Summer Sun Exposure Research Articles

The physiology of vitamin D-far more than calcium and bone.

The physiology of vitamin D-far more than calcium and bone.

Interdependence and contributions of sun exposure and vitamin D to MRI measures in multiple sclerosis

PurposeTo assess the relationships of sun exposure history, supplementation and environmental factors to vitamin D levels in multiple sclerosis (MS) patients and to evaluate the associations between sun exposure and...

Modelling vitamin D status due to oral intake and sun exposure in an adult British population

A mathematical model is described for estimating changes in plasma 25-hydroxyvitamin D (25(OH)D) levels throughout the year as a consequence of varying the oral intake of vitamin D and the behaviour outdoors of white British adults resident in different regions of the UK. The model yields seasonal and geographical patterns of 25(OH)D concentrations that agree closely with observational studies. Use of the model allows estimates to be easily made of the sun exposure and oral intake necessary to avoid vitamin D deficiency in defined proportions of the population, as well as strategies that would lead to vitamin D sufficiency throughout the year. The analysis demonstrates that addressing concerns about insufficient vitamin D levels, especially during the winter, may be achieved by modifying oral vitamin D intake over the winter, increasing summer sun exposure or a combination of both.

“Prescribing sunshine”: a national, cross-sectional survey of 1,089 New Zealand general practitioners regarding their sun exposure and vitamin D perceptions, and advice provided to patients

BackgroundThe health effects of ultraviolet radiation vary according to wavelength, timing and pattern of exposure, personal characteristics and practices. Negative effects include skin cancers, eye diseases and immune suppression; positive effects primarily relate to endogenous vitamin D production which protects against bone disease. Drafting comprehensive guidelines regarding appropriate sun protective behaviours and vitamin D sufficiency is challenging. Advice given by general practitioners is potentially influential because they are widely respected.MethodsA survey instrument was developed, pre-tested and provided to practising GP’s, either by on-line link or mailed, reply paid hard-copy. Odds ratios, differences in means, or ratios of geometric means from regression models are reported for potential predictor variables with 95% confidence intervals.ResultsData (demographic, training, practicing, information accessing, confidence in vitamin D knowledge) suitable for analysis were obtained from 1,089 GPs (32% participation). Many (43%) were ‘not at all confident’ about their vitamin D knowledge. Recent information led 29% to recommend less sun protection during winter months and 10% less all year. Confidence was positively associated with non-‘Western’ medical training, information sources read and practising in a metropolitan centre with a medical school. Reading the Melanoma Clinical Practice Guidelines was associated with lower estimates of the amount of summer sun exposure required to obtain adequate vitamin D. Increasing years in practice was negatively associated with provision of recommended advice about summer and winter sun protection. Greater concern about vitamin D than skin cancer was expressed by females and those in practice longer.ConclusionsConcern about the potentially negative impact of skin cancer prevention on vitamin D status may undermine appropriate sun protective recommendations. Reading some educational resources was associated with confidence about vitamin D knowledge and a perception that significantly less summer sun exposure was required for those with high sun sensitivity to achieve adequate vitamin D, suggesting a potentially positive impact of such resources. Education could be targeted towards groups least likely to promote existing recommendations. Authoritative guidelines about vitamin D and sun protection would be a valued resource among GPs. Study findings are potentially valuable to help guide public policy and target interventions.

Use of nonsteroidal anti-inflammatory drugs and risk of basal cell carcinoma in the United States radiologic technologists study

Nonsteroidal anti-inflammatory drugs (NSAIDs) have been associated with reduced risk of colorectal and other cancers, but the association with basal cell carcinoma (BCC) is unclear. Previous epidemiological studies have been small in size, conducted in especially vulnerable populations, or have not accounted for solar ultraviolet exposure, a major risk factor for BCC. In the United States Radiologic Technologists cohort, we followed subjects to assess NSAID use on risk of first incident BCC. We included Caucasian participants who responded to both second and third questionnaires (administered from 1994 to 1998 and 2003 to 2005, respectively), and who reported no cancer at the time of the second questionnaire, N = 58,213. BCC, constituent risk factors (e.g., eye color, complexion, hair color) and sun exposure history were assessed through self-administered survey. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated using Cox proportional hazards models. Of the 58,213 people in the study population, 2,291 went on to develop BCC. Any NSAID use was not associated with subsequent incidence of BCC (HR = 1.04, 95% CI: 0.92-1.16) after adjusting for age, sex and estimated lifetime summer sun exposure. Neither association was observed when stratified by NSAID type (aspirin and other NSAIDs), nor did dose-response patterns emerge by frequency of use (average days per month). Further analyses did not reveal interaction with sex, birth cohort, smoking, alcohol consumption, sun exposure, occupational radiation exposure or personal risk factors for BCC. In this large nationwide study, we observed no association between NSAID use and subsequent BCC risk.

