Part of the vitality that we feel when we are fully healthy and well-nourished comes from the functioning of our body systems at their full capacity and the successful balancing and integration of all metabolic processes taking place within our cells. We feel our best only when our diet optimally supports our physiology and our metabolism, and it is this same great physiological and metabolic support that allows our diet to lower our risk of chronic disease.
All natural food groups provide outstanding support for multiple body systems, and regular intake of foods each group can definitely help us feel our best. But it would be hard to find a single food group that provides broader "all systems" support than the cruciferous vegetables. This food group shines not only in terms of conventional nutrients (including vitamins, minerals, proteins, carbohydrates, and fats) that support optimal functioning of most body systems, but also in terms of phytonutrients (including carotenoids and flavonoids) that play a more subtle but equally important role in our vitality. Also present in this food group is one group of phytonutrients — the glucosinolates — that is virtually exclusive to this group. When combined with their extremely low-calorie, nutrient-rich nature, these features of cruciferous vegetables make them a uniquely "whole body" support food group.
This whole body impact of the cruciferous vegetables as a group has its parallel in the disease prevention category for which cruciferous vegetables have become most famous. No area of chronic disease is more dependent on a breakdown in our whole body functioning than cancer. In order for cells to become cancerous, a variety of interconnected body functions become compromised. While it is true that metabolic "errors" typically build up inside the genetic machinery of "pre-cancerous" cells and play a primary role in the development of cancer, it is also true that dysfunction in our immune system, inflammatory system, hormonal system, detoxification system, and antioxidant system — and problematic interactions between these five systems — can significantly increase the risk of cells becoming cancerous. What is particularly striking about cruciferous vegetables as a group is their ability to provide nutrient support for all five of the body systems listed above. But given this unique ability on behalf of cruciferous vegetables, it is not surprising that no other group of foods has a better track record when it comes to cancer prevention. Understanding how cruciferous vegetables support basic physiological systems in the body and understanding the role of cruciferous vegetables in cancer prevention are highly interconnected topics and will be reviewed together in the next sections of this article.
For all types of cancer and all types of cruciferous vegetables, the total number of high-quality research studies is fast approaching 600 studies. The chart below shows the number of studies identified in the U.S. Library of Medicine's PubMed database when the terms "cruciferous vegetables" and "cancer" (or a specific cruciferous vegetable, like "broccoli," and a specific cancer (like "bladder cancer") are used as search terms:
|Cancer Type||All Cruciferous Vegetables||Broccoli||Cabbage||Kale|
Prior to 1996, about two-thirds of all research studies showed protective effects for cruciferous vegetables in prevention of certain cancer types. This proportion has increased over the past 10 years, with researchers becoming more and more precise about the mechanisms of cancer prevention provided by intake of cruciferous vegetables.
Among all cancer-preventive mechanisms studied by nutritional researchers, none have been as far-reaching as the mechanisms involving one unique family of phytonutrients — the glucosinolates — found in no other food group to the same extent that they are found in cruciferous vegetables.
Nutritionists don't always study nutrients in cruciferous vegetables called glucosinolates — but they should! From a chemistry point of view, glucosinolates are sugar-based molecules that contain a modified form of sugar (glucose) together with sulfur and nitrogen. Glucosinolates aren't found exclusively in cruciferous vegetables, but they are overwhelmingly absent from most other food groups. Over 100 different glucosinolates have been identified in cruciferous vegetables, and all of the glucosinolates studied to date have been shown to have anti-cancer properties.
What is it about glucosinolates that makes them such prominent anti-cancer compounds? In and of themselves, nothing at all! It takes the work of an enzyme called myrosinase to convert glucosinolates into anti-cancer compounds. All plants in the cruciferous vegetable family that contain glucosinolates also contain myrosinase enzymes. When the plants are alive, their myrosinase enzymes are also active and functioning. In freshly harvested plants, myrosinase enzymes continue to function, but this function decreases over time. Cooking for more than 10 minutes will typically end all myrosinase function in cruciferous vegetables. Between 1-10 minutes, loss of myrosinase activity will become greater and greater with each additional minute of heating and with each increase in cooking temperature.
