Study finds vitamin D2 may lower D3 levels, raising fresh fears over supplement choices

For decades, vitamin D supplements were seen as a straightforward safeguard against deficiency. Health advisers urged people in northern climates to top up their levels during the darker months, when the sun’s rays are too weak to trigger the body’s natural production of vitamin D. But new findings suggest the story is far more complicated, and in some cases unsettling.

Vitamin D exists in two forms most commonly sold in supplement aisles: vitamin D3, or cholecalciferol, and vitamin D2, or ergocalciferol. Traditionally, both were assumed to be equally effective. D3 is typically sourced from lanolin, a waxy substance found in sheep’s wool, while D2 comes mainly from mushrooms, making it the preferred option for vegans. Pharmacies continue to place them side by side, with the expectation that either will help strengthen bones, muscles and the immune system.

The assumption has been badly shaken by a University of Surrey study led by Emily Brown. Her team analysed 11 trials covering 655 people and made a startling discovery: D2 supplements actually lowered concentrations of vitamin D3 in the body. In some cases, participants who took D2 ended up with less vitamin D3 than those who took nothing at all. Why this happens remains a mystery, and researchers do not yet know whether D3 also reduces D2. Still, the effect overturns long-standing beliefs and raises urgent questions about supplement safety.

Vitamin D deficiency is notorious for its subtlety. Fatigue, bone pain, frequent illness and muscle weakness are common, but many sufferers attribute these symptoms to stress or age. That makes deficiency one of the most underdiagnosed public health problems worldwide. Against this backdrop, the new research is particularly troubling. A substance intended to help may actually be interfering with the body’s own protective reserves.

The immune system appears to be where the difference between D2 and D3 is most critical. A 2022 study suggested that only D3 activates the type-I interferon pathway, a frontline defence against viral infections. With respiratory illnesses typically spiking in winter, the implications are stark: choosing the wrong supplement could blunt the body’s ability to resist disease at precisely the moment it is most vulnerable.

Even so, scientists stop short of declaring D2 worthless. According to Brown, D3 may be more effective for most people, but D2 still prevents outright deficiency and remains valuable for vegans and those unable to access D3. The real concern lies in the nuanced effects: while D2 maintains overall vitamin D levels, it may lack the added immune benefits of D3 and could even suppress them.

The debate grows more complicated with age. Research shows the body becomes less efficient at converting D3 into its active form, calcitriol, in older adults. In those cases, D2 might offer some compensatory benefits. This suggests that the best supplement is not universal but shaped by age, health status and dietary choices.

Veganism adds another layer. Until recently, plant-based consumers had little option beyond D2. Now, however, advances in biotechnology are producing vegan-friendly D3 derived from lichen, and even experimental tomatoes engineered to manufacture D3. These could eventually provide plant-based alternatives that combine ethical sourcing with the more robust health benefits of D3.

The University of Surrey’s Susan Lanham-New stresses that the real lesson is not to ignore vitamin D at all. In the UK and other northern countries, mild vitamin D deficiency leading to osteomalacia is widespread during winter. Tiredness, bone aches and recurring infections are often the result. While public campaigns focus on iron, calcium and vitamin C, vitamin D quietly underpins immunity, energy and even mood.

The strange conclusion of the latest research is clear: the form of vitamin D you take may matter as much as whether you take it at all. Supplements remain vital, but they are not all created equal.