SP2-4 Impact assessment of institutional campaigns compared to summer months on the search for information about self-examination of the skin

IntroductionThe analysis of log files of qualified websites has become recently a simple way to estimate the collective demand for health information. We analysed the access to the Brazilian National...

Sufficient Vitamin D from Casual Sun Exposure?

Next to the adverse effects of solar UV exposure, the beneficial effects mediated by vitamin D(3) have come into the limelight. The question then is "how much sun exposure do we actually need?" Estimates have been made, but the data are not quite adequate. The groups of Drs. Rhodes and Webb bridged the gap between experiments and everyday life by a study in which 109 volunteers were exposed in mid-winter to simulated solar UV radiation in summertime clothing at dosages of 1.3 SED three times a week. Thus, 90% reached sufficiently high vitamin D statuses (>50 nmol L(-1)). In this issue, these researchers transpose these experimental exposures in a cabinet to summertime noon exposures of people walking around for about half an hour in open terrain on a clear day in Manchester, UK. This result is an improvement over earlier estimates and shows that casual mid-day summer sun exposure should indeed suffice.

Predictors of clock drawing test (CDT) performance in elderly patients attending an internal medicine outpatient clinic: A pilot study on sun exposure and physical activity

Influence of sun exposure and physical activity on cognition has not been evaluated simultaneously. We aimed to evaluate predictors of clock drawing test (CDT) performance on n = 125 patients attending an internal medicine outpatient clinic. Interview data was gathered on sociodemographic, health-related and lifestyle factors referring to the last year. Factors associated with obtaining a score >0 and a full score (10/10) were analyzed by univariate (UVA) and multivariate (MVA) logistic regression analyses. Mean age of the participants was 72 ± 5, 58% were women and 17% were illiterate. Mean CDT score was 4.70 ± 2.27, 61.6% scored >0 and 21.6% scored 10/10. Both duration of walking and summer sun exposure predicted a CDT score >0 in UVA. However only summer sun exposure was an independent predictor (odds ratio = OR = 1.73, 95% confidence interval = CI = 1.16–2.57). Other factors independently associated with obtaining a score >0 were education level (OR = 2.70, 95%CI = 1.77–4.12) and cerebrovascular disease (CVD) history (OR = 0.08, 95%CI = 0.008–0.78). Factors independently associated with obtaining a full score were weight (OR = 1.05, 95%CI = 1.00–1.10), education level (OR = 2.04, 95%CI = 1.38–3.00) and visiting the clinic alone (OR = 3.92, 95%CI = 1.354–11.39). Our study shows that CDT can be utilized to unravel the lifestyle factors associated with cognitive function. To our knowledge, this is the first study to suggest an association between sun exposure and cognition.

Sun exposure questionnaire predicts circulating 25-hydroxyvitamin D concentrations in Caucasian hospital workers in southern Italy

Introduction Recent sun exposure should correlate with circulating 25-hydroxyvitamin D [25(OH)D] due to ultraviolet B (UVB)-catalyzed cutaneous synthesis of vitamin D. Methods A Sun Exposure Score was calculated for healthy adults using a recall questionnaire assessing daily Time in Sun (<5 min, 5–30 min, >30 min) and Skin Exposure (face/hands; face/hands and arms; face/hands and legs; and “bathing suit”) for 1 week in each of the winter and summer ( n = 47 and 23, respectively; n = 18 participated in both). Concentrations of 25(OH)D were measured by DiaSorin RIA on end-of-week sera. Results Mean serum 25(OH)D was higher in summer than winter (58.6 ± 16.5 nmol/L vs. 38.8 ± 29.0 nmol/L, respectively, P = 0.003 unpaired). The calculated Sun Exposure Score correlated strongly with serum 25(OH)D during summer (Spearman's rho = 0.59, P = 0.003); based on the Pearson coefficient of determination, summer Sun Exposure Score explained 38% of the variability in summer serum 25(OH)D. The Sun Exposure Score did not correlate with 25(OH)D in the winter (rho = 0.19, P = 0.210). The summer correlation was largely explained by the Time in Sun (rho = 0.58, P = 0.004) rather than area of Skin Exposed (rho = 0.10, P = 0.660). Although there was a correlation between winter and summer Sun Exposure Scores (rho = 0.63, P = 0.005), there was no summer vs. winter correlation in serum 25(OH)D (rho = 0.08, P = 0.76). Conclusion This simple 1-week sun exposure recall questionnaire predicted summer serum 25(OH)D concentrations, accounting for 38% of the variability in 25(OH)D among healthy Italian adults.

