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Vitamin D May Explain the Flu … and Fight it, Too
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New review makes a compelling case that seasonal lack of the “sunshine vitamin” causes the sharp wintertime rise in flu rates

by Craig Weatherby

The air of deep apprehension created by the possibility that a deadly form of flu (H5N1) might leap from birds to humans peaked a few months back, and has already receded in the media and in most people’s minds.
But a team of nutrition researchers have now published a probing scientific paper that challenges conventional wisdom concerning the causes and cures of the flu.
Physicians have long known that influenza sweeps the Northern Hemisphere during the winter months and usually peaks in America, Canada, and Europe between late December and March.
None of the numerous theories floated to explain the seasonal flu spikefrom the flood of frigid air to the wintertime tendency of people to huddle indoorsreally meets the scientific test.

Key Points

  • New review finds much evidence that flu is related to seasonal deficiencies of vitamin D.
  • CDC agrees that conventional explanations for the seasonal nature of flu don’t make sense.
  • Harvard Public Health head calls for higher vitamin D intake in Newsweek column; cites fish as leading source.
Dr. Scott Dowell is director of the Global Disease Protection Program at the Centers for Disease Control and Prevention. In a recent interview in LOHAS journal he called the conventional explanations “…astonishingly superficial and full of inconsistencies.”
Dr. Dowell and others find merit in a provocative new hypothesis: one that attributes the winter-focused nature of flu epidemics in the northern third of the globe to the season’s obvious shortage of vitamin-D-generating sunshine.
Influenza kills about 36,000 people in the U.S. every winter. As Dowell said, should the new hypothesis prove true, “…the potential impact would be far greater than the current influenza vaccine.”
Paper’s authors offer radically different view of the flu
The authors of the new evidence review include leading vitamin D researchers Michael C. Holick of Boston University and Edward Giovanucci of Harvard University, as well as William B. Grant, the NASA scientist turned vitamin D researcher, who we interviewed last year for an articleon the topic of vitamin D and cancer (To view all of our vitamin D articles, go to our archiveand enter “vitamin D” in the Search Articles box).
Many mysteries attend to the seasonal nature of influenza, and the simultaneous occurrence of local epidemics in widely separate regions: characteristics that make it hard to explain influenza epidemics as typical person-to-person viral infections, like the common cold or measles.
The study’s authors make a strong case that the mystery factor behind the seasonal rise in flu rates in the winter is a lack of sunshine-generated vitamin D.
If they’re right, high doses of supplemental vitamin D could provide a safe, effective, natural remedy for garden variety flu, and could even help prevent and treat far more dangerous forms, such as the flu virus that killed about 20 million people worldwide in 1918.

Fish fit the vitamin D bill; Sockeye salmon stand out

Certain fish rank among the very few substantial food sources of vitamin D.
Among fish, wild sockeye salmon may be the richest source of all, with a single 3.5 ounce serving surpassing the US RDA of 400 IU by about 70 percent:

Vitamin D per 3.5 ounce serving*

Sockeye salmon  687 IU

Albacore tuna  544 IU

Silver salmon  430 IU

King salmon  236 IU

Sardines  222 IU

Sablefish  169 IU

Halibut  162 IU

*For our full test results, click here.

