In one pass Heat has no universal rule for nutrients: in the same pot it takes some nutrients apart and releases others so the body absorbs them better.
Educational content, not medical advice — consult a clinician.
Heat has no universal rule for nutrients: in the same pot it takes some nutrients apart and releases others so the body absorbs them better.
The line "raw is healthier" squeezes this into a slogan. The answer depends on which nutrient, which food, and how you cook it. The next four chapters split heat into four actions: releasing nutrients locked inside cells, destroying heat-sensitive vitamins, carrying off some antinutrients, and knocking out certain useful enzymes.
One group of foods has no raw option at all: raw or undercooked common beans such as green beans and red kidney beans contain a plant lectin that causes nausea, vomiting, and diarrhea. They must be cooked through.
Myth · Where raw is healthier goes wrong
"Raw is healthier" sounds reasonable because it treats heat as a single job: destruction. More than that one thing happens.
When heat opens cell walls, the fat-soluble pigments locked inside can finally get out. Water-soluble vitamins are taken apart by heat and also leach into the pot of water you pour away. Soluble oxalate leaves with the blanching water. Certain enzymes are knocked out by heat, so some useful compounds never get made. All four can happen in the same vegetable at once; which one you see depends on which molecule you are watching.
So this story does not declare raw or cooked the winner. It takes apart the four things heat actually does, so you can choose a method for each food.
Mechanism · Which foods each of the four affects
Release: lycopene and other carotenoids need their cell walls broken, plus a little fat, before they get in (see Tomato / carrot).
Destroy: vitamin C and folate are sensitive to heat and also leach into water you pour away (see Vitamin C / folate).
Remove antinutrients: blanch and pour off the water, and soluble oxalate goes with it (see Spinach); the plant lectin in common beans is a protein, and cooking them through inactivates it.
Knock out enzymes: broccoli's myrosinase can no longer make sulforaphane, and garlic's alliinase can no longer make allicin; the fix is to cut first, wait a little, then heat (see Broccoli / garlic).
Keep these four in mind and the raw-or-cooked slogan stops pushing you around.
Chapter 2
Heat releases · lycopene & carotenoids
Some nutrients are locked inside plant cell walls. Heat opens those walls, and with a little fat alongside, they are absorbed better, not worse.
Lycopene and beta-carotene, both carotenoids, work this way: eaten raw, much of them passes straight through the gut; cooked and eaten with a little oil, more gets absorbed. This is the most direct counterexample to "raw is healthier" (see Tomato / carrot).
A CLOSER LOOK
Heat opens a plant's nutrient package
Carotenoids are a counterexample: cooking with a little fat can improve absorption.
1Nutrients are enclosed
Carotenoids such as lycopene and beta-carotene can be enclosed by plant cell walls; some pass through the gut unchanged when eaten raw.
2Release and uptake are separate steps
Heat opens the walls, and a little fat helps more carotenoid absorption. This concerns these nutrients, not a rule that all nutrients favor heat.
Illustration for understanding; not to scale. Saved figures include explanations and sources.
Evidence · Why paste beats fresh tomato for lycopene
Lycopene is locked inside the structure of tomato cells, so when you eat tomatoes raw, much of it passes straight through. Heat breaks the cell walls and releases the pigment.
Gärtner, Stahl, and Sies compared the two directly in 1997: both groups got 23 mg of lycopene, and the tomato-paste group ended up with a higher lycopene curve in the blood. The spatial forms of the molecule (its isomers) were the same in both foods, so what the trial supports is processing opening the cell walls, not heat flipping the molecule from trans to cis.
That is why tomatoes made into a sauce with a little fat are absorbed better than raw ones. For each step lycopene takes from a bite of tomato to your blood, see Tomato.
Evidence · Broken walls still need a little fat
Carotenoids (such as the beta-carotene in carrots, bell peppers, and dark leafy greens) are fat-soluble, and they are often trapped behind tough plant cell walls. Two conditions clearly raise absorption: a little cooking and chopping to break the walls, then a bit of fat so they can dissolve in it first.