Seasonally Dynamic Habitat Use by Spotted (Clemmys guttata) and Blanding's Turtles (Emydoidea blandingii) in Maine

We used radio-telemetry to investigate the seasonal dynamics of wetland use by Spotted Turtles (Clemmys guttata) and Blanding's Turtles (Emydoidea blandingii) in southern Maine. Habitat use was examined in a temporally segregated manner, comparing wetland use among seasonally discrete activity periods. Distinct seasonal movement patterns were detected and logistic regression revealed significant differences in wetland characteristics across seasons for both species. Spotted Turtles exhibited a positive association with wetlands hosting abundant Wood Frog (Lithobates sylvaticus) egg masses in spring, and a negative association with forested wetlands from spring through late summer. Blanding's Turtles were closely associated with forested wetlands in spring, wetlands with abundant Wood Frog egg masses and good sun exposure in early summer, and deep-water wetlands in late summer and fall. The seasonal differences in habitat use found in this study highlight the complex and dynamic landscape required to sustain these rare turtles. Spotted and Blanding's Turtles' diverse habitat requirements require frequent terrestrial movements, exposing them to threats for which mitigation requires understanding spatial and temporal shifts in habitats use.

Estimation of the dietary requirement for vitamin D in free-living adults ≥64 y of age

BackgroundOlder adults may be more prone to developing vitamin D deficiency than younger adults. Dietary requirements for vitamin D in older adults are based on limited evidence. Objective:The objective was to establish the dietary intake of vitamin D required to maintain serum 25-hydroxyvitamin D [25(OH)D] concentrations above various cutoffs between 25 and 80 nmol/L during wintertime, which accounted for the effect of summer sunshine exposure and diet. Design:A randomized, placebo-controlled, double-blind, 22-wk intervention was conducted in men and women aged ≥64 y (n = 225) at supplemental levels of 0, 5, 10, and 15 μg vitamin D3/d from October 2007 to March 2008. Results:Clear dose-related increments (P < 0.0001) in serum 25(OH)D were observed with increasing supplemental vitamin D3 intakes. The slope of the relation between total vitamin D intake and serum 25(OH)D was 1.97 nmol · L−1 · μg intake−1. The vitamin D intake that maintained serum 25(OH)D concentrations >25 nmol/L in 97.5% of the sample was 8.6 μg/d. Intakes were 7.9 and 11.4 μg/d in those who reported a minimum of 15 min daily summer sunshine exposure or less, respectively. The intakes required to maintain serum 25(OH)D concentrations of >37.5, >50, and >80 nmol/L in 97.5% of the sample were 17.2, 24.7, and 38.7 μg/d, respectively. Conclusion:To ensure that the vitamin D requirement is met by the vast majority (>97.5%) of adults aged ≥64 y during winter, between 7.9 and 42.8 μg vitamin D/d is required, depending on summer sun exposure and the threshold of adequacy of 25(OH)D. This trial was registered at http://www.controlled-trials.com/ISRCTN20236112 as ISRCTN registration no. ISRCTN20236112.

Athletic Performance and Vitamin D

Activated vitamin D (calcitriol) is a pluripotent pleiotropic secosteroid hormone. As a steroid hormone, which regulates more than 1000 vitamin D-responsive human genes, calcitriol may influence athletic performance. Recent research indicates that intracellular calcitriol levels in numerous human tissues, including nerve and muscle tissue, are increased when inputs of its substrate, the prehormone vitamin D, are increased. We reviewed the world's literature for evidence that vitamin D affects physical and athletic performance. Numerous studies, particularly in the German literature in the 1950s, show vitamin D-producing ultraviolet light improves athletic performance. Furthermore, a consistent literature indicates physical and athletic performance is seasonal; it peaks when 25-hydroxy-vitamin D [25(OH)D] levels peak, declines as they decline, and reaches its nadir when 25(OH)D levels are at their lowest. Vitamin D also increases the size and number of Type II (fast twitch) muscle fibers. Most cross-sectional studies show that 25(OH)D levels are directly associated with musculoskeletal performance in older individuals. Most randomized controlled trials, again mostly in older individuals, show that vitamin D improves physical performance. Vitamin D may improve athletic performance in vitamin D-deficient athletes. Peak athletic performance may occur when 25(OH)D levels approach those obtained by natural, full-body, summer sun exposure, which is at least 50 ng x mL(-1). Such 25(OH)D levels may also protect the athlete from several acute and chronic medical conditions.