The new hypothesis holds that the various flu viruses lie dormant in people and emerge during the dimmer days of winter in northerly climes and during the rainy seasons in tropical regions: especially in people whose darker skin coloror sun avoidance or heavy sunscreen useleaves them very low in vitamin D.
For example, studies ranging from Malaysia and South India to China and Japan show that women there have surprisingly low levels of vitamin D, especially in their winter months.
Researchers attribute the lack of vitamin D in these Asian nations in part to women’s indoor careers and a cultural preference for pale skin that leads them to use clothes, parasols, and sunscreens to avoid tanning. (This unhealthful prejudice is common among East and South Asian women, where skin-lightening creams are increasingly popular.)
The US recommended daily allowance (RDA) for vitamin D is 400 international units (IU).
And on sunny days, a fair-skinned person generates the equivalent of 20,000 international units (IU) in just 15-20 minutes.
In contrast, one cup of fortified milk contains fewer than 100 IU.
Virtually every vitamin D researcher now believes that people should be taking at least 1,000 IU per day, rather than the US RDA of only 400 IU. And most urge older, heavier, and dark-skinned peoplewho make and absorb less vitamin D--to take 3-5,000 IU every day.
The biological link between vitamin D and flu
Why would vitamin Dwhich is usually seen as an anti-cancer nutrient that keeps bones strongbe associated with risk of the flu?
It turns out that vitamin D plays a key role in the body’s immune-system defense against infection. This is especially true in the lungs, where flu viruses tend to settle first, and, as was the case in the virulent worldwide “Spanish” flu of 1918, where flu viruses pose the greatest risk.
During the 1918 pandemic, the highest death rate occurred among young, healthy people whose robust immune systems released excessive swarms of messenger proteins called cytokines: a reaction that caused raging, system-wide inflammation and filled their lungs with fluid. (Ironically, greater percentages of children and the elderly survived the 1918 flu, probably because their immune systems could not mount as strong a counterproductive reaction.)
Excerpts from “Epidemic influenza and vitamin D”
The following quotes come directly from the authors’ summary of their findings and conclusions. (We’ve added some clarifications in brackets [ ].) Note: E.R. Hope-Simpson, M.D. is the British doctor who conducted pioneering research that undermined conventional wisdom concerning the causes of flu outbreaks:
  • “In 1981, R. Edgar Hope-Simpson proposed that a 'seasonal stimulus' intimately associated with solar radiation explained the remarkable seasonality of epidemic influenza.… We conclude that vitamin D, or lack of it, may be Hope-Simpson's ‘seasonal stimulus’.”
  • “Solar radiation triggers robust seasonal vitamin D production in the skin; vitamin D deficiency is common in the winter, and activated vitamin D… a steroid hormone, has profound effects on human immunity.”
  • “[vitamin D] acts as an immune system modulator, preventing excessive expression of inflammatory cytokines [messenger proteins released by immune-system cells] and increasing the ‘oxidative burst’ potential of macrophages.” [Macrophages are immune system cells that spew oxygen free radicalsi.e., an “oxidative burst”— intended to kill invading bacteria and viruses].”
  • “Perhaps most importantly, it [vitamin D] dramatically stimulates the expression of potent anti-microbial peptides, which exist in [immune system cells called] neutrophils, monocytes, natural killer cells, and in epithelial cells lining the respiratory tract where they play a major role in protecting the lung from infection.”
  • “Volunteers inoculated with live attenuated [weakened] influenza virus are more likely to develop fever and serological [blood-test] evidence of an immune response in the winter."[In other words, the dimmer sunlight and consequent low vitamin D levels in winter months allow flu viruses to infect cells in significant numbers, and this causes the body to mount an immune response. But in the summer, blood levels of vitamin D rise, which allows the body to produce enough anti-microbial peptides to cripple the ability of influenza virus to infect significant numbers of cells, thereby reducing or eliminating the body’s need to mount a major, cytokine-mediated, inflammatory immune response to the virus.]
  • “Vitamin D deficiency predisposes children to respiratory infections.”
  • “Ultraviolet radiation (either from artificial sources or from sunlight) reduces the incidence of viral respiratory infections, as does cod liver oil (which contains vitamin D).”
  • “An interventional study [clinical trial] showed that vitamin D reduces the incidence of respiratory infections in children.”
British country doctor challenged conventional wisdom
Dr. John J. Cannell, one of the authors of the provocative new review, highlighted the pioneering work of Dr. Hope-Simpson in a recent edition of his vitamin D newsletter.
Hope-Simpson, who died in 2003 at the age of 95, marshaled his maverick research on influenza in a 1992 book titled “The Transmission of Epidemic Influenza.”

