[Progress in health effects and measurement standards of ultraviolet radiation in the workplace].

Ultraviolet radiation affects human health. On the one hand, moderate amounts of UV radiation can promote human health and have the effect of promoting vitamin D production; but on the other hand, excessive UV radiation can also cause adverse effects on human skin and eyes, such as causing skin photoaging, skin cancer, electrophthalmia and cataracts to occur. Therefore, the measurement of ultraviolet radiation is extremely important. This paper mainly reviews the health effects of ultraviolet radiation and the progress of measurement standards in the workplace, and puts forward suggestions on the revision of the existing standards from five aspects including use new measuring instruments and methods, improve the existing measuring instruments, specify the number of measurements, expand the scope of application of the standards and consider the influence of the sun on the measurement of artificial ultraviolet radiation, so as to provide reference for the revision of new standards.

Sun-induced production of vitamin D3 throughout 1 year in tropical and subtropical regions: relationship with latitude, cloudiness, UV-B exposure and solar zenith angle.

This study evaluated the differences in vitamin D3 synthesis in two different latitudes throughout 1 year using an in vitro model, which simulates cutaneous vitamin D photoproduction. Borosilicate ampoules containing 7-dehydrocholesterol (7-DHC) were exposed to sunlight hourly throughout the daylight hours, 1 day per month for a year, in Fortaleza (latitude 03° 43' 01" S-LAT3° S) and Sao Paulo (latitude 23° 32' 53" S-LAT23° S). Later, vitamin D3 and photoisomers of 7-DHC (tachysterol and lumisterol) were measured by a high-performance liquid chromatography system (HPLC). Vitamin D synthesis weighted UV radiation (UVBVitD) and solar zenith angle (SZA) were calculated during the same periods for both latitudes. Vitamin D3 synthesis occurred throughout the year in both locations, as expected in latitudes lower than 35°. Median of photoconversion to vitamin D3 through the year was higher in LAT3°S [median (IQR): LAT 3°S 4.1% (6.0); LAT 23°S 2.9% (4.5); p value = 0.020]. Vitamin D3 production strongly correlated with UV-B (LAT3° S, r = 0.917; p < 0.0001 and at LAT23° S, r = 0.879; p < 0.0001) and SZA (LAT3° S, r = - 0.924; p < 0.0001 and in LAT23°S, r = - 0.808; p < 0.0001). Vitamin D3 production starts later in LAT23° S, especially in winter. Lowest percentages were observed in June in both cities, although, compared to LAT3° S, in LAT 23° S the conversion was over 50% lower in the winter period. Cloudiness impaired photoproduction of Vitamin D3 even in summer months in both latitudes. Our results provide data to help guide medical recommendations for sensible sun exposure to promote the cutaneous production of vitamin D3 at different latitudes, seasonality, time of day and cloudiness status in Brazil.

Idiopathic Myocardial Fibrosis in Captive Chimpanzees (Pan troglodytes).

Cardiovascular disorders and predominantly idiopathic myocardial fibrosis are frequently associated with mortality among zoo-housed chimpanzees (Pan troglodytes). Formalin-fixed whole hearts of deceased chimpanzees housed in zoos (n = 33) and an African sanctuary (n = 2) underwent detailed macroscopic and histopathologic examination using a standardized protocol. Archived histological slides from the hearts of 23 additional African sanctuary-housed chimpanzees were also examined. Myocardial fibrosis (MF) was identified in 30 of 33 (91%) of the zoo-housed chimpanzees but none of the 25 sanctuary-housed chimpanzees. MF was shown to be characterized by both interstitial and replacement fibrosis. Immunophenotyping demonstrated that the fibrotic lesions were accompanied by the increased presence of macrophages, alpha smooth muscle actin-positive myofibroblasts, and a minimal to mild T-cell-dominant leukocyte infiltration. There was no convincing evidence of cardiotropic viral infection or suggestion that diabetes mellitus or vitamin E or selenium deficiency were associated with the presence of the lesion. However, serum vitamin D concentrations among zoo-housed chimpanzees were found to be lower in seasons of low ultraviolet light levels.

