A recent study has shown that levels of the beneficial, cancer-fighting compound sulforaphane in broccoli are reduced by 90 percent when the vegetable is cooked.
The research was published in the Journal of Agricultural and Food Chemistry, and the abstract is given below.
"The aim of this study was to determine the bioavailability and kinetics of the supposed anticarcinogen sulforaphane, the hydrolysis product of glucoraphanin, from raw and cooked broccoli. Eight men consumed 200 g of crushed broccoli, raw or cooked, with a warm meal in a randomized, free-living, open cross-over trial. Higher amounts of sulforaphane were found in the blood and urine when broccoli was eaten raw (bioavailability of 37%) versus cooked (3.4%, p = 0.002). Absorption of sulforaphane was delayed when cooked broccoli was consumed (peak plasma time = 6 h) versus raw broccoli (1.6 h, p = 0.001). Excretion half-lives were comparable, 2.6 and 2.4 h on average, for raw and cooked broccoli, respectively (p = 0.5). This study gives complete kinetic data and shows that consumption of raw broccoli results in faster absorption, higher bioavailability, and higher peak plasma amounts of sulforaphane, compared to cooked broccoli."
Vermeulen M, et al. Bioavailability and Kinetics of Sulforaphane in Humans after Consumption of Cooked versus Raw Broccoli J. Agric. Food Chem., 2008, 56 (22), pp 1050510509
The research showed that the bioavailability of sulforaphane dropped from 37 percent in raw broccoli, to only 3.4 percent when the vegetable was cooked. Furthermore, it took longer for the sulforaphane from cooked broccoli to be absorbed by the body. Optimal levels of sulforaphane were found 1.6 hours after eating raw broccoli, but these levels were not reached among consumers of cooked broccoli for six hours.
All cruciferous vegetables contain plant compounds known as glucosinolates, which are metabolized by the body into cancer-fighting isothiocyanates. One of the plant compounds identified as partially responsible for this protective effect in this study is sulforaphane, a member of the isothiocyanate family. broccoli and broccoli sprouts have the highest amount of these isothiocyanates.
Sulforaphane is believed to help activate the production of antioxidants which remove dangerous free radicals from the body. Studies have also shown that it reduces inflammation, which is linked to chronic health problems like cancer and diabetes.
Numerous studies, some outlined in previous articles below, have linked the intake of broccoli and other cruciferous vegetables to lowing cancer risk and other health problems, especially when eaten raw or lightly cooked. Exmaples of common cruciferous vegetables include: cabbage, cauliflower, Brussels sprouts, garden cress, horseradish, kale, kohl rabi, mustard, radish, rocket, turnip, wasabi and watercress.
To obtain maximum nutritional benefit from your vegetables, especially the cruciferous ones, eat them raw, or lightly steam them for a few minutes. Several studies have shown that of all the methods of preparation, steaming causes the least loss of nutrients. A co-author of one of these studies noted that:
"Most of the bioactive compounds are water-soluble; during heating, they leach in a high percentage into the cooking water. Because of this, it is recommended to cook vegetables in the minimum amount of water (as in steaming) in order to retain their nutritional benefits."
Vallejo F, Tomás-Barberán F A, García-Viguera C. Phenolic compounds content in edible parts of broccoli inflorescences after domestic cooking. J Sci Food Agric, 2003 Volume 83(14) 2003.