Vitamin D can be synthesised in the skin from exposure to UVB rays in sunshine.  However, the ability to synthesise vitamin D is dependent on a variety of different factors, such as the season, latitude, skin pigmentation and clothing.   Northern Europe lies at a latitude between 40-60ºN, with Cork at a latitude of 51ºN.  In these regions, from October to March, the sun is not strong enough to ensure cutaneous synthesis of vitamin D ( Webb et al, 1988 ).  For this reason, people in northern European countries, such as Ireland, achieve their highest levels of vitamin D towards the end of the summer and have their lowest vitamin D concentrations towards the end of winter and beginning of spring ( Hill et al, 2008 ).

The colour of a person’s skin can influence the amount of vitamin D produced from sunshine.  Melanin is the pigment that gives skin a darker colour; it is a natural sun-block, so people with darker skin produce less vitamin D with the same sunlight exposure. Clothing and sunscreen lotions also block UVB rays from reaching the skin, decreasing cutaneous synthesis of vitamin D.

Vitamin D deficiency is a global health issue, potentially resulting in  rickets in children and  osteomalacia or  osteoporosis  in adults. Concentrations of serum 25(OH) D below 100nmol/L have been linked to an increased risk of chronic diseases such as  CVD ,  certain cancers , certain inflammatory related diseases  and  diabetes .  Serum 25(OH)D concentration is the generally accepted biomarker of vitamin D status, which is the major form of circulating vitamin D in the body. In general, vitamin D deficiency has been defined as 25(OH)D concentrations less than 10-25nmol/L.  Although concentrations ranging from >25-100nmol/l have been classified as being sufficient, concentrations of >50nmol/l or >80nmol/l are the most commonly used values for sufficiency ( Lips et al, 2004 ;  Heaney, 2005 ).  Studies are ongoing to determine what thresholds of 25(OH)D are most appropriate to denote sufficiency in population groups.

Vitamin D toxicity is extremely rare.  Excessive sunlight exposure cannot cause vitamin D toxicity, as there is a limited ability by the skin to produce vitamin D ( Holick, 2004 ). The toxic threshold for vitamin D consumption is unknown but a daily dietary intake of up to 50ug is considered safe. Extreme intakes of vitamin, in the range of >250ug/d, has been found to cause adverse effects such as nausea, vomiting, poor appetite, constipation, weakness and weight loss.  The main symptom of vitamin D toxicity is hypercalcemia, primarily resulting from intestinal calcium hyperabsorption and to a lesser degree from calcium release from the bone ( Zittermann, 2003 ). Vitamin D toxicity can cause other serious adverse outcomes such as elevated serum calcium, which can result in severely high blood pressure and calcification of soft tissue such as the kidneys, heart, lungs and blood vessel walls. Such high circulating levels are generally reached only by excessive dietary intake and supplement use. However, such cases are extremely rare and there is no link between vitamin toxicity and commonly consumed foods.