Freshly harvested cruciferous vegetables eaten in raw form will typically have a significant percentage of their glucosinolates converted into isothiocyanates by still active myrosinase enzymes and these isothiocyanates will become available in the upper digestive tract (small intestine) for absorption into the bloodstream. Freshly harvested cruciferous vegetables not consumed in raw form, but chopped and allowed to sit for several minutes prior to cooking will typically have some of their glucosinolates converted into isothiocyanates by still active myrosinase enzymes, and these isothiocyanates will also be available for absorption in the upper digestive tract. Other glucosinolates remain intact all the way through the cooking process, and through most of digestion as well. Freshly harvested cruciferous vegetables that are not chopped and left to sit but immediately cooked will typically provide few isothiocyanates for early absorption in the upper digestive tract and more glucosinolates left to travel the full length of the digestive tract all the way down to the colon. When glucosinolates from cruciferous vegetables reach the lower intestine (colon) intact, colonic bacteria often proceed to convert the glucosinolates into other compounds, including isothiocyanates.
For every specific glucosinolate, there are corresponding anti-cancer compounds produced following action of the myrosinase enzyme. The molecules formed from enzymatic activity on glucosinolates are called isothiocyanates (ITCs). While most cruciferous vegetables contain several dozen different glucosinolates in significantly health-supportive amounts, some glucosinolates have been better researched than others and/or appear to be more concentrated in commonly eaten foods. The chart below shows five very well-studied glucosinolates, the best cruciferous vegetables for obtaining them, and some key isothiocyanates that form after the glucosinolates have been acted upon by myrosinase enzymes:
|Cruciferous Vegetable||Glucosinolate||Derived Isothiocyanate||Isothiocyanate Abbreviation|
|broccoli*, red cabbage*, kale*, collard greens*, Brussels sprouts, kohlabi||Glucophanin||Sulforaphane||SFN||Brussels sprouts*, savoy cabbage*, cauliflower, broccoli, kale, Chinese cabbage||Glucobrassicin||Indole-3-carbinol**||I3C||Brussels sprouts*, savoy cabbage*, white cabbage*, kale*, collard greens*, mustard greens*, cauliflower||Sinigrin||Allyl-isothiocyanate||AITC||Turnip greens*, watercress*, kale, collard greens, mustard greens, broccoli, Brussels sprout||Gluconasturiian||Phenethyl-isothiocyanate||PEITC||Watercress*, garden cress*, collard greens*, kale, mustard greens, cabbage||Glucotropaeolin||Benzyl-iothiocyanate||BITC|
** Indole-3-carbinol (I3C) is not an isothiocyanate. It's a benzopyrrole, and it is only formed when isothiocyanates made from glucobrassicin are further broken down into non-sulfur containing compounds.
Isothiocyanates (ITCs) formed from glucosinolates are known to help prevent cancer by several different mechanisms. Sometimes the ITCs deactivate potential cancer-causing substances before they can damage a cell's genetic material (DNA). At other times, they alter communications between cells in such a way that healthy cells are prevented from becoming cancerous. In the case of hormone-sensitive cancers, ITCs can modify the metabolism of hormones like estrogen in such a way that risk of cancer is decreased. ITCs can also increase detoxification of toxic substances that might otherwise increase cancer risk.
There is also evidence that ITCs may be able to help change the course of cancers in cells once those cancers have occurred. One in-depth series of studies in the cruciferous vegetable research has shown the ability of ITCs in cruciferous vegetables to "sensitize" cancer cells. Many cancer cell types are resistant to chemotherapy and cannot be treated using drugs because of their "chemoresistance." Because ITCs are able to help inactivate certain metabolic events in cancer cells (for example, Akt phosphorylation), they may be able to sensitize cancer cells and make them more responsive to chemotherapeutic drugs. In the case of one particular ITC in cruciferous vegetables (sulphoraphane), a cell cycle (the G1 cell cycle) normally participated in by ovarian cancer cells has been shown to be disrupted in the presence of this ITC. Many other cell cycle events have also been shown to be altered by ITCs. And in a fascinating finding of several studies, the impact of ITCs on healthy cells has been shown to be different than the impact of ITCs on cancerous cells.
Taken as a whole, we are not surprised by the hallmark impact on cancer prevention made by cruciferous vegetables, or the potential of this food group to modify activities in cells that have already become cancerous. What makes these cancer-directed impacts seem logical to us are the many findings about cruciferous vegetables and their support for three body systems that are so critical to the development or non-development of cancers. These three body systems are the detoxification system, the inflammatory system, and the antioxidant system. It's worth taking a little closer look at cruciferous vegetables and their support of these three body systems in order to understand how this food group helps keep us optimally healthy.
