It has been a common presumption that eating raw fruit and vegetables is better for you. But now experts are saying that this is not always the case, and some vegetables actually benefit from cooking.
Nutritionists say that by heating some vegetables actually allows them to release more vitamins and nutrients than by leaving them in a raw state. However, they do warn not to overcook them as these essential nutrients are then lost.
Heating vegetables also helps to break down tough fibres and release minerals. Experts state that certain foods, including tomatoes, spinach, carrots, asparagus and mushrooms are better for you when cooked.
Mel Wakeman, nutrition expert and Senior Lecturer in Applied Physiology at Birmingham City University said: “Many of the nutrients found in plants are often less readily absorbed in the gut compared to nutrients derived from animal products.
“The fibre found in plants often binds particularly to minerals and makes them less available for the body to use (their bioavailability).
“Heating can help to breakdown the fibre and so release some of the minerals for absorption, and can often increase the phytochemical content of plants which can provide additional non-nutrient benefits to our health.”
With tomatoes for instance, heating them actually increases the levels of lycopene in the fruit. Lycopene has been shown to reduce the risk of prostate cancer.
Wakeman said: “Tomatoes are rich in many nutrients but one is of particular interest to men’s health – lycopene has been shown to reduce the risk of prostate cancer.
“Although inconclusive at the moment there are also suggestions lycopene may also improve heart health. Heating tomatoes increases the levels of lycopene in tomatoes. I would advise against only eating cooked tomatoes as heating does destroy other important vitamins (such as Vit C) so simply have a mixture of cooked and raw tomato products.’
With vegetables such as spinach, the high fibre and oxalate content can make it difficult to absorb the iron, and eating raw spinach allows us to only ingest around 5% of this iron. By cooking spinach briefly it reduces the oxalate level by about 15%.
Carrots are another vegetable that benefit from cooking. Rich in carotenoids, heating carrots actually increases the carotenoid content. Carotenoids are powerful anti-oxidants which can help to prevent heart disease and cancer.
The problem with cooking vegetables in boiling water is that most of the nutrients are leeched into the boiling water and then poured away.
Ms Wakeman advised to either steam vegetables or even microwave them: “Microwaving and steaming often help to retain the nutrient content of veg, rather than boiling. Eat a mixture of raw and cooked carrots too.”
Studies have shown that heating asparagus may help to increase the polyphenol content.
“Polyphenols – also found in tea, red wine and chocolate – have strong antioxidant properties and provide beneficial effects in terms of risks of heart disease and cancers for example,” said Ms Wakeman.
Mushrooms also benefit from heating but as they can soak up fats and oils, Ms Wakeman warned about using too much fat when cooking.
Professor Rui Hai Liu, who works in the department of food science at Cornell University, and studies how heat affects food, agreed that people have a perception that cooking foods reduces the nutrients.
Professor Liu told the Washington Post: “Common wisdom says cooked food has lower nutritional value compared to fresh produce, but that’s not always true.”
The professor explained that many nutrients in vegetables are trapped within cell walls: “Cooking helps release them, so they’re more bioavailable and absorbed by the body,” he said.