In a small human trial (Brown 2004), the same salad eaten with fat-free dressing delivered almost no carotenoids; reduced-fat dressing let a bit more in, and full-fat dressing more still. Without fat, little gets in even after the walls are broken.
So for this class of pigment nutrients, mild heat plus a little fat suits better than eating them raw. For how to cook carrots, see Carrot.
Chapter 3
Heat destroys · vitamin C & folate
Vitamin C and folate are vulnerable to two things: high heat taking the molecule apart, and dissolving into water that then gets poured away.
This is the half of "raw is healthier" that holds, and it does not contradict the lycopene chapter: in the same vegetable, lycopene likes heat while vitamin C does not (see Vitamin C / folate). Long boiling in a big pot of water, then pouring the water away, stacks both losses; a quick stir-fry, steaming, or eating the broth keeps more.
Mechanism · Heat breaks it, water carries it off
Vitamin C and folate are both water-soluble and sensitive to heat. The US National Institutes of Health Office of Dietary Supplements (NIH ODS) lists vitamin C as unstable under heat: high temperatures destroy the molecule directly, and the longer and hotter you cook, the less remains.
The second path is leaching. Because they dissolve in water, long boiling soaks them out into the broth; pour the broth away and the nutrients go with it. Heat and water stacked together are why long boiling in a lot of water is a double loss.
So the loss is not captured by the word heat alone. Two things matter: whether the molecule is still intact, and whether it is still in the vegetable or in the pot of water you poured out.
In practice · How to keep vitamin C and folate
For vegetables rich in vitamin C (bell peppers, broccoli, and others), the better approach is shorter and gentler heat: a quick stir-fry or steaming beats long boiling; use little water, or eat the cooking water and broth too; eating these vegetables raw or lightly cooked keeps more vitamin C. Broccoli also has a heat-sensitive enzyme of its own; see Broccoli.
For the full story on vitamin C, see Vitamin C; for folate, see Folate.
Oxalate binds calcium and iron in the gut, and in some people it adds to the load behind calcium-oxalate kidney stones. Most of it dissolves in water: blanch the vegetable, pour off the water, and soluble oxalate leaves with it. So for high-oxalate vegetables such as spinach, a quick blanch often suits better than a raw salad (see Spinach).
Common beans are the more extreme case: raw or undercooked green beans and red kidney beans contain a plant lectin that causes nausea, vomiting, and diarrhea. It is a protein, and cooking the beans through inactivates it. Eating these beans raw carries a risk and brings no nutritional gain.
Mechanism · Soluble oxalate leaves with the water
Antinutrients are compounds that occur naturally in plants and get in the way of absorbing nutrients or cause other trouble; oxalate is the classic example. It is fairly high in spinach, beet greens, and other dark leafy greens. It binds calcium and iron and lowers how much of them you absorb, and high oxalate is a risk factor for kidney stones (calcium-oxalate stones) in some people. If you have a history of kidney stones, ask your doctor how to handle high-oxalate vegetables; this is not a substitute for medical advice.
Most oxalate dissolves in water. Blanching means boiling a vegetable briefly, lifting it out, and pouring off the water, which carries soluble oxalate away. Chai and Liebman measured high-oxalate vegetables in 2005: boiling clearly reduced soluble oxalate. How much depends on the vegetable and the method, so this records the direction rather than one blanket percentage.
In practice · What blanching costs and what it buys
Blanching also carries off some water-soluble vitamins. That cost is real, not a detail to skip. What you lose are water-sensitive nutrients such as vitamin C and folate; what you gain is a lower oxalate load, and by the mechanism, less calcium and iron tied up by oxalate.
So for vegetables such as spinach, a raw salad is not necessarily the better choice; blanched and then dressed or stir-fried often strikes a better balance. For people at risk of calcium-oxalate stones, this deserves priority over saving a little vitamin C. For more on spinach, see Spinach.
