In a paper published in the British Medical Journal this month, 21 experts from the U.K., Ireland and the U.S. conclude that while vitamin D is “essential for good health” (especially for bones and muscles) and may bolster the immune system, it can also be dangerous in high doses.

 “Many people have low blood levels of vitamin D, especially in winter or if confined indoors, because summer sunshine is the main source of vitamin D for most people,” the authors write. “Taking a daily supplement ... and eating foods that provide vitamin D is particularly important for those self-isolating with limited exposure to sunlight.” Foods high in vitamin D include fatty fish (such as salmon and tuna), portobello mushrooms, fortified milk and yogurt and eggs. But high doses of the vitamin — which the authors refer to as “mega doses” can be extremely dangerous.

 The paper comes on the heels of a study suggesting that vitamin D deficiency may be linked to higher rates of mortality with COVID-19. But the report warns individuals not to simply start taking those mega doses. “The continued spread of the novel SARS-CoV-2 virus, and the disease COVID-19 that is caused by SARS-CoV-2, has led to calls for widespread high-dose vitamin D supplementation,” the authors write. “These calls are without support from pertinent studies in humans at this time, but rather based on speculations about presumed mechanisms.”