Different vegetables contain different nutrients. Some days ago I was adding some cabbage to a stew for the first time in months and felt my body rejoice. Maybe some nutrient I was lacking? Could be a case of that if you tend to only eat meat and potatoes.
Great and cheap vegetables to add are onions and leeks, and also all sorts of cabbage, and carrots.
Every body is different and it depends on metabolism and physical activity, find the amount you can add to your stew that makes you feel good!