Researchers tested these off-the-shelf supplements from 12 different brands to ascertain the volumes of vitamin D present in each. They found that each type differed from the dose that was stated, with them having anywhere between 52 percent and 135 percent of the stated dose.
Vitamins that were mixed together by compounding pharmacies had an even greater range of variation: they were found to have anywhere from 23 to 146 percent of the dose stated on the bottle.
One of the lead researchers, Dr Pieter Cohen from Harvard Medical School, an expert who specialises in the study of dietary supplements, admitted that he was not at all surprised by the results of the research.
The expert, who was not involved in the study himself, blamed a lack of regulation for the discrepancies, adding that it is very hard to find a supplement that works because of these variations in dose.
Vitamin D is thought to contribute to wellness by improving bone health, and there are some claims that it can boost immunity and lower the blood pressure too.
The research was carried out by a team from the Kaiser Permanente Centre for Health Research, based in Portland, Oregon. Only one brand of pills, a brand that is verified by a U.S. Pharmacopeial Convention (USP) was found to be accurate, as all pills in this batch were within six percent of the dose listed on the bottle.