Previous studies have indicated that vitamin D synthesis is affected by seasons and geographical latitude.¹⁾ For example, a Japanese study reported that vitamin D synthesis is affected by seasons and geographical location.²⁾ In Korea, some studies showed that vitamin D levels varied with geographic location³⁾; however, measurement of vitamin D levels during each season did not reveal any significant differences.

Vitamin D deficiency is associated with cardiovascular events, metabolic syndrome, diabetes mellitus (DM), chronic kidney disease, infection, osteoarthritis, low muscle strength, urinary incontinence, inflammatory bowel disease, and sleep apnea.⁴⁾

Biological actions of activated vitamin D (1,25-dihydroxyvitamin D [1,25(OH)2D]) are mediated by the vitamin D receptor (VDR). The VDR is expressed in reproductive tissues, such as testes, Sertoli cells, Leydig cells, prostate, seminal vesicles, and sperm.⁵⁾ A recent study suggested that vitamin D supplementation increases testosterone levels.⁶⁾ Other studies have also suggested a positive association between vitamin D levels and testosterone levels in men⁷⁾; however, such studies have not been conducted in Korean men.

Testosterone performs several beneficial functions in men, but total testosterone levels decrease with age. Lower testosterone is associated with osteoporosis, sexual dysfunction, and poor quality of life. Recently, lower testosterone levels were implicated in other health issues such as depressed mood, fatigue, cognitive impairment, insulin resistance, type 2 DM, hypertension, obesity, and metabolic syndrome.⁸⁾ Therefore, increasing testosterone levels improves the quality of life and helps prevent low testosterone-associated health issues in men.

Deficiencies in either vitamin D or testosterone can cause cardiovascular events, metabolic syndrome, DM, and emotional instability. Vitamin D and testosterone deficiency have similar symptoms, and therefore, may be closely related to one another. Considering the crucial functions mediated by both vitamin D and testosterone, it is clinically important to identify the nature of the association in their levels.

Considering that vitamin D and testosterone deficiencies have very similar symptoms and that vitamin D levels undergo seasonal variations, it is possible that testosterone levels also vary according to the season. This hypothesis is supported by the results of recent studies, which showed that testosterone levels vary with the seasons.^9,10) One Korean study that attempted to identify seasonal variations in testosterone levels found that free testosterone levels, but not total testosterone levels, underwent seasonal variations.¹¹⁾ Because total testosterone levels are used for the diagnosis of andropause by many primary hospital doctors, it is clinically important to evaluate seasonal variations in total testosterone levels.

The aim of this study was to determine whether serum vitamin D levels are positively correlated with testosterone levels in Korean males, and whether both vitamin D and testosterone levels undergo seasonal variations.

In this study, we found no correlation between serum 25(OH)D and total testosterone levels, even after adjustment for potential confounding variables. However, we found a significant association between seasonal variations in 25(OH)D levels, and a correlational tendency in seasonal variations of total testosterone levels. In addition, after adjustment for confounding variables a significant correlation was found in seasonal variations in total testosterone levels.

A recent study reported a positive correlation between vitamin D and total testosterone levels.^7,8) Moreover, seasonal variations in vitamin D and testosterone levels have also been reported.^1,2,9) Our study results support the occurrence of seasonal variations in vitamin D and total testosterone levels. However, our results do not support a correlation between total testosterone and vitamin D levels, which was reported in previous studies.

During sex hormone synthesis, vitamin D regulates calcium metabolism.¹²⁾ Moreover, calcium supports male reproductive function,^13,14) and the VDR in expressed in reproductive organs such as the testes. Thus, vitamin D could affect testosterone synthesis. Wehr et al.¹⁵⁾ reported an association between androgen levels and 25(OH)D levels in men, and many other study findings support the existence of a relationship between vitamin D and testosterone levels.

In the present study, serum 25(OH)D levels were not correlated with testosterone levels even after adjustment for confounding variables. This result indicates that a different confounding variable, other than the ones considered in this study, could be involved in both vitamin D and testosterone metabolism. Tak et al.¹¹⁾ reported that a higher level of 25(OH)D was associated with a higher total testosterone level in Korean men. However, the sample size in their study was different from that in the present study (652 versus 1,559), as were the average laboratory values, such as those of 25(OH)D and testosterone (25[OH]D: 21.23 versus 15.9 ng/mL, testosterone: 4.7 versus 5.1 ng/mL). A larger scale study is needed to resolve this discrepancy.

Recent studies that investigated seasonal variations in vitamin D yielded controversial results. Hagenau et al.¹⁶⁾ reported that vitamin D levels depend on age, sex, and skin color. However, Oleson et al.¹⁷⁾ reported seasonal variations in vitamin D levels in white individuals. This finding was supported by other studies suggesting that vitamin D levels are affected by seasons.^2,18) Our results also support the theory that vitamin D levels vary by season, with the highest levels occurring in the summer and the lowest levels in the spring. One of the reasons for this variation could be that during the colder seasons, people wear heavier clothing, which decreases the amount of skin exposed to sunlight, and results in a decrease in vitamin D synthesis. The average temperatures during the spring have decreased in recent years, which could contribute to greater seasonal variation in vitamin D levels.

Similarly, a recent study investigating the effect of seasons on testosterone levels yielded controversial results. González-Sales et al.⁹⁾ reported that testosterone levels during the winter and spring were higher than the levels during the summer and fall. Svartberg et al.¹⁰⁾ reported that testosterone levels were the highest in October and November, and the lowest in June. In contrast, Maes et al.¹⁹⁾ and Tancredi et al.²⁰⁾ reported that total testosterone and free testosterone levels, respectively, showed no significant seasonal variability.

Our result supports the theory that testosterone levels show seasonal variations, with the highest levels in the spring and the lowest levels in the summer. One possible reason for this variation may be the increased consumption of seafood such as oyster, shrimp, and blue crab during winter and spring (September–February) because of the increased availability of these foods during these seasons. Seafood contains zinc, a trace element known to be important for normal male reproductive function²¹⁾; thus, this seasonal dietary variation could contribute to the seasonal differences in testosterone levels.

On the basis of our results, we suggest that the reference values of serum 25(OH)D and testosterone levels should be adjusted for seasonal variations after reviewing the results of other studies.

Our study has several limitations. We enrolled subjects who underwent a medical examination at a university hospital. Therefore, our study sample does not reflect the general population, and our findings cannot be generalized. Furthermore, our study has a cross-sectional design, which made it difficult to evaluate prospective changes or the relationships among variables over time. In addition, participants could have self-medicated with vitamin D supplements, which would have significantly affected our results. Another limitation is that, in some men, blood samples were collected in the afternoon, and thus, differences in the time of sample collection could have contributed to differences in total testosterone levels. Finally, the total testosterone level, which was measured in this study, does not represent the level of the active sex hormone.

In conclusion, the serum 25(OH)D level was not correlated with the total testosterone level in Korean men. This result is inconsistent with results of recent studies that found a correlation between vitamin D and testosterone levels; additional studies with a larger general patient population are required to resolve this discrepancy.

In the present study, we found seasonal variations in serum 25(OH) D and total testosterone levels. These associations persisted after adjusting for age, total cholesterol, LDL, HDL, WC, BMI, FG, physical activity, alcohol use, and smoking. Although additional studies are required to confirm these findings, the reference values for vitamin D and testosterone levels in Korean men should be adjusted according to the season during sample collection.