For common beans there is no trade-off to weigh; the point is to cook them through. Methods with water and enough time, such as braising, stewing, and steaming, are the most reliable, while a fast high-heat stir-fry or a quick blanch followed by a quick stir-fry is most likely to leave the inside of the pod undercooked. To judge doneness, look at the pod: a deeper color, a softer texture, and no raw, grassy taste. The Green Beans story covers this in detail.
Chapter 5
Heat kills enzymes in broccoli, garlic
Broccoli's sulforaphane and garlic's allicin are not sitting there ready-made. Only when you cut or chew do the enzymes stored in a separate compartment of the cell come out and convert a precursor into them. Both enzymes, myrosinase and alliinase, are sensitive to heat.
Put the vegetable straight into long, high heat and the enzyme is knocked out first, so conversion stops. The fix is to cut first, wait a little, then heat gently (see Broccoli / garlic).
Mechanism · Why broccoli's enzyme fears heat
In broccoli and other cruciferous vegetables, sulforaphane is not ready-made. In intact tissue, the precursor glucoraphanin and the enzyme myrosinase are kept apart; only when you chop or chew do they meet, and the enzyme turns the precursor into sulforaphane. Fahey, Zalcmann, and Talalay laid out this pathway in 2001: myrosinase is sensitive to heat, and long, high heat knocks it out, so conversion stops.
In cell and animal experiments, sulforaphane can switch on the cell's own antioxidant program (a pathway called Nrf2); whether eating broccoli makes people less likely to get sick because of this has not been shown in humans.
Kitchen fixes: steam lightly, chop and wait a while before cooking, or sprinkle a little mustard powder to add back some myrosinase. The story on broccoli goes through these in detail; see Broccoli.
In practice · Crush garlic, wait, then cook
Allicin in garlic is not ready-made either. In an intact clove, the precursor alliin and the enzyme alliinase sit in separate compartments; only when you cut or crush the clove do they meet and form allicin. Alliinase is also sensitive to heat: put garlic straight into a hot pan and the enzyme is inactivated before it can work. The fix is to crush or chop it, wait until you smell the sharp pungency, and then heat it, so the enzyme makes the allicin first (see Garlic).
Put the four actions side by side and raw versus cooked was never a one-answer question: tomatoes want cooking, vitamin C dislikes long boiling, spinach does well blanched, beans must be cooked through, and broccoli and garlic want to be cut and left to wait. A smart kitchen picks the method for each food instead of trusting any one universal slogan.
Gärtner, C., Stahl, W., & Sies, H. (1997). Lycopene is more bioavailable from tomato paste than from fresh tomatoes. The American Journal of Clinical Nutrition, 66(1), 116-122. Cooking and processing (cell-wall breakdown), plus fat, raise lycopene bioavailability above raw tomato. 10.1093/ajcn/66.1.116
Brown, M. J., Ferruzzi, M. G., Nguyen, M. L., Cooper, D. A., Eldridge, A. L., Schwartz, S. J., & White, W. S. (2004). Carotenoid bioavailability is higher from salads ingested with full-fat than with fat-reduced salad dressings as measured with electrochemical detection. The American Journal of Clinical Nutrition, 80(2), 396-403. Dietary fat markedly raises absorption of fat-soluble carotenoids. 10.1093/ajcn/80.2.396
Chai, W., & Liebman, M. (2005). Effect of different cooking methods on vegetable oxalate content. Journal of Agricultural and Food Chemistry, 53(8), 3027-3030. Boiling markedly reduces soluble oxalate in high-oxalate vegetables such as spinach. 10.1021/jf048128d
Fahey, J. W., Zalcmann, A. T., & Talalay, P. (2001). The chemical diversity and distribution of glucosinolates and isothiocyanates among plants. Phytochemistry, 56(1), 5-51. Foundational chemistry of glucoraphanin conversion to sulforaphane by the enzyme myrosinase; myrosinase is heat-sensitive. 10.1016/S0031-9422(00)00316-2