Vitamin D: The One Supplement Most People Genuinely Should Take

Nutrition writers spend most of their time telling people that supplements are unnecessary. Here is the exception, and it is a real one: vitamin D deficiency is genuinely common, genuinely consequential, and genuinely hard to fix with food.

Why food can't solve it

Vitamin D barely exists in the food supply. Oily fish has a decent amount, egg yolks a little, and fortified products whatever the manufacturer added. Everything else has essentially none. Human beings are designed to make it in skin exposed to ultraviolet B radiation from sunlight — which works well if you live near the equator and spend time outside, and works poorly if you live at a northern latitude, work indoors, cover your skin, have darker skin, or are over sixty-five.

In much of the northern hemisphere, the sun is too low in the sky between roughly October and March for the skin to produce any vitamin D at all, regardless of how long you stand in it.

Who is most likely to be short

• Anyone living far from the equator, in winter — which is a very large number of people.
• People with darker skin. Higher melanin means substantially longer sun exposure is needed for the same production.
• Older adults, whose skin synthesises it less efficiently.
• People who are mostly indoors, or who cover their skin.
• People with obesity — vitamin D is fat-soluble and is sequestered in adipose tissue.
• People with conditions affecting fat absorption (coeliac, Crohn's, after certain surgeries).

The practical guidance

• Standard adult dose: commonly 10 µg (400 IU) to 25 µg (1,000 IU) a day. UK guidance suggests 10 µg daily for everyone through autumn and winter, and year-round for people at higher risk.
• Form: D3 (cholecalciferol) is generally preferred over D2. Vegan D3, made from lichen, is widely available.
• Take it with a meal containing fat. It is fat-soluble; absorption is meaningfully better with food.
• Don't megadose. Very high intakes over time can cause hypercalcaemia — too much calcium in the blood — leading to nausea, kidney stones and worse. More is not better. There is an upper limit and it is easy to exceed with high-strength products bought online.

Should you test?

A blood test (25-hydroxyvitamin D) tells you where you stand and is worth doing if you have risk factors, symptoms such as bone pain or muscle weakness, or if you plan to take a high dose. For a healthy adult taking a standard 10–25 µg supplement through winter, routine testing is usually unnecessary.

If you are pregnant, have kidney disease, sarcoidosis, or take medication that affects calcium, speak to your doctor before supplementing.