Inflammation is one of our body's most complicated physiologic systems that we constantly rely on for protection of our health. Any time our body is faced with injury (whether it be physical or chemical), our inflammatory system kicks in and increases blood flow to the injured area, sends healing substances to the site of injury, and coordinates the response of other body systems in dealing with the threat to our health. Sometimes the injury to our body is not as specific or immediate as described in the examples above, however. Sometimes we develop metabolic imbalances that last for years and that worsen very gradually over time. It's also the job of the inflammatory system to try and minimize the damage to our health that is caused by chronic, lasting injury of this kind. There can be a risk, however, involved with constant overactivation of inflammatory processes. Some problems in our health can be the direct result of chronic inflammatory processes. In short, the job of our body's inflammatory system is to protect our health without "overdoing things" and becoming a problem in and of itself.
No set of events is more central to regulating inflammation than the set of events surrounding a molecule called Nuclear factor- B (NF- B). NF- B is a molecule that initiates many inflammatory responses. It operates at the genetic level, helping take instructions found in our genes and apply those instructions to the production of substances that will increase our inflammatory response. (In technical terms, NF- B is described as a proinflammatory transcription factor.) What's remarkable about the cruciferous vegetables is their ability of their glucosinolates (after being converted into isothiocyanates, or ITCs) to block the activity of NF- B. Especially well-studied and potent in this regard is the ITC called indole-3-carbinol (I3C).
In addition to the anti-inflammatory impact of ITCs from cruciferous vegetables and their ability to block activity of NF- B is the nutrient richness of this food group in terms of vitamin K. Vitamin K is a direct regulator of our inflammatory system response, and optimal intake of vitamin K helps avoid chronic, excessive responses by our body's inflammatory system.
A third contributing factor to the inflammation-regulating ability of the cruciferous vegetables is their omega-3 fatty acid content. In a common, 100-calorie serving from this unique food group, we can obtain about one-half gram of omega-3s in the form of alpha-linolenic acid (ALA). ALA is the initial building block for all other omega-3 fats found in the body. One omega-3 fat made from ALA is called eicosapentaenoic acid, or EPA. EPA is one of the premiere anti-inflammatory compounds in the body because it is the source of numerous messaging molecules (like prostaglandin H3, prostaglandin I3, and thromboxane A3) that send signals to decrease the intensity of inflammatory events. The combination of ITCs, vitamin K, and omega-3s in cruciferous vegetables make them a unique food group for supporting our body's inflammatory system.
When researchers list critical antioxidant nutrients, some of the first nutrients they name are vitamin C and beta-carotene. These two conventional nutrients play a critical role in our body's regulation of oxygen metabolism, and they are widely known for their role in helping lower our risk of oxidative stress. As a food group, the cruciferous vegetables are outstanding providers of these two antioxidant nutrients. In fact, we cannot find another food group (even a fruit subgroup) that is as nutrient-rich in these antioxidants as the cruciferous vegetables. Combined with the vitamin C and beta-carotene in cruciferous vegetables are four other key antioxidant nutrients: vitamin E, manganese, zinc, and selenium. While the concentration of these four antioxidants in cruciferous vegetables is not as great as their concentration of vitamin C and beta-carotene, it is still substantial and combines with vitamin C and beta-carotene to provide the body with outstanding antioxidant support.
Conventional nutrients are not the only means by which cruciferous vegetables support the body's antioxidant system, however. This unique food group also contains a wide variety of antioxidant phytonutrients. Flavonoid antioxidants like isorhamnetin, quercitin, and kaempferol are plentiful in cruciferous vegetables, as are antioxidants like caffeic acid and ferulic acid.
In addition to these conventional and phytonutrient antioxidants are other substances in cruciferous vegetables that may be able to regulate the antioxidant response process itself. One such substance is a sulfur-containing compound called D3T (which stands for 3H-1,2-dithiole-3-thione). Researchers look to future study results to determine exactly how cruciferous vegetable compounds like D3T may be able to optimize responses by our body's antioxidant system.
Most toxins that are present in our body must be detoxified and eliminated from our body in order for us to stay healthy. Prolonged exposure to toxins is a risk factor for many chronic diseases, including cancers. Many different cell types in our body have detoxification systems for neutralizing toxins. These detoxification systems have two basic components, called Phase I and Phase II. Both phases are important for elimination of toxins, and both phases call for substantial nutritional support. There can almost never be too much activity in Phase II — it's a part of detoxification that is almost always necessary and required for health protection. But there can sometimes be too much activity in Phase I, and it's important for the body to keep Phase I activity in balance.
