PROF BLOG
VITAMIN D - NO LONGER JUST THE SUNSHINE VITAMIN Simon Langley-Evans Professor of Human Nutrition, University of Nottingham
Simon has 25 years’ experience in nutrition research, with expertise in maternal and infant nutrition. He is Chair in Human Nutrition and Head of School of Biosciences at the University of Nottingham and is the Editor-inChief of The Journal of Human Nutrition and Dietetics.
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In the last decade, understanding of the role of vitamin D in not just bone health, but in many other aspects of physiology, metabolic regulation and health, has exploded and vitamin D is very much a hot topic in nutrition research. Vitamin D has long been recognised as one of the essential nutrients and the contribution of vitamin D deficiency to rickets in children was determined early in the 20th century. Indeed, in the history of public health nutrition, measures to prevent rickets through administration of cod liver oil to children and improving access to milk were early success stories. Since then, the role of vitamin D in maintaining calcium homeostasis and hence bone health has been a major focus of interest. Vitamin D is most unlike the other vitamins in that it is largely derived from non-dietary sources. The action of UVB rays from sunlight upon 7-dehydrocholesterol in the skin leads to the formation of pre-vitamin D which is then metabolised to vitamin D. Vitamin D formed in the skin is metabolised to 25-hydroxy vitamin D in the liver. This undergoes a further hydroxylation step to form the active form of vitamin D; calcitriol. It is also more appropriate to think of vitamin D as a pro-hormone rather than a vitamin, as it has many of the properties of steroid hormones and like those hormones is ultimately synthesised from cholesterol. The process of synthesis of vitamin D is little impacted by dietary factors, although it is becoming clear that in individuals taking statins to lower cholesterol, vitamin D status may be compromised.
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Determining vitamin D status in human subjects has been a major research challenge for many decades. The best biochemical marker is 25-hydroxy vitamin D, but as described by New and Wilson,1 there has long been a challenge with standardising biochemical measurements so that comparisons can be made between laboratories and robust assessment of vitamin D insufficiency and deficiency (25-hydroxy vitamin D below 25nmol/l) can be made. For a long time, the value of measuring dietary vitamin D intake was considered to be questionable given the major contribution of synthesis within the body to overall vitamin status. However, with high levels of populationwide insufficiency of vitamin D, there is increasing use of fortification of foods and clear recommendations for certain sub-populations to consume vitamin D-rich foods. As a result, robust methodology is required to assess intake in research studies and dietary surveys. Weir and colleagues,2 for example, have validated a food frequency questionnaire for the estimation of vitamin D intakes in adults aged 18-64 years. Other researchers have used detailed dietary records to assess vitamin D in the diets of preschool children in the UK3 and have estimated that whilst major sources were fat spreads, milk and fortified cereals, all children in a sample of 755 18- to 40-month-olds were consuming vitamin D at a level below
US and UK dietary recommendations. This disturbing finding suggested that fortification of cereals should be more widespread with a level of 2Îźg per 100g of cereal being necessary to provide most children with adequate intakes. Vitamin D deficiency is common all around the world and risk factors include living in northern latitudes, increasing age, low consumption of dairy produce, low socioeconomic status and overall metabolic health. With morbid obesity, for example, severe deficiency has been observed in more than a third of people.4 As noted above, the contribution of vitamin D to maintaining calcium homeostasis and bone mineralisation is well-understood. When circulating calcium is low, vitamin D contributes to release from bone, but when calcium is adequate in the diet, vitamin D promotes gastrointestinal uptake and bone remineralisation. However, most studies suggest that whilst calcium intake is important for increasing bone mineral, vitamin D has been reported to have more of an effect on reducing fractures due to prevention of falls in elderly people with osteoporosis. This has been proposed to stem from beneficial effects of vitamin D on muscle strength and neuromuscular tone. A recent systematic review on the effects of vitamin D upon muscle strength in elderly people5 contradicts this view however, suggesting that vitamin D improves mobility with no clear impact on measures of muscle strength such as hand-grip.
Our understanding of the hormonal actions of vitamin D is increasing rapidly. With this new understanding comes the realisation that this vitamin can impact upon many aspects of human physiology at all stages of the lifespan. For example, Samimi and colleagues found that treatment of pregnant women who were at high risk of pre-eclampsia with a vitamin D and calcium supplement between weeks 20 and 32 of gestation, improved several measures of cardiovascular function, glycaemia and antioxidant status.6 Vitamin D is increasingly being linked to cancer and other non-communicable diseases in adulthood. As we make improvements in the direct measurement and non-invasive estimates of vitamin D status, it is becoming clear that insufficiency of this key nutrient is widespread in all populations, but particular among those living in the northern latitudes where formation from ultraviolet exposure is low during the winter months. As a result, the achievement of robust dietary intakes is of importance among particular population sub-groups, including the elderly and pregnant women. For more information on the fascinating science of vitamin D it is worth noting that The Journal of Human Nutrition and Dietetics and Nutrition Bulletin released a joint virtual issue on the subject in 2016.7 There is so much more to learn about this fascinating nutrient and the days of vitamin D just being the sunshine vitamin that is good for our bones are well and truly over.
You can read more Prof Blogs from Simon Langley-Evans in the Student zone at www.NHDmag.com
www.NHDmag.com July 2017 - Issue 126
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