Summary

There increasing concern over intakes of vitamin D and whether these are too low to ensure optimum health. Exposure to sun is the main source of vitamin D and in many countries, the sun does not sign often enough. And it seems that diet is not making up the shortfall. While vitamin D deficiency is usually linked with bone health problems, there is evidence that it could also be a factor in immune functioning, cancer, heart disease, and diabetes. It might, therefore, be time to revise vitamin D recommended intakes upwards, according to a major new review.

Introduction

Vitamin D is a fat soluble vitamin with many functions in the body. The majority of our vitamin D comes from exposure to the sun, with uv light converting previtamin D to active vitamin in the skin. It also comes from dietary sources such as fortified margarine, eggs, oily fish and supplements. There is increasing evidence that vitamin D is vital for physical and mental wellbeing and that levels may be too low among the population in the UK and elsewhere.

What was done

Dr Carrie Ruxton, an independent dietitian, carried out an extensive survey of the medical literature on vitamin D. Her aim was to investigate dietary intakes and vitamin D status, comparing these with national recommendations. She also looked at the links between vitamin D and health. The findings were to shed light on the question of whether we are actually getting enough vitamin D.

What was found

The vast majority of UK children and adults have vitamin D intakes below the US recommendation of five micrograms per day (there is no corresponding recommendation in the UK for children and most adults). In the US, there have been calls for an ten-fold increase in the vitamin D recommendation to bring it to 50 micrograms a day, based upon the most recent scientific evidence on the importance of vitamin D.

The review also looks at vitamin D status – that is, levels in blood – which is more accurate than dietary intake data. This is measured in nanomoles/liter but it is not clear what the optimal level is – some experts say more than 50 nmol/l and others say 80 nmol/l. We do know that levels less than 8 nmol/l are associated with rickets and levels less than 25 nmol/l are linked with osteoporosis.

In the UK, a significant proportion of the population have potential vitamin D deficiency by having levels less than 25 nmol/l. This is a particularl problem for those living in institutions, who may not be able to get out and whose diets may be poor.

Vitamin D has an impact in many areas of health – although the strength of the evidence varies. Most people associate vitamin D with bone health because the evidence is strong. But recent studies point to vitamin D being a factor in diabetes, heart disease, cancer, immunity, infection, cell aging and even cognitive function.

For instance, in a trial of vitamin D supplementation, those not given supplements had a 7% greater risk of earlier death than those who did get supplements. There is also evidence for a link between vitamin D deficiency and colorectal and breast cancer and maybe cancers in other sites. Adults with the highest intake of vitamin D had a 60% lower risk of colorectal cancer, in a meta analysis.

There is also evidence for a link between vitamin D deficiency and high blood pressure, inflammation and blood lipids, which could increase the risk of heart disease. High vitamin D status can also halve the risk of diabetes.

What this study means

Many of us do not get enough vitamin D, according to the emerging evidence on its importance for health. A combination of lack of sun exposure and a diet low in vitamin D is responsible for this. Ruxton says that scientists must try to harmonize their recommendations on vitamin D and these must be promoted so people know what to do. This might include sun exposure without sunscreen for 20 minutes or so to get the skin to make enough, maybe fortifying more foods and boosting the vitamin D content of supplement.

Source

Ruxton CS and Derbyshire E Health impacts of vitamin D: are we getting enough? Nutrition Bulletin May 2009; 34: 185-197