Due to many factors, (e.g., decrease in non-protected sun exposure, decreased consumption of vitamin D fortified foods, changing racial and age composition of the population, and increase in percentage of adults unable to use vitamin D efficiently due to advanced age, obesity, medication use, medical treatments, and health conditions), the prevalence of vitamin D insufficiency and deficiency in the population is increasing
[32–34, 40, 45]. Based on our survey, we estimate that 40% of women aged 25–50, over 25% of women aged 51–85, and approximately 50% of men in both age groups in a relatively well-educated, insured health plan population are getting no vitamin D from any dietary supplementation. This suggests that the substantial portion of adults who are not getting adequate vitamin D from sun exposure and fortified food sources to meet the IOM’s current RDA for vitamin D are unlikely to be making up the difference with vitamin D supplements. Age- and gender-related differences in vitamin D supplementation were primarily due to differences in use of calcium with D supplements.
Similar to the NHANES-based findings of race-ethnic differences in calcium and multivitamin supplement use
[50, 51], we found that Black and Latina women in the 25–50 and over 50 age groups and Black and Latino men over the age of 50 were significantly less likely than nonHispanic Whites to be getting any vitamin D from dietary supplements. This is of clinical and public health concern because of the documented high prevalence of vitamin D deficiency in Black and Latino populations
Because the results of epidemiologic studies of risks associated with vitamin D deficiency will continue to reach the public a long time before definitive recommendations based on clinical trial results are available, manufacturers of multivitamins, calcium with D supplements, and singular vitamin D, as well as producers of foods that are fortified with vitamin D, need guidance about the appropriate dosage of vitamin D to put into these types of supplements. Also, because manufacturers are likely to increase the amount of vitamin D and calcium in their multivitamin and calcium supplements based on the new IOM recommendations, people who have been taking both a daily multivitamin and calcium with D supplement may suddenly find that they are exceeding the recommended intake, although evidence suggests that dosages as high as 4,000 IU/day are not toxic
. In the short term, serum 25-hydroxyvitamin D (25[OH]D) samples from the most recent cycle of NHANES could be analyzed separately for males and females in different age and race-ethnic groups and for these groups by different parts of the U.S to determine extent of variation in vitamin D insufficiency and deficiency. This information about the general population could then be augmented by clinical studies to determine how high a dose of vitamin D3 is required to bring D-insufficient and D-deficient people up to what is considered adequate levels, resulting in more tailored DRIs for vitamin D based not only on age, but also race-ethnicity, season, and geographic location.
Our study has several limitations. First, because the overall survey response rate to this general health survey was under 50%, there is a possibility that response bias might limit the accuracy of the results. However, previous studies have found that respondents to self-administered surveys are more likely to be better educated than nonrespondents, and because numerous studies have found that health promoting behaviors are more prevalent among better educated adults, any response bias would likely result in our findings over-estimating the prevalence of vitamin D supplementation in the general population.
Another limitation of the study sample is that the numbers of Blacks, Latinos, Filipinos, and Chinese respondents used to estimate vitamin D supplementation in different race-ethnic groups were relatively small after being split across the four age-gender groups. This resulted in relatively wide confidence intervals around the estimated prevalence of supplement use. However, we obtained the same results with narrower confidence intervals in preliminary analyses using a sample of pooled 2005 and 2008 member survey respondents with nearly double the size of all the race-ethnic subgroups. The reason we decided to restrict our analyses to the 2008 survey was that the 2005 survey did not enable us to differentiate people who used calcium with D vs. calcium without D. Because the nonHispanic White subgroup was relatively large, we had sufficient power to identify several statistically significant race-ethnic differences in vitamin D supplementation.
A different type of limitation resulted from assumptions we could make about vitamin D intake from supplements. We could only estimate number of sources of vitamin D based on indication of taking multivitamins, calcium with D tablets, and singular vitamin D, not actual vitamin D intake as relates to IOM recommendations. While the normative amount of vitamin D in an adult multivitamin and calcium with D tablet in 2007–2008 was 400 IU, we do not know whether people taking calcium with D took one tablet (400 IU) or two (800 IU). Also, our estimates of the percentages of women and men getting vitamin D from ≥ 2 sources was based on the assumption that people who were taking calcium and multivitamins were doing so daily. However, when we analyzed frequency of calcium and multivitamin use data from a separate 2008 survey of members of the same health plan to test that assumption, we found that it did not hold (unpublished data). Among those who reported using both calcium and multivitamin supplements, only 45% of women and 32% of men aged 25–49, 68% of women and 55% of men aged 50–69, and 80% of women and 68% of men aged 70–84 were using both of these supplements on a daily basis, and only about 5-7% more were using both supplements at least five times a week. We also found that across both age groups, Black, Latino, and Filipino men and women users of both calcium and multivitamin supplements were significantly less likely than nonHispanic Whites to take both of them daily or at least five times a week. This suggests that our study results actually overestimates the percentages of adults who were getting vitamin D daily from at least one source, and that the extent of race-ethnic differences in vitamin D supplementation is probably underestimated.
A final limitation of our study is that we could not place vitamin D supplementation in the context of the extent to which individual respondents were in need of vitamin D from supplements based on their serum 25-hydroxyvitamin D (25[OH]D) concentration and amount of vitamin D they were getting from sun exposure and food intake. The survey did not include dietary recall data to enable analysis of potential vitamin D availability from food sources, nor did it ask about sun exposure practices. At the time of the survey, serum vitamin D tests were not routinely done, limiting the number of survey respondents for whom serum vitamin D status data would have been available. However, a recent national survey found that over 70% of nonHispanic White adults and nearly all Black and Latino adults in the U.S. were vitamin D insufficient
. Further, the aim of this study was to examine patterns of vitamin D intake from dietary supplements, not vitamin D intake from all sources nor the extent to which supplementation was actually desirable.