Nutrition
Many New Zealanders have low levels of vitamin C and overcooking vegetables is one of the reasons.
Many New Zealanders have low levels of vitamin C and overcooking vegetables is one of the reasons.
Question:
If vitamin C is destroyed by oxygen, heat and water, does cooking reduce the levels of this essential nutrient in our food?
Answer:
One in 10 New Zealanders over the age of 50 have low vitamin C levels, according to a 2017 study conducted by University of Otago researchers. And those levels are so low they’re likely to result in decreased mood and energy levels. Worse still, 2.4% of those over 50 have such low vitamin C status that it puts them at risk of developing scurvy – a condition that results in joint pain, lethargy, bleeding and ulceration. Notably, in the 18th century, many sailors affected by scurvy suffered bleeding, swollen gums.
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 2009, Chinese researchers studied the effects of steaming, microwaving, boiling and stirfrying on the nutrient content of broccoli – a rich source of vitamin C. They found all cooking treatments except steaming caused significant vitamin C losses. Steaming also retained more glucosinolates – the sulphur-containing compounds found in broccoli that are credited with protecting us from cancer and cardiovascular disease.
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
The shorter the cooking time, the greater the retention of vitamin C, whether you steam or microwave.
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 fiveplus-a-day Ministry of Health guidelines are largely ineffective.
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