The reason for this recommendation was that any vitamins and minerals lost from cooking the vegetables would end up in the water. This cooking water could then be added to homemade soups or sauces with the nutrition benefiting those that consumed them.
This notion took hold and has not let go to this day. Unfortunately, reserving vegetable cooking water can do more harm than good. If the vegetables that are cooked are not organic, pesticides and nitrites from commercially produced fertilizers can end up in the cooking water. Even low to no spray veggies such as asparagus would not be safe bets as these crops are still typically fertilized with commercial preparations that are high in nitrites.
Cooking cruciferous vegetables such as broccoli, cauliflower, cabbage, Brussels sprouts and kale would add goitrogenic (thyroid blocking) substances to the cooking water which should be discarded. Water used to boil potatoes would contain chemicals called hemagglutinins that disrupt red blood cell formation.
Dark green, leafy vegetables such as beet greens, spinach and chard contain oxalic acid that blocks calcium and iron absorption. This irritating substance also can cause distress to the sensitive mucous membranes in the mouth and intestinal tract and contributes to the formation of kidney stones.
Consuming cruciferous or dark, leafy greens raw is not an alternative either as the substances that cause problems when in the cooking water also cause problems if consumed directly with the vegetable in an uncooked state.
As a result, only the cooking water from organic root vegetables (carrots, turnips, parsnips, and beets), organic squash (including zucchini), and organic vegetables from the lily (onions, leeks, and garlic) and nightshade (tomatoes, eggplant and peppers) families can safely be used.
If you find it confusing as I do to remember the distinctions of which organic veggies are safe to use the cooking water for and which are not or if you mix you veggies together when you steam them, it is best to just adopt the practice of not using the cooking water at all.
It is of particular importance not to use the vegetable cooking water for use in pureeing homemade baby food.
One final word of caution: All vegetables tend to form nitrates after cooking and during storage. These nitrates can transform into strong carcinogens in the intestines, so it is best to avoid refrigerating and reheating vegetables, particularly leafy, green vegetable which concentrate nitrates when grown commercially.
Source: Nourishing Traditions Cookbook