Though we call it a vitamin, Vitamin D is in fact a powerful hormone that influences how hundreds of systems in the body work together. We tend to think of it as the nutrient for bones, but its reach goes far beyond that. Vitamin D helps direct the activity of hundreds of genes, quietly shaping how the body functions every single day.

Vitamin D is fat-soluble, meaning it dissolves in fat rather than water, so the body tucks it away in fat cells, called adipose cells, for later use. However, when Vitamin D is stored more deeply in body fat, it can be harder for the body to access and use efficiently. This means that people with higher amounts of body fat sometimes have lower circulating levels of active Vitamin D, even with similar intake.

After sunlight or food sources provide vitamin D₃ (cholecalciferol), the body converts it in the liver and kidneys into its active form, calcitriol. This activated form then attaches to the Vitamin D Receptor (VDR) found on many cells in the body. The receptor acts like a dimmer switch, turning hundreds of genes on or off to help manage growth, repair, energy production, and immune balance. When Vitamin D binds to the VDR, it can influence how your DNA expresses itself, guiding key enzymes and proteins that keep the body’s internal systems running smoothly. This explains why Vitamin D affects so many different areas of health at once.

Above roughly 50 degrees north, which includes the UK and Ireland, the sun never climbs high enough in the sky between October and late March for the right type of ultraviolet light (UVB) to reach our skin. That means even on bright, clear winter days, we can’t make meaningful amounts of Vitamin D. This is why public health guidance recommends dietary or supplemental support during those months.

As ancient populations moved away from the equator, they encountered weaker sunlight. Over generations, lighter skin allowed more UVB to penetrate and trigger Vitamin D synthesis, a striking example of how our biology adapted to the environment.

Vitamin D receptors are found throughout the nervous system, including regions linked with mood, motivation and daily resilience. Vitamin D supports enzymes that help maintain normal levels of brain messengers such as serotonin. It also helps activate genes that produce neurotrophic factors, which are protective proteins that encourage brain cell growth, repair, and connection. When Vitamin D levels are optimal, the brain is better equipped to support cognitive wellbeing.

Serotonin, made during the day, is the starting point for melatonin, the hormone that signals sleep at night. Vitamin D supports the enzymes within that pathway, helping maintain a healthy sleep–wake rhythm. Consistent daylight, movement and a steady eating pattern all support this rhythm naturally. Adequate Vitamin D adds another layer by ensuring those enzymes, and the genes that control them, have what they need to function well.

Getting enough Vitamin D from food alone would mean eating about fifteen eggs a day or several pounds of cheese, which is clearly not realistic. Fatty fish, eggs, and fortified foods help, but most of us rely on a combination of diet, daylight, and supplementation to maintain optimal levels of Vitamin D.

Inside every cell, tiny structures called mitochondria convert nutrients into energy. Vitamin D influences how these mitochondria function by regulating genes involved in energy production and calcium balance. This helps sustain steady physical and mental energy rather than short bursts. 

Vitamin D receptors are found in the retina of the eyes. Research suggests Vitamin D helps maintain the health of eye tissues by supporting genes involved in cell protection and antioxidant defence. As we age, maintaining adequate Vitamin D alongside a diet rich in colourful plants, omega-3 fats, and regular movement creates a supportive environment for long-term eye function.

Vitamin D relies on key partners to do its job properly. It helps move calcium into the bloodstream, while Vitamin K₂ directs that calcium into bones and teeth, and magnesium activates and transports Vitamin D so it can be used effectively. Low magnesium can make it harder to maintain healthy Vitamin D levels, even with regular intake. Fermented foods like natto and certain cheeses provide Vitamin K₂, while nuts, seeds, beans, whole grains, and leafy greens supply magnesium. Together, these nutrients form a quiet network that helps Vitamin D support the body as intended.

Mushrooms are one of the few plant foods that can make Vitamin D, but only when exposed to sunlight. Like our skin, they convert UVB light into Vitamin D₂ (ergocalciferol), while humans produce Vitamin D₃ (cholecalciferol) from sunlight or animal foods such as eggs and oily fish. Both forms contribute to overall Vitamin D levels, though D₃ is generally more effective at maintaining them. Leaving mushrooms on a sunny windowsill for an hour or two, gills facing up, can noticeably increase their Vitamin D₂ content and turn an everyday ingredient into a natural nutrient booster.

*The advice in this article is for information only and should not replace medical care. As with any nutritional supplement please consult your qualified healthcare professional before use, especially if pregnant, breastfeeding, if you have a known medical condition or are taking medication. Food supplements must not be used as a substitute for a varied and balanced diet and a healthy lifestyle.