These findings from a nationally representative US sample indicate that regular outdoor physical activity is associated with higher levels of serum 25-hydroxyvitamin D. In particular, consistent with previous research (17
), it is the place of physical activity (outdoor rather than indoor) and the frequency of activity rather than the intensity that are related to vitamin D status ().
Previous research has shown that the capacity to synthesize vitamin D from solar ultraviolet B exposure decreases with age (6
). A somewhat surprising observation in the present study was that the 25-hydroxyvitamin D increase with daily outdoor activity in people aged 60 or more years was as great as that in people aged 20–39 years (). One explanation is that older people, who are mostly retired, may spend more time outside each time they participate in physical activity than younger and middle-aged people who are mostly working. Another explanation is that adults aged less than 60 years, who are mostly working, are less able to do physical activity in the middle of the day when ultraviolet radiation from the sun is strongest (30
). The elevated serum 25-hydroxyvitamin D level in people aged 60 or more years who participated in daily outdoor physical activity indicates that this activity pattern can minimize and potentially prevent the age-related decline in body vitamin D status. This finding is consistent with that of a recent survey of people aged 60–87 years who were living in a Dutch town (i.e., that outdoor activities were associated with significantly higher plasma 25-hydroxyvitamin D levels) (21
The increase in serum 25-hydroxyvitamin D with daily outdoor physical activity occurred in all seasons (). This result was also surprising because solar radiation is strongest in summer and weakest in winter when prolonged sun exposure (>4 hours) is required for skin to synthesize vitamin D at latitudes above 25 degrees, particularly above 42 degrees North where no synthesis takes place (30
). However, interviews in NHANES III were carried out mainly in the southern states during winter and in the northern states during summer to ensure optimal weather conditions for interviewing (9
). Thus, the NHANES III sample will have overestimated solar radiation exposure in winter and underestimated it in summer, since the intensity of solar radiation is inversely associated with latitude (30
The greater increase in 25-hydroxyvitamin D with daily outdoor activity in non-Hispanic Whites, compared with Mexican Americans and non-Hispanic Blacks, was expected because the capacity to synthesize vitamin D from sun exposure is inversely related to the degree of skin pigmentation (10
). The maximum mean level of serum 25-hydroxyvitamin D of 52 nmol/liter in non-Hispanic Blacks observed among those who engaged in daily outdoor activity () is of concern, given recent evidence suggesting that serum 25-hydroxyvitamin D levels need to be above 75 nmol/liter for optimum health (32
). Other strategies, such as vitamin D supplementation or fortification of food, as already occurs with milk, are required to elevate 25-hydroxyvitamin D in African Americans to the above desired level during winter months at higher latitudes. The lack of an association between cereal consumption and serum 25-hydroxyvitamin D () contrasts with results for African American and White women aged 15–49 years from the same study that showed significantly higher unadjusted vitamin D levels for women in the highest cereal consumption category, although the significant association between serum 25-hydroxyvitamin D and cereals remained only in Black women after adjusting for covariates that included milk consumption (23
). In our analyses, there was a significant positive association (p
< 0.001) between the frequency of cereal intake and serum 25-hydroxyvitamin D when milk was excluded from the model (data not shown), indicating that milk consumption was a positive confounder. Thus, cereal consumption in the United States during 1988–1994 may have been associated with vitamin D status in subgroups such as young Black women (23
), but not in the total population.
Daily outdoor physical activity was associated with an increase in 25-hydroxyvitamin D similar to that of daily consumption of milk, both about 10 nmol/liter (), indicating that both are likely to be effective strategies for modest increases in body levels of vitamin D. Daily consumption of at least 400 IU of vitamin D supplements was associated with a smaller increase in 25-hydroxyvitamin D of about 5 nmol/liter (), a little lower than the reported increase of 1.75 nmol/liter per 100 IU of vitamin D3
or 7 nmol/liter per 400 IU supplement (33
). A possible explanation is that many participants may have used vitamin D2
that does not increase 25-hydroxyvitamin D levels as much as vitamin D3
). More importantly, recent studies indicate that much higher levels (e.g., 4,000 IU per day) are required to increase serum 25-hydroxyvitamin D levels up to 100 nmol/liter (35
). These and other data (33
) suggest that the larger-dosed vitamin D supplements would assist individuals living at higher latitudes to have optimal levels of 25-hydroxyvitamin D.
The finding of a strong association between outdoor physical activity and vitamin D status has implications for public health. The data in show that 22 percent of all US adults did not participate in any outdoor physical activity in the past month at the time NHANES III was carried out. Of concern, those groups with the lowest vitamin D levels—the elderly, Mexican Americans, and non-Hispanic Blacks—were least likely to have reported engaging in any outdoor physical activity in the past month. Public health interventions supporting outdoor activities probably would improve the vitamin D status of these communities. Previous research suggests that a 10 nmol/liter increase in 25-hydroxyvitamin D from daily outdoor physical activity () could result in modest but important reductions in disease rates. For example, a recent meta-analysis of randomized clinical trials has shown that a weighted vitamin D dose of 528 IU/day, estimated to have increased 25-hydroxyvitamin D by 1.75 nmol/liter per 100 IU (33
) or about 9 nmol/liter, reduced total mortality by 7 percent (2
). A cohort study of US health professionals observed that a 25 nmol/liter increase in predicted plasma 25-hydroxyvitamin D was associated with a 17 percent reduction in total cancer incidence (37
), equivalent to a 7 percent reduction for a 10 nmol/liter increase in 25-hydroxyvitamin D. The association between outdoor physical activity and vitamin D status provides another mechanism for how physical activity may lower chronic disease risk. The well-documented inverse association between physical activity and various diseases, such as coronary heart disease, diabetes, and breast and colon cancers (38
), may partly be due to increased vitamin D levels arising from outdoor physical activity, since epidemiologic evidence suggests that low vitamin D status increases the risk of these diseases after controlling for physical activity (39
This study has a few limitations. Its cross-sectional design precludes determination of cause and effect. Moreover, measurement error is likely to be present, particularly with regard to physical activity measures. For example, the place of physical activity (outdoor or indoor) was not recorded during the interview and had to be inferred from the type of activity. Information was collected only on the frequency of physical activity and not on the time participating in activity or the time of day when it was carried out. Both of the latter are likely to have influenced serum 25-hydroxyvitamin D levels. Further, the determination of vitamin D status from a single measure of serum 25-hydroxyvitamin D also is likely to have contributed to measurement error. If this measurement error was random, it may have weakened the observed association between outdoor physical activity and vitamin D, in which case, the true effect of outdoor physical activity on vitamin D status maybe stronger than what is reported in this paper.
In summary, nationally representative data demonstrate a strong association between the frequency of outdoor physical activity and serum levels of 25-hydroxyvitamin D in the US population. Daily outdoor activities may be able to redress the differences in vitamin D status associated with age and sex, but not those associated with race/ethnicity. Public health strategies that support daily outdoor activities that are safe (e.g., by avoiding sun burn) probably will increase the vitamin D status of the general population. However, additional strategies, such as higher-dosed vitamin D supplements, are required for African Americans in order to achieve optimal vitamin D levels.