In our sample of well-functioning men and women aged 70–81 years, 84% of blacks had 25(OH)D <30 ng/mL, while the prevalence among whites was 57%. Furthermore, blacks were more than four times as likely to have 25(OH)D <30 ng/mL compared to whites. The prevalence of 25(OH)D <30 ng/mL was lower in Health ABC compared to black and white men and women ≥70 years old in NHANES 2001–2004, in which the prevalence of 25(OH)D <30 ng/mL was approximately 93% in blacks and 73% in whites30
. This may be due, in part, to the improved functional status and overall health of Health ABC participants.
Among blacks who reported taking a multi-vitamin or vitamin D-containing supplement, more than 60% had serum 25(OH)D <30ng/mL, while 38% of whites who reported taking these supplements had serum 25(OH)D <30ng/mL. Dietary supplement use did not appear to have been sufficient enough to increase serum 25(OH)D levels ≥30 ng/mL, especially among blacks It is important to note that supplement use was queried in 1998, when most multivitamins contained 400 IU or less of vitamin D; these levels have increased and many multivitamins currently contain 800 IU. Dose-response studies in blacks and whites suggest an intake of 400 IU/day (10 µg/d) vitamin D3
will increase circulating 25(OH)D by 2.8 ng/mL in the absence of sun exposure 31;32
. It was recently reported that 800 IU/day of vitamin D3
increased circulating 25(OH)D to 35.4 ng/mL (from a mean baseline value of 12.1 ng/mL) in older adults 32
. However, outcomes of other studies suggest higher doses may be necessary to achieve 25(OH)D ≥ 30 ng/mL in older adults 33
, and higher doses of vitamin D given daily, monthly, or as a single bolus have been shown to be safe and effective for repletion of 25(OH)D in this age-group 34–37
. The baseline level of 25(OH)D, which is usually lower in blacks, is an important determinant of the response to supplementation, so it is plausible higher supplemental doses would have been required to raise serum levels to 30 ng/mL in the blacks in our cohort, as demonstrated by Aloia et al 38
. Considering a recent report that a single annual dose of 500,000 IU of vitamin D3
increased hip fracture risk in older women 39
, the optimal dosing regimen of vitamin D supplementation for older adults merits investigation.
Consistent with younger age groups, being female and/or obese increased the odds of being vitamin D insufficient in older black adults in Health ABC 17;19;40
. We also identified several additional risk factors for vitamin D insufficiency among whites. Whites who reported a dietary intake <400IU/day were more likely to have 25(OH)D <30 ng/mL. Season and participation in physical activity were also predictive of vitamin D insufficiency among whites. Because the cutaneous production of vitamin D is greater in whites compared to blacks, whites tend to be more susceptible to seasonal variation in vitamin D status 41
. Participation in physical activity was defined according to self-reported walking; thus, it is likely that those who reported walking spent more time outdoors which could influence the cutaneous synthesis of vitamin D. Being female, obese, or having type 2 diabetes was also associated with vitamin D insufficiency among whites, so these characteristics may help discriminate white older adults who are at greater risk for low vitamin D status.
Low dietary vitamin D intake (<400 IU/day) was identified as an important determinant of 25(OH)D <30 ng/mL among whites in Health ABC, but not as a primary determinant among blacks, although dietary intake has been identified as determinant of 25(OH)D in blacks by others 17;19;40
. In our analyses, we dichotomized vitamin D intake as <400 or ≥400 IU/day because only 6 black and 8 white participants reported dietary intakes ≥800 IU/day and less than 2% of Health ABC participants (15 black and 30 white) reported dietary intakes ≥600 IU/day. In NHANES III (1988–1994), no more than 2% of adults over age 70 met the requirement for vitamin D intake from food alone, which at the time was 600IU/day 42
. The prevalence of dietary vitamin D intake <400 IU/day was similar among blacks (90%) and whites (92%). The IOM recently recommended adults over 70 years old consume 800IU/day of vitamin D and 1200 mg/day of calcium4
. The overall median (interquartile range) of vitamin D intake in Health ABC was 198(164) IU/day in whites and 159(157) IU/day in blacks and of calcium intakes was 739(444) mg/day in whites and 681(480) mg/day in blacks. Furthermore, 63% of blacks and 51% of whites in Health ABC reported a dietary vitamin D intake <200IU/day. Therefore, to meet the 800IU/day recommendation it is likely black and white older adults will require dietary supplementation.
Whites with type 2 diabetes and CVD were more likely to have 25(OH)D <30ng/ml. These results are consistent with previous analyses of NHANES III that found vitamin D status to be associated with prevalence of type 2 diabetes among whites and Hispanics, but not in blacks, which the authors attributed to blacks being less sensitive to the effects of vitamin D and related hormones 25
, as has been suggested by others 15
. It is plausible the associations between vitamin D status and disease outcomes differ by race because low vitamin D levels are reported to contribute to the increased risk for certain cancers (such as colorectal) and cancer-related mortality in blacks, but not in whites 24;43;44
. In Health ABC, both black and white participants with osteoporosis were less likely to have insufficient vitamin D. The majority (70%) of participants diagnosed with osteoporosis were on medication to treat osteoporosis, which likely included advisement to take a vitamin D-containing supplement.
