Results from a recent study at the national meeting of the American Chemical Society indicated that cooking vegetables for about 10-20 minutes increases the amount of iron that our body can absorb from them. Tung-Ching Lee, a food scientist at Rutgers University in New Jersey, tested 37 vegetables and found that 25 of them benefited from cooking of some sort, including boiling, steaming, or stir-frying.
The iron content in the vegetable is the same before and after cooking; heating simply causes the iron stored in the food to be released, making more of it available to the body for absorption. For instance, Lee found that the available iron in cabbage increased from 5% to 15% through cooking and the available iron in broccoli increased from 6% to more than 30%. However, some vegetables, such as lettuce, were found to be equally nutritious whether consumed raw or cooked.
They also found that storing cooked vegetables overnight, even in the refrigerator, causes a significant drop in the available iron content. To get the most iron, people should consume the vegetables soon after they have been cooked. Lee found that cooking tomatoes with other vegetables enhances the nutritional value of those other vegetables however, that vegetables that are blended before cooking have available iron levels almost as low as those in raw vegetables, while vegetables that are blended after cooking retain the higher iron levels.