National Health Departments around the world urge the population to increase the consumption of fruits and vegetables. The American Institute of Cancer Research estimates that if the only dietary change made was to increase the daily intake of fruits and vegetables to 5 servings per day, cancer rates could decline by as much as 20%. Among the reasons cited for this health benefit are that fruits and vegetables are excellent sources of fibre, vitamins, and minerals. In addition, they contain non-nutritive components that may provide substantial health benefits beyond basic nutrition.
The family of cruciferous vegetables that includes broccoli, Brussels sprouts, cabbage, cauliflower, rucola, radishes, watercress, wasabi, mustard and other plants, has been studied extensively for their cancer-protective properties. Cruciferous vegetables are high in flavonoids such a quercetin, selenium and glucosinolates, which have a potent odour and taste. When chopped, masticated and digested, glucosinolates are chemically transformed to isothiocyanates, such as sulforaphane, which is the bioactive form. The total amount of bioactive glucosinolates absorbed from a meal depends on the type of cruciferous vegetable, the food preparation conditions and gut microflora of the person consuming the meal.
Studies of isothiocyanates in test tubes and rodents have demonstrated their potent anticarcinogenic properties. This has been attributed to their ability to alter detoxification pathways in the body, leading to a decreased activation of potential carcinogens and an increased excretion of carcinogens. In cases where cancer is already present, cruciferous vegetables can slow down the growth of tumours and even induce cancer cell death. Some studies have shown that consuming cruciferous vegetables 3 times per week can cut the risk of colon cancer and prostate cancer by 40-50%. Sulforaphane has also been shown to reduce sensitivity to UV radiation by reducing oxidative stress.
How to eat cruciferous vegetables for maximum benefit
How much sulforaphane you absorb from your food depends on a number of factors. Sulforaphane is released only when the vegetable is chopped or chewed, in which case the glucosinolates mix with an enzyme in the vegetable called myrosinase, forming sulforaphane. Excessive cooking can cause glucosinolates to be lost in the water (studies show 18-59%) and deactivate the myrosinase enzyme. While cooking does reduce the concentrations of sulforaphane to some extent, eating cooked cruciferous vegetables is still better than not eating any at all. Prolonged storage, even at optimal refrigeration, reduces the amount of sulforaphane released as well.
Chewing raw cruciferous vegetables that have been stored for a short period of time is likely to provide the most sulforaphane. Even when the myrosinase enzyme is deactivated during cooking, bacteria in the intestinal tract still make the formation and absorption of sulforaphane possible.
The degree to which people benefit from an increased intake of cruciferous vegetables may be a matter of genetics. People inherit different capacities to metabolize and eliminate sulforaphane, though at this point, there’s no way to know who may benefit the most.
Besides possessing these potential cancer-protective properties, cruciferous vegetables are also high in fibre, low in calories and rich in nutrients, a good source of vitamins C, E and K, folate and minerals.
How Much to Eat?
The National Cancer Institute recommends consuming five to nine servings of fruits and vegetables each day. The 2010 Dietary Guidelines for Americans recommend that one-half of each plate at a meal consist of fruits and vegetables. But no official recommendations have been established specifically for the consumption of cruciferous vegetables.
People taking blood-thinning medications should watch their cruciferous vegetable intake, since the vegetable’s vitamin K content may interfere with the medication’s effectiveness. Those with hypothyroidism should also limit their intake. Juicing raw cruciferous vegetables is not recommended in such cases. Some experts recommend not exceeding half a cup of cooked cruciferous vegetables per day in patients with underactive thyroid. This is still a considerable amount for many people that can be safely consumed.
Tips for selection and preparation:
- Ensure green leaves are fresh, with no wilting or yellowing
- Wash with cool running water just before use
- Steam, microwave, stir-fry, or sauté to retain glucosinolates, folate, a vitamin C.
- Cook just until tender/crisp, with greens still bright to avoid bad odours
- Try roasting with olive oil, garlic, and your favourite herbs
- Enjoy raw with hummus. If the flavour is too strong to enjoy raw, steam or blanch briefly, cool in ice water, and serve cold.
- Add fresh to green salads