Raw, boiled or steamed: what cooking does to vitamins

“Raw vegetables are healthier” — we hear it so often it sounds like an axiom. In fact, cooking both destroys and releases: some vitamins vanish into the pot, while others become easier to absorb. Let’s look at what actually happens to vegetables on the stove and how to cook them to lose the least.

The main culprit is water

Vitamin C and most B vitamins dissolve in water and don’t handle long heating well. When vegetables simmer in a big pot, some of the vitamins move into the cooking water, which then goes down the drain. Korean researchers compared several cooking methods for vegetables: anywhere from 0 to 91% of vitamin C remained afterwards — the most after microwaving, the least after boiling (Lee et al., 2018).

Folate (vitamin B9) behaves the same way. In a study by McKillop and colleagues (2002), typical boiling left only 49% of the folate in spinach and 44% in broccoli. Steaming, by contrast, caused no significant loss in either vegetable, even at the longest times — 4.5 minutes for spinach and 15 for broccoli. Losses also depend on the food itself: whole potatoes boiled for a full hour lost no significant folate, with or without their skins.

Broccoli is probably the best-studied vegetable here. When it was cooked five common home ways, all of them except steaming caused significant losses of vitamin C. The biggest losses came from stir-frying and from stir-frying followed by boiling — the two most popular methods in Chinese home cooking (Yuan et al., 2009).

Vitamin C in vegetables before cooking

These figures are for raw vegetables. If you log boiled broccoli in the diary, the calculator accounts for the vitamin C that was destroyed: we assume an average of 55% retained for boiling and 80% for steaming. These are approximate values for the nutrient group rather than measurements for each vegetable: real losses depend on cooking time, the amount of water and how finely the vegetables are cut.

When cooking helps

With carotenoids — beta-carotene in carrots, lycopene in tomatoes — the story is the opposite. They’re locked inside plant cells whose tough walls our gut breaks down poorly. Chopping and heating open those walls up: according to a review by van het Hof and colleagues (2000), processing increases carotenoid absorption from vegetables by anywhere from 18% to sixfold. Carotenoids also need a little fat to be absorbed: roughly 3–5 g per meal, less than a spoonful of oil.

Lycopene is the clearest example. People got the same amount of lycopene (23 mg) from fresh tomatoes or from tomato paste, each time with 15 g of oil. After the paste, peak blood lycopene was 2.5 times higher, and overall 3.8 times more was absorbed (Gärtner et al., 1997). So borscht with tomato paste or stewed tomatoes beat a fresh salad when it comes to lycopene.

Italian researchers compared boiling, steaming and frying for carrots, courgettes and broccoli. Cooking in or over water better preserved antioxidants, especially carotenoids, in all three vegetables, while frying lost the most. The authors say their findings defy the notion that processed vegetables are necessarily less nutritious, and that each vegetable has its own best cooking method (Miglio et al., 2008).

How to cook to lose less

• Cook vegetables rich in vitamin C and folate — broccoli, cabbage, spinach, peppers — by steaming or in the microwave with a spoonful of water, rather than boiling them in a big pot.
• If you do boil, keep it short and use little water. The vitamins that moved into the cooking water aren’t gone: use it for soup or sauce.
• Feel free to stew, roast or add carrots, tomatoes and pumpkin to borscht — and don’t forget a spoonful of oil or sour cream.
• The simplest rule is variety: eat some vegetables raw (salad, peppers, kiwi) and some cooked.

In the diary, note how each food was cooked — the calculator adjusts both the weight and the vitamins. Where the retention percentages come from is explained in How we calculate, and the richest sources are listed on the vitamin C and folate pages.