Vitamin Supplements Frequency
Most daily multivitamins have close to 100% of the RDA for each element in the vitamin, and I believe that the nutrients get into your blood in a short time frame.
Does this daily spike in vitamins have any known health consequences? It would seem more natural to have them split up into smaller doses and be taken with meals each day.
For example, in Japan many vitamins are smaller and designed to be taken a few times over the course of a day. The one-a-day approach most Americans use seems arbitrary and makes me wonder, for example, why we don't take one bigger vitamin every three days (let me ignore the problem of forgetting the day on which we last took vitamins even though it is important here).
So are there any health consequences to taking a single multivitamin once a day? Or is a split approach more conducive to absorption and utilization? At what frequency will vitamin blood concentrations start to have unhealthy spikes?
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In addition to Jan's answer it is important to recognize that the biological half-life of most vitamins is quite long, on the order of several days.
This means that you do not really have a spike when you take a multivitamin: the amount of each vitamin already in your body is much higher than the daily intake amount. It is not actually necessary to take the daily recommended intake every day, that's just an amount that, if taken daily, will not result in any long-term deficiencies (assuming otherwise normal absorption/metabolism).
To give an example of one particular vitamin from this book, the estimated body concentration of vitamin B6 is somewhere between 60-160 mg depending on sex and which model they use. Compare this to the recommended daily intake of 1.3 mg (hardly a spike relative to even 60 mg), and the tolerable intake of 100 mg/day (which would indeed be a 'spike', but amounts less than that can be cleared without negative effects in a normal individual).
RDA is the recommended dietary allowance. Vitamins and minerals should not cause side effects when you exceed 100% RDA, but they can cause them when you exceed "the tolerable upper intake" (UI), which is usually considerably higher than RDA.
According to Office of Dietary Supplements, the RDA for vitamin C for adults is 90 mg/day and the UI is 2,000 mg/day. You can also see from the table 2 that a single serving of certain foods can exceed the RDA for vitamin C. Excessive amounts of vitamin C and other water soluble vitamins and minerals are excreted in the urine and the fat-soluble vitamins are stored in the body fat. For a complete list of recommended and upper intakes of all nutrients, you can check these charts on nationalacademies.org. They use Dietary Reference Intakes (DRI) instead of RDA, but they are very similar.
Concluding from the big difference between the RDA and UI for most supplements, I don't think you need to split a 100% RDA dose of a supplement into smaller doses to make it "less harmful." Splitting the dose can increase the percent of a supplement that is absorbed, though. For example, RDA for calcium for adults is 1,300 mg, but the maximal absorption occurs when you take less than 500 mg at once.
Splitting the dose can be more important with prescribed supplements, which usually come in multiple RDA doses. The entire dose of iron can irritate the stomach, calcium can cause constipation, magnesium lose stools, etc. Again, this should not often happen with multivitamins in 100% RDA doses.
Currently, there is no convincing evidence that taking multivitamin supplements would be harmful; healthy people do not need supplements, anyway, and can get all the essential nutrients in appropriate amounts from food (PubMed, 2012).
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