Part of an occasional series on new health information.
Broccoli. Garlic. Tomatoes.
Several diet pyramids encouraging consumption of fruits and vegetables have been discussed in the media lately. Certain foods have received particular attention, including the three listed above.
Health benefits from crucifers (includes broccoli, cauliflower, cabbage, Brussels sprouts, kale, chard, mustard, radish and others), allium vegetables (includes garlic, onions, scallions, leeks, chives and shallots) and tomato products appear in food columns, magazines, broadcast media, books and advertisements. These possible benefits may be real. However, as I discussed in an earlier column, most of what you hear and see about prevention through nutrition is based on a weak type of evidence that cannot actually prove that benefits exist.
Readers familiar with this column know that one recurrent theme is “it isn’t so simple.” This turns out to be very true about vegetables — in fact, about plant foods altogether. Plant are chock full of “phytochemicals” which simply means plant chemicals. Plants can be thought of as complex packages of bioactive chemicals. Many medicinal drugs are phytochemicals or have origins in phytochemicals.
In matters like stability, absorption, metabolism, potential benefits or adverse effects, plant chemicals may be as complex as drugs.
Recently I attended some meetings where the handling of foods like garlic and broccoli was discussed. I was intrigued by indications that storage, preparation and cooking make a difference in what is present in the food. Chopping, crushing and heating may cause the gain or loss of some of the bioactive components thought to have important benefits.
To complicate matters further, there is evidence that how your broccoli is grown and what genetic variety it is affects what’s in it.
Cruciferous vegetables contain a number of compounds thought to have important biological properties. Broccoli is particularly well known as a good source of sulforaphane. However, broccoli doesn’t contain sulforaphane, rather it has a precursor compound that can be converted to sulforaphane when broccoli is chopped or crushed. Depending on the conditions, a different reaction can occur which converts the precursor to another compound, which is 10,000 times LESS potent than suforaphane. The interaction of preparation and heating affects these conversions so what you end up with can depend on how it’s prepared. Furthermore, there are other potentially important compounds in broccoli that may be affected differently.
What about garlic? Garlic has had a reputation as a potent medicinal agent in many cultures for thousands of years. Studies have demonstrated a variety of effects including antibacterial, antifungal, anticancer, anti-clotting, anti-oxidant and pro-oxidant.
The chemistry of garlic is complex; the amounts of active constituents in raw garlic vary considerably and how you prepare and cook it affects what you end up ingesting. The composition of garlic preparations is another matter altogether.
Furthermore, serious toxic effects of garlic have been observed; it may not be safe at all doses. There is evidence that some actions of garlic overlap with actions of aspirin, so it’s possible that ingesting both can be additive and problematic.
The availability of lycopenes from tomatoes seems to be enhanced by mechanical processing and by heating. The outcome may vary among juice, sauce, ketchup, puree and other tomato products.
You have probably seen or heard stories about wine, or other alcoholic beverages, being associated with a reduced risk for heart disease and stroke. A similar association has been suggested for tea. Both wine and tea are rich in polyphenols, particularly flavonoids, thought to be important beneficial compounds in fruits and vegetables, including grapes, citrus fruits, apples, pears, onions, garlic, broccoli and even cocoa. Particular plant chemicals, then, may be found in multiple food sources. It may not be necessary to focus on one food or beverage to get a desired benefit.
The health effects of food components are immensely complicated. Delving into the recent scientific literature on a particular topic leaves my head swimming with the complexity, contradictions and unknowns. It seems that many, many factors are involved in the final effects of our eating habits, including our genetics, what we ingest, processing and preparation of foodstuff, the combinations of foods, our state of health and so on.
It’s just not so simple; don’t expect simple answers. Try venturing past broccoli.
Mark H. Zweig, M.D., previously a staff physician at NIH, has more than 35 years of experience in medical research and 25 years in patient care.