Overview on vitamin D and sunbed use.

Vitamin D seems to be associated with a protective effect in a vast range of diseases, including cardiovascular, autoimmune and oncologic conditions. Since ultraviolet (UV) B light is the most important prerequisite for the cutaneous synthesis of vitamin D, sunbeds are able to increase serum vitamin D levels, although only transiently in most cases. In this scenario, the artificial tanning industry relentlessly tries to promote the use of sunbeds as a 'safe' therapeutic measure to achieve an adequate serum vitamin D status. The World Health Organization classified UV-emitting tanning devices, as well as the whole UV spectrum, as group-1 carcinogens, as they significantly increase the risk of melanoma and non-melanoma skin cancer. In case of vitamin D deficiency or insufficiency, the current risk-benefit ratio is therefore in favour of vitamin D supplementation instead of sunbed use. Artificial tanning devices should never be considered as an option to achieve an appropriate vitamin D status. Their supposedly beneficial effects, vastly publicised by the artificial tanning industry, are not worth the carcinogenic risk associated with sunbed use.

The role of UV radiation and vitamin D in the seasonality and outcomes of infectious disease.

The seasonality of infectious disease outbreaks suggests that environmental conditions have a significant effect on disease risk. One of the major environmental factors that can affect this is solar radiation, primarily acting through ultraviolet radiation (UVR), and its subsequent control of vitamin D production. Here we show how UVR and vitamin D, which are modified by latitude and season, can affect host and pathogen fitness and relate them to the outcomes of bacterial, viral and vector-borne infections. We conducted a thorough comparison of the molecular and cellular mechanisms of action of UVR and vitamin D on pathogen fitness and host immunity and related these to the effects observed in animal models and clinical trials to understand their independent and complementary effects on infectious disease outcome. UVR and vitamin D share common pathways of innate immune activation primarily via antimicrobial peptide production, and adaptive immune suppression. Whilst UVR can induce vitamin D-independent effects in the skin, such as the generation of photoproducts activating interferon signaling, vitamin D has a larger systemic effect due to its autocrine and paracrine modulation of cellular responses in a range of tissues. However, the seasonal patterns in infectious disease prevalence are not solely driven by variation in UVR and vitamin D levels across latitudes. Vector-borne pathogens show a strong seasonality of infection correlated to climatic conditions favoring their replication. Conversely, pathogens, such as influenza A virus, Mycobacterium tuberculosis and human immunodeficiency virus type 1, have strong evidence to support their interaction with vitamin D. Thus, UVR has both vitamin D-dependent and independent effects on infectious diseases; these effects vary depending on the pathogen of interest and the effects can be complementary or antagonistic.

Ultraviolet B Radiation: The Vitamin D Connection.

Vitamin D is known as the sunshine vitamin. During exposure to sunlight the skin transforms 7-dehydrocholesterol into vitamin D3. Throughout evolution vitamin D3 has played a pivotal role in the evolution of vertebrates. Vitamin D is not only critically important for bone health but has a multitude of other biologic functions to help reduce chronic illnesses. This Chapter reviews how vitamin D is produced in the skin, factors that affect its production and a prospective on how to obtain vitamin D from sensible sun exposure.

Role of Vitamin D in Rheumatoid Arthritis.

Vitamin D is a fat soluble hormone, with a well described role in bone health and calcium/phosphate metabolism. Recent evidences have related vitamin D to other physiological functions and pathological conditions. Specifically, vitamin D has widely proven activities on immune system and evidences suggest that it may be implicated in the pathogenesis of rheumatoid arthritis (RA). The relationship between vitamin D and RA is complex, also because a deficitary vitamin D status, which is very common in RA patients, can contribute to the increased risk of osteoporosis typical of RA. In this chapter, will be described and discussed the main aspects of the relationship between RA and vitamin D.