What's remarkable about cruciferous vegetables is their ability to strongly support Phase II detoxification activity, while simultaneously helping to regulate Phase I. For example, some of the isothiocyanates (ITCs) in cruciferous vegetables have been determined to increase certain Phase I activities, like the activity of cytochrome P450 1A2 (CYP 1A2). Others have been found to decrease certain Phase I activities, like the activity of cytochrome P450 3A4 (CYP 3A4). In the case of CYP 3A4, researchers have shown that the blocking activity of one ITC in cruciferous vegetables — sulforaphane — occurs at fundamental level. Sulforaphane actually locks onto a receptor on the cell's nuclear membrane to help shut down the genetic machinery that produces CYP 3A4.
Support of Phase II detox activity by cruciferous vegetables may be unsurpassed by any other food group. Researchers have determined that isothiocyanates (ITCs) from cruciferous vegetables can activate a wide variety of enzymes that are essential in Phase II detox, including heme oxygenase, aldoreductase, glutamate cysteine ligase, and quinone reductase. And once again, this support of Phase II detox operates at a fundamental molecular level in which ITCs from cruciferous vegetables latch onto nuclear factors like nuclear factor Nrf2.
Bolstering our knowledge about cruciferous vegetables and detox are studies involving human genetics. Some people are born with genetic predispositions that tend to alter their detox system activity. For example, there is a fairly common genetic change related to Phase II detox that is called a null mutation of GSTM1. This genetic change changes the body's ability to neutralize toxic substances by combining them together with a Phase II molecule called glutathione. (The "G" in "GSTM1" stands for "glutathione.") When this genetic mutation occurs, scientists see changes in the pattern of detox events. They also see changes in the impact of cruciferous vegetables on detox events. Sometimes a genetic mutation can provide a greater role for cruciferous vegetables in altering detoxification. At other times, a genetic mutation may restrict the role of cruciferous vegetables in this regard. However, regardless of any specific consequences, the study of genetics has firmed up our understanding of cruciferous vegetables and their important role in regulation of detoxification.
Unique Three Systems Support by Cruciferous Vegetables: Detoxification + Antioxidant + Inflammatory System Support
It's impossible to overstate the potential role of cruciferous vegetables in cancer prevention because of their unique ability to support the three body systems described above. When scientists have examined risk factors for the development of cancer in healthy cells, they have repeatedly been drawn to imbalances in these three body systems. Inadequate detoxification of toxins through lack of well-supported and well-regulated Phase I and Phase II detox activity has clearly been shown to increase risk of cancer development. Similarly, unsupported oxygen metabolism due to lack of antioxidant nutrients has been shown to result in oxidative stress, excessive formation of free radicals, and a metabolic environment that increases risk of cancer development. Chronic, excessive inflammatory response is yet another body system imbalance that increases the risk of cancer development. Combined dysfunction in these three body systems is a hallmark of increased cancer risk. For this reason, the cruciferous vegetables have a unique role to play not only in our physiological health, but in cancer prevention as well.
Although research in this area is still in the early stages, anti-inflammatory substances found in cruciferous vegetables are becoming the topic of increasing interest with respect to heart disease. One particular focus here involves the anti-inflammatory properties of sulforaphane, one of the isothiocyanates (ITCs) derived from cruciferous vegetables. In some individuals susceptible to high blood sugar, sulforaphane may be able to prevent (or even reverse) some of the damage to blood vessel linings that can be caused by chronic blood sugar problems. Decreased risk of heart attacks and strokes may also eventually be linked in a statistically significant way to the intake of cruciferous vegetables and their unique anti-inflammatory compounds.
B-complex vitamins play a special role in cardiovascular health. Especially with respect to excessive formation of homocysteine — an event which raises our risk of atherosclerosis, stroke, and heart attack — deficient B-complex vitamin intake can be a major problem. Vitamins B6, B12, and folate are especially important for lowering our risk of hyperhomocysteinemia (excessive formation of homocysteine), and by making a large contribution to our folate intake, cruciferous vegetables can help us lower our risk of all three cardiovascular diseases.
The concentrated fiber content of cruciferous vegetables, in and of itself, makes this food group a natural for digestive system support. It's just unusual to see 100 calories of any food providing nearly half of the Daily Value for dietary fiber. But that is precisely the case for most cruciferous vegetables. Few foods can make as outstanding a contribution to your daily fiber intake as foods like broccoli, Brussels sprouts, cauliflower, and kale.
