Summer is here. Beach holidays are being planned, and many of us may be hoping to sit out in the warm sun and top up our vitamin D levels.  Or are we?  Whether due to an indoor lifestyle, cultural dress, a fear of burning or skin ageing, many people are vitamin D deficient.

Vitamin D can be found in some food such as eggs and fortified foods.  There is evidence that exposing mushrooms to sunlight increases the levels of vitamin D they contain. Indeed, one supermarket in the UK already sells vitamin D enhanced mushrooms. However, for most of us, vitamin D intake from food is limited. The main source of Vitamin D for most of us is sunshine.  Vitamin D is synthesized in our skin from the UVB rays from sunlight.  If we do not get sufficient sunlight, we can become vitamin D deficient. 

But is vitamin D deficiency really a problem?
Vitamin D is involved in calcium homeostasis, therefore is vital for bone health.  Deficiency can cause rickets in children and osteomalacia (soft bones) and increased risk of osteoporosis in adults1.  This relationship is well understood however the beneficial effects of vitamin D for overall health are thought to go way beyond bones.

One meta-analysis reviewed 18 randomised control trials (RCTs).  The review reported that ordinary doses of vitamin D supplements (400 - 800 IU per day) reduces all-cause mortality in subjects over 50 years old2.  Additionally, analysis of the Third National Health and Nutrition Examination Survey (NHANES III) showed increased mortality in subjects with low vitamin D status3.  Vitamin D has also been positively associated with muscle strength and function; both in the elderly4 and in adolescence1.

There is also evidence on the relationship between cancer and vitamin D status, although findings are less clear.  Ecological and observational studies have suggested that vitamin D may be protective against a wide range of cancers.  However caution must be taken when interpreting these studies as they do not prove causality5.  The International Agency for Research on Cancer (IARC) carried out a meta-analysis on vitamin D and cancer risk.  The review found evidence for increased risk of colorectal cancer with low vitamin D levels.  They also investigated the relationship between breast and prostate cancer and vitamin D status.  The conclusions drawn for breast cancer were limited and no evidence was found for prostate cancer.  Further randomised trials are needed to investigate further.

So vitamin D may be beneficial against cancer, but isn’t the sun a carcinogen?  Is it really wise to stop wearing sunscreen?
Yes, and no.  The link between the sun and skin cancer is well documented and caution should always be taken in the sun.  However, it is the burning of the skin which is linked with skin melanoma.  The optimum time for UVB rays is midday when the sun is highest and it is reported that those with fair skin can reach their optimum daily limit in 5-10 minutes6.  Darker skins need longer sun exposure as pigmentation inhibits synthesis7.  If you are very fair-skinned and susceptible to burning, you may want to consider vitamin D supplementation.

The emerging links of vitamin D on overall health have led many to call for a revision on sun advice, in particular around the use of sunscreen8.  If you are concerned about sun exposure, technology is one step ahead of public health.  There is a bracelet and app which senses sun exposure and sends warning alerts to your smart phone to prevent burning.

This is just a brief summary of some of the evidence.  If you are keeping your fingers crossed for sun drenched days or heading off to warmer climes, take advantage of the potential health benefits and enjoy the sunshine but make sure you follow Cancer Research UK guidelines and take care not to burn. 

1. Carson, Pourshahidi , Boreham, Hill , Cashman , Strain and Mulhern (2015).  The association between vitamin D status, muscle strength, power and cardiorespiratory fitness in adolescents from the Young Hearts Study.  Proceedings of the Nutrition Society, 74, 163.
2. Autier & Gandini (2007).  Vitamin D Supplementation and Total Mortality: A Meta-analysis of Randomized Controlled Trials.  Arch Intern Med.,10;167(16):1730-7. 
3. Looker, Johnson, Lacher, Pfeiffer, Schleicher & Sempos.  Vitamin D Status: United States, 2001–2006 (2011).  NCHS Brief Data, Number 59.
4. Dawson-Hughes (2011).  Vitamins and cognitive development and performance Serum 25-hydroxyvitamin D and muscle atrophy in the elderly.  Proceedings of the Nutrition Society, 71, 46-49.
5. Ashwell, Stone, Stolte, Cashman, Macdonald, Lanham-New, Hiom, Webb & Fraser.  UK Food Standards Agency Workshop Report: an investigation of the relative contributions of diet and sunlight to vitamin D status.  British Journal of Nutrition, 104, 603–611.
6. World Health Organization International Agency For Research On Cancer, Working Group Report, (2008).  ISBN 978 92 832 2446 4. 
7. Moore, Radcliffe & Liu (2014).  Vitamin D intakes of adults differ by income, gender and race/ethnicity in the USA, 2007 to 2010.  Public Health Nutrition, 17(4):756-63.
8. Moan, Baturaite, Juzeniene and Porojnicu (2011).  Review Article Vitamin D, sun, sunbeds and health.  Public Health Nutrition, 15(4), 711–715.

Photo courtesy of Gary Stevens, Flickr