Wintry Mix

The science behind the season's multiple maladies.

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If only we were so well-adapted to winter.
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As a native Californian deprived of real winters, I most definitely romanticize the season. I expect to sing “Silver Bells” while dancing down the street of town, past shops decorated with Christmas lights and snow. Truly.

Alternately, I imagine ice skating on our local pond and wandering the nearby woods through quiet, soft snow. In my head, its like the rural winter scene captured by biologist Bernd Heinrich in Winter World. Bernd tells of wandering in the snowy woods of Maine, and finding hints of life and beauty everywhere: the call of the great horned owl and the coo of doves, the tracks from moose and big cats and wolves, a scampering chipmunk and a hidden den of porcupines, and snow-frosted trees. Of course, since I live in New Jersey, I don’t really expect the moose.

Winter World documents the amazing evolutionary innovations that animals have developed to survive winter, such as developing snowy white, camouflaging fur; hoarding food and slowing metabolisms; bedding down in tree trunks and well-designed snow dens; and huddling together and settling in for lengthy hibernations. Humans, unlike other animals, do not adapt so well to the winter conditions. We rely on fireplaces and indoor heat, hot baths and hot coffee, heavy coats and fuzzy hats, and slippers and blankets to survive the season in comfort.

Nevertheless, these crutches are also often less cozy and protective than expected (just as my anticipation of majestic snow often runs into the reality of messy sleet). The wonderland of winter quickly crumbles when disease, depression, lethargy, and overall lumpiness arrive on the scene.

Increasingly, science is proving that the seasonality of such discomforts and dangers: People really are sicker, sadder, and perhaps even a little slower — mentally — during the colder, darker seasons than they are in the warmer, brighter ones. Research has started explaining why.

Sicker: Researchers have confirmed that low temperatures contribute significantly to influenza’s rapid spread. Now they have identified a reason why: While a protective gel surrounding the influenza virus melts and makes the virus vulnerable to the elements (and thus weaker) at warm temperatures, at cold temperatures, this outer covering hardens to a “rubbery gel” that shields it and enhances its passage from person to person. In cold conditions, the protective covering is like the shell on an M&M candy, only melting — and becoming capable of infecting a cell — once it has already entered the respiratory tract, according to senior scientist Joshua Zimmerberg, Ph.D., chief of the laboratory of cellular and molecular biophysics at the National Institute of Child Health and Human Development.

Of course, the flu is not winter’s only sickness-related misery. Shorter days and colder temperatures also mean less sun and less activity. A growing body of literature implicates decreasing vitamin D levels as a player in high blood pressure and heart disease. Some studies have found that rates of severe disease or death increase by 30% to 50% among sun-deprived individuals with heart disease. Scientists have also linked reduced physical activity and cold temperatures that narrow blood vessels to a wintertime seasonal surge in deep vein thrombosis and pulmonary embolism, according to scientists. Pulmonary embolism causes 200,000 deaths in the United States annually.

Finally, and not at all surprisingly, science continues to show that winter conditions compel people to eat less healthfully. No big surprise, since winter is certainly the time for weight gain. Most recently, researchers conducting animal studies found that shorter days unwittingly stimulated higher sugar and carbohydrate intake.

Sadder: Approximately 10 percent to 20 percent of Americans experience some form of winter blues, or seasonal affective disorder (so aptly abbreviated SAD). Although scientists have learned that it generally occurs when winter’s late dawn and early dusk throw off the body’s natural rhythms, just what happens physiologically to make this occur has been unclear.

Recently scientists from Canada — where the more northern latitude increases the likelihood of SAD — found a possible culprit in serotonin transporters, proteins in the brain that deactivate serotonin. They made the novel discovery that humans have higher levels of serotonin transporters in the brain in winter than in summer, thus more serotonin is likely to be deactivated during winter. Since serotonin plays a critical role in mood regulation, sleep, appetite, and pain inhibition, the findings might explain why some people experience low mood and energy in the winter, and why depression reoccurs every winter in some vulnerable individuals.

Stupider: Some of may also even, legitimately, blame winter for mental mistakes as well. Yes — cold, dark days may make us stupider, I mean slower, at least about some things. A series of studies on the effects of cold from scientists at the University of Oulu in Finland have shown that people exposed to cold temperatures and dim light tend to function more slowly on simple cognitive tests and remember basic tasks less accurately. On the upside, though, they show improved accuracy on tests measuring complex cognitive tasks.

With all this being considered now, I am beginning to recognize why many of my neighbors complain so much about the wintertime. Still, more than 15 years of cold, dark, and mostly snow-less Northeast winters haven’t dampened my spirit yet. Just yesterday, I walked out of Starbuck’s humming Christmas carols, even though there wasn’t a speck of snow to be found. And I’m looking out right now at the pond across that street that I just know I’ll ice skate across again, at least one more time before I die. • 15 December 2008

REFERENCES: Winter World: The ingenuity of animal survival, by Bernd Heinrich (HarperCollins Publishers: New York. 2003); Polozov IV, Bezrukov L, Gawrisch K, & Zimmerberg J. Progressive ordering with decreasing temperature of the phospholipids of influenza virus. Nat Chem Biol. 2008;4:248-55; Sinitskaya N, Schuster-Klein C, Guardiola-Lemaitre B, et al. Short day-length increases sucrose consumption and adiposity in rats fed a high-fat diet. Psychoneuroendocrinology. 2008;33:1269-78; Praschak-Rieder N, Willeit M, Wilson AA, Houle S, Meyer JH. Seasonal variation in human brain serotonin transporter binding. Arch Gen Psychiatry. 2008;65:1072-8; Palinkas LA. Mental and cognitive performance in the cold. Int J Circumpolar Health. 2001;60:430-9.

 

Jennifer Fisher Wilson is the science reporter for Annals of Internal Medicine. Her stories are available at www.annals.org.
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