Research suggests that the original estimates of the adult requirements for vitamin D were too low. This is because they were based largely on medical cases of children being supplemented with small amounts of cod liver oil to prevent rickets. New research suggests that the actual requirement for an adult may be between 2 to 10-fold larger than original estimates. Sunlight exposure is one way that vitamin D levels can be increased, as the ultraviolet light in sunlight catalyses a reaction in the skin that converts cholesterol to vitamin D. However, at latitudes above 40 °N or 40 °S the angle of incidence becomes too great in the winter months and sunlight is therefore too weak to cause this reaction to occur. As a result many individuals living in areas where sunlight is not strong year round may be at increased risk of developing vitamin D insufficiency during winter months. Because relatively few foods contain vitamin D, supplements or fortified foods may be required by at risk populations in order to maintain year round vitamin D levels.
Studies have been performed in northern latitude populations in order to investigate the effects of vitamin D fortification to foods. In one randomised controlled trial1, researchers recruited subjects from 201 families including individuals between 4 and 60 years of age and randomly assigned to them either vitamin D fortified foods, or identical foods with no vitamin D fortification. The foods included milk that was fortified at concentrations of ~0.38 μg per 100 mL (compared to 0.004 μg/100 mL naturally in the same milk) and a range of breads including white, brown, rye loaves as well as buns baked from rye and wheat flour (containing 5.2 μg per 100 grams vitamin D). Subjects consumed the fortified products for 6 months starting from september and the fortified foods (and their controls) replaced the subject’s normal foods. The results showed that the intake of vitamin D was 9.4 μg (376 IU) per day in those subjects eating the fortified foods and 2.2 μg ( 88 IU) per day in those consuming the non-fortified alternatives.
The researchers then assessed the effects the higher intake of vitamin D in those subjects receiving the fortified foods. Serum levels of 25-hydroxyvitamin D (the accepted marker of vitamin D status) decreased from 71.1 to 41.7 nmol/L (28.5 to 16.7 ng/mL) in the control group showing that over the winter months the levels of sunlight and the dietary intake were not able to maintain plasma levels of vitamin D. However in those subjects receiving the vitamin D fortified foods, serum levels of 25-hydroxyvitamin D decreased from 73.1 to only 67.6 nmol/L (29.3 to 27.1 ng/mL), suggesting that the fortified foods had helped maintain a better vitamin D status. At the end of the 6 month trial, the final vitamin D concentration was higher in those subjects receiving the vitamin D fortified foods, compared to those receiving the non-fortified foods. This was reflected in the fact that less than 1 % of the fortified group had serum 25-hydroxyvitamin D serum levels below the recommended 30 nmol/L, whereas 25 % of the non-fortified group were below this level.
Dr Robert Barrington’s Nutritional Recommendation: Evidence suggests that the vitamin D status of those living in high latitude regions becomes poor during the winter months. Fortified foods are a solution to this, but such foods are not widely available yet. Therefore it is recommended that a vitamin D supplement of around 2000 IU is taken everyday (by adults) during the winter months in order to maintain optimal plasma levels of vitamin D.
RdB