In his essay, Dr. Cannell summarized some of Dr. Hope-Simpson’s cogent but inconvenient observations as follows (key points underlined for emphasis):

  • “Going back to 1945, he discovered that influenza epidemics above 30 degrees latitude in both hemispheres occurred during the six months of least solar radiation. Outbreaks in the tropics almost always occur during the rainy season.”
  • “Hope-Simpson concluded, ‘Latitude alone broadly determines the timing of the epidemics in the annual cycle, a relationship that suggests a rather direct effect of some component of solar radiation acting positively or negatively upon the virus, the humans host or their interaction.’”
  • “Hope-Simpson rejected the theory that this year's virus is only transmitted from actively infected persons to well persons, concluding instead the facts were more consistent with transmission by symptom-less carriers who become contagious when the sun is either in the other hemisphere or obscured by the rainy season.”
  • “He theorized that annual movement of the sun caused a ‘seasonal stimulus that reactivates latent virus in the innumerable carriers who are everywhere present, so creating the opportunity for epidemics to occur in the wake of its passage.’”
We’ll have to await the results of controlled clinical studies to know whether vitamin D really fights the flu.
But leading vitamin D researchers believe that it makes good sense to take ample daily doses of vitamin D (i.e., 1-2,000 IU) to help prevent cancer and osteoporosis, and the authors of the new paper say it can’t hurt to start taking take larger doses temporarily if you feel a flu coming on.
While vitamin D can be toxic if taken in very large doses over a period of weeks or months, all of the available evidence indicates that it is safe to take daily doses that are at least several times higher than the US government’s established safe upper level dose (Vieth 1999, 2001, 2006).
Authors propose preventive and therapeutic use of vitamin D
In their new paper, some of American’s preeminent vitamin D researchers went beyond hypothesizing to recommend prudent anti-flu use of vitamin D.
Dr. Cannell noted the hypothetical nature of his team’s findings, saying “Like all theories, our theory must withstand attempts to be disproved with dispassionately conducted and well-controlled scientific experiments.”
But their research certainly suggests the possibility of some immediate practical implications:
  • “physiological [i.e., maintenance] doses of vitamin D (5,000 IU a day) may prevent colds and the flu…”
  • “physicians might find pharmacological [i.e., therapeutic] doses of vitamin D (2,000 IU per kilogram of body weight per day for three days) useful…”
  • “We recommend that enough vitamin D be taken daily to maintain 25-hydroxy vitamin D levels at levels normally achieved through summertime sun exposure (50 ng/ml). For many persons, such as African Americans and the elderly, this will require up to 5,000 IU daily in the winter and less, or none, in the summer, depending on summertime sun exposure.”
Newsweek provides forum for vitamin D pitch
Meir Stampfer, M.D., chair of the epidemiology department at the Harvard School of Public Health, wrote a column in last week’s issue of Newsweek magazine, in which he decried the vitamin D deficiencies common among Americans.
These were some of his arguments for raising the public profile of vitamin D, to ensure that Americans to get much more of it via foods and supplements:
  • “If applied adequately to protect against sun-induced skin damage and to reduce the risk of skin cancer, sunscreen lowers the skin's ability to form vitamin D by more than 95 percent. More of us are older and fatter; age and obesity also reduce the amount of vitamin D we produce. An average 70-year-old can produce only about a quarter of the vitamin D of a 20-year-old. Obese people generally have substantially lower blood levels of vitamin D.”
  • “…in Boston between November and March, the ultraviolet radiation from the sun is insufficient to produce vitamin D, even with abundant skin exposure on a sunny day. The farther you go away from the equator, the greater the effect. One study found that among white girls in Maine, 48 percent had low vitamin D blood levels at the end of the winter, while only 17 percent were deficient at the end of the summer. Clothing can play a big role as
  • “Skin pigmentation also affects the way we process vitamin D.… Indeed, the majority of African-Americans have low levels of vitamin D.
  • “Finding ways to counteract these barriers to getting enough vitamin D is the next challenge. … The only foods with high levels of vitamin D are fatty fish and certain kinds of mushrooms.”
  • “Among nutritionists working on vitamin D, there is general agreement that the current recommended intake of 400 IU per day (600 for those over the age of 70) is too low, and should be re-evaluated. Most believe that 1,000 IU per day would be a reasonable dose for a typical adult in the United States, and I agree. … The current official upper limit is 2,000 IU, although many experts think this is too low and should be raised, perhaps to 4,000 IU.”

Stay tuned… we’ll keep you posted on developments regarding vitamin D and human health.


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