Assessing the status of vitamin D in a family medicine clinic in the fall

ObjectiveTo determine the status of Vitamin D in patients presenting to a suburban family medicine clinic among life‐stage groups in the fall.BackgroundVitamin D deficiency has been reportedly low in most of the US population with the majority, up to 78% of the population having inadequate levels due to low dietary intake and limited sun exposure. Depending on the cutoffs, the low status is found to vary and 25 ‐hydroxyvitamin D levels of 35‐40 ng/ml are hence considered adequate and a reasonable goal.MethodsRetrospective review of 73 ambulatory patients presenting to a suburban family medicine clinic in a Midwest city at latitude 41° were evaluated for their vitamin D status using the 25‐hydroxyvitamin D concentration and were grouped into 3 lifestages of 20‐49, 50‐69 and greater than 70 years of age. The Vitamin D levels were recorded in the fall when the level after the summer sun exposure should be at their highest. The results of Vitamin D levels were delineated as 0‐11 ng/mL, 12‐19 ng/mL, 20‐31 ng/mL, and above 32 ng/mL. None of the patients were housebound or incapacitated.ResultsA majority of the patients were found to have Vitamin D levels that were low‐ only 14 out of the 73 or 19.17%, had a level of optimum Vitamin D of greater than 32 ng/mL, with the large majority, 80.93%, being low.ConclusionVitamin D deficiency was confirmed to be endemic and universal across the life‐stages with the older cohort having less deficiency, probably as a result of more supplementation with increasing age and/or time for sun exposure. Younger cohorts had surprisingly the most severe deficiency probably as a result of limited sun exposure. The prevalence of low levels in the fall, following usual maximum sun exposure over the summer months indicates that the prevalence probably can be suspected to be more pronounced and that programs for widespread supplementation will be required across all age groups to optimize vitamin D status.

Melanocortin 1 receptor genotype, past environmental sun exposure, and risk of multiple sclerosis

Low past sun exposure, fair skin type, and polymorphisms of the MC1R gene have been associated with multiple sclerosis (MS) risk. We aimed to investigate the interplay between melanocortin 1 receptor gene variants, red hair/fair skin phenotype, and past environmental sun exposure in MS. Population-based case-control study in Tasmania, Australia, involving 136 cases with MS and 272 controls randomly drawn from the community and matched on sex and year of birth. Measures included past sun exposure by calendar and questionnaire, spectrophotometric skin type, and MC1R genotype, with any MC1R Arg151Cys, Arg160Trp, or Asp294His alleles present denoted as red hair color (RHC) variant. The association between RHC variant genotype and MS was more evident for women (odds ratio 2.02 [1.15-3.54]) than for men (odds ratio 0.65 [0.27-1.57]) (difference in effect, p = 0.03). The RHC variant genotype was associated with behavioral sun avoidance. In addition, increasing summer sun exposure at ages 6 through 10 years was associated with reduced MS risk among those with no RHC variant (p = 0.03), but not among those with RHC variant genotype (p = 0.15; difference in effect, p = 0.02). Similar findings were evident for other past sun exposure measures and when the sample was restricted to women only. The interplay between red hair color variant genotype, red hair/fair skin phenotype, and multiple sclerosis (MS) is complex. The modification of past sun exposure by MC1R genotype provides further support that ultraviolet radiation or derivatives such as vitamin D may be causally related to a reduced MS risk.

