A Brief Guide to the Vitamin D, the Sunshine Vitamin

Ellie Holder | Published 07 September 2026

Why is it called the sunshine vitamin?

When bare skin is exposed to UVB light from the sun, the body produces vitamin D3, or cholecalciferol.

This vitamin D is then converted by the liver and kidneys into forms the body can use. Once activated, it interacts with vitamin D receptors found throughout the body.

In the UK and many similar climates, sunlight is our main natural source of vitamin D during the warmer months. From around October to early March, however, sunlight is not strong enough for most people to make sufficient vitamin D through their skin.

The NHS recommends that everyone should consider taking a daily vitamin D supplement during autumn and winter.

Who is most at risk of low vitamin D?

Some people are more likely to have low vitamin D levels because they produce less through sunlight or have limited opportunity for sun exposure.

The NHS advises that people at greater risk should consider taking a vitamin D supplement throughout the year. This includes people who:

spend very little time outdoors during the day, including those who are housebound or live in a care home, and those who work night shifts
usually cover most of their skin when outdoors
have darker skin, including people from African, African-Caribbean and South Asian backgrounds

Age, season and lifestyle can also affect how efficiently we produce vitamin D from sunlight, while certain medical conditions and medicines can affect vitamin D status.

Vitamin D and the immune system

Vitamin D plays an important role in normal immune function. Many immune cells have vitamin D receptors, including macrophages and T and B lymphocytes, allowing vitamin D to influence how these cells function and communicate.

It is involved in both major parts of the immune response. The innate immune system provides a rapid first line of defence, while the adaptive immune system provides a more targeted response. Vitamin D helps support the normal activity of both.

Rather than thinking of vitamin D as something that “boosts” immunity, it is more accurate to think of it as one of the nutrients the immune system needs to function normally.

What else does vitamin D do?

Vitamin D has several important roles beyond immune health.

It helps the body absorb and use calcium and phosphorus, two minerals that are essential for maintaining normal bones and teeth. It also contributes to normal muscle function.

This is another reason that adequate vitamin D intake is important throughout life, not just during the winter months.

Vitamin D2 vs Vitamin D3

Vitamin D occurs in two main forms:

Vitamin D2, or ergocalciferol, is primarily derived from fungi and yeast.

Vitamin D3, or cholecalciferol, is the same form naturally produced by the skin in response to sunlight.

Both can raise vitamin D levels, but research generally shows that D3 is more effective at raising and maintaining blood levels of vitamin D than D2.

Although D3 has traditionally been sourced from lanolin, it can also be obtained from sources such as lichen, making plant-based and vegan forms available.

Can you get vitamin D from food?

Only a relatively small number of foods naturally contain significant amounts of vitamin D.

Sources include:

oily fish such as salmon, sardines, trout and mackerel
egg yolks
liver and red meat
some fortified cereals and spreads
UV-exposed mushrooms, which can provide vitamin D2

Because food sources are limited, it can be difficult to obtain enough vitamin D from diet alone, particularly during autumn and winter.

Overview

Vitamin D is essential for normal immune function, bone health and muscle function. The body can produce it when skin is exposed to sunlight, but how much we make varies with season, skin pigmentation, age, time outdoors and other factors.

In the UK, sunlight is not strong enough for most people to produce enough vitamin D during autumn and winter, and some groups may be at greater risk of low vitamin D throughout the year. With relatively few foods providing substantial amounts, diet and supplementation can play an important role in maintaining adequate intake.

Vitamin D3, or cholecalciferol, is the same form produced by the skin in response to sunlight and is generally more effective than vitamin D2 at raising and maintaining vitamin D levels in the blood.