Also, it will destroy thiamine. It is desirable to retain the original flavor of the vegetable as much as possible. Flavors are diminished by dissolving into the cooking water and by volatilization. The loss of flavor can be minimized in various ways. The cooking period should be as short as possible, and it is best to add vegetables to water that is already boiling.
Salt also helps to retain flavor.
Strongly flavored vegetables contain sulfur compounds. Vegetables in the cabbage family are mild when raw and can become much stronger in flavor when cooked. The onion family is strong when raw and mild when cooked. These vegetables should be cooked quickly with the lid off to allow the strong flavors to volatize. There are ways to cook vegetables to retain the original color as much as possible.