The main source of vitamin D is sunlight, but in the Ireland we can't make any vitamin D from winter daylight between November and March, meaning vitamin D deficiency is common in men and women of all ages. For others, the kidneys have trouble converting the nutrient to its active form or their being overweight makes vitamin D less available for use in the body.

Vitamin D is essential for building strong bones, maintaining your immune system and may help prevent some cancers. Symptoms of low vitamin D can include muscle weakness, pain, fatigue and depression.

A vitamin D level of 20 ng/ml to 50 ng/ml is considered adequate for healthy people.