Improved human gingival fibroblast response to titanium implants coated with ultraviolet-irradiated vitamin D precursor and vitamin E.

BACKGROUND AND OBJECTIVE: Ultraviolet (UV)-irradiated 7-dehydrocholesterol (7-DHC) and vitamin E (VitE)-coated titanium (Ti) implants have a beneficial effect on bone cells. Human gingival fibroblasts (HGFs) are the most abundant cells in periodontal tissues and are involved in the wound healing and repair. The objective of this study was to evaluate the response of HGFs to Ti implants coated with UV-irradiated 7-DHC and VitE, for improved soft-tissue integration of dental implants. MATERIAL AND METHODS: Ti surfaces were coated with 7-DHC and VitE, irradiated with UV light and incubated for 48 h at 23°C to allow cholecalciferol (D3 ) synthesis from 7-DHC onto the Ti surface. HGFs were cultured on the modified surfaces and the influence of the coating on these cells was evaluated through the analysis of: (i) biocompatibility; (ii) the mRNA levels of genes involved in the composition and turnover of the extracellular matrix, the inflammatory response, periodontal bone resorption and wound healing; and (iii) the levels of MMP-1 and TIMP-1 proteins. RESULTS: We found a beneficial effect of UV-irradiated 7-DHC:VitE-coated Ti implants on HGFs. Besides being biocompatible with HGFs, the UV-irradiated 7-DHC and VitE coating increased the levels of collagen III α1 and fibronectin mRNAs. and decreased the level of interleukin-8 mRNA. TIMP-1 was increased at both mRNA and protein levels in HGFs cultured on UV-irradiated 7-DHC:VitE-coated Ti implants. Finally, the UV-irradiated 7-DHC and VitE coating decreased the level of RANKL mRNA in HGFs. CONCLUSION: UV-irradiated 7-DHC:VitE-coated Ti implants have a positive effect on HGFs in vitro by reducing the inflammatory response and extracellular matrix breakdown.

Serum 25-hydroxyvitamin D increases with NB-UVB and UVA/UVB phototherapy in patients with psoriasis and atopic dermatitis in Western Australia.

BACKGROUND/OBJECTIVE: The benefit of NB-UVB phototherapy on serum 25-hydroxyvitamin D [25(OH)D] levels in patients with inflammatory skin conditions has been reported in the northern hemisphere. Vitamin D status is known to differ between geographical latitudes. The objective of this study was to investigate the influence of NB-UVB and UVA/UVB phototherapy on the 25(OH)D serum levels in patients with psoriasis and atopic dermatitis in Western Australia. METHODS: A total of 35 patients with psoriasis or atopic dermatitis requiring phototherapy thrice weekly for a minimum of 4 weeks were enrolled. Of these, 20 patients completed the study. Serum vitamin D levels were measured at baseline and at approximately 6 weeks into phototherapy. Data were adjusted for season, patients' age, sex, skin condition and Fitzpatrick skin phototype. RESULTS: There was a statistically significant increase in serum 25(OH)D from pre- to post-NB-UVB and UVA/UVB phototherapy (P < 0.0001), with a mean raw increase of 34.6 (25) nmol/L; or 45.1 (7.5) nmol/L when adjusted for covariates. This was also true for patients receiving NB-UVB phototherapy with a baseline vitamin D of <80 nmol/L (P < 0.05) and >80 nmol/L (P < 0.004). CONCLUSIONS: NB-UVB and UVA/UVB phototherapy significantly increased 25(OH)D serum level in patients with psoriasis and atopic dermatitis in Western Australia. Our study cohort had a higher baseline vitamin D level and a lower percentage increase of serum 25(OH)D post-phototherapy than the increases reported in the literature from cohorts in the northern hemisphere.

Effects of ultraviolet light on human serum 25-hydroxyvitamin D and systemic immune function.

