The numbers from this study of Long-Haulers are rather stark:
84% had a deficiency of vitamin D (< 20 ng/dL)
11.4% had an insufficiency (20-30 ng/dL)
Only 4.9 % were at a level considered acceptable.
Prevalence of vitamin D deficiency among patients attending Post COVID-19 follow-up clinic: a cross-sectional study. European Review for Medical and Pharmacological Sciences, April 2022.
Among the 219 people they studied who developed lingering problems after a covid infection, levels were low. But there was no clear relationship between levels of Vitamin D and the severity of symptoms. Based on this study, the researchers could not conclude that higher Vitamin D levels are protective. That is an open question. It is still an open question whether raising vitamin D levels in Long Haulers will help. This one study wasn’t designed to test either idea.
But the fact that 95% of these people are low on Vitamin D is itself an issue that should be remedied! People can suffer from nutritional deficiencies in a variety of ways. Fixing this one problem ~might~ help people recover, or maybe it won’t. But if the idea of a nutritional deficiency has any meaning, people should try to avoid being below the recommended level.
Vitamin D deficiencies and insufficiencies are the norm among modern western people. I have friends and family in Florida, the Midwestern U.S., and Iceland who were tested. All were below the ideal level - many were alarmingly low.
Even in the Sunshine State, most people live an indoor lifestyle; time is spent mostly shielded from the sun. And unless a person is downing a tablespoon or two of cod liver oil each morning or regularly eats some other form of liver, they probably aren’t getting enough from food. My Vitamin D level was below the norm (around 22 ng/ml) until we got a small pool in the backyard. It has since doubled and is at the respectable level of 44.
For decades, there has been a medical campaign to limit sun exposure. Sun exposure does increase the risks of skin cancer. But skin cancers are relatively easy to detect and treat compared to other forms of cancer … and sun exposure appears to limit the overall risk of dying from cancer. One analysis concluded that having vitamin D levels at 80 or above could prevent 70% of all cancers. 1
Vitamin D is also essential for immune function, and seems to play a role in tamping down autoimmunity.
Vitamin D levels don’t change quickly. Bringing the numbers up is a process that takes months or years - especially when the path is based on food or a vitamin D capsule.
Full-body exposure to the sun that leads to a bit of temporary redness (but not a burn) can generate 100,000 International Units of Vitamin D. The dietary guidelines suggest only 2,000 I.U. per day for most people.
“Vitamin D and Cancer: An Historical Overview of the Epidemiology and Mechanisms.” Nutrients, 30 Mar 2022. PMID 35406059.