It is harder to produce enough vitamin D as we get older and the rate of vitamin D production reduces significantly as we age.
Spending time indoors during daylight hours reduces our exposure to UV & decreases the production of vitamin D.
Living in the UK, with shorter days during the winter, our exposure to UV is reduced making us more susceptible to vitamin D deficiency.
Certain diseases such as Crohn’s and cystic fibrosis are more susceptible to vitamin D deficiency as they cannot efficiently absorb vitamin D from diet or supplements.
Due to the reduced exposure to natural sunlight and UV in the winter we are unable to produce enough vitamin D but if we are exposed to the sun between May and October then the excess vitamin D produced is stored in the body and released during the winter.