BACKGROUND: Many immune-mediated diseases are associated with low levels of vitamin D and sunlight. UV light or supplementation with vitamin D can increase regulatory T-cell activity and prevent animal models of autoimmune disease. Increasing population vitamin D levels may therefore alleviate the burden of human immune-mediated disease. OBJECTIVE: To determine the responses of circulating 25-hydroxyvitamin D [25(OH)D] levels, regulatory T-cell numbers, and immune function to UV light exposure in patients being treated for skin disease. METHODS: Twenty-four subjects with skin disease from the North of Scotland were recruited between December and March. At baseline, and after 2 and 4 weeks of narrowband UV light exposure, we measured peripheral blood 25(OH)D level, numbers of regulatory T cells (CD4(+)CD25(hi)FoxP3(+)), and T-cell proliferative and cytokine responses to anti-CD3/CD28 stimulation. RESULTS: Median (interquartile range) narrowband UV-B received during the study was 39.1 (30.9) as standard erythema dose, comparable to a quarter of the median summer sunlight exposure received locally. This increased the 25(OH)D level from a mean ± SD of 34 ± 17 nmol/L to 58 ± 16 nmol/L after 2 weeks and 78 ± 19 nmol/L after 4 weeks. The mean proportion of circulating regulatory T cells increased from 0.5% to 1.6% CD3(+) cells, which significantly correlated with the increased 25(OH)D level. UV treatment was also followed by reduced proliferative and IL-10 responses to anti-CD3/CD28 independent of the 25(OH)D level. CONCLUSION: Narrowband UV light reduces systemic immune responsiveness via the induction of regulatory T cells. Light and 25(OH)D levels may affect particular immune functions independently. The levels of serum 25(OH)D over which these effects are apparent should guide future interventions.

Ultra-low Ultraviolet Radiation in Office Lighting Can Moderate Seasonal Vitamin D Cycle: A Pilot Study.

BACKGROUND/AIM: Ultraviolet-B (UV-B) radiation initiates vitamin D synthesis in the skin, making sun exposure a major source of vitamin D. We aimed to determine whether office lighting containing ultra-low levels of UV-B radiation could modify the winter decline in vitamin D status in the UK, while being safe and well tolerated. PATIENTS AND METHODS: Twenty commercial office desk lamps were modified with the addition of UV-B LEDs. Ten hospital office administrative staff received UV-modified lamps with UV-on, and 10 staff received identical placebo lamps with UV switched off, in a double-blind, cross-over pilot study during the winter of 2021/22. Circulating 25-hydroxyvitamin D [25(OH)D] was measured every 4 weeks for 20 weeks: at baseline and during an 8-week trial period, 4-week washout, and a cross-over 8-week trial period. RESULTS: The linear regression combining the complete datasets for phase 1 and 2 of the trial showed that an 8-week UV light intervention significantly increased 25OHD by 7.13 nmol/l with a p-Value=0.02, compared to the placebo group. Similar results were confirmed by cross-over analyses using the datasets of those completing both phases of the trial both with and without using the inverse probability weighing method to handle dropouts. CONCLUSION: The UV-B-modified lighting was well-tolerated and safe with weekly doses of UV-B of 0.5 - 0.9 Standard Erythema Dose [SED=100 Jm-2 erythema weighted UV radiation] measured at chest level. This ultra-low dosing was effective in reducing the winter decline in vitamin D status.

Prostate cancer incidence in Australia correlates inversely with solar radiation.

