As a general rule, water soluble nutrients, like vitamin C and the B vitamins, found mostly in fruits and vegetables, are best eaten raw.
The reason why is two-fold.
First, when these nutrients are heated, they tend to degrade; this is from any heat, be it steaming, boiling, roasting, or frying. Vitamin C and the B vitamins are a bit more “delicate” and susceptible to heat than many other nutrients.
Of course, the obvious way to combat these nutrient losses is to eat foods high vitamin C and B vitamins in their raw form (like in an awesome salad) or to cook them for as short a time as possible (like quickly steaming or blanching).
Fun fact: Raw spinach can contain three times the amount of vitamin C as cooked spinach.
The second reason why foods high in vitamin C and the B vitamins are best eaten raw is that they’re “water soluble.” So, guess where the vitamins go when they’re cooked in water? Yes, they’re dissolved right into the water; this is particularly true for fruits and veggies that are boiled and poached but even for foods that steamed as well.
Of course, if you’re a savvy health nut, you’ll probably keep that liquid to use in your next soup or sauce to preserve those nutrients that are left after cooking. Just don’t overheat it or you may lose what you were aiming to keep.
But, how much loss are we talking about? Well, of course, it ranges but can go from as low as 15%, up to over 50%.
In short, the water soluble vitamins like vitamin C and the B vitamins degrade with heat and some of what’s left over after they’re heated dissolves into the cooking water. So be sure to cook your fruits and veggies as little as possible, and keep that cooking water to use in your next recipe.