Eating an alchemy ingredient shows you it's effect(s) so then you can use it to make potions/poisons...cool...got that part.
And I do notice that when I eat samples of a lot of new stuff all at once I get all hopped up on their combined effects -- Nothing beats mixing meds - wooohoo! LOL

So how about if I eat an ingredient after that initial time, say when I'm fighting, will I get the effect on me of the ingredient?
Say for example I eat an ingredient that is supposed to fortify health, will my health get fortified, and if so how much and for how long?
And if that ingredient has two effects or more listed as "known" how do I know which of them will be taking effect, or do they all take effect at once?
Not seeing the answer in the UESP...

Um ... you could always just eat something again and then you could let us know.

You can see the effects in your magic menu. Press P and scroll to the bottom where it says Active Effects.

Know that ingredients on their own have only weak effects and that it is only the first of the four effects that will effect you.

And I would imagine that it's similar to potions in that only one of each type will have an effect on you. You won't get cumulative effects from eating 100 of the same ingredient. But I have not tested this, and so I may be wrong about it.

i think what he is saying is that each ingredient has four listed known effects, he is asking before you take it, how will you know which effect will kick in? all of them? i am also curious...

yes you get the effect but the effect of a single item is tiny as far as im aware

When you eat two different ingredients, both with a different first effect, then both first effects of each ingredient will be applied. They will only last for a few seconds and only potions will last longer and have stronger and combined effects.

Eating two ingredients, either two of the same ingredient or with only the same first effect, will not stack.