UNLABELLED: What's known on the subject? and What does the study add? Increased sun exposure and blood levels of vitamin D have been postulated to be protective against prostate cancer. This is controversial. We investigated the relationship between prostate cancer incidence and solar radiation in non-urban Australia, and found a lower incidence in regions receiving more sunlight. In landmark ecological studies, prostate cancer mortality rates have been shown to be inversely related to ultraviolet radiation exposure. Investigators have hypothesised that ultraviolet radiation acts by increasing production of vitamin D, which inhibits prostate cancer cells in vitro. However, analyses of serum levels of vitamin D in men with prostate cancer have failed to support this hypothesis. This study has found an inverse correlation between solar radiation and prostate cancer incidence in Australia. Our population (previously unstudied) represents the third group to exhibit this correlation. Significantly, the demographics and climate of Australia differ markedly from those of previous studies conducted on men in the United Kingdom and the United States. OBJECTIVE: • To ascertain if prostate cancer incidence rates correlate with solar radiation among non-urban populations of men in Australia. PATIENTS AND METHODS: • Local government areas from each state and territory were selected using explicit criteria. Urban areas were excluded from analysis. • For each local government area, prostate cancer incidence rates and averaged long-term solar radiation were obtained. • The strength of the association between prostate cancer incidence and solar radiation was determined. RESULTS: • Among 70 local government areas of Australia, age-standardized prostate cancer incidence rates for the period 1998-2007 correlated inversely with daily solar radiation averaged over the last two decades. CONCLUSION: • There exists an association between less solar radiation and higher prostate cancer incidence in Australia.

Availability of vitamin D photoconversion weighted UV radiation in southern South America.

Ultraviolet radiation (UVR) plays a key role in several biological functions, including human health. Skin exposure to UVR is the main factor in vitamin D photoconversion. There is also evidence relating low levels of vitamin D with certain internal cancers, mainly colon, breast and prostate, as well as other diseases. Several epidemiological studies have shown an inverse relationship between the above-mentioned diseases and latitude, in accordance with the ultraviolet radiation latitudinal gradient. The aim of this study is to determine whether UV irradiance levels in the southern South America are sufficient to produce suitable levels of vitamin D year around. For this purpose, vitamin D photoconversion weighted-irradiance was analyzed between S.S. de Jujuy (24.17°S, 65.02°W) and Ushuaia (54° 50'S, 68° 18'W). In addition to irradiance, skin type and area of body exposed to sunlight are critical factors in vitamin D epidemiology. Due to a broad ethnic variability, it was assumed that the skin type in this region varies between II and V (from the most to the less sensitive). All sites except South Patagonia indicate that skin II under any condition of body area exposure and skin V when exposing head, hands, arms and legs, would produce suitable levels of vitamin D year round (except for some days in winter at North Patagonian sites). At South Patagonian sites, minimum healthy levels of vitamin D year round can be reached only by the more sensitive skin II type, if exposing head, hands, arms and legs, which is not a realistic scenario during winter. At these southern latitudes, healthy vitamin D levels would not be obtained between mid May and beginning of August if exposing only the head. Skin V with head exposure is the most critical situation; with the exception of the tropics, sun exposure would not produce suitable levels of vitamin D around winter, during a time period that varies with latitude. Analyzing the best exposure time during the day in order to obtain a suitable level of vitamin D without risk of sunburn, it was concluded that noon is best during winter, as determined previously. For skin type II when exposing head, exposure period in winter varies between 30 and 130 min, according to latitude, except for South Patagonian sites. During summer, noon seems to be a good time of day for short periods of exposure, while during leisure times, longer periods of exposure without risk of sunburn are possible at mid-morning and mid-afternoon. At 3 h from noon, solar zenith angles are almost the same for sites between the tropics and North Patagonia, and at 4 h from noon, for all sites. Then, in these cases, the necessary exposure periods varied slightly between sites, only due to meteorological differences.

Factors influencing vitamin D status.

