Dr Sarah Brewer, medical nutritionist, GP and Healthspan medical director, says:
“You can make vitamin D in your skin when the UV index is greater than 3. As a result, vitamin D levels tend to plummet during autumn and winter in people living in northern latitudes such as the UK. Food sources then become important, but if you don’t eat much oily fish, liver products, eggs, butter or fortified foods, you are likely to remain deficient during winter.
“Public Health England found that 23% of adults aged 19-64 years, 21% of adults aged 65-plus, and 22% of children aged 11-18 have low blood levels of vitamin D. They therefore advise everyone should consider taking a supplement supplying 10 micrograms of vitamin D per day during autumn and winter months.
“However, this is the minimum needed to prevent vitamin D deficiency conditions, such as muscle and bone aches and pains, osteomalacia or rickets. For optimum health, there is increasing evidence that higher doses of 25mcg to 50mcg vitamin D are needed, especially for older people as the ability to synthesise vitamin D declines in later life.
“Even when the sun is shining, many people fail to synthesise sufficient vitamin D, because they cover up, use high SPF sunscreens, or stay in the sun for more than 20 minutes which causes inflammation (sunburn) that breaks down the vitamin D they’ve already produced. Age also plays a role – we make four times less vitamin previtamin D3 in the skin over the age of 60 than in our 20s.
“The synthesis of active vitamin D is quite complicated and the pre-vitamin D made in the skin must undergo two further conversions in the liver, then in the kidneys/other tissues, to become active. [Another study found] among 93 fit, healthy surfers in Hawaii, for example, more than half had low vitamin D status despite achieving 29 hours of sunshine exposure per week. This may partly be due to genetic inefficiencies of vitamin D3 metabolism, and partly because longer sun exposures cause the rapid breakdown of previtamin D3 in the skin.
“Take a supplement supplying at least 10mcg vitamin D3 (and preferably 25mcg) per day. Those aged over 50 may need 50mcg per day to offset lower production in the skin and reduced dietary absorption. The European Food Safety Authority has suggested a tolerable upper safe level for long-term use from supplements of 100mcg vitamin D3 per day. Select supplements providing vitamin D3, which is more effective than the plant form known as vitamin D2 for maintaining vitamin D status.”