D*action is a study cohort with 7,809 members who have completed at least one questionnaire and blood test. Of these, 972 have completed 5 or more tests (over the span of up to 5 years). While the study is open to anyone, roughly 2/3 (64%) of the cohort is female, and it is mostly Caucasian (92%). The average age at sign-up is 51. The number 51 also represents the number of countries our participants call home, although 90% reside in the US or Canada.
Serum Levels are not Affected by Gender
While there is a big difference in the number of males and females enrolled, gender does not play a role in determining serum levels. The median serum level of males in our cohort is 45 ng/ml and for females is 43 ng/ml.
Intake Methods are not Affected by Gender
As with serum levels, there is no difference by gender in how 
participants get their vitamin D. Of the D*action participants, 80% of both females and males take supplements. This matches what our scientists tell us - that most people will need a daily supplement, along with other sources (sensible sun exposure and food), to meet their daily requirement for vitamin D. It is recommended to use a combination of all three of these methods of vitamin D intake, to test periodically, and make adjustments as necessary to keep your serum level between 40-60 ng/ml.
How Much to Take?
Despite a difference in median weight, the median intake of supplements is the same for both genders, 4000 IU/day. This tells us what many of our scientists have shown in the past, that the RDA of 600 IU/day is simply not enough for most people to attain a serum level of 40-60 ng/ml. Many studies that have looked for the positive effects of vitamin D for preventive medicine have been negative simply because the dose was too low (200 IU/day up to 1000 IU/day). In fact, one of the groundbreaking studies led by Dr. Bruce Hollis from the Medical University of South Carolina in Charleston, found that a dose of 4000 IU/day in healthy pregnant women was safe and effective in raising circulating vitamin D to a level in the range of 40-60 ng/ml.
High Intake Levels and Kidney Stones
Some doctors remain wary of taking more than 2000 IU/day and they quote what they learned in medical school, that too much vitamin D causes kidney stones. According to wha
t we found with the D*action cohort, this simply isn't the case.
Our analysis, published in the American Journal of Public Health, found no statistically significant association between 25(OH)D serum levels and kidney stone risk. In fact, the researchers found a non-significant trend towards lower incidence of kidney stones for those with higher 25(OH)D serum levels.
