Heat adversely affects vitamin C. Vitamin C, like many other nutrients, breaks down and becomes more unstable in the presence of high heat. Combine that with heating the vitamin C containing food in water, and it is possible that all vitamin C content could be lost before the food even enters your mouth. The reason for that is because vitamin C is a water-soluble vitamin. The heat causes the vitamin to break down, and the water causes the vitamin content to leach out into the water. The vitamin is still somewhat present in the water; however, more than likely, a person isn't consuming the water that is cooking their vegetables. This means that the best way to cook your vitamin C-containing foods would be to do it quickly with minimal exposure to water. For example, steaming them works well.