Heat damage and leaching vary sharply by nutrient
Vitamin C is highly unstable in heat, dissolving into cooking water and oxidizing in air. Boiling vegetables can destroy 50 percent of their vitamin C in 10 minutes, while steaming preserves more because vegetables don't sit in hot liquid. Microwave cooking sits in the middle, retaining more C than boiling but less than steaming.
B vitamins, except niacin, are water-soluble and leach into cooking water during boiling. Cabbage loses half its folate when boiled but loses much less when steamed. Minerals like iron, calcium, and potassium don't degrade from heat but do leach, so boiling potato water discards significant minerals while baking potatoes retains them.
Cooking methods ranked by nutrient preservation
Steaming and microwaving preserve the most nutrients overall because vegetables stay intact and don't sit in water. Stir-frying ranks next, combining brief heat with minimal liquid and quick absorption of any liquid by the vegetables. Boiling ranks worst for water-soluble vitamins because prolonged contact with hot water leaches them away.
Roasting and baking occupy a middle ground: minerals and some heat-stable nutrients remain, but vitamin C and some B vitamins degrade from prolonged dry heat. Pressure cooking, counterintuitively, preserves more nutrients than boiling despite the heat because cooking times are much shorter.