Vitamin D deficiency symptoms: and how to put it right.

The main vitamin D deficiency symptoms are tiredness that won't lift, weak muscles in the thighs and shoulders, aches in different places, infections that keep coming back and low mood in the dark months. No single sign proves anything, because each one looks like many other things. So vitamin D is not guessed: it is measured with a blood test.

November arrives and you feel flat. You leave home in the dark, spend the day in offices, cars and airports, and come back in the dark. Then your blood test marks vitamin D as low, with an asterisk and no explanation. I see it every autumn in the people I work with.

Low vitamin D: what are the symptoms?

The most common sign is tiredness that rest does not fix, with muscles that respond less. The weakness shows up first in the muscles closest to the trunk, in the thighs and shoulders.

• Strength missing where you don't expect it. Getting up from a chair and climbing stairs cost more than a year ago.
• Aches here and there, and less steady balance. Sore muscles for no reason, some discomfort in the bones, your footing less sure in the dark.
• Your defences and your mood drop together. More colds and sore throats, and longer ones, plus a mood that follows the short days.

What causes low vitamin D?

In most cases the cause is one thing: too little sun on your skin. About 80% of your vitamin D is made by you, in your skin, and the starting material is cholesterol.

The rest is in your schedule. Indoor work, long sleeves, and sun cream that blocks exactly that process. Then there is the season. In a group of professional footballers, who train outdoors, the level dropped clearly from autumn to winter. And as the years pass your skin makes less of it.

Food brings little, because nobody eats cod liver oil any more. In one review, inadequate levels were found in around one in three healthy young adults.

How to increase vitamin D: sun, plate and a blood test

Get the sun on your skin, not through glass. When there is sun, have 15-20 minutes with your face and arms uncovered, outside the strongest hours, and leave the cream off. Office and car glass filters out part of the useful light.

Use the moments you already have. Take the long phone call while walking. Finish lunch with a short walk outside. Drink your coffee outside the cafe.

On your plate, animal fats. Vitamin D lives in fat: salmon, mackerel, sardines, eggs and butter. One hundred grams of fresh salmon carries more than three times as much as the same weight of eggs, and milk almost none. Have oily fish several times a week.

Train your strength as the level comes back up. Your muscles have their own receptors for vitamin D, and strength work and the vitamin act on the same fast fibres, the ones you lose first. In one comparison of older people with low levels, correcting it improved their balance by 9% in eight weeks.

Have the level measured. 25-OH vitamin D is read in an ordinary blood test, and the most useful moment is the end of winter, when the level is at its lowest. If your doctor corrects it, the test is repeated after about three months.

The mistake almost everyone makes. Buying something at the pharmacy without having measured anything. The opposite is a mistake too: counting on an August tan to carry you to February.

Book your end-of-winter blood test, and start your fifteen minutes in the sun tomorrow.