Brassica vegetables include broccoli, cauliflower, and cabbage, and other closely related, famously healthy and cancer-fighting vegetables. (See below for a list of Brassica and cruciferous vegetables, also called crucifers.)
You might be as surprised as I was to learn of the controversies surrounding these vegetables. They have anti-cancer effects but can also suppress thyroid function. How much of these vegetables should we eat? Should we eat them raw or cooked?
There are many scientific studies on both anti-cancer and anti-thyroid effects, but the arguments are technical.
However, the conclusions from both sides are about the same: a relatively small amount of these vegetables is beneficial!
Perhaps the best strategy for the average person might be to:
And, as with every food "family," the cruciferous vegetables are not for everyone. Some people's metabolisms are unsuited for so-called "sulfur foods." Likewise, cultured (fermented or pickled) crucifers aren't for everyone.
The controversy has to do with these vegetables' unique phytochemicals ("plant chemicals") called glucosinolates. When a Brassica vegetable is cut or chewed, an enzyme (myrosinase) within the vegetable breaks down its glucosinolates into various unique chemicals. Different cruciferous vegetables yield different amounts of some of these chemicals.
These chemicals, which collectively can be called glucosinolate products or glucosinolate metabolites, have been the focus of much scientific study. (See below for a list of glucosinolate products.) Some have anti-thyroid effects. One, nitrile, is perhaps simply a toxin. But others appear to be potent anti-carcinogens.
Glucosinolates (or the enzyme that creates the beneficial breakdown chemicals) can be inactivated by cooking. The beneficial bacteria in the human gut can activate glucosinolates from cooked crucifers, but antibiotics have eliminated the beneficial bacteria in most people.
This neglected fact has caused confusion in cancer studies and caused conflicting recommendations.
And, studies on the effects of cooking have been somewhat contradictory.
In animal and human studies, glucosinolate products have been shown to:
A study by the Roswell Park Cancer Institute (Buffalo, New York) showed that just three servings a month of raw broccoli, cauliflower, or cabbage reduced the risk of bladder cancer by about 40%.
There was no effect from fruits, total vegetables, or cooked Brassica vegetables in the diet. The study concluded that the anti-cancer factor was uniquely in Brassica vegetables and was destroyed by cooking.
This study was of 1000 patients treated at Roswell Park; about one third had bladder cancer and the rest had non-cancer conditions. An unusually good study: every diagnosis was definite, many different foods were individually considered, and it's expected that people can clearly remember whether they eat these vegetables and whether they eat them raw.
This study may be the best evidence on the value of raw Brassica vegetables over cooked.
However, the glucosinolate products also act as goitrogens, that is, as chemicals that inhibit thyroid function. They slow the thyroid gland by making it more difficult for it to absorb iodine.
With small amounts of crucifers, this effect can be countered by getting enough iodine in the diet. However, some large amount of glucosinolate products will suppress the thyroid regardless of the amount of iodine in the diet.
Organic vegetables contain up to 40% more glucosinolates.
Raw vegetables retain all of the glucosinolates.
Ideas: salads, shredded cabbage, cole slaw, slaw made from broccoli or cauliflower, grated daikon radish
Cultured crucifers make the glucosinolates more available; reduce nitrile, one of the most toxic glucosinolates, by half; provide beneficial bacteria to create glucosinolate products in the gut.
Cultured vegetables should be raw, not pasteurized; and should be traditionally made without vinegar. Pasteurized vegetables are essentially cooked and have no beneficial bacteria.
There are instructions in the cookbook Nourishing Traditions, by Sally Fallon of the Weston A. Price Foundation, for homemade cultured Brassica vegetables, including sauerkraut, cortido (Mexican), kimchi (Korean), and tsukomono (Japanese).
Traditionally made sauerkraut and kim chee (or kimchi) are available in natural food stores.
According to the background information in a study in Food and Chemical Toxicology, heating these vegetables to "high temperatures":
Steaming for up to 20 minutes showed no loss of glucosinolates in the Food and Chemical Toxicology study. The study did not address the loss of myrosinase. The Weston A. Price Foundation review, however, reports that steaming eliminates two thirds of the glucosinolates.
Most roots and greens steam marvelously; however, bok choy and some others are bitter when steamed. See How to steam vegetables.
Stir frying also showed no loss of glucosinolates in the Food and Chemical Toxicology study, although the loss of myrosinase was not addressed.
Boiling eliminates most glucosinolates. According to the Weston A. Price Foundation review, boiling for five minutes eliminates one third of the glucosinolates; boiling for thirty minutes eliminates 87%. The Food and Chemical Toxicology study showed a loss of 58% to 77% after thirty minutes of boiling.
Here's a list of cruciferous vegetables: the four species of Brassica (a genus in the Cruciferous family) and the other cruciferous vegetables.
Notice that broccoli, cauliflower, and cabbage are the same species! They differ only in how they have been bred, and thus are classified as different cultivars.
Chinese cabbage (Napa cabbage)
Rapeseed oil (canola)
Other cruciferous vegetables
The most studied phytochemicals that derive from glucosinolates are:
Diindolyl methane (DIM)
Phenyl ethyl isothiocyanate (PEITC)
Cancer Modulation by Glucosinolates: A Review
A literature review of the scientific research about glucosinolates, their toxic and anti-cancer effects, and how they are believed to work.
Cancer Epidemiology Biomarkers & Prevention:
Consumption of Raw Cruciferous Vegetables is Inversely Associated with Bladder Cancer Risk
The Roswell Park Cancer Institute study discussed above.
Raw Broccoli, Cabbage Slash Bladder Cancer Risk by 40 Percent: Cooking Destroys Benefits
Discusses the Roswell Park Cancer Institute study and promotes benefits of raw vegetables and juices.
Food and Chemical Toxicology:
Effect of storage, processing and cooking on glucosinolate content of Brassica vegetables (PDF)
Life Extension Magazine: How cruciferous vegetables prevent cancer
Life Extension Magazine: The coming age of vege-medicine
Advances in Experimental Medicine and Biology: Brassica vegetables and cancer prevention: epidemiology and mechanisms (abstract)
Weston A. Price Foundation:
Bearers of the Cross: Crucifers in the Context of Traditional Diets and Modern Science
A contrarian view: the case against Brassica vegetables. Emphasizes goitrogens and is skeptical about cancer prevention. Good information about how glucosinolates are affected by cooking and fermentation.
Life Extension Magazine: Broccoli: Providing Cancer Protection, Liver Support, and Essential Nutrients
Life Extension Magazine: Kale: Powerful Cancer Protection and Healthy Eye and Heart Benefits
Life Extension Magazine: Watercress: Benefits for Cancer Protection, Vision, and Heart Health
Weston A. Price Foundation: Sauerkraut--The Miracle Cabbage
New Zealand Food Safety Authority:
Glucosinolates Information Sheet (PDF)
Life Extension Magazine: Natural Prevention: I3C and Cancer
Life Extension Magazine: I3C and DIM
Life Extension Newsletter: New research contributes to the understanding of how I3C blocks cancer cells