Study assesses dietary requirements for vitamin D in the winter

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.

10 Responses to Study assesses dietary requirements for vitamin D in the winter

  1. Peter Silverman 8 December 2008 at 6:10 pm #

    Wm Davis, MD, who tests a lot of people for D says that the tablets don’t raise your blood level, only the gel caps do. He writes the HeartScan blog and is a cardiologist.

  2. Anna 8 December 2008 at 10:41 pm #

    My 10 you son needed to have some blood drawn for another reason a few weeks ago, so I asked that his Vit D levels get checked too. I was eager to see, because since school started in late August, he gets far less time outside in prime Vit D sunlight hours) so he’s been taking Vit D3 of about 3,000iU per day, per the Vitamin D Council’s recommendation of 1,000 iU per 25 pounds of body weight in winter.

    My son’s level was 72, so I was happy to see his levels are great and that the Vit D3 supplementing wasn’t too much. We live in So Cal, so he does get perhaps more sun than others do in winter, but even so, school and time at home indoors, plus cooler weather clothing during the shorter prime D times still limit Vit D3 production from sunlight. I’ll back off the supplements in the spring when his sunlight exposure increases.

    He also didn’t catch “the cold that turned into a lingering, awful cough” that spread around the school population last month, creating lost of empty desks in the classrooms. Then again, he also consumes little immune-suppressing sugar.

  3. Dave 8 December 2008 at 10:44 pm #

    Hi Dr. Briffa. How was the Vitamin D administered? Thanks.

  4. Dr John Briffa 8 December 2008 at 10:54 pm #

    The D3 was administered in capsule form (produced by Banner Pharmacaps in the Netherlands).

  5. Dave 8 December 2008 at 11:06 pm #

    Did they happen to mention if it was powdered or in an oil base?

  6. Dr John Briffa 8 December 2008 at 11:17 pm #

    ‘fraid not.

  7. Anne 9 December 2008 at 2:07 am #

    The optimal lab value of 25-hyroxyvitamin D at Kingston-upon-Thames NHS hospital is from 75 to 200 nmol/L. I take 2,000ius of D3 (a gel cap) per day and my levels vary between 155 nmol/L and 200 nmol/L – my endocrinologist checks it regularly.


  8. Liz 12 December 2008 at 2:33 pm #

    Hi Dr Briffa
    Do you think it is a good idea for people with Hypoparathyroidism to take a high dose D3 as well as alfacalcidol/calcitriol ? There are currently opposing schools of thought on this.

  9. John Ivens 12 December 2008 at 9:25 pm #

    Was it here I read that as soon as your shadow is longer than your body, you will be getting no appreciable Vitamin D production via the UVB rays?

  10. Lilly Adams 12 March 2010 at 12:17 am #

    This article was very informative. I believe getting the essential vitamins that your body needs is very important to living a very long and healthy life. I found this really awesome website that lists every dietary vitamins Check out the website. I really hope this helps you.

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