Answer: It's amazing what food science achieves when using its powers for good, like turning nutritional zeroes into heroes. With a flash of harmless UV rays, low-nutrient mushrooms become a vitamin D powerhouse. We checked the USDA Nutrient Database and found that cooking slightly affects content: 2 ounces of UV-treated grilled portobellos supply about 319 IU of vitamin D, two-thirds of your daily needs. (The same amount of UV-treated fresh mushrooms contains 261 IU, about half of your needs for the day.) Such UV-treated mushrooms are available across the nation—check labels for vitamin D content.