Vitamin D, Sunlight, and Seasonal Supplementation: A Hormonal Perspective

Vitamin D sits at a unique intersection between nutrition, hormone biology, and environmental light exposure. Unlike most nutrients, it is produced in the skin in response to sunlight, and its levels fluctuate strongly with the seasons. This has led many people to question whether year-round supplementation is necessary, particularly during summer months when sunlight exposure is higher.

To understand this properly, it helps to look at sunlight not as a single input, but as a biological signal that affects the body in two distinct hormonal systems: one mediated through the skin and one mediated through the eyes.

Vitamin D production: the skin-based sunlight system

Vitamin D is produced when ultraviolet B (UVB) radiation from sunlight hits the skin. This triggers a photochemical reaction in which 7-dehydrocholesterol is converted into previtamin D3, which is then metabolized into vitamin D3 and eventually activated in the liver and kidneys.

This system is well established in the scientific literature and is considered the primary natural source of vitamin D for humans (Holick, 2007).

However, UVB exposure is highly dependent on geography and season. In higher latitudes such as Canada, winter sunlight is too weak for meaningful vitamin D production in the skin for several months of the year. In contrast, summer months provide sufficient UVB intensity for cutaneous synthesis to occur efficiently (Webb et al., 1988).

Because vitamin D is fat-soluble, the body can store it in adipose tissue and the liver, creating a buffer that helps smooth seasonal fluctuations. Population studies consistently show that vitamin D levels rise in summer and fall in winter, reflecting this cycle of production and depletion (Cashman et al., 2016).

Vitamin D storage and seasonal dynamics

The body does not rely solely on daily production. Instead, vitamin D status behaves more like a reservoir system.

During months of high sun exposure, levels tend to increase as production exceeds usage. During winter, when UVB exposure drops, the body gradually draws on stored reserves. However, studies show that these reserves are often not sufficient to fully prevent seasonal declines in many northern populations.

This creates a natural seasonal rhythm: higher levels in late summer, lower levels in late winter.

The eye–light hormonal system: a separate pathway

While vitamin D is produced in the skin, sunlight also affects the body through a completely different pathway involving the eyes.

Light entering the retina is detected not only for vision but also by specialized cells called intrinsically photosensitive retinal ganglion cells (ipRGCs), which contain melanopsin. These cells send signals directly to the brain’s master clock, the suprachiasmatic nucleus (SCN) in the hypothalamus.

This system regulates hormonal rhythms such as:

• Melatonin (sleep hormone), suppressed by morning light

• Cortisol (alertness hormone), which follows a daily rhythm

• Sleep–wake cycles and circadian timing

• Metabolic and temperature regulation

In this way, the eyes act as a light-sensitive hormonal signalling gateway, helping synchronize internal biology with the external day–night cycle (Berson et al., 2002).

However, this pathway is entirely separate from vitamin D biology. Light through the eyes influences timing and endocrine rhythm, but it does not contribute to vitamin D production.

Why this distinction matters

A common misconception is that all beneficial effects of sunlight are part of the same system. In reality, there are two parallel but independent mechanisms:

• UVB → skin → vitamin D production

• Visible light → eyes → circadian hormone regulation

This means that sunlight can influence hormonal balance and physiology even when vitamin D production is not occurring.

Seasonal supplementation and summer sunlight

Given this physiology, there is a reasonable argument that vitamin D supplementation may not need to be constant year-round for everyone.

During summer months, UVB exposure increases significantly, and individuals who spend regular time outdoors with exposed skin can produce meaningful amounts of vitamin D. In these conditions, endogenous production combined with existing body stores may be sufficient to maintain adequate levels, reducing the need for supplementation in some people.

A dynamic system, not a fixed requirement

Vitamin D status is not static. It is a dynamic system shaped by environmental light exposure, storage capacity, and individual behavior.

• The skin uses UVB light to produce vitamin D

• The body stores it and releases it over time

• Seasonal sunlight determines how much is produced

• The eyes regulate circadian and endocrine timing, but not vitamin D

Within this framework, summer may represent a period where some individuals can rely more on natural production and potentially take a structured break from supplementation.

Conclusion

Sunlight affects human biology through two distinct hormonal systems: one through the skin that governs vitamin D production, and one through the eyes that regulates circadian and endocrine rhythms. Understanding this distinction clarifies why sunlight has such broad effects on health, but also why vitamin D cannot be replaced by light exposure alone.

Rather than a fixed year-round requirement, vitamin D behaves more like a seasonal biological system. During months of higher UVB exposure, some individuals may be able to rely on endogenous production and stored reserves to take a structured break from supplementation.

References

Berson, D. M., Dunn, F. A., & Takao, M. (2002). Phototransduction by retinal ganglion cells that set the circadian clock. Science, 295(5557), 1070–1073.

Cashman, K. D., Dowling, K. G., Škrabáková, Z., et al. (2016). Vitamin D deficiency in Europe: pandemic? American Journal of Clinical Nutrition, 103(4), 1033–1044.

Holick, M. F. (2007). Vitamin D deficiency. New England Journal of Medicine, 357(3), 266–281.

Webb, A. R., Kline, L., & Holick, M. F. (1988). Influence of season and latitude on cutaneous vitamin D3 synthesis. Journal of Clinical Endocrinology & Metabolism, 67(2), 373–378.