It's been six or seven months since I've had a cold.
But according to a review of cold remedies published in the Canadian Medical Association Journal last week, vitamin C (one of the supplements I take regularly) has little if anything to do with warding off the common bug.
With so much conflicting evidence out there, it's hard to know what fights colds and flus and what doesn't.
Here are some of some of my favourite supposed bug-busters and what scientists say about them:
What it is: Zinc is an essential mineral found naturally in foods such as oysters, red meat, crab and fortified cereals. It aids in healing wounds.
Cold- and flu-fighting power: A 2010 analysis of 15 randomly controlled trials published by the Cochrane Collaboration found that when healthy people take zinc within 24 hours of the onset of cold symptoms, the severity and duration of their colds are reduced. The same analysis found that zinc, when taken for five months, can prevent colds. At the same time, many trials for over the past three decades have shown less promising results.
Safety record: Most scientists familiar with the subject say zinc lozenges are safer than zinc pills. (A common dose in successful trials was a 23 milligrams of zinc gluconate every two hours). Zinc has been linked to nausea and, in nasal spray format, it has been linked to loss of sense of smell. Some researchers say using zinc over long periods can lead to immune suppression.
What it is: Also known as ascorbic acid, vitamin C is a water-soluble vitamin scientists say prevents cell oxidation while boosting tissue repair, wound-healing and gum health. Fruits and vegetables -- particularly citrus fruits -- are good sources of vitamin C. Because the human body cannot make or store this compound, people need a continuous supply of it.
Cold- and flu-fighting power: Late American chemist and Nobel Prize winner Linus Pauling was the world's most respected vitamin C proponent. His 1970 book, Vitamin C and the Common Cold, suggested that taking doses of 1,000 mg of vitamin C daily could boost the immune system and reduce incidences of the cold by 45 per cent. He later revised that book saying that even higher doses of vitamin C could prevent the flu.
Pauling and his wife took extremely large doses of vitamin C, saying that doing so reduced their cold and flu incidences. Most of the scientific world says there is no hard science -- in the form of multiple large, randomized, double-blind, placebo-controlled trials -- supporting this theory. Nevertheless, thousands of people around the world swear that their daily dose of vitamin C has lessened and even prevented their instances of cold and flu.
Safety: Health Canada says vitamin C deficiency in North America is rare. The federal agency recommends a maximum dose for adults of 2,000 mg. (That includes vitamin C from foods and supplements.) Side-effects of taking too much vitamin C include nausea, gastric symptoms and kidney stones. People with impaired kidney function should watch their vitamin C intake. Experts say that dividing your supplemental dose of vitamin C may allow your body to use it more efficiently. Chewing on vitamin C tablets can erode teeth and cause mouth irritation; combining with calcium can limit its acidity.
What it is: Known as the sunshine vitamin, vitamin D is a fat-soluble compound that we get mostly from sun exposure. It's also found in salmon, eggs and fortified milk. Scientists agree that vitamin D helps the body absorb calcium and phosphorus and therefore prevents diseases such as rickets. More recent studies show that vitamin D may play a role in protecting against cancer and autoimmune diseases such as Type 1 diabetes and multiple sclerosis (although research published in The Lancet Diabetes & Endocrinology in January says evidence is lacking to back up claims that vitamin D supplements have a causal effect on heart attack, heart disease, stroke, cancer or bone fractures). Experts say as little as 10 minutes of sun exposure to bare skin daily can prevent vitamin D deficiency. Those who live in northern climates as well as those with darker skin (people with darker skin don't absorb the sun as well as their paler counterparts) should make sure they get adequate amounts of vitamin D, say experts.
Cold and flu connection: A study published in a February 2009 issue of Archives of Internal Medicine found that people with the most severe vitamin D deficiency were 36 per cent more likely to suffer respiratory infections than those with adequate levels. Other studies suggest the same type of phenomenon.
The reason? Scientists say that vitamin D increases the production of an antimicrobial peptide called hCAP-18, a protein that helps the immune system kill viruses.
Safety: Health Canada says that the tolerable upper intake level of vitamin D for adults is 2,000 IU. Vitamin D advocates suggest taking a daily supplement of at least 800 IU. Vitamin D is fat-soluble and therefore toxic at high doses. Too much vitamin D is linked to increased risk for premature heart attack, atherosclerosis and possibly kidney stones in people who are predisposed to kidney problems. However, some vitamin D researchers challenge government health agencies about what constitutes too much vitamin D.
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