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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.

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    Do you really think science has teased out all the intricacies of nutrition and that it's really as simple as the dozen or two compounds they know we need now? I hope not because I think that's a naive viewpoint. Check again in 50 years and I think they'll have a whole new list with all new RDAs. That doesn't matter much to most people because most people get most of their nutrients from food, but it's going to matter to you because you'll be depending on science being perfect, which it is not. – Carey Gregory May 23 '17 at 17:39
  • @CareyGregory see my edit. – Alex Hall May 23 '17 at 21:11
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    Your edits don't change my comment. My point is that food has hundreds of compounds in thousands of combinations and science is far from fully understanding what effect all those combinations have. Simply making sure you get enough of a dozen of them is unlikely to be adequate, and the definition of "enough" is likely to change over time. You recognized this yourself when you said Pretty much all the information I can find suggests this is a bad idea. – Carey Gregory May 23 '17 at 21:21
  • @CareyGregory I can construct an extremely varied and nutritious diet that everyone would agree is very healthy except for the fact that it contains no calcium. Everyone would also agree that a similar diet with enough calcium is better. The point is that I need calcium. I can get it from supplements, or from kale and such. It doesn't seem sensible to assume that I should eat lots of kale as opposed to lots of other veggies just in case the kale also contains some other mysterious nutrients - the same could be said for the other veggies that I'd eat instead but don't contain calcium. – Alex Hall May 23 '17 at 21:32
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    You can construct an extremely nutritious diet that contains no calcium. My point is what else might it be missing? You're paying attention to calcium because it's one of those couple dozen compounds identified (so far) as being important, but my contention is there are others you just don't know about yet. – Carey Gregory May 23 '17 at 23:18
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The biggest issue with using supplements instead of food is that studies don't always measure what they think they're measuring. A lot of near-magical powers were ascribed to Vitamin C, for example, that have not held up when done with supplements. It may turn out that many of the benefits of "Vitamin C" are really be related to fibre, since people were getting that Vitamin C from fruits and veg. Or to some other phytonutrient. Or to being the kind of person who spontaneously eats a lot of fruit and veg, or at least doesn't get kicked out for noncompliance when in a study that asks you to eat a lot of fruit and veg.

Example:

The present systematic review included 78 randomised clinical trials. In total, 296,707 participants were randomised to antioxidant supplements (beta-carotene, vitamin A, vitamin C, vitamin E, and selenium) versus placebo or no intervention. Twenty-six trials included 215,900 healthy participants. Fifty-two trials included 80,807 participants with various diseases in a stable phase (including gastrointestinal, cardiovascular, neurological, ocular, dermatological, rheumatoid, renal, endocrinological, or unspecified diseases). A total of 21,484 of 183,749 participants (11.7%) randomised to antioxidant supplements and 11,479 of 112,958 participants (10.2%) randomised to placebo or no intervention died. The trials appeared to have enough statistical similarity that they could be combined. When all of the trials were combined, antioxidants may or may not have increased mortality depending on which statistical combination method was employed; the analysis that is typically used when similarity is present demonstrated that antioxidant use did slightly increase mortality (that is, the patients consuming the antioxidants were 1.03 times as likely to die as were the controls). When analyses were done to identify factors that were associated with this finding, the two factors identified were better methodology to prevent bias from being a factor in the trial (trials with ‘low risk of bias’) and the use of vitamin A. In fact, when the trials with low risks of bias were considered separately, the increased mortality was even more pronounced (1.04 times as likely to die as were the controls). The potential damage from vitamin A disappeared when only the low risks of bias trials were considered. The increased risk of mortality was associated with beta-carotene and possibly vitamin E and vitamin A, but was not associated with the use of vitamin C or selenium. The current evidence does not support the use of antioxidant supplements in the general population or in patients with various diseases.

There was also the whole "oat bran fibre" thing that turned out to be mostly about what participants didn't eat because they were so full from eating three huge fibre-filled muffins every day. Choosing a sweetened breakfast cereal that happens to have a little oat fibre in it isn't going to have that effect.

When there is an observation that "people who eat a lot of food X have less Y", reasoning about exactly the reason for it - a compound in food X, a compound in a thing people often eat with food X, a food they don't eat instead, a cultural habit -- is more of a challenge than you might think. To use a non-food example, studies about whether circumcision affects the spread of STDs was complicated by different rates of being piously religious and monogamous in the two groups. As a result, just taking a Vitamin C pill may not help you at all, if you're not generally eating good food.

For the specific case of vegans, Dieticians of Canada seems to be in favour of supplements-in-disguise by recommending fortified products a lot:

Vitamin B12: Vitamin B12 is important for making red blood cells and helping the body use fats.   Good sources of vitamin B12 include: Red Star nutritional yeast, fortified soy beverages and other fortified non-dairy beverages like rice and almond beverage, fortified meat alternatives like TVP, veggie burgers and meatless chicken, fish and meatballs.

Vitamin D: Vitamin D helps the body to absorb and use calcium and phosphorus for strong bones and teeth.  Good sources of vitamin D include: fortified soy beverages and other fortified non-dairy beverages like rice and almond beverage, non-hydrogenated margarines.

Calcium: Calcium helps bones to grow and stay healthy. It also helps muscles to contract, including making the heart beat.  Good sources of calcium include: soy yogurt, fortified soy beverages and other fortified non-dairy beverages like rice and almond beverage, soybeans, navy beans, white beans and tofu prepared with calcium sulfate, almonds, sesame butter (tahini), blackstrap molasses, some vegetables, such as bok choy, okra, collard greens and turnip greens, some fruit, like figs and fortified orange juice.

They do include sources that are not supplements though, and these probably are still the wiser approach.

A long article on vegans and calcium (among other nutrients) includes this conclusion:

Conclusion on Calcium and Vegan Diets: There is no reason to think that vegans are protected from osteoporosis more than other diet groups, and they should strive to meet calcium recommendations. Although it is possible to meet the calcium recommendations by eating greens alone (see chart below), the average vegan probably will not meet recommendations without drinking a glass of fortified drink each day, eating calcium-set tofu, or taking a 250 - 300 mg supplement (in addition to eating an otherwise balanced diet). Although it is important to get enough calcium, do not get more than 1,400 mg of calcium per day.

The article itself includes links to studies if you need the science behind the conclusions.

  • I am generally eating good food, but not necessarily the right food to get nutrients that I need. See my edit. – Alex Hall May 23 '17 at 21:12

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