30 Apr 2018

Fransplaining science: Will eating vitamin C cure my cold?

12:18 pm on 30 April 2018

Dr Frances-Rose Schumacher looks into whether loading up on oranges and vitamin C supplements will help you get over that nasty cold.


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Photo: Rawpixel.com

My good friend has had a bad cold for a few days now. She just can’t seem to shake it. 

I love this girl, and I want her to be well enough to do vodka shots and dance around the lounge with me in four days’ time, so I’ve been sending her daily reminders to stay hydrated and eat loads of feijoas.

The reasoning behind my advice, I thought, was that feijoas are high in vitamin C and “everyone knows” that a diet high in vitamin C is a good way to hasten your recovery or defend yourself from a cold. 

But like many things I used to say aloud with the utmost confidence, since starting this column I now find myself wondering “is this even true?”  

Vitamin C (also called ascorbic acid) is a compound we need to get from our diet that is necessary for human survival. It acts as an antioxidant to protect our cellular machinery from the harm caused by free-radicals, and as an essential cofactor (a "helper molecule") within our cells. 

Many of the enzymes within our cells that perform essential maintenance tasks - like absorbing iron, repairing damaged tissue, or forming new neurotransmitters - require vitamin C to be active. The cells of our immune-system, which act to help clear infections also need vitamin C to function. Our phagocytes, the cells that engulf bacteria to clear them from our system, accumulate vitamin C to be primed to perform their task. If we are deficient in vitamin C these cells can not perform their function. Without them functioning as they should, we are likely to get sick more often. 

A second type of immune cell, the T-cell, also requires vitamin C. T-cells are crucial for our adaptive immunity - the immunity we acquire as we are exposed to new cold strains and viruses. T-cells function to detect infection and up-scale our cellular response, effectively launching a natural immune response to help us neutralise infection. 

This known role of vitamin C, as an essential component in the functioning immune system, is where my confidence in telling my friend to eat (vitamin C rich) feijoas to speed up her recovery came from. But I hadn’t until now, looked at the research surrounding the hypothesis that a higher intake of vitamin C can prevent or cure a cold.

The recommended daily allowance of vitamin C varies from country to country. Interestingly, New Zealand has one of the lower recommended doses (45 mg of vitamin C per day), while in the USA the recommendation is double that. The allowances are somewhat historic and are based on a mean daily vitamin C intake of 30 mg per day required to prevent scurvy.  

There have been many calls to raise this limit, based on the increasing evidence surrounding the benefits of a diet richer in fruit and vegetables (and thus higher in vitamin C). The most comprehensive study, performed in 2007, compared a group of people with a base-line intake of 90 mg per day to a matched group of people who consistently consumed 200 mg of vitamin C  (to give you an idea of what we’re talking intake wise; two medium sized feijoas contain about 32 mg of vitamin C, while an ‘average orange’ contains about 45 mg, and a kiwifruit has 64 mg). 
The evidence is robust. Having a long-term diet consisting of heightened levels of vitamin C (200 mg or more daily) does not stop you from getting a cold in the first instance, but it does appear to reduce the length of time (by 8 to 14 percent) you experience the symptoms of a cold. So instead of a cold dragging on for seven to ten days, people who consumed higher levels of vitamin C had colds that typically lasted one or two days less. 

So can we up the dose of vitamin C and reduce the cold time by even more? Apparently not. Our bodies absorb vitamin C through transporters that line our intestine. The vitamin C levels in our blood are raised, enabling our cells to then absorb vitamin C as required. Once our system reaches saturation, we excrete any extra vitamin C in our urine. That old trope about vitamins doing nothing but leaving you with expensive wee is accurate in this instance. 

Possibly for this reason, no added benefit in terms of cold prevalence and duration was found when people consumed more than 200 mg of the vitamin.

Interestingly, in studies investigating vitamin C in the context of people who put their body through repeated extreme physical stress, (namely, marathon runners) a higher daily vitamin C intake of 200 mg (vs 90 mg) was shown to reduce the risk getting a cold by 50%.  So if you’re likely to be running a marathon any time soon, or you think you could become super stressed for whatever reason, it seems like a good ideas to keep your vitamin C levels high all year round to protect yourself from catching a cold.

But what about my advice - upping your Vitamin C intake when your cold symptoms come on? Can eating loads of feijoas act as a treatment to shorten the length of time you experience symptoms? The evidence suggests not. There have been seven studies looking into this, and in each case, there was no benefit observed when people increased their dietary intake of vitamin C at the onset of cold symptoms appearing. 
So what does this mean? Well ultimately it means my advice to my friend was not based in scientific fact.  

To reduce the length of time you are unwell with a cold, you can make sure you have around 200 mg of vitamin C per day, best achievable by eating a diet rich in fruit and vegetables. Going on a crash-diet of foods high in vitamin C, or taking a supplement when you get cold symptoms will not mean you recover any quicker. 

And while this means me telling my friend to eat loads of feijoas was not founded in scientific evidence, I still have a feeling it is the right thing to do. When your body is under stress it makes sense to me that you would aim to provide as much nourishment as possible by eating well and giving yourself all the help you can. If nothing else, this might get you into the habit of eating more vitamin C and if you maintain this your next cold will be shorter in duration.

Cover photo: Sean Mungu / Unsplash.