Hypovitaminosis D in British adults at age 45 y: nationwide cohort study of dietary and lifestyle predictors.
Hyppönen E, Power C.
Am J Clin Nutr 2007; 85: 860-868.
Centre of Paediatric Epidemiology and Biostatistics, Institute of Child Health, London, UK.
This is an extremely important paper, investigating the extent of the problem of vitamin D insufficiency in adults in the UK, and associated factors. Vitamin D was measured as calcidiol, 25(OH)D. Although the study was exclusively of British subjects and describes British experience, the paper was published in the USA medical press and so the blood levels of vitamin D are expressed as nmol/L. In this review I will convert them into ng/ml so as to conform with UK practice.
16,751 people were born in Great Britain during the same week in March 1958 and therefore form a birth cohort. The authors of the study were able to contact and obtain blood samples from 7591 of the cohort between September 2002 and April 2004. Questionnaires were completed by all participants. Whites only were included in the study, and the 154 ethnic minorities were excluded.
The main points from the study follow.
Blood vitamin D levels are maximal in September and minimal in February, conforming with previous studies; winter and spring average 16.4ng/ml, summer and autumn average 24 (ideal >30). During the summer and autumn vitamin D levels tend to be slightly lower in women than in men(23 vs 25), but there is no difference during the winter and spring (both 16). Perhaps men spend more time out of doors than women in the summer and perhaps expose more of their skin to the sun.
Vitamin status of the cohort is shown in the following figure. 15.5% have a profoundly low vitamin D level in the winter and spring (<10ng/ml) and 3.2 % in the summer and autumn.
The study demonstrates social variation with lower vitamin D levels in the socio-economically deprived. Winter and spring average 17ng/ml, summer and autumn average 24 in social classes I & II; 14 and 22 respectively in the unemployed. This could explain the similar social gradient of illness in the UK and in other northern European countries. Further data is provided to illustrate the frequency of profound vitamin D deficiency in the social groups. We can see once again the social gradient. The prevalence of profound vitamin D deficiency is particularly high, especially in the winter and spring, in the unemployed, who are know to have the worst health profile within the UK.
A major geographical variation is demonstrated. In all months the vitamin D levels are highest in the south of England and lowest in Scotland, 17 vs 14ng/ml in the winter and spring, 25 vs 20 in the summer and autumn. The authors suggest that profound deficiency of vitamin D is most importance, and this is also displayed. There is a clear geographical gradient, with northern regions of residence showing the highest prevalence of profound vitamin D deficiency.
A greater amount of time spent outdoors in the the summer is associated with higher vitamin D levels in the summer and autumn, but not in the winter and spring. This is not unexpected, but it does identify the importance of being outdoors.
The obvious antithesis of spending time out of doors is spending time indoors. The study noted that those who spent the most hours per day watching television or using a PC had lower vitamin D levels.
It is noted in the study that vitamin D levels are lower in those who declare themselves to be obese. This might imply that obese persons are less inclined to spend time outdoors or to expose their skin to the sun. The observation is true in all seasons, and therefore it is possible that vitamin D insufficiency is causally linked to the development of obesity. Once again we are able to see the association of obesity with profound vitamin D deficiency, blood level less than 10ng/ml.
The study also assesses the effect of vitamin D supplements, usually taken in the form of cod liver oil. Those taking supplements have higher vitamin D levels at all times of the year.
The consumption of fortified margarine (the only mandatory fortification with vitamin D in the UK) has no effect on blood levels of vitamin D, indicating the inadequacy of this form of food fortification, which was designed to bring the vitamin D content of margarine to that of butter. In contrast, those subjects eating oily fish most frequently have the highest level of vitamin D in all months, clearly demonstrating the effectiveness of this form of diet.
It is known that sun protection creams reduce vitamin D synthesis in the skin. The question arises as to whether this in practice leads to vitamin D insufficiency. The data presented in this study indicate that subjects using sun protection creams most often have the highest blood vitamin D levels. The conclusion is that sun protection creams are used appropriately, at least in 45 years olds.
The average vitamin D levels in this study of 45 year-olds in Great Britain are lower than adult populations in the USA and Canada (Calgary). The use of vitamin supplements (13% of men, 20% of women) is about half hat in the USA (30% of men and 40% of women). The 154 non-whites of the cohort who were excluded from the study have a higher prevalence of profound vitamin D deficiency, 50% less than 10ng/ml.
“It is disturbing .... that nearly 90% of the current study population was affected by hypovitaminosis D during the winter and spring, and 60% had sub-optimal concentrations year-round.”
“The high rates of hypovitaminosis D reported in this study suggest that immediate action is needed to improve the vitamin D status of the British population.”
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