Vegetables on their own are healthy foods, but how you cook vegetables can affect their nutritional value. Most vegetables are healthiest when eaten raw, and boiling them can leech out the vitamins and minerals that have you eating veggies in the first place.
While raw vegetables do have the highest concentration of vitamins and minerals, there are a couple exceptions. Garlic and onions are actually healthier when cooked because heat helps activate their inherent sulfur compounds that provide health benefits.
Research shows that vegetables lose a small amount of their nutritional value for every 30 seconds of steam exposure. However, this loss is so minimal, that steaming veggies for a minute or two isn’t going to render them useless, but overcooking will drastically reduce the nutrition quality of vegetables. In addition, steaming provides you with a crisper, tastier finished product that also retains more flavor.
Boiling is a common means of doing damage to healthy vegetables. Any veggie that you would consider boiling can be steamed instead. Boiling results in greater nutrient loss than surrounding a vegetable with hot steam. There is no good reason to boil, so don’t do it! Even large vegetables, like butternut squash, can be fully cooked by steam in 10 minutes.
Stir-frying is another means of locking in vegetable nutrition. Short exposure to high heat always works to retain as much nutrition as possible, and the small amount of oil used when stir-frying helps seal in vitamins and minerals.
Also keep in mind that vegetables naturally lose nutritional content as they age or are exposed to light and air. Fresher is better. To get the maximum health benefits from your veggies, eat them soon after picking.