Vitamin D Newsletter


Influenza in Hong Kong

Deborah from Hong Kong writes:

Dr. Cannell: I live in Hong Kong. We are near the equator where sunshine is strong all year. If vitamin D prevents the flu, why do we get the flu?

Dr. Cannell replies:

Great question! If vitamin D is antivral and antibiotic, then influenza  should be rare around the equator due to the high intensity of year-round ultraviolet B light. However, the incidence of influenza in Hong Kong is comparable to other countries. Understanding influenza in Hong Kong is particularly important because Hong Kong is close to what many consider to be the epicenter of new influenza strains in southern china and the next highly lethal influenza strain, destined to cause a devastating pandemic, may first be detected in Hong Kong. However, Hong Kong, in spite of being just south of the Tropic of Cancer at latitude 22 degrees, has a significant incidence of year-round vitamin D deficiency due to five factors: avoidance of sunlight, use of sunscreens, high rise buildings, terrible air pollution, and rainy seasons.

In 1990, a study reported that 55% of Hong Kong patients with fractured hips, 36% of healthy elderly, and 15% of healthy young people had 25(OH)D levels below 20 ng/mL. None of the subjects, even the healthy young adults, had levels above 40 ng/mL at any time of the year. In a 1988 study, 25% of subjects had levels less than 20 ng/mL in September and close to 50% had such levels in January. A 1989 study of Hong Kong infants showed almost half had levels less than 20 ng/mL in January through April. None of the infants had adequate levels (higher than 40 ng/mL), not even at the height of the Hong Kong summer.

Unfortunately, their are no recent studies of vitamin D levels in Hong Kong. However, a recent study in Miami, just a little further north of the equator at latitude 25 degrees North, found that 40% of adults had levels of less than 20 ng/mL at the end of winter—22% had these levels at the end of summer! Few had levels above 40 ng/mL, even at summer's end. Living in tropical climates simply does not prevent vitamin D deficiency. (This fact is important for all the sunlight and cancer studies because it suggests such studies significantly, perhaps dramatically, underestimate the effect of vitamin D on cancer.)

In a recent study of attitudes about sunlight, 62% of Hong Kong women did not like going in the sun. Sunscreens and parasols are commonly used, even among those women who know sunlight to be important for adequate vitamin D.

Surface UVB radiation in Hong Kong reaches its lowest level in January. This is interesting, because a 2002 article in the New England Journal of Medicine found that pediatric hospitalizations for influenza in Hong Kong peaked in January and February for two of the three years studied. A 2004 study found influenza deaths at all ages peaked in Hong Kong during January through March for three of the four years studied.

What is urgently needed is a study assessing vitamin D levels in patients with influenza. Blood from patients infected during the 1997 Avian influenza outbreak is stored in Hong Kong. Six Hong Kong residents died terrible deaths from Avian flu in 1997 while others caught it but had few symptoms. Did 25(OH)D levels decide who lived and who died? Will 25(OH)D levels decide who lives and who dies from influenza this winter?

Page last edited: 03 June 2011