fat by bacteria, or you can just eat fat directly and skip the middle man. This is the whole "fiber feeds the gut" argument you hear, gut research is still early stages. Its not clear what the ideal biome is, but whatever you eat will determine what lives in the gut. The bacteria will adapt to your diet and stabilize in about 9 months.

Some people believe fiber helps with constipation because of the reduced transit time, that isn't actually true. Fiber is actual causal of constipation (in one RCT, the only fiber RCT that exists...)