“The reason for this is obscure possibly caused by the baking process of the bread or a potentially indigestible form of D2 in the yeast preparation,” the authors wrote in the British Journal of Nutrition.
The study conducted as part of the project ODIN (food-based solutions for optimal vitamin D nutrition and health through the lifecycle) by scientists at universities in Finland, Denmark and Ireland also strengthened previous data suggesting vitamin D2 supplementation decreases vitamin D3 concentrations.
"The randomized controlled trial showed that consumption of the UV yeast-vitamin D2-fortified bread did not affect serum 25-hydroxyvitamin D concentration. This was despite the fact that the bread, even following the baking process, was analysed and shown to actually contain the amount 25 μg (micrograms) vitamin D2 that was given daily," Dr Suvi Itkonen, author and researcher from the University of Helsinki, told NutraIngredients.
"Thus, this new data suggests that vitamin D2 from UV-irradiated yeast in bread, despite being present post baking, was not bioavailable in humans."
Vitamin D3 is the most common form of vitamin D used to fortify foods, but D2 may be more cost effective and has the advantage of being acceptable to people who avoid animal products in food.
UVB irradiation of foods such as mushrooms and baker’s yeast has been shown to stimulate production of vitamin D2 within the food itself.
This present study was the first of its kind to test the bioavailability of vitamin D2 in humans using enriched bread in an effort to provide cheaper, ecological alternatives for improving vitamin D statuses in the population.
The researchers looked at 33 healthy women in Helsinki over eight weeks in February and April when sunlight is insufficient for vitamin D production within the body.
The subjects were blindly randomised to receive regular bread and either a daily placebo pill, vitamin D2 supplement or vitamin D3 supplement, or vitamin D2-fortified bread and a daily placebo pill.
All daily doses of vitamin D were 25 μg which corresponds to 2.5 times the national Finnish recommendation for vitamin D in the age group. Vitamin D concentrations in blood were measured at the beginning, midpoint and end of the trial.
Vitamin D2 and D3 supplements increased total serum 25-hydroxyvitamin D by 14.7% and 26.2%, respectively. Total vitamin D levels remained the same as at baseline with the fortified bread. The D3 concentration decreased in the D2 supplement group relative to the D2-bread.
Previous research with fortified bread and UV-irradiated mushrooms has suggested effectiveness may only be evident in vitamin D deficient subjects, and as the women in the present study had relatively good vitamin D status this may have impacted on the results.
It is estimated 56% of Finnish females use vitamin D supplements, meaning recruiting study participants with low levels would be difficult.
Bring me sunshine
The health benefits of vitamin D are now recognised to go beyond bone health, with research suggesting a role in the immune system and mental and cardiovascular health.
With average European intakes of 50 kg bread per person annually, this is an ideal food for enrichment as part of a public health strategy for vitamin D improvement.
Nordic countries such as Finland, Denmark and Sweden fortify foods like margarine and dairy products and recommend these in addition to vitamin D supplementation, particularly in at risk groups such as pregnant women, children, dark skinned people and the elderly.
A review of the Nordic Nutrition Recommendations in 2013 increased the recommended daily vitamin D intake for adults from 7.5 μg to 10 μg daily for under 75 years of age and to 20 μg daily for over 75’s.
Last year the UK retailer Marks and Spencer rolled out its fortified bakery range in an effort to make up for lack of vitamin D due to sunlight exposure in six months of the year.
The national Institute for Health and Care Excellence (NICE) estimates one in five adults in the UK have low vitamin D status.
Source: British Journal of Nutrition
Published online ahead of print, http://dx.doi.org/10.1017/S0007114516000192
“Effects of vitamin D2-fortified bread v. supplementation with vitamin D2 or D3 on serum 25-hydroxyvitamin D metabolites: an 8-week randomised-controlled trial in young adult Finnish women”
Authors: S. T. Itkonen et al