Study assesses dietary requirements for vitamin D in the winter

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Last week one of my blogs focused on a recent study which explored the relationship between vitamin D levels and risk of cardiovascular disease. This issue is timely for those of us who are in our winters, and who may therefore be running short on sunlight and vitamin D. This issue is perhaps even more timely when one considers that a study just out (published in the December edition of the American Journal of Clinical Nutrition) has found some evidence that vitamin D levels are on a bit of a downward trend in American men [1]. This study is accompanied by an editorial which points out that if this research reflects reality, then we have cause for concern [2]. The editorial points our attention to the fact that vitamin D affects a multitude of systems and processes in the body including bone health, cardiovascular disease and cancer risk, as well functions that relate to immune, pancreatic, muscle and brain function.

One question we might ask is what level of vitamin D is required to ensure optimal health, particularly in the winter when sunlight is highly unlikely to meet our needs in this respect. The issue was addressed by the authors of the study I covered last week, and they quoted research suggesting that 1000-2000 IU is required for many individuals to meet their vitamin D requirements. As it happens, December’s edition of the American Journal of Clinical Nutrition contains a study which addressed this issue directly [3].

In this study, individuals from Ireland and Northern Ireland aged 20-40 had their vitamin D (25-hyroxyvitamin D) levels monitored From October of one year to March of the following year (the winter). Individuals were, during this time, treated with 0 (placebo), 5, 10 or 15 micrograms of vitamin D (D3) each day (0, 200, 400 or 600 IU per day). From this data the researchers were able to calculate the dietary requirement of D3 to maintain blood vitamin D at certain levels. Below, I have listed the four vitamin D thresholds chosen by the authors, and the intake of vitamin D required to ensure that 97.5 per cent of the population would have these levels. A range of cut-offs were chosen because there is really no consensus on what is regarded as the ‘right’ vitamin D levels. Values of 30 ” 80 nmol/L have been suggested as lower limits, for instance. In one study that I reported on here, there was evidence that optimal levels of vitamin D are something above 75 nmol/L.

> 25 nmol/L: 8.7 micrograms/day (348 IU per day)
> 37.5 nmol/L: 19.9 micrograms/day (796 IU per day)
> 50 nmol/L: 28.0 micrograms/day (1120 IU per day)
> 80 nmol/L: 41.1 micrograms/day (1644 IU per day)

The results of this study suggest that during the winter in Ireland, individuals need to take what would be traditionally regarded as generally high dosages of vitamin D to ensure good levels of vitamin D in their bodies. Of course, some of this need can be offset by getting more sunlight exposure. The researchers looked into this too. Here are the nutritional doses required to maintain a vitamin D level of > 25 nmol/L according to sunlight exposure:

Those enjoying sunshine: 7.2 micrograms/day (288 IU/day)
Those who sometimes have sun exposure: 8.8 micrograms/day (352 IU/day)
Those avoiding sun exposure: 12.3 micrograms/day (492 IU/day)

It seems from this that those who enjoy the sun can indeed offset some of their nutritional needs for vitamin D in the winter. However, perhaps even more importantly, those who avoid the sun (or perhaps can’t get out enough to get decent sunlight exposure) have increased requirement for vitamin D. Compared to those who enjoy the sun, those who avoid the sun need about 50 per cent more vitamin D in terms of what they put into their mouths.

Overall, the results show that the recommended daily amounts of vitamin D (400 IU in the UK, and in the USA 200, 400 and 600 IU per day for individuals aged <50 years, 50-70 years and older than 70 years respectively) are way too low, especially in the winter.


1. Looker AC, et al. Serum 25-hydroxyvitamin D status of the US population: 1988″1994 compared with 2000″2004. Am J Clin Nutr 2008;88: 519-1527.

2. Norman AW. A vitamin D nutritional cornucopia: new insights concerning the serum 25-hydroxyvitamin D status of the US population. Am J Clin Nutr 2008;88:1455-1456.

3. Cashman KD, et al. Estimation of the dietary requirement for vitamin D in healthy adults. Am J Clin Nutr 2008;88:1535-1542.

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