Green peppers are very high in vitamin C- 1 medium pepper has almost double the amount you need in a day. They also contain small amounts of vitamin A, fiber, iron and calcium. Onions are high in vitamin C, a good source of fiber, and contain small amounts of iron and calcium. Mushrooms are high in riboflavin (one of the B vitamins), and a good source of niacin, pantothenic acid (other B vitamins) and copper. They also have small amounts of iron and fiber.
All three are low in calories, contain little or no sodium, and no fat. Like all fruits and vegetables, they are cholesterol free and contain phytochemicals, plant compounds that are good for health.