Assuming they’re comprising a reasonable portion of the diet (<20%), adding vegetables to increase the bulk of the dog's diet isn't going to result in nutrient absorption issues. These issues tend to arise when "isolated fibers" (non-whole food fibers) are fed in high quantities – i.e.) fruit pectin, beet pulp, pea fiber, cellulose, peanut hulls, bran, etc. It also appears that rapidly fermentable fibers (such as guar gum and pectins) are the fibers that decrease the availability of certain nutrients (namely zinc, calcium, iron and phosphorus), while fibers that contain more cellulose have little effect on mineral absorption. Vegetables are vitamin and mineral rich whole foods, to add a few spoonfuls of pumpkin or a small amount of steamed low glycemic vegetables to a meal is not going to result in a mineral deficiency – if anything it would enhance the nutrient value of the meal in addition to allowing the dog to eat a greater volume of food without having a significant impact on caloric intake.
