Given the new Sars-CoV-2 virus comes from the same family as those that cause the common cold and flu, there has been speculation that high-dose vitamin C supplements may prevent or treat Covid-19.
However, taking vitamin C has not been shown to reduce the incidence of standard coronaviruses in the general population, according to a 2013 Cochrane review. Though some evidence suggests regular supplementation may reduce the duration of colds, clinical trials have found no evidence that taking vitamin C at the onset of a cold reduced the duration.
Nevertheless, the Cochrane review concluded it may be worthwhile for patients to be individually tested to see whether taking vitamin C supplements at the onset of a cold is beneficial.
A blinded randomised clinical trial is under way at Wuhan University’s Zhongnan Hospital to determine whether a high-dose vitamin C infusion may help the treatment of patients with severe cases of Covid-19 and pneumonia. The results are yet to be determined or published.
Why many New Zealanders have low levels of vitamin C
In the first randomised trial documented in history, Dr James Lind discovered in 1747 that adding two oranges and one lemon to the daily diet of sailors with scurvy resolved their symptoms within a week, as compared with other trialled solutions such as a quart of cider or a half-pint of sea water a day. Pity the poor men who were “treated” with sea water …
Cats and dogs can synthesise their own vitamin C, but humans need a regular dietary source of this essential vitamin and antioxidant, which plays a critical role in enzyme systems in the body. Added to that, vitamin C greatly enhances our ability to absorb iron and copper from food.
Along with oranges and lemons, many other fruits and vegetables are rich in vitamin C such as kiwifruit, blackcurrants, guava, citrus, broccoli and sprouts.
However, vitamin C is indeed easily damaged by light, heat and air. So, food handling, preparation and cooking processes including storage time, cutting, bruising, heating, exposure to iron and copper and leaching from foods into cooking water can reduce the vitamin C content of food.
In contrast, another study, published in 2018, found that more vitamins were retained when a variety of vegetables were microwaved rather than being blanched, steamed or boiled. Boiling was found to cause the greatest losses.
So why the difference? The researchers in each trial used different amounts of water and shorter or longer cooking times – the earlier study steamed the broccoli for five minutes (half the time of the 2018 study) and microwaved it for five minutes (more than double the 2018 study).
The general theme here, as you’ve probably noted, is the shorter the cooking time, the greater the retention of vitamin C regardless of whether you choose steaming or microwaving. Other factors will reduce vitamin C losses, too, such as using less water in cooking, leaving the vegetables as intact as possible and the type of vegetable.
Generally speaking, the fibre and fat-soluble vitamins in vegetables, such as vitamin A and E, aren’t greatly affected by either boiling or steaming. The bioavailability of some nutrients, such as the lycopene found in tomatoes and the beta-carotene in carrots, may actually improve with cooking.
Of course, none of this matters if you’re not eating five-plus servings of vegetables and fruit each day.
Indeed, only 7% of New Zealand adults over the age of 50 actually had fully replete levels of vitamin C, which suggests, according to the Otago University research team, the five-plus-a-day Ministry of Health guidelines are largely ineffective.
This article was first published in the March 28, 2020 issue of the New Zealand Listener.