There are several nutrients which I'm not getting enough of from food, in some cases hardly any, due to dietary restrictions. I can get the nutrients I need if I carefully structure my diet, but I don't want to. I'm really lazy. I'm still trying to eat a varied diet including plenty of fruits and veggies, I just know it's not enough. So I'm taking a multivitamin and a couple of other pills.
Pretty much all the information I can find suggests this is a bad idea, but none of it seems to directly answer why. The biggest apparent reason is that supplements are not absorbed as well as food. But I can't see any indication of how well supplements are absorbed. I know it depends on a lot of factors (including the dosage) and is probably hard to measure, but don't we have at least a rough idea in specific situations? If I take, say, 100% of the RDA of iron in a pill, is the amount my body absorbs probably more or less than 50%? How much do caffeine, vitamin C, calcium, etc. affect the absorption? The information always seems to be qualitative, not quantitative.
Furthermore, if a certain dosage won't be sufficiently absorbed to fulfill my requirements, why can't I just take more? I know that this leads to diminishing returns and high dosages can be problematic, but that doesn't quite explain it. The amounts found in supplements are generally on the order of 100% of the RDA, much less than the amounts which are considered risky which are more like 1000% (with the exception of calcium, where there are some concerns about supplementing around 100-200%). It's hard to believe that the cost of manufacturing the pills or the difficulty of swallowing them would significantly increase in many cases, especially when we're talking about adding a few milligrams or less.
Other than that, many of the fears seem irrelevant. Don't take certain dangerous supplements like the dirty dozen. Don't make certain combinations. Don't try to use supplements to treat diseases like cancer. And so on.
Is the problem uncertainty? Does it just seem safer to tell people to get their vitamins from food? Or perhaps do some people know what can and should be done but others are still catching up?
OK, that's a lot of rambling, let me try and make a coherent, SE style question:
Suppose I'm a healthy adult between the ages of 20 and 40. I don't know of any medical conditions that I have relevant to supplements. I am not currently deficient in anything. I can afford to buy as many supplements as I need, I remember to take them, and I don't mind doing so.
- For which nutrients can I safely obtain X% from supplements (where X is ideally 100 but can be as low as 50 if necessary), and under what conditions (e.g. don't drink coffee)? How much of the supplement should I take to reach this amount?
- For which nutrients is it a really bad idea to lean on supplements, and why? Are there some supplements that just don't seem to work? Or are the risks too great?
EDIT: it looks like I need to clarify. I'm vegan. There are several nutrients that I can't easily get from food, particularly B12, calcium, and vitamin D. My diet is generally fairly good - plenty of fruits, veggies, nuts, and grains - but that doesn't help enough with those nutrients. Yes some vegetables have calcium but I don't want to eat, say, 600g of kale every day. It seems safe to say that I need calcium in my body, so I want to know if pills work for that. I'm not suggesting living on junk food and pills.