To better absorb the vitamins from vegetables, it is essential to preserve them during cooking.
When cooking, wash vegetables before cutting them, and do not soak them in water after cutting, as many vitamins are water-soluble. Some people like to blanch vegetables in hot water before stir-frying, but this leads to a significant loss of vitamins.
Additionally, avoid overcooking vegetables and do not deep-fry them. Vitamin C, in particular, is very unstable and can be destroyed by prolonged cooking and high heat. Vinegar can help protect vitamins, so you may add a small amount when cooking.
For some vegetables with skins, such as radishes, the skin contains far more vitamins than the flesh. Be sure not to peel them to prevent unnecessary nutrient loss.