Broccoli in a Diet to Reduce Cancer Risk: Raw or Cooked?

How can you best take advantage of the potential broccoli and other cruciferous vegetables offer as part of a diet to lower cancer risk? Research on health benefits is growing, as are studies on the difference it makes how – and whether – you cook these vegetables. (Hint: don’t assume raw is always better.)  Here, in Part 1 of a series, Elizabeth Jeffery, PhD, provides an update on how we can prepare cruciferous vegetables like broccoli for its optimal part in a healthy diet. Dr. Jeffery, Professor in the Department of Food Science and Human Nutrition at the University of Illinois at Urbana-Champaign, is widely recognized for her research in this field.

After you watch the video interview, scroll down for an update on research about why we care about cruciferous vegetables… and more on what happens to the glucosinolate compounds in broccoli and other crucifers when we eat them in raw or cooked form.

Cancer-fighting Oomph of Broccoli & Company

Cruciferous vegetables constitute a family that includes at least two dozen vegetables. Vitamin and mineral content varies among family members, though many are good sources of nutrients that research suggests can reduce cancer risk and promote overall health. Many are good sources of vitamin C, beta-carotene, magnesium, folate (a B vitamin important for healthy DNA) and a group of phytochemicals called flavonoids that provide a bounty of health-protective benefits.

What’s truly unique about cruciferous vegetables like broccoli, Brussels sprouts, kale, mustard greens, bok choy and cabbage is their glucosinolate content.  Glucosinolates are broken down to isothiocyanates and indoles, compounds that seem to:

• Decrease inflammation
• Inhibit enzymes that activate carcinogens
• Stimulate enzymes that detoxify carcinogens
• Turn on tumor suppressor genes, which slow cell growth so that cell damage can be repaired and stimulate the process called apoptosis in which damaged cells self-destruct
• Induce genes that regulate the body’s antioxidant defense system and protect DNA from damage

Scientists are learning more all the time about the compounds in cruciferous vegetables that seem key to their unique potential. Here’s the bottom line: the “parent” compounds found in cruciferous vegetables, which are known as glucosinolates, have no protective effects themselves. However, when cells are disrupted by chewing or chopping, the glucosinolates are exposed to an enzyme stored elsewhere in the plant food, which converts the inactive glucosinolate compounds to isothiocyanate and indole compounds that have significant anti-cancer effects.

Does broccoli reduce cancer risk?

Cruciferous vegetables have been linked with reduced risk of lung, colorectal, stomach and breast cancers.  Further research is underway, including intervention trials investigating potential for isothiocyanates in our foods to slow prostate cancer progression through effects on gene expression.

More recent well-designed population studies do not show as strong and consistent a link between cruciferous vegetable consumption and reduced cancer risk as earlier studies did. However, part of that may be because of individual differences in genetics and in the colon bacteria that metabolize these compounds.

• People may differ genetically in how rapidly they metabolize the protective isothiocyanate compounds, meaning that some keep them in their body longer than others.
• Differences in colon bacteria known as the “microbiota” mean people may differ in how effectively they break down whatever glucosinolates make it through the digestive tract intact to form the protective isothiocyanates.
• The same amount and frequency of cruciferous vegetable consumption could provide vastly different amounts of protective isothiocyanates (like sulforaphane) depending on how they are prepared. So benefits from cruciferous vegetables likely vary with how people cook them, as Dr. Jeffery noted in our video interview.

How to Maximize the Cancer Protection

How you prepare broccoli and other cruciferous vegetables will significantly change the nutritional benefits you receive.

photo courtesy of National Cancer Institute (NCI)

♦ Boiling broccoli and other cruciferous vegetables is a triple-whammy. First, glucosinolates are water-soluble compounds that can leach out into cooking water, decreasing total glucosinolate content by 18-59%. Second, only limited amounts of the glucosinolates that do remain will be converted to the active isothiocyanates since, as Dr. Jeffery explained, the myrosinase enzyme will be destroyed. That means that you’ll only get whatever amount your colon bacteria are able to produce for you. Finally, boiling also leaches out the other water-soluble nutrients in these vegetables, including vitamin C, folate, potassium (your blood pressure’s friend) and flavonoid phytochemicals.

♦ Broccoli and cauliflower are terrific options for a raw vegetable appetizer or snack tray. They retain many nutrients, but Dr. Jeffery explains that the latest research shows that you will get more of the active protective isothiocyanates (such as sulforaphane from broccoli) if you briefly blanch the vegetables first. That means just a quick dip in boiling water, and then immediately cooling them.

♦ Cooking inactivates the enzyme that converts the inactive glucosinolates to active compounds.  However, if you steam these vegetables for three or four minutes (just until crisp-tender), the cooking temperature is low enough that you destroy the protein “co-factor” that would have allowed some of the glucosinolates to form inactive nitrile compounds, yet stop before the myrosinase enzyme needed to form protective isothiocyanate compounds (like sulforaphane) is destroyed.

Bottom line: Cruciferous vegetables do seem to offer excellent potential as part of an overall plant-focused eating pattern to reduce cancer risk. To get the most from them: blanch first to serve “raw”, and steam them up to four minutes to serve cooked. Don’t boil.

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Come back for the next sections of my interview with Dr. Jeffery to hear about how other cooking and preparation options influence what you are getting from your broccoli.

Resources

Check the American Institute for Cancer Research’s website section called Foods that Fight Cancer for more on research underway on cruciferous vegetables and links to delicious recipes to enjoy them.

References

Navarro SL, et al. Mechanisms of action of isothiocyanates in cancer chemoprevention: an update. Food Funct. 2011 Oct;2(10):579-87.

Singh SV, Singh K. Cancer chemoprevention with dietary isothiocyanates mature for clinical translational research. Carcinogenesis. 2012 Oct;33(10):1833-42.

Yang, G., et al., Isothiocyanate exposure, glutathione S-transferase polymorphisms, and colorectal cancer risk. Amer J Clin Nutr, 2010. 91(3): p. 704-11.

McNaughton SA, and Marks GC. Development of a food composition database for the estimation of dietary intakes of glucosinolates, the biologically active constituents of cruciferous vegetables. Br J Nutr. 2003;90(3):687-697.

Wang GC, Farnham M, Jeffery EH. Impact of Thermal Processing on Sulforaphane Yield from Broccoli ( Brassica oleracea L. ssp. italica). J Agric Food Chem. 2012. 60(27):6743–6748.