Queimadura solar em jovens: estudo de base populacional no Sul do Brasil

To assess the prevalence and risk factors for sunburn in young people. Population-based cross-sectional study using a multiple-stage sampling carried out with people living in the urban area of Pelotas, Southern Brazil, between October and December 2005. Data was collected from interviews with 1.604 subjects using a standardized pre-coded questionnaire about their family and another questionnaire applied to those aged between ten and 29 years for assessing the occurrence of sunburn episodes. Sunburn was defined as skin burning after sun exposure. Chi-square test with Yates' correction was used to compare proportions and Poisson regression with design effect control and robust adjustment of variance was applied in the multivariate analysis. Of those aged between 10 and 29 years, 1,412 reported sun exposure in the last summer. Losses and refusals were 5.5%. A total of 48.7% of the interviewees reported sunburn in the last year. The following variables were associated with sunburn in the multivariate analysis: white skin (PR=1.41; 95% CI: 1.12;1.79); higher skin sensitivity to sun exposure (PR=1.84; 95% CI: 1.64;2.06); age between 15 and 19 years (PR=1.30; 95% CI: 1.12;1.50); belonging to the higher quartile of income (PR=1.20; 95% CI: 1.01;1.42); and irregular use of sunscreens (PR=1.23; 95% CI: 1.08;1.42). The prevalence of sunburn in the population studied was high mainly among white young people with higher skin sensitivity, higher income and who used sunscreens irregularly. Sun exposure during safe times and with adequate protection should be promoted.

Sun-related behavior after a diagnosis of cutaneous malignant melanoma

Cutaneous malignant melanoma is a significant public health problem in Western countries, and the probability that patients with one melanoma will develop a second one is high. This study is an attempt to assess and understand sun-related behavior in patients subsequent to a melanoma diagnosis. We recruited 35 melanoma patients diagnosed in 2001 or 2002, who resided in the Greater Vancouver Regional District of British Columbia, Canada, and 35 controls frequency matched by broad age-group, sex, ethnicity, and area of residence to patients. All participants were interviewed over the telephone in the autumn of 2003 to ascertain their outdoor activities for the previous summer months (June-August) of 2003, along with their clothing preferences, sunscreen use, sunburn frequency, and knowledge and attitudes regarding sun exposure and tanning. Patients were less involved in outdoor recreational activities than controls (mean monthly hours 23.0 vs. 31.0, P=0.023). In addition, patients protected themselves from sun exposure using clothing and sunscreen more often than controls. However, the data also revealed that patients still engaged in outdoor activities to a substantial degree, often without sun protection. Furthermore, 27% still had a positive or neutral attitude toward tanning, even after melanoma diagnosis. The continuing presence of unprotected episodes of summer sun exposure in patients with melanoma, coupled with sunburn prevalence similar to normal controls, suggests that better education and behavior modification programs are needed for patients with melanoma. The results may indicate the need for more emphasis on post-treatment counseling by physicians.

Adolescent Girls in Maine Are at Risk for Vitamin D Insufficiency

The objective was to determine the seasonal fluctuations in serum 25-hydroxyvitamin D (25-OHD) in a group of healthy adolescents living in a northern climate. Twenty-three 9- to 11-year-old girls participated in the study from September 2000 to March 2003. Serum 25-OHD and parathyroid hormone levels were measured each September and March. Dietary intake of vitamin D was assessed each summer and winter. Summer-sun exposure was evaluated using reports of time spent outdoors. The mean decrease in serum 25-OHD from September to March was 28%. Vitamin D insufficiency (at least one serum 25-OHD level <50 nmol/L) was observed in 11 of 23 (48%) subjects. Four of 23 subjects (17%) exhibited vitamin D insufficiency in both September and March. Mean parathyroid hormone levels increased 4 pg/mL (15%) from September to March. Vitamin D intakes need to be increased in winter at northern latitudes.

Sunlight and the 10-year incidence of age-related maculopathy: the Beaver Dam Eye Study.

