MayoClinic.com lauds roasting as one of the healthiest cooking methods you can employ, since it uses dry heat to soften vegetables and doesn't require any added butter or oil, which can contribute significant amounts of calories and fat to cooked foods. Although there is a risk of losing some beneficial enzymes and nutrients in veggies with any cooking method, the health benefits of eating any cooked vegetable outweigh the costs.
Specific nutritional information for roasted vegetables will vary based on whether you add other ingredients to the veggies before or after cooking, but dry roasting won't significantly change their nutrition facts. For example, a dry roasted green bell pepper has about 33 calories, 0.3 g of fat, 8 g of carbohydrates, 1.4 g of protein, 2.8 g of fiber and 3.9 g of natural sugar. Four ounces of a roast veggie mixture that includes zucchini and squash has only about 39 calories and 3.5 g of fat, 2 g of carbohydrates, 0.7 g of protein, 0.7 g of fiber and 1 g of sugar.
It can be useful to compare the nutrition facts of roasted and raw vegetables to get an idea of the slight differences between the two. The nutrition database from the U.S. Department of Agriculture states that a raw green bell pepper has 24 calories, 1 g of protein, 0.2 g of fat, 5.5 g of carbohydrates and 2 g of fiber. Two ounces of raw zucchini has 10 calories, 0.7 g of protein, 0.2 g of fat, 1.75 g of carbohydrates and 0.6 g of fiber, while 2 oz. of raw butternut squash has 26 calories, 0.6 g of protein, no fat, 6.5 g of carbohydrates and 1 g of fiber.
When cooking any vegetable, it is likely that some of the veggie's nutrients, vitamins, minerals and enzymes will be lost due to the heat's effects. However, different cooking methods have different impacts. Boiling, for example, causes a greater nutrient loss than roasting. "Cooking [vegetables] in water robs them of some of their nutritional value because the nutrients leach out into the cooking water," states the Harvard Medical School Family Health Guide. In truth, however, any cooking method that employs heat results in a nutrient loss. In a 2009 issue of the "Journal of Zhejiang University Science," researchers noted that boiling, stir-frying and microwaving "caused significant losses of chlorophyll and vitamin C and significant decreases of total soluble proteins" in broccoli.
In some cases, however, roasting veggies or cooking them in another fashion may actually improve their nutrition values. In a 1999 BBC article, for example, food scientists noted that cooking carrots increases the bioavailability of the carotenoids they contain. According to "The Asia Pacific Journal of Clinical Nutrition," you can minimize nutrient loss with roasting by avoiding overripe vegetables, roasting them with peels, keeping pieces large and minimizing added water.