How Much Vitamin D Do I Need?

Vitamin D is unusual among nutrients in that the main source is not food but sunlight, and in the UK that source stops working for around five months of the year. That single fact drives all the guidance.

What it does

Vitamin D regulates the absorption of calcium and phosphate, which is what makes it essential for bones, teeth and muscles. Severe deficiency causes rickets in children and osteomalacia, a softening of the bones, in adults.

It also has a role in immune function, though claims made for it in that area have often run ahead of the evidence.

The recommendation

UK guidance is 10 micrograms a day for everyone over the age of one, including pregnant and breastfeeding women. Ten micrograms is the same as 400 international units, which is how many supplement labels express it.

Babies under one need 8.5 to 10 micrograms, usually as drops, unless they are formula fed and taking more than 500 ml a day, since formula is already fortified.

Why the UK is a special case

The skin makes vitamin D when exposed to ultraviolet B radiation. Whether any reaches the ground depends on the angle of the sun, which depends on latitude and season.

At UK latitudes, from roughly October to early March, the sun never climbs high enough for meaningful UVB to penetrate the atmosphere. During those months the skin makes essentially none, however long you spend outside and however bright the day.

This is the point people find most surprising. A crisp, brilliantly sunny January day produces no vitamin D at all. Cloud cover is not the issue; the angle is.

Summer sunlight

From late March to September, short daily exposure is usually enough. Guidance suggests short periods with forearms, hands or lower legs uncovered, without sunscreen, and without burning.

How short depends on skin tone, time of day and how much skin is exposed. Someone with very pale skin might need only a few minutes around the middle of the day; someone with darker skin needs considerably longer, because melanin reduces vitamin D synthesis.

Deliberately burning is never the route. The risk of skin cancer rises with burning and the vitamin D benefit plateaus long before that point.

Who should supplement year round

• People with darker skin, including those of African, African-Caribbean and South Asian background, because higher melanin substantially reduces synthesis.
• People who cover their skin for cultural or religious reasons.
• People rarely outdoors, including those in residential care or housebound.
• Everyone else, from October to March.

Food sources, and why they rarely suffice

Very few foods contain useful amounts.

• Oily fish: salmon, mackerel, sardines, herring
• Red meat and liver
• Egg yolks
• Fortified products: some breakfast cereals, fat spreads, plant milks

A portion of oily fish is a genuinely good source. The difficulty is that typical UK diets contain little of it, and reaching 10 micrograms daily from food alone would require eating oily fish most days.

Mushrooms exposed to ultraviolet light contain vitamin D2, a different and somewhat less effective form than the D3 the skin produces.

Upper limits

Vitamin D is fat soluble, meaning the body stores rather than excretes the excess, so overdose is genuinely possible in a way it is not for vitamin C.

Guidance sets an upper limit of 100 micrograms a day for adults. Sustained intake above that can raise blood calcium to harmful levels, causing nausea, weakness, kidney damage and, over time, harm to the heart.

The recommended 10 microgram supplement is ten times below that limit, so ordinary supplementation carries no risk. The concern applies to high-dose products taken daily over months, sometimes on the assumption that more must be better.

You cannot overdose from sunlight. The skin regulates production and stops once it has made enough.

Signs of deficiency

Often nothing at all, which is why testing rather than symptoms drives most diagnoses.

Where symptoms occur they tend to be vague: fatigue, general aches, bone pain, muscle weakness, low mood. Because those describe a great many conditions, deficiency is frequently found incidentally rather than sought.

More specific signs are bone pain and muscle weakness, particularly difficulty rising from a chair or climbing stairs.

Testing

A blood test measuring 25-hydroxyvitamin D is available, though it is not offered routinely to people without risk factors or symptoms, on the reasonable grounds that the supplement is cheap and safe enough that testing first is often unnecessary.

If you have persistent unexplained fatigue or bone pain, it is a reasonable thing to raise with a GP.

The practical answer

Take a 10 microgram supplement daily from October to March, and year round if you fall into one of the groups above. It costs a few pounds a year, it is one of the few supplements with clear population-level evidence behind it, and it is the only nutrient in UK guidance where supplementation is recommended for essentially everybody.