Does cooking destroy nutrients: raw vs steamed vs fried?

Heat does two opposite things to plant foods. It breaks some compounds down, and it softens the plant's cell walls so other compounds are easier to release and absorb⁹. Which effect wins depends on the nutrient, the vegetable and the method.

Vitamin C and folate fall, mostly into the water. These water-soluble vitamins degrade with heat and leach into cooking water. Boiling kept only 49% of the folate in spinach and 44% in broccoli, while steaming caused no significant loss; grilling beef and boiling whole potatoes did not significantly change folate¹. In broccoli, every cooking method except steaming caused significant losses of vitamin C, and stir-frying hurt glucosinolates most². If you drink the cooking liquid, as in soup, you get back much of what leached out.

Carotenoids rise. Heat frees lycopene and beta-carotene from tough cell structures. Tomatoes heated at 88 °C for 15 to 30 minutes held more than two and a half times the lycopene of raw tomatoes, even though vitamin C dropped³. Women who ate processed, pureed carrots and spinach had about three times the rise in blood beta-carotene of those who ate them raw, although that result was borderline¹⁰. Fat matters too: with a fat-free dressing, almost no carotenoids were absorbed from a salad⁴.

Sulforaphane needs a raw or barely cooked crucifer. Broccoli's sulforaphane forms when the enzyme myrosinase, which heat destroys, meets its precursor. Sulforaphane bioavailability was 37% from raw broccoli and 3.4% from cooked in eight men⁵, and steamed broccoli gave about a third of the urinary excretion of fresh in twelve men⁶. Very gentle heat (60 °C) actually raised sulforaphane in the lab by disabling a protein that diverts the reaction¹¹. Our sulforaphane page has more.

Some foods need cooking. Cooked egg protein was about 91% digested, against 51% for raw⁸.

High, dry heat makes new compounds. Dry-heat cooking raised AGEs 10- to 100-fold, while vegetables, fruit, whole grains and milk stayed low even when cooked. Moist heat, shorter times, lower temperatures and acidic ingredients such as lemon juice or vinegar all reduced AGE formation⁷. See dietary AGEs and acrylamide.

Putting it together. Steaming comes out best for most vegetables^9,2. Eat a mix of raw and cooked, cook tomatoes and carrots with oil, and go easy on charring and deep-frying.

The big picture

A 2014 review of more than 100 articles found great variability. Steaming generally preserved phenols and glucosinolates best, because the food does not touch water or oil and so loses less by leaching; carotenoids behaved differently, with severe processing increasing their extraction⁹. Italian researchers cooked carrots, zucchini and broccoli by boiling, steaming and frying. Water cooking preserved antioxidants better than frying, especially carotenoids (and ascorbic acid in carrots and zucchini), and antioxidant capacity rose in all cooked vegetables on three assays; steamed vegetables kept firmer texture than boiled ones¹².

Folate and vitamin C

In the folate study, typical cooking habits were set by a survey of 100 shoppers, then folate was measured by microbiological assay. Spinach fell from 191.8 to 94.4 µg/100 g when boiled and broccoli from 177.1 to 77.0 µg/100 g. Steaming (up to 4.5 minutes for spinach, 15 for broccoli) caused no significant decrease. Grilled beef (54.3 to 51.5 µg/100 g) and potatoes boiled for 60 minutes (125.1 to 102.8 µg/100 g) did not change significantly¹. A Chinese study compared steaming, microwaving, boiling, stir-frying and stir-frying followed by boiling in broccoli: all but steaming caused significant losses of chlorophyll and vitamin C, and all but steaming significantly changed glucosinolates, with stir-frying worst². Not every study agrees. The Italian work found water cooking kept ascorbic acid in carrots and zucchini better than frying¹². Different vegetables, times and assays give different answers, so no one method is best for every food.

Carotenoids

In tomatoes, trans-lycopene rose from 2.01 mg/g raw to 5.45 mg/g after 15 minutes at 88 °C and 5.32 mg/g after 30 minutes, while vitamin C fell from 0.76 to 0.54 µmol/g and total antioxidant activity rose from 4.13 to 6.70 µmol vitamin C equivalents/g³. In a crossover study, eight women ate about 9.3 mg of beta-carotene a day from raw or thermally processed, pureed carrots and spinach for four weeks each. Blood beta-carotene rose significantly above baseline only with the processed vegetables, and the average response was three times larger, but the raw-versus-processed difference had P = 0.09¹⁰. Seven people ate salads of spinach, romaine, cherry tomatoes and carrots with dressings containing 0, 6 or 28 g of canola oil. Absorption was negligible with no fat, higher with reduced fat, and highest with the full-fat dressing⁴.

Sulforaphane

Eight men ate 200 g of crushed broccoli, raw or cooked, in a crossover trial. Sulforaphane peaked in plasma at about 1.6 hours after raw broccoli and about 6 hours after cooked, with 37% versus 3.4% bioavailability (P = 0.002)⁵. In twelve men eating 200 g of fresh or steamed broccoli, the isothiocyanate content after adding myrosinase was almost identical (1.1 vs 1.0 µmol/g), yet 24-hour urinary excretion was 32.3% of the dose after fresh broccoli and 10.2% after steamed⁶. Heating whole broccoli florets to 60 °C before crushing raised sulforaphane and lowered the inactive nitrile form, paralleling the loss of a heat-sensitive protein; at 70 °C and above, formation of both fell in florets¹¹.

Protein and AGEs

Five ileostomy patients ate 25 g of isotope-labeled egg protein, cooked and raw, on separate occasions. True ileal digestibility was 90.9% for cooked and 51.3% for raw⁸. A database of dietary AGEs found dry heat increased new AGE formation more than 10- to 100-fold above the uncooked state, mainly in animal foods high in fat and protein, and moist heat, short times, low temperatures and acidic ingredients cut it⁷.

Where the studies leave gaps

The human trials are tiny (5 to 12 people), crossover in design, and measure blood or urine markers, not disease. The raw-versus-processed beta-carotene result was not statistically firm. The sulforaphane authors suggested cooking "may markedly reduce" broccoli's benefit⁶, but we found no evidence that cooked rather than raw broccoli changes disease risk, and steaming keeps glucosinolates intact. Antioxidant-capacity assays rise after cooking, yet they are lab measures, not health outcomes¹².

Bottom line

Confidence is 4 of 5. The direction of change is consistent across a large review and several independent trials: boiling costs water-soluble vitamins, heat improves carotenoid availability, and raw or barely cooked crucifers deliver more sulforaphane. The size of each effect varies by vegetable and method, and none of it has been tied to disease outcomes.