New research shows that less exposure to sunlight in the winter months leads to lower levels of vitamin D in spring, with the effects lasting longer and affecting more people than previously thought.
As far as vitamins go, vitamin D is unusual in that we make most of it ourselves (the word vitamin usually refers to nutrients we have to eat because our bodies can’t manufacture them).
A substance in our skin, called 7-dehydrocholesterol, creates vitamin D when exposed to ultraviolet light from the sun – specifically, UVB. The liver then turns the vitamin D into calcidiol, and the kidney turns part of that into calcitriol, which is a hormone that controls levels of calcium and phosphate, and is essential for bone growth.
Vitamin D was first identified because a lack of it can cause rickets, an early childhood disease involving soft or weakened bones. But a deficiency can also lead to osteoporosis and other bone and muscle pain, and has been linked to multiple sclerosis and cancer, as well as general mortality.
This latest study, by Professor Steven Boyages and PhD student Kellie Bilinski from the Sydney Medical School, looked at vitamin D levels in 24,000 samples taken in 2008-2010 from New South Wales hospital patients (Boyages S & Bilinski K 2012, “Seasonal reduction in vitamin D level persists into spring in NSW Australia: implications for monitoring and replacement therapy”, Clinical Endocrinology, vol. 77, no. 4, pp. 515–523, DOI: 10.1111/j.1365-2265.2012.04398.x).
They found vitamin D deficiencies in up to 58 per cent of the population, and lasting most of the year, but greatest between autumn and the end of spring. In fact, spring was the worst season despite rising levels of sunshine, due to the time taken to replenish the body’s stores of vitamin D.
In summer, 33 per cent of patients had a deficiency, even though this was when vitamin D levels were highest. Interestingly, women reach their maximum levels in January and men in February.
The comparison of demographic groups revealed a previously unknown risk for females between 20 and 39 years of age – but the cause of this is still unknown.
According to Professor Boyages, the results of this study mean there is a need to review guidelines for vitamin D testing and the use of supplements:
“Ideally testing would occur in spring when vitamin D levels reach their lowest concentration. If an individual is found to be deficient a subsequent test three months afterwards would see if they have been able to replenish their vitamin D.
“Similarly use of vitamin D supplements currently fail to address this factor of seasonal variation. A modified approach would see the use of supplements commence, or increase, at the end of summer and be maintained until the end of spring when they would either be stopped or reduced depending on an individual’s sunlight exposure.”
For more information, see The University of Sydney.