Recap of an oral intake study led by Dr Cedric Garland
When Professors Cedric Garland and Robert Heaney team up, it would be wise to listen to what they say. This paper, published early in 2011, is unique. The authors studied vitamin D levels of 3,667 people who had at least one vitamin D blood level done.
Garland CF, French CB, Baggerly LL, Heaney RP. Vitamin D supplement doses and serum 25-hydroxyvitamin D in the range associated with cancer prevention. Anticancer Res. 2011 Feb;31(2):607-11.
First, the authors cite evidence that cancer protection by vitamin D require vitamin D levels between 40 and 60 ng/ml, about the same levels the Vitamin D Council recommends. Besides a vitamin D blood level, many of these people were taking vitamin D, anywhere from 500 to 10,000 IU/day. By correlating intake with vitamin D levels, the authors were able to make some startling conclusions.
- The wide variance in human vitamin D levels after a 100,000 IU mega dose “spanned a six-fold range from 4.9 ng/ml to 30.8 ng/ml.” Obesity “failed to account for most of the between-participant variance.” It appears genetics plays a major role.
- Intakes of 6,100, 9,600 and 14,100 IU per day resulted in average blood levels of 64, 75, and 85 ng/ml respectively.
- However, for public health reasons, the dose that ensures 97.5% of the population has healthy levels is quite important and is much higher than the average. “The points where that lower band crosses 30, 40, and 50 ng/ml (75, 100, and 125 nmol/L) are the supplemental intake values that ensure 97.5% of the population would be above the specified serum 25(OH)D concentrations concerned. These are, respectively, 6,100, 9,600, and 14,100 IU/d for this population.” This is the first time scientists published such data.
- The response to an extra 1,000 IU/day depends heavily on what the vitamin D blood level is to begin with. “For example, at a starting value of 10 ng/ml, the mean increment that would be expected to be produced by and an additional 1,000 IU/day is 11 ng/ml, whereas at 30 ng/ml it is 8 ng/ml, and at 50 ng/ml only 5 ng/ml. Above a starting value of 90 ng/ml, the response is nearly flat at about 1.6 ng/ml/1,000 IU/day. . . The very slow rise in serum 25(OH)D concentration for each 1,000 IU increment at serum levels above 80-100 ng/ml is firm expression of the general safety of even relatively high doses.”
- “Although an order of magnitude higher than currently recommended oral intakes, these calculated daily intakes are of the same magnitude as produced by a single, minimal erythemal dose of UVB radiation, such as would be obtained during a few minutes of solar UVB exposure near noon in midsummer, assuming nearly complete skin exposure.”
- No published data exist comparing assay methods above 100 ng/ml.
This is evidence that knowing your vitamin D level is important, as no single dose fits everyone, although 5,000 IU/day is okay for 75% of the population. Check with your insurance company if they cover a 25(OH)D test. If they do not, an in-home test is often significantly cheaper than paying for a doctor’s order and blood test.