Why should water added to food not through away?

Cooking water contains nutrients and flavor. Especially if you're cooking vegetables, a lot of the nutrients leach out into the water, making this "vegetable broth" a good base for soups, stews, and sauces.

It's a waste of water.

By dumping the cooking water used in pasta, you're losing the 2 cups of drinking water you could have boiled in your pot or electric kettle. The water on your plate won't hydrate you anyway; its saltiness will actually further dehydrate you.

Salted pasta cooking water can be used to season and thin tomato sauce. Adding it, a ladleful at a time, to your tomato sauce will thicken it and bring the flavors together.

It's the perfect replacement for water in homemade bread dough. This way you can sneak veggies into your bread by using veggie water (especially potato water).

Some cooking water can even be used as plant fertilizer. When using starchy foods like rice or potatoes, the excess starch in the water can provide valuable nutrients for plants without harming them (assuming you don't use too much salt or other seasonings in your cooking).

You should throw out pasta cooking water only when you've done one of these two things:

- Overcooked the pasta to the point of disintegration

- Added something you regret to your pasta boiling water, such as a few glugs of oil