Vitamin D is an extremely important vitamin that has powerful effects on several systems throughout your body. Unlike other vitamins, vitamin D functions similar to a hormone, and every single cell in your body has a receptor for it. Our body mostly makes vitamin D from cholesterol, when the skin is exposed to sunlight. It's also found in foods such as fatty fish and some fortified dairy products, though it's very difficult to get enough from diet alone. The recommended daily intake (RDI) is considered to be 400–800 IU, but many experts recommend aiming to get even more than that, as it is estimated that over 50% of people can have levels that would be considered deficient. Most people don't realize that they’re deficient, as symptoms are generally subtle (sleep difficulties, fatigue/ tiredness, mood disturbances including depression symptoms, bone loss, muscle pain). We see this regularly in the StriveStonger lab with more than half clients assessed with suboptimal Vitamin D blood levels.
One of vitamin D's most important roles is keeping the immune system strong to fight off viruses and bacteria that cause illness. It directly interacts with the cells that are responsible for fighting infection (such as T and B cells). The importance of Vitamin D has recently been highlighted in 2 preliminary studies that have reported relationships between low Vitamin D and both susceptibility to and prognosis from COVID-19. Although these studies demonstrate associations and not necessarily causation in cohorts of patient with COVID-19, higher blood Vitamin D levels has been reported to either improve clinical outcomes or mitigate worst (severe to critical) outcomes, while lower levels considered deficient seem associated with worsened clinical outcomes, even after controlling for potential confounders such as sex, age, and comorbidity (chronic diseases).
You can read these full preliminary studies at:
Fortunately, maintaining or promoting vitamin D in the body is usually easy to fix. You can either increase your sun exposure, eat more vitamin-D-rich foods such as fatty fish (salmon, tuna, mackerel and beef liver), fortified dairy products and egg yolks.