For example my womens vitamin has 18 mg iron as 100% daily value. However we only absorb about 20% of iron so do the vitamins take this into account?
Yes it does, however supplements generally speaking aren’t a good way to get the nutrients you need. You won’t get the health benefits of fruits and vegetables by taking a multivitamin. In foods, absorption isn’t the same across the board, but %DV is still a valuable guideline.
Yes. Recommendations have historically been based on the observed intake of people who exhibit no signs of deficiency vs the observed intake of people who become deficient (plus statistically derived wiggle room so they should cover IIRC 97% of people).
However, the assumption is that you are consuming any given nutrient from an average range of sources. If your only sources of any given nutrient are ones where that nutrient is much less bioavailable than in other sources, you may need to consume more than 100% DV to actually absorb the amount that someone with a more average diet would get from consuming 100% DV.
Reduced bioavailability in many supplements plus the fact that supplements are intended for deficient people who need to over-consume to return to baseline are why supplements often have much higher than 100% DV of many nutrients. Sometimes this is harmless (B12, Vit C, and many others). Other times this can be dangerous if you are not deficient (iron, potassium). And supplements have such little regulation in the US, that you should not assume that just because something is on the shelf that it’s safe for you to consume, so people really should consult their doctor and get bloodwork done to see if there’s any supplements they actually need instead of just buying what internet nutritionists tell you that “everyone” is deficient in or popping a multi and calling it a day.
Absolutely not. Most vitamin supplements on the market are not very bioavailable to begin with. The supplement Industry is sadly ran by the pharmaceutical industry, purely out for profits. Pick your supplements wisely that’s all I’m going to say.