Taking vitamin D2 supplements could actually reduce the levels of vitamin D3 in your body, according to a surprising new study published in Nutrition Reviews.
The finding comes just as much of the Northern Hemisphere heads into autumn – the time when health experts recommend people start supplementing with vitamin D.
Vitamin D is essential for regulating calcium and phosphate in the body, which in turn keeps bones, teeth and muscles healthy. Deficiency is common worldwide: in the US, rates are as high as 31 per cent among non-Hispanic Black adults.
But not all vitamin D is created equal. It comes in two main forms: Vitamin D2, which is found in plants and mushrooms, and Vitamin D3, which is produced in the skin when exposed to sunlight, and also found in animal products such as oily fish.
During the summer, people living in higher latitudes can usually make enough vitamin D through sunlight exposure. In autumn and winter, however, the Sun never rises high enough for this to happen.
“People often think that on a sunny day in October, November and December they’ll make vitamin D,” Prof Susan Lanham-New, head of the Department of Nutritional Sciences at the University of Surrey and study co-author, told BBC Science Focus. “But they just won’t make any.”
A simple rule of thumb: unless your shadow is shorter than your height, the Sun is not strong enough for vitamin D3 production.
To explore the effects of vitamin D2 supplementation, Emily Brown, a PhD research fellow in Lanham-New’s group, analysed 20 randomised controlled trials. In 18 of them, participants who received vitamin D2 had lower levels of vitamin D3 compared with placebo or control groups.
“We don’t want people to think vitamin D2 is in any way dangerous. It’s not,” Brown told BBC Science Focus. “Vitamin D2 still raises your total vitamin D levels, but we think vitamin D3 is preferential.”
Previous research has shown that vitamin D3 is converted more efficiently into the active form of the vitamin than vitamin D2, suggesting it may be a better choice for supplementation, particularly during the darker months.
Brown now plans to investigate the reverse effect: what happens to D2 when people take D3.
The results are especially relevant for vegans, who cannot obtain D3 from traditional dietary sources. Vegan-friendly D3 can be made from lichen, but it is less widely available than D2.
In 2022, scientists even engineered a genetically modified tomato capable of producing D3, and Lanham-New’s team is now testing whether it can boost vitamin D levels in humans.
“We were really astonished that when you give D2, the D3 levels drop below even the placebo,” Lanham-New said.
“There’s a lot more work that we now need to do because the long-term impact could be that you don’t reach the required vitamin D levels or, dare I say it, you could make the situation worse.”
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