A recent study reveals that taking vitamin D2 supplements can actually lower vitamin D3 levels in the body, according to a research published in Nutrition Reviews.

This finding comes at a time when health experts advise individuals to start replenishing vitamin D, especially as the Northern Hemisphere transitions into autumn.

Vitamin D plays a critical role in regulating calcium and phosphate levels, which are essential for maintaining healthy bones, teeth, and muscles. The deficiency of this vitamin is prevalent globally; for instance, in the US, the rates are 31% among non-Hispanic Black adults.

Not all vitamin D is alike; it comes in two primary forms: Vitamin D2, which is derived from plants and mushrooms, and Vitamin D3, which is synthesized in the skin when exposed to sunlight and can also be found in animal products like oily fish.

During summer months, individuals living in higher latitudes can typically produce sufficient amounts of vitamin D through sun exposure. However, as autumn and winter approach, the sun’s angle is often insufficient for this to occur.

“People often produce vitamin D on sunny days in October, November, and December,” said Professor Susan Lanham-New, the Director of Nutrition Science at the University of Surrey and co-author of the study. This was reported by BBC Science Focus. “But they aren’t actually doing anything.”

A simple rule of thumb: If your shadow is not shorter than your height, the sun is not strong enough for vitamin D3 production.

We analyzed 20 randomized controlled trials to assess the impact of vitamin D2 supplementation, as noted by Emily Brown, a doctoral researcher in the Lanham-New group. Of these trials, 18 participants receiving vitamin D2 exhibited lower levels of vitamin D3 when compared to placebo or control groups.

“We don’t want people to think that vitamin D2 is somehow harmful. That’s not the case,” Brown commented to BBC Science Focus. “While vitamin D2 does elevate overall vitamin D levels, vitamin D3 should be prioritized.”

A prior study indicated that vitamin D3 is converted more efficiently into its active form compared to vitamin D2, making it a preferable option for supplementation, particularly during the darker months.

Brown is currently planning to explore the reverse effect, specifically what happens to D2 levels when individuals take D3.

The findings are especially pertinent for vegans who cannot obtain D3 from conventional dietary sources. While vegan-friendly D3 is derived from lichens, it is not as widely accessible as D2.

In 2022, scientists also developed a GMO tomato capable of producing D3, and the Lanham-New team is currently investigating whether this can effectively boost human vitamin D levels.

“I was genuinely surprised to find that when I administered D2, my D3 levels fell even when compared to placebo,” Lanham-New said.

“There is a lot we need to investigate now, as the long-term implications might hinder our ability to meet necessary vitamin D levels, potentially worsening the situation,” she added.

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