The steroid hormone vitamin D is required for normal calcium and phosphorus metabolism and is thus an important contributor to musculoskeletal health. Recent data have linked low vitamin D levels to a wide range of diseases, including cancer, cardiovascular disease, autoimmune disease and infection. Adequate levels of vitamin D are maintained through its cutaneous photosynthesis and oral ingestion. By some estimates, one billion people worldwide have vitamin D deficiency or insufficiency. A number of factors influence the photosynthesis and bioavailability of vitamin D and contribute to risk of impaired vitamin D status. These factors include variation in sun exposure due to latitude, season, time of day, atmospheric components, clothing, sunscreen use and skin pigmentation, as well as age, obesity and the incidence of several chronic illnesses. This review will focus on factors that influence vitamin D status and contribute to the prevalence of low vitamin D levels.

[Radiated drugs, the way of health]. / Médicaments irradiés, sources de santé.

During the inter-war years, the word "radiated" did not only suggest radioactivity, but it was also used to indicate exposure to others radiations, such as ultraviolets. The actinotherapy, a new therapy in vogue, was applied to many pathologies and tried on many substances. "Radiated drugs" result of those experimentations. Their therapeutical characteristics were found during searches on rickets. Our study relates the story of fight against rickets in France, from the use of cod liver oil to the synthesis of Vitamine D.

Air pollutants are negatively associated with vitamin D-synthesizing UVB radiation intensity on the ground.

Atmospheric levels of pollutants may reduce the UVB intensity at the earth's surface, with a subsequent reduction in cutaneous vitamin D synthesis. We investigated the association of various pollutants with UVB intensity on the ground. Four-year data obtained from four weather stations from across Kuwait were analyzed by median regression. Pollutants that were negatively associated with UVB were [ß (95% CI)]: benzene [- 2.61 (- 4.13, - 1.09)], ethyl-benzene [- 2.20 (- 3.15, - 1.25)], ozone [- 0.23 (- 0.28, - 0.17)], nitric oxide [- 0.11 (- 0.15, - 0.06)], sulfur dioxide [- 0.10 (- 0.17, - 0.04)] and particulate matter PM10 [- 0.002 (- 0.003, - 0.002)]. Pollutants that were negatively associated with the UVB/UVA ratio were [ß (95% CI)]: benzene [- 15.57 (- 24.94, - 6.20)], nitric oxide [- 0.53 (- 0.81, - 0.25)], ozone [- 0.38 (- 0.70, - 0.06)], and total hydrocarbon [- 0.02 (- 0.04, - 0.01)]. Furthermore, benzene and nitric oxide levels were higher in the morning and evening hours, which are the times of most solar exposure in this region due to high temperature during midday. In addition to other known factors, attenuation of UVB by these pollutants may contribute to lower vitamin D levels in populations. In addition to direct public health hazard, these pollutants may contribute to the very high prevalence of VDD in this region.

Narrowband ultraviolet B treatment improves vitamin D balance and alters antimicrobial peptide expression in skin lesions of psoriasis and atopic dermatitis.

BACKGROUND: Narrowband ultraviolet B (NB-UVB) is a routine treatment for psoriasis and atopic dermatitis (AD) but its effect on vitamin D balance is not well studied. OBJECTIVES: To examine whether NB-UVB treatment in winter improves vitamin D balance in psoriasis and AD, and to study the effects of NB-UVB on antimicrobial peptide and cytokine expression in the skin. METHODS: Eighteen adult patients with psoriasis, 18 with AD and 15 healthy subjects received a total of 15 NB-UVB exposures on the whole body, given three times a week. Serum calcidiol (25-hydroxyvitamin D) was measured by radioimmunoassay. Antimicrobial peptide and cytokine expression in skin lesions was examined by real-time quantitative polymerase chain reaction. RESULTS: At onset 16 (89%) patients with psoriasis, 17 (94%) patients with AD and eight (53%) healthy subjects had vitamin D insufficiency (calcidiol < 50 nmol L(-1)). NB-UVB treatment significantly increased (P < 0.001) serum calcidiol. The increase was 59.9 nmol L(-1) (95% confidence interval, CI 53.5-66.9) in psoriasis, 68.2 nmol L(-1) (95% CI 55.4-80.1) in AD and 90.7 nmol L(-1) (95% CI 63.8-123.4) in healthy subjects. Psoriasis Area and Severity Index and SCORAD improved significantly (P < 0.001) but no correlation to the increase of serum calcidiol was found. Cathelicidin and human beta-defensin 2 (HBD2) expression was high in skin lesions of psoriasis. After six NB-UVB treatments cathelicidin increased further while HBD2 expression decreased. A similar trend was observed in AD lesions. NB-UVB caused a marked but nonsignificant decrease of interleukin (IL)-1beta and IL-17 in psoriasis lesions. CONCLUSIONS: The present study shows that in addition to a significant improvement of psoriasis and AD, NB-UVB treatment effectively corrects vitamin D insufficiency. It also increases cathelicidin and decreases HBD2 levels in healing skin lesions of psoriasis and AD. This effect might be mediated by improved vitamin D balance and the local cytokine network.

