Eating fresh is your best bet in many circumstances, yet researchers have found heating vegetables boosts the availability of plant chemicals, like lycopene found in tomatoes and beta carotene in carrots and vegetables such as broccoli.
Carrots should be boiled until tender, and broccoli is best washed and cut right before cooking, then steamed rather than boiled to retain vitamin C and phytochemicals.
Steam asparagus vertically to retain water-soluble nutrients, and keep tips up as they're fragile. Steam or stir fry Brussels sprouts to preserve their cancer-fighting compounds. Boiling will leach away goodness.
When it comes to tomatoes, roasting with a bit of olive oil is best, as fat helps the body absorb vitamin A and lycopene.
Onions are one of the best sources of quercetin, an anti-inflammatory plant chemical that helps control allergies and shows promise in the area of treating Alzheimer's and Parkinson's disease.
Bake thick chunks of onion in foil for about 5 minutes to preserve almost all compounds while diminishing odor.
Surprisingly, even chopping can affect foods both pro and con. Finely chopping vegetables results in a loss of vitamin C, yet crushing or mincing garlic and letting it sit out for about 10 to 15 minutes increases the plant-chemical availability that's associated with heart health and reducing cancer.