Millions of people take vitamin D supplements during winter months to protect their bones and immune systems, but new research suggests a widely used form may actually undermine the body's natural defenses.
Scientists from the University of Surrey, John Innes Centre, and Quadram Institute Bioscience have found that vitamin D2 supplements can reduce concentrations of vitamin D3, the more potent form that the body produces naturally from sunlight. The finding, published in Nutrition Reviews, challenges the assumption that all vitamin D supplements work equally.
The researchers analyzed data from randomized controlled trials and discovered that people taking vitamin D2 experienced drops in vitamin D3 levels compared to those not taking any supplement. In many studies, vitamin D3 concentrations fell below those in control groups.
Emily Brown, lead researcher at the University of Surrey's Nutrition, Exercise, Chronobiology and Sleep Discipline, emphasized the timing of the finding. "Vitamin D supplements are important, especially between October and March, when our bodies cannot make vitamin D from sunlight in the UK," she said. "However, we discovered that vitamin D2 supplements can actually decrease levels of vitamin D3 in the body, which is a previously unknown effect of taking these supplements."
The distinction matters because vitamin D3 appears to offer immune advantages that vitamin D2 does not provide. Earlier research led by Professor Colin Smith at the University of Surrey found that only vitamin D3 activates the type I interferon signaling system, a critical first line of defense against bacteria and viruses.
"Vitamin D3, but not vitamin D2, appears to stimulate the type I interferon signalling system in the body," Smith said. "A healthy vitamin D3 status may help prevent viruses and bacteria from gaining a foothold in the body."
The UK government recommends 10 micrograms of vitamin D daily, particularly during darker months when natural production is impossible. Most people have access to two supplement options: vitamin D2, typically derived from plant sources, and vitamin D3, which can come from animal products or increasingly from plant-based sources.
Researchers suggest that vitamin D3 may be the more effective choice for most individuals, though they acknowledge that personal circumstances should guide supplementation decisions. Professor Cathie Martin from the John Innes Centre said the research underscores the need to make plant-based vitamin D3 more widely available in the UK.
Vitamin D deficiency remains a significant public health concern across Britain, particularly during winter. The new findings add weight to arguments that the form of supplement matters as much as the dose.
Author Jessica Williams: "If a cheaper supplement is actively working against your immune system, it's not a bargain at any price."
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