To examine the association of sunlight exposure and indicators of sun sensitivity with the 10-year incidence of age-related maculopathy (ARM). Population-based cohort study. We included persons aged 43 to 86 years at the baseline examination from 1988 to 1990, living in Beaver Dam, Wis, of whom 3684 persons underwent 5-year follow-up and 2764 underwent 10-year follow-up. Data on sun exposure and indicators of sun sensitivity were obtained from a standardized questionnaire administered at baseline and/or follow-up. We determined ARM status by grading stereoscopic color fundus photographs using the Wisconsin Age-Related Maculopathy Grading System. Incidence and progression of ARM. While controlling for age and sex, we found that participants exposed to the summer sun for more than 5 hours a day during their teens, in their 30s, and at the baseline examination were at a higher risk of developing increased retinal pigment (risk ratio [RR], 2.99; 95% confidence interval [CI], 1.18-7.60; P =.02) and early ARM (RR, 2.20; 95% CI, 1.02-4.73; P =.04) [corrected] by 10 years than those exposed less than 2 hours per day during the same periods. In participants reporting the highest summer sun exposure levels in their teens and 30s, the use of hats and sunglasses at least half the time during the same periods was associated with a decreased risk of developing soft indistinct drusen (RR, 0.55; 95% CI, 0.33-0.90; P =.02) and retinal pigment epithelial depigmentation (RR, 0.51; 95% CI, 0.29-0.91; P =.02). Participants who experienced more than 10 severe sunburns during their youth were more likely than those who experienced 1 or no burn to develop drusen with a 250-microm diameter or larger (RR, 2.52; 95% CI, 1.29-4.94 [corrected] P =.01) by the 10-year examination. No relationships were found between UV-B exposure, winter leisure time spent outdoors, skin sun sensitivity, or number of bad sunburns experienced by the time of the baseline examination and the 10-year incidence and progression of ARM or its associated lesions. Few significant relationships between environmental exposure to light and the 10-year incidence and progression of ARM were found in the Beaver Dam Eye Study. Consistent with results from the baseline and 5-year follow-up examinations, significant associations were found between extended exposure to the summer sun and the 10-year incidence of early ARM and increased retinal pigment. A protective effect of hat and sunglasses use by participants while in their teens and 30s against the 10-year incidence of soft indistinct drusen and retinal pigment epithelial depigmentation was also found, but only in those who reported the highest amount of sun exposure during the same periods.

PAST SUN EXPOSURE, SKIN PHENOTYPE AND RISK OF MULTIPLE SCLEROSIS

The striking latitudinal gradient of multiple sclerosis (MS) prevalence may reflect variation in ultraviolet radiation (UVR). A Th1 cell mediated attack on myelin proteins appears to be important in the pathogenesis of MS and recent immunological work shows that UVR can suppress the Th1 cell mediated activity. We conducted a population based case-control study in Tasmania, Australia, to examine whether high past sun exposure was associated with a reduced risk of MS. Interviews were conducted with 136 cases with MS and 272 controls randomly drawn from the community and matched on sex and birth year. Measures included reported sun exposure, dermal actinic damage (a marker of cumulative lifetime sun exposure), and skin spectrophotometry. Higher (2.5 hours a day on average) sun exposure in summer on weekends and holidays over the age span of 6–15 years was associated with a decreased risk of MS (adjusted odds ratio, 0.31; 95 percent confidence interval, 0.16 to 0.59). High (grade 4–6) actinic damage was also independently associated with a decreased MS risk (adjusted odds ratio, 0.32; 95 percent confidence interval, 0.11 to 0.88). A dose response relationship was observed in both sun exposure over the age span of 6–15 years (p < 0.01) and actinic damage (p < 0.01). Higher sun exposure appears to be associated with a reduced risk of MS. These findings are consistent with insufficient UVR influencing the development of MS.

Effects of above average summer sun exposure on serum 25-hydroxyvitamin D and calcium absorption.

The purpose of this study was to examine the effects of summer sun exposure on serum 25-hydroxyvitamin D [25(OH)D], calcium absorption fraction, and urinary calcium excretion. Subjects were 30 healthy men who had just completed a summer season of extended outdoor activity (e.g. landscaping, construction work, farming, or recreation). Twenty-six subjects completed both visits: after summer sun exposure and again approximately 175 d later, after winter sun deprivation. We characterized each subject's sun exposure by locale, schedule, and usual attire. At both visits we measured serum 25(OH)D, fasting urinary calcium to creatinine ratio, and calcium absorption fraction. Median serum 25(OH)D decreased from 122 nmol/liter in late summer to 74 nmol/liter in late winter. The median seasonal difference of 49 nmol/liter (interquartile range, 29-67) was highly significant (P < 0.0001). However, we found only a trivial, nonsignificant seasonal difference in calcium absorption fraction and no change in fasting urinary calcium to creatinine ratio. Findings from earlier work indicate that our subjects' sun exposure was equivalent in 25(OH)D production to extended oral dosing with 70 micro g/d vitamin D(3) (interquartile range, 41-96) or, equivalently, 2800 IU/d (interquartile range, 1640-3840). Despite this input, at the late winter visit, 25(OH)D was less than 50 nmol/liter in 3 subjects and less than 75 nmol/liter in 15 subjects.