Voluntary exposure of some western-hemisphere snake and lizard species to ultraviolet-B radiation in the field: how much ultraviolet-B should a lizard or snake receive in captivity?

Studies of voluntary exposure to ultraviolet-B (UVB) radiation from the sun in the field were conducted in the southern US and Jamaica for 15 species of lizards and snakes occupying various habitats. Species were sorted into four zones of UVB exposure ranging from a median UV index of 0.35 for zone 1 to 3.1 for zone 4. Guidelines for UVB exposure in captivity of these and species occupying similar light environments are presented. Data for most species were collected during mid-day during the spring breeding season, which appeared to be the time of maximum exposure. For two species of Sceloporus studied more intensively there was significant variation of exposure among times of the day and among seasons. So, all-day studies over the entire active season are necessary to fully understand the pattern of natural exposure for a particular diurnal species. Environmental and body temperature and thermoregulation as well as UVB/vitamin D photoregulation influences exposure to UVB. Regressions allowing the inter-conversion of readings among some meters with different detector sensitivities are presented. Readings of natural sunlight predict the same photobiosynthetic potential for vitamin D as the same reading from artificial sources whose wavelength distribution within the UVB band of the source is comparable to that of sunlight. Research approaches to further increase our understanding of vitamin D and UVB use and requirements for squamate reptiles in captivity are outlined.

Vitamin D status is seasonally stable in northern European dogs.

BACKGROUND: Numerous studies in veterinary species have recently linked vitamin D status with nonskeletal health disorders. Previous studies have indicated that dogs cannot produce endogenous vitamin D via cutaneous production and rely solely on dietary intake of vitamin D. The seasonal variation of vitamin D seen in humans due to changes in ultraviolet (UV) exposure, therefore, is unlikely to be replicated in these animals. OBJECTIVES: The objective of this study was to investigate the natural variation in 25-hydroxyvitamin-D concentrations in dogs subject to seasonal UV exposure. METHODS: This longitudinal study followed 18 healthy dogs fed a standardized diet over 1 year, with blood samples obtained monthly. Two key vitamin D metabolites, 25-hydroxyvitamin-D2 and 25-hydroxyvitamin-D3 , were assessed by liquid chromatography-tandem mass spectrometry in serum samples. Various other biochemical parameters were also measured. Seasonality was assessed using cosinor statistical analysis. RESULTS: Although the dogs were subject to seasonally varying UV radiation, 25-hydroxyvitamin-D and related biomarkers (including calcium and parathyroid hormone) remained stable over time and did not follow a seasonal pattern. 25-hydroxyvitamin-D was not positively correlated with exposure to UV radiation. Nonetheless, variation in 25-hydroxyvitamin-D concentrations between individual dogs was detected. CONCLUSIONS: Given the standardization of diet, we concluded that the seasonal stability of 25-hydroxyvitamin-D concentration (vitamin D status) was likely a direct result of lack of cutaneous vitamin D production in this species and highlights the importance of dietary intake. The variation in 25-hydroxyvitamin-D concentration between animals warrants further investigation.