例如，前疾病控制与预防中心主任汤姆·弗里登（Tom Frieden）博士在3月23日 在福克斯新闻网站上发表了 专栏文章，暗示维生素D可以降低人的COVID-19风险。
但是专家提醒不要过度解释初步的相关性或假设机制。最初的州立大学 营养学家 A. 凯瑟琳·罗斯告诉我们的，关联与因果关系并不相同，支持或反对维生素D和COVID-19的证据“极其”。薄弱”。
“这实际上是一种激素，在我们拥有的所有营养素中，它是唯一的非饮食来源的营养素，” 英国萨里大学维生素D专家Susan Lanham- Newsay 。
相反，她告诉我们，大多数人的维生素d是在皮肤暴露在阳光下，这就是为什么深色皮肤的人做 更容易 有维生素水平升高，为什么人谁去外面那么交替，包括那些年龄严重或健康状况较差的人容易出现缺陷。
但是，维生素D可以脂溶并存于体内，因此，添加更多并不一定总能带来好处。Lanham -Newsay：“如果过多的维生素D，您会得到所谓的高钙血症，”这是指血液中钙的含量升高 ，这对高血压疾病的人来说尤其是危险。
The authors identified a protective effect for those taking vitamin D supplements daily or weekly, with the greatest benefit going to those who had the lowest levels of the vitamin to start. Periodic large doses, or boluses, of vitamin D were not effective.
An accompanying editorial, however, noted that the absolute risk of coming down with at least one respiratory infection when taking vitamin D supplements dropped by only 2 percentage points — from 42% to 40% — and that given differences between the studies that were analyzed, large randomized controlled trials were still needed.
Lack of Evidence for Vitamin D and COVID-19
Because the coronavirus is so new, little rigorous research has been done specifically on vitamin D and COVID-19.
Oxford’s rapid review, which was posted last month and reflected literature searches performed in April, did not identify any clinical evidence that vitamin D is beneficial for COVID-19. The report concluded that “well-masked randomized trials” were needed before specifically recommending the nutrient for COVID-19, but that Britons should already be taking vitamin D supplements anyway, per national guidance.
Since then, a variety of published and unpublished studies investigating potential links between vitamin D and COVID-19 have appeared, but Dr. Joseph Lee, a general practitioner and co-author of the report, told us that he was not aware of any subsequent studies that would alter his group’s recommendation that “people should take vitamin D, but not because of COVID-19.”
Lanham-New, who was the lead author of the BMJ Nutrition, Prevention & Health review, also said her conclusions had not changed.
One U.K. study, published on May 6 in Aging Clinical and Experimental Research, identified a crude association between the average vitamin D level reported in 20 European countries and the number of per-capita COVID-19 cases and deaths in those nations.
Another similar paper, appearing in the Irish Medical Journal, found an inverse relationship between the vitamin D levels in older people in different European countries, as reported in past studies, and a country’s COVID-19 mortality rate, with fish-loving Nordic countries generally faring better than those in southern Europe.
But these so-called ecological epidemiology studies can only hint at any effect of vitamin D, since they’re simple correlations at the population level. “This is not a design suited to identifying causal effects,” Lee said, “and I would not consider them as evidence of a role for vitamin D in COVID-19.”
Other reports also claim to identify links between lower levels of vitamin D and COVID-19 infection or disease severity in different cohorts of people, but most of these have not yet been vetted by other scientists through peer review.
One unpublished report of 780 confirmed COVID-19 cases in Indonesia found that the majority of deaths occurred in patients with abnormally low vitamin D blood levels, and claimed to have found an association between vitamin D deficiency and COVID-19 mortality after controlling for other factors, such as other preexisting health conditions.
But as Lanham-New noted in an interview, the paper “doesn’t in any way prove cause and effect.” Lee, too, said that while the authors attempted to control for other health conditions, the group did so in an odd way, lumping all of the conditions together. And even if the team had controlled for them individually, that doesn’t necessarily eliminate bias, so the relationship could reflect the fact that people who are in poorer health generally fare worse with COVID-19.
“It is relatively easy to calculate associations, and that is what most of these papers have done,” Penn State’s Ross said. “But associations do not show cause and effect, and in fact, ‘reverse causation’ is not carefully considered.” The disease itself could cause a reduction in a person’s vitamin D levels, she said, noting that there are negative associations of serum vitamin D with diabetes, obesity and other infections. “I place little confidence on any of these studies,” she said.
More credible, Ross said, were two analyses of U.K. biobank data, neither of which support the idea that less vitamin D leads to a higher risk of COVID-19 infection. Given concerns that the disproportionate number of coronavirus infections in blacks and South Asians in the U.K. could be due to lower vitamin D levels in darker-skinned people, the researchers checked to see if there was any connection between a person’s vitamin D levels, which had been measured in participants about a decade ago, and testing positive for COVID-19.
While the published report affirmed that black and South Asian participants were several times more likely to test positive for COVID-19 than whites, there was no association with vitamin D. The authors concluded that vitamin D “is unlikely to be the underlying mechanism for the higher risk observed in black and minority ethnic individuals and vitamin D supplements are unlikely to provide an effective intervention.”
The other analysis, which Lanham-New said has since been submitted to a journal with additional participant data, found no difference in vitamin D status among those who tested positive versus negative for COVID-19. She said additional work was planned once COVID-19 severity and mortality data became available.
Another consideration when interpreting many observational studies, Lee said, is something called collider bias, which can sometimes result in spurious correlations when the people included in a dataset aren’t representative of the wider population. Some researchers have already noted that COVID-19 studies may be especially prone to collider bias, making it hard to identify risk factors and medications that work.
Consider a study analyzing outcomes among people who are tested for COVID-19. Some people are likely getting tested because they are quite ill and have been admitted to the hospital, while others may be tested because they’re a health care worker or because they’re more privileged and have access to testing.
In this scenario, Lee said, people with good levels of vitamin D will be less likely to test positive, compared to the sick people. “This selection will in itself induce an association between low vitamin D and COVID-19 positive tests or severity of disease,” he said, “even if it isn’t true in the general population.”
In the end, Ross is skeptical that vitamin D will prove to be beneficial for COVID-19, although she hesitated to entirely discount the possibility, given how much is still unknown about the disease.
“[I]t is hard for me to conceive that vitamin D has much chance of being as effective as other kinds of treatments, if at all,” she said. “We don’t know however.”
Her recommendation, irrespective of COVID-19, is to consume vitamin D-rich foods or take a supplement to provide the recommended dietary amount, or RDA, of 600-800 International Units per day. This matches the Institute of Medicine’s national guideline, which Ross helped write, and which assumes minimal sun exposure.
One cup of fortified milk, for example, contains 120 IUs of vitamin D, while a 3-ounce serving of cooked salmon provides 570 IUs.
Lanham-New emphasized that excessive doses of vitamin D should not be used. But especially for those who have been cooped up indoors while social distancing, it may be a good idea to take regular supplements.
“If you’re in self-isolation, definitely be taking a vitamin D supplement according to your government guidelines,” she said.
Lee also pointed to following state or national vitamin D recommendations. “There is no reason to think it will help with COVID-19, but that might change when the trials report,” he said of vitamin D. “Our advice is to take vitamin D in accordance with local guidelines.”
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