Levels of the beneficial, cancer-fighting compound sulforaphane in broccoli are reduced by 90 percent when the vegetable is cooked, according to a study conducted by researchers from TNO Quality of Life in the Netherlands, and published in the Journal of Agricultural and Food Chemistry.
"Consumption of raw broccoli resulted in faster absorption, higher bioavailability, and higher peak plasma amounts of sulforaphane, compared to cooked broccoli," the researchers wrote.
Eight male participants were fed 200 grams of crushed raw or crushed cooked broccoli as part of a warm meal; researchers then measured the men's blood and urine levels of sulforaphane. Based on these measurements, the researchers calculated that while the sulforaphane in raw broccoli had a bioavailability of 37 percent, this dropped 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 observed in the blood and urine of participants 1.6 hours after eating raw broccoli, but these levels were not reached among consumers of cooked broccoli for six hours.
The cruciferous vegetables, also known as Brassicaceae, include broccoli, cabbage, cauliflower, arugula, Brussels sprouts, collard greens, daikon, garden cress, horseradish, kale, kohlrabi, mustard, radish, rape (canola), rapini, rutabaga , tatsoi, turnip, wasabi and watercress. Numerous studies have linked higher intake of these vegetables to lower rates of cancer and other health problems, particularly when the vegetables are consumed raw.
One of the plant compounds identified as partially responsible for this protective effect is sulforaphane, the main member of the isothiocyanate family that is found in broccoli. All cruciferous vegetables contain plant compounds known as glucosinolates, which are metabolized by the body into cancer-fighting isothiocyanates.
Studies have suggested that sulforaphane may help activate genes that produce antioxidants to clear dangerous free radicals from the body. This effect is believed to be partially responsible for the observed lower rates in breast, bladder, cervix, colon, endometrium, liver and lung cancers among those who eat large quantities of cruciferous vegetables. It is also believed to help protect the immune and other bodily systems from age-related decline.
Sulforaphane is also believed to reduce inflammation, which can transform precancerous cells into tumors and has also been linked to other chronic health problems such as heart disease and diabetes. At least one study has suggested that the chemical can even prevent the blood vessels of diabetics against the damage caused by high blood sugar.
The current study is not the first to suggest that most of broccoli's health benefits are destroyed by cooking. Recent research from the International Agency for Cancer Research found lower cancer rates among those who consumed at least three servings of raw cruciferous vegetables per month. This mirrors the results of an earlier study by researchers from the Roswell Park Cancer Institute in Buffalo, N.Y., who found a 40 percent lower risk of bladder cancer among those who ate that many raw cruciferous vegetables.
There was no protective effect observed, however, among those who ate cooked vegetables.
The researchers in the current study noted that other forms of processing besides cooking might also lead to the degradation of sulforaphane or its chemical precursors.
"The sulforaphane content of cooked broccoli was lower than the glucoraphanin content of raw broccoli, 9.92 and 61.4 micromoles, respectively," the researchers noted. "It seems that the conversion from glucosinolate to isothiocyanate was incomplete or that another reaction occurred."
Glucoraphanin (a glucosinolate) is the chemical precursor to sulforaphane (an isothiocyanate).
"In future research," they said, "care should be taken that glucoraphanin is not hydrolyzed into other metabolites when broccoli is crushed."
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