We're told repeatedly to eat more vegetables – and for good reason. Hundreds of studies have linked a high intake of vegetables to protection from heart attack, stroke, high blood pressure and certain cancers thanks to their generous supply of vitamins, minerals, fibre and phytochemicals.
And while raw food enthusiasts contend that cooking your vegetables destroys vitamins and minerals – as well as enzymes that aid digestion – it turns out that raw vegetables aren't always more nutritious.
Recent studies suggest that cooking can actually increase the antioxidant content of some vegetables, even microwave cooking.
The latest study, published last month in the Journal of Food Science, demonstrated that microwave cooking without water and only until tender maintained the highest antioxidant levels. Baking and grilling also preserved antioxidants while boiling and pressure cooking led to the greatest losses.
An often cited study from 2002 found that microwaving caused a significant loss of antioxidants. But this study measured antioxidants in broccoli that was submerged in water and cooked in a microwave for an extended period of time.
In fact, cooking your vegetables can actually boost their antioxidant content.
The new study found that all cooking methods increased antioxidants in carrots, celery and green beans. It's thought heating vegetables releases antioxidants by breaking down cell walls.
Studies have found that eating cooked spinach and carrots – versus raw – results in much higher blood levels of beta-carotene, an antioxidant that when consumed from foods (not supplements) is thought to guard against heart disease and lung cancer.
Cooking tomatoes also increases the amount of lycopene that's available for absorption in the body. A higher intake of lycopene from cooked tomato products is associated with a lower risk of prostate cancer.
You'll also get more lutein, a phytochemical that helps prevent cataract and macular degeneration, if you eat your leafy greens cooked instead of raw.
When it comes to certain minerals, you're better off eating your vegetables cooked. Green vegetables such as spinach, beet greens and Swiss chard are rich in calcium. But they also contain elevated levels of a compound called oxalic acid that binds calcium and reduce its absorption.
Cooking releases some of the calcium that's bound to oxalic acid. Three cups of raw spinach, for example, has 90 milligrams of calcium whereas one cup of cooked has nearly triple the amount – 259 milligrams.
Cooking vegetables also increases the amount of magnesium and iron that's available to the body.
When it comes to cooking, water is your enemy. Boiling vegetables causes a significant amount of nutrients that dissolve in water – vitamin C, folate and thiamin (vitamin B1) – to be leached away. The longer you cook your veggies, and the more water you use, the more vitamins you'll lose.
Steaming is much gentler on nutrients because vegetables don't come in contact with cooking water. Dry cooking methods like grilling, roasting, stir-frying and microwave cooking without water also preserve a greater amount of nutrients than boiling or pressure cooking.
Some vegetables, however, are a better source of other phytochemicals when eaten raw. Cruciferous vegetables such as cabbage, broccoli, cauliflower, bok choy kale and turnip are best known for their high concentration of glucosinolates, compounds that are converted to anti-cancer chemicals called isothiocyanates. (One of the most famous isothiocyanates is sulforaphane, a phytochemical that's plentiful in broccoli, broccoli sprouts and kale.)
When you chop or chew cruciferous veggies, glucosinolates are converted to isothiocyanates by an enzyme called myrosinase. However, this enzyme that's easily destroyed by heat. So, cooking cruciferous vegetables reduces the formation of isothiocyanates, which may reduce their cancer-fighting potential.