Although several have suggested ≥30 ng/mL are needed to meet skeletal and non-skeletal needs for vitamin D 5;8–10
, ≥20 ng/mL have been considered sufficient to maintain skeletal health 4
, and the concentration of 25(OH)D that is considered sufficient remains controversial5;8;9
. We also examined the prevalence and correlates of 25(OH)D <20 ng/mL. Over half of black participants in Health ABC had 25(OH)D <20 ng/mL, while the prevalence among the whites was 18%. Overall, the determinants of 25(OH)D <20 ng/mL were similar to those of 25(OH)D <30 ng/mL. However, some differences were noted. Among blacks, season was associated with 25(OH)D <20 ng/mL, but not <30 ng/mL. Those measured during winter were over twice as likely to have 25(OH)D <20 ng/mL, supporting others who have suggested that winter season contributes to more severe vitamin D insufficiency among blacks 40
. Among whites, the odds of having 25(OH)D <20 ng/mL were 85% higher among those who did not participate in any physical activity and among those who consumed <1000 mg/day of calcium. Although 25(OH)D <20 ng/mL is not considered to be frank vitamin D deficiency, 9% of black and 2% of white participants had 25(OH)D <10ng/mL which has been considered deficient 13
. Approximately 80% of blacks and whites with 25(OH)D <10 ng/mL were female (Chi square p<0.001). Sixty-eight percent of blacks with 25(OH)D <10ng/mL were measured in spring or winter (Chi square p=0.01) and 96% of whites below 10 ng/mL reported calcium intakes < 1000 mg/day (Chi square p=0.02). Although Health ABC participants with 25(OH)D <10 ng/mL did not significantly from those with ≥10ng/mL in any other way, it is plausible additional differences would have been detected with a larger number of participants with 25(OH)D below this threshold. Since the evidence of vitamin D’s role in age-related disease continues to mount, identifying older adults at risk for more severe vitamin D insufficiency or deficiency may be important clinically.
This study has several notable strengths and certain limitations to consider. Importantly, we were able to describe the prevalence and determinants of vitamin D insufficiency in a large sample of black and white adults aged ≥70 years to provide insight into which older adults are at risk for vitamin D insufficiency. Although the determinants of vitamin D insufficiency among older adults have been previously described 17;45;46
, the factors that are associated with specific thresholds of serum 25(OH)D in black and white adults aged ≥ 70 years has not yet been reported. Our results may assist geriatricians in identifying older adults to screen and treat for vitamin D insufficiency as well as researchers who need to identify vitamin D insufficient participants for supplementation trials. In Health ABC, dietary supplement use was not based on self-report. Rather, all participants were asked to bring all medications and supplements they were currently taking to the yearly clinic visit, which were then inventoried and recorded by study personnel. However, the quantity of vitamin D and/or calcium contained in the supplements is not available, so we are unable to determine associations between supplement dose and vitamin D insufficiency. It is likely that the high prevalence of vitamin D insufficiency even among those who reported taking a multivitamin or vitamin D-containing supplement may be because the supplement dose was not high enough to improve 25(OH)D concentrations to ≥30 ng/mL. However, we were unable to test this formally. Although we were able to adjust for latitude/location (Memphis vs Pittsburgh) and participation in physical activity, we did not have more specific measures of sunlight exposure and/or use of sunscreen, which have been identified as determinants of 25(OH)D in older adults in other population-based studies 45;46
. Such measures would have likely explained more of the variability in 25(OH)D in Health ABC and would provide additional pertinent information with respect to additional risk factors for vitamin D insufficiency in older adults. Also, fewer blacks were recruited during the summer (June–August) at the Memphis field center which may have contributed to the association between season and vitamin D insufficiency we observed in whites compared to blacks. Substantial variation in the measurement of serum 25(OH)D, because of different assay methodology, laboratory experience, and differences between assays to recognize the vitamin D3
form equally 47
make it difficult to make comparisons across studies.
The importance of vitamin D’s role in health and disease is becoming more evident, and older adults have been identified as a group for whom the prolonged effects of vitamin D insufficiency may be exacerbated 14;14;48
. We have identified that the determinants of vitamin D insufficiency differ in black and white older adults. Knowledge of supplement use, dietary intake, physical activity and/or prevalence of co-morbidities such as obesity, type 2 diabetes and/or cardiovascular disease may provide additional pertinent information to identify older adults who are at risk for vitamin D insufficiency and, thus, for whom 25(OH)D screening and/or vitamin D supplementation might be appropriate.