I have heard that cutting or grating vegetables with metal tools somehow reduces the vitamin content (or as it is sometimes stated "it kills the vitamins"). There are also commercials, which advocate glass or ceramic tools for preserving vitamins.
I have seen similar claims on the Internet (e.g., provided by @ChrisW: How to keep the vitamins in the food – STEEL KNIFES DESTROY VITAMINS), but I couldn't find any scientific studies.
Are vitamins in food significantly affected by contact with metal utensils?