Seasonal Effects on Vitamin D Production and Availability
Seasonal changes influence vitamin D physiology primarily through variations in sunlight intensity and duration. As the angle of the sun shifts across the year, the amount of ultraviolet B (UVB) radiation reaching the skin changes, altering the body’s capacity to produce vitamin D through sun exposure.
During periods of lower UVB availability, vitamin D synthesis in the skin is reduced or absent, regardless of time spent outdoors. In contrast, seasons with higher UVB exposure allow more efficient vitamin D production, although this process remains regulated and self-limiting within the skin.
Because seasonal variation affects vitamin D input at the very start of the pathway, it can influence downstream availability, activation, and response. These effects interact with other factors such as storage, body composition, and regulation, contributing to predictable yet variable patterns across the year.
Understanding seasonal effects helps explain why vitamin D handling changes over time even when behaviour remains constant. Seasonal variation reflects environmental constraints rather than individual choice alone.
This page focuses on season as an environmental modifier of vitamin D physiology. Later sections explore how season interacts with geography, lifestyle, and biological regulation to shape vitamin D behaviour across populations and individuals.