1. Buying fresh tomatoes instead of canned.
Cooking tomatoes makes them more nutritious, and the longer you cook them, the better. Heat changes the lycopene into a form our bodies can more readily absorb and — surprise! — canned tomatoes are much higher in phytonutrients, thanks to the heat of the canning process. Tomato paste, being more concentrated, is even better.
2. Storing lettuce wrong.
You might think that damaging your vegetables before storing them is a mistake, but when it comes to lettuce, tearing the leaves triggers a protective blast of phytonutrients that you can take advantage of by eating the greens within a day or two. Lettuce that is torn before storing can have double the antioxidants of whole lettuce leaves.
3. Boiling spinach — or any vegetable really.
You may have heard that boiling vegetables is a no-no because water-soluble vitamins like vitamin C leach out of the food and into the cooking water, but you might not know that boiling also reduces the antioxidant content. The difference in spinach is especially dramatic: after 10 minutes of boiling, three-quarters of its phytonutrient content is in cooking water, not in the vegetable itself. (Of course, if you consume the cooking liquid, as you do when making soup, you consume all the nutrients in the water as well.)