Calories and Fat Like most vegetables, mushrooms have a low energy density, meaning they contain few calories in a large portion. A half-cup serving of cooked mushrooms contains 20 calories, and a half-cup of raw mushrooms contains only 10 calories. Substituting mushrooms for higher energy-density foods, such as cheese, can help you reduce your caloric intake. Mushrooms also contain no fat, cholesterol or sodium.Overview Raw and cooked mushrooms are naturally low in calories and fat, and provide good sources of fiber and potassium. Add mushrooms to pasta dishes, omelets, salads and other meals for a healthy burst of flavor that won’t contribute to caloric excess.Carbohydrates Mushrooms have a lower total carbohydrate content than many other vegetables. A 1/2-cup serving of cooked mushrooms contains 4 g of total carbohydrates, while the same serving of raw mushrooms contains only 1 g of carbohydrates. A half-cup of raw mushrooms does not supply a significant amount of dietary fiber, but a serving of cooked mushrooms provides 2 g of dietary fiber. Fiber, a non-digestible carbohydrate, provides a feeling of fullness and promotes healthy digestion.Vitamins Mushrooms do not provide a significant source of vitamin A, and only a small amount of vitamin C. A 1/2-cup serving of raw mushrooms provides 2 percent of the recommended daily vitamin C intake, while a similar serving of cooked mushrooms provides 6 percent of the recommended daily vitamin C intake.Other Nutrients A half-cup serving of raw mushrooms provides 1 g of protein, and a half-cup of cooked mushrooms provides 2 g of protein. Mushrooms do not provide a significant amount of calcium, and provide only a small amount of iron. A 1/2-cup serving of raw mushrooms provides 2 percent of the recommended daily iron intake, while the same-sized serving of cooked mushrooms provides 8 percent of the recommended daily iron intake. Mushrooms are a good source of potassium, with 300 mg in a serving of five medium raw mushrooms.