Yet, the fiber content of cruciferous vegetables is only one of their digestive support mechanisms. Researchers have determined that the isothiocyanates made from glucosinolates in cruciferous vegetables (especially sulforaphane) help protect the health of our stomach lining by helping prevent bacterial overgrowth of Helicobacter pylori or problems resulting from overgrowth of this bacterium or too much clinging by this bacterium to our stomach wall. Broccoli sprouts appear to have especially strong stomach support properties in this regard.
Most of the research on dietary intake of cruciferous vegetables and their physiological benefits has focused on intake of glucosinolates from this food group. For total glucosinolates, we've seen reliable estimates that cover a wide range of daily intake values. At the lower end of this range, we've seen studies in which individuals average only 6 milligrams of total glucosinolates from cruciferous vegetables each day. At the higher end of the range, we've seen studies showing an average of about 50 milligrams. Health benefits have been seen across this wide range of glucosinolate intake levels. Depending on the specific cruciferous vegetables in question and the method of preparation, we're talking about daily cruciferous vegetable intake levels beginning at approximately one-half cup and ranging upward to approximately 2 cups as being necessary to provide these glucosinolate intake levels. At the lower end of this range, it would be possible for a person to consume a moderate amount of cruciferous vegetables 2-3 times per week and come out with a one-half cup per day average. At the higher end of the range, it would take more like 4-5 servings per week to reach the glucosinolate levels involved. Yet, that's really not that much if you think about it; it's just including one serving of cruciferous vegetables almost every day. For persons who enjoy the textures, aromas, and flavors of cruciferous vegetables, foods in this unique group can usually be enjoyed on a daily basis. For individuals who aren't as fond of foods in this group, working them into the diet 2-3 days per week would still be highly recommended in most cases.
When myrosinase enzymes convert glucosinolates found in cruciferous vegetables into thiocyanates (including isothiocyanates), some of the thiocyanates formed have the ability to bind together with free iodine found in the body. Iodine is an important body mineral for a wide variety of reasons, but included in these reasons is its role in thyroid hormone production. Since thyroid hormones always require at least one atom of iodine in their structure, iodine availability can be a rate limiting factor in the production of hormones by our thyroid gland. Some research has raised the possibility that when excessive amounts of thiocyanate ions are formed from glucosinolates, these thiocyanate ions can bind unwanted amounts of iodine and prevent the iodine from being optimally available to the thyroid for production of its thyroid hormones.
In principle, this possibility is definitely a consideration for anyone consuming large amounts of cruciferous vegetables. However, studies have also shown that fairly large amounts of cruciferous vegetables containing substantial amounts of glucosinolates fail to damage thyroid function. For example, in one study, subjects consumed 5 ounces of Brussels sprouts every day for 4 weeks and did not have their thyroid function compromised. We've seen one published report involving an 88-year-old woman who developed severe hypothyroidism (low production of thyroid hormones) and a related coma following consumption of an estimated 2-3 pounds of raw bok choy for several months. However, in this situation we are talking about several pounds of a raw cruciferous vegetable on a daily basis over a prolonged period of time, rather than high-but-balanced consumption of cruciferous vegetables as a group within the context of an overall healthy diet.
If you are an individual with a pre-existing history of thyroid problems and you would like to consume cruciferous vegetables in generous amounts on a daily basis, we recommend a consultation with your healthcare provider to determine the best intake level for you. For individuals with no history of thyroid problems, however, routine dietary intake of cruciferous vegetables along the guidelines of several ounces per day, 3-4 days per week should not be regarded as posing a health risk to the thyroid based on published research in this area.
Because cruciferous vegetables provide such integrated nourishment across a wide variety of nutritional categories, they provide broad support across a wide variety of body systems as well. Their showcase amounts of antioxidant, anti-inflammatory, and detox-related nutrients make them unique supporters of the body's antioxidant system, inflammatory system, and detoxification system. And because this 3-system combination plays such a pivotal role in our risk of cancer development, cruciferous vegetables are unsurpassed as a food group in terms of their cancer risk-reducing properties. The anti-inflammatory properties of cruciferous vegetables — together with their unusual concentration of folic acid — combine to make this food group protective against atherosclerosis, heart attack and stroke. The fiber and glucosinolates in cruciferous vegetables also give them unique digestive support properties, including the ability to help regulate interactions between Helicobacter pylori bacteria and the stomach wall. Without regular intake of cruciferous vegetables in your diet, your body systems will be missing out on some of the best-researched pathways for disease prevention.
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