No dessert until you finish your vegetables! Health experts probably wish they could use that line on the recalcitrant American public.
Fruits and vegetables are packed with vitamins, minerals, antioxidants, fiber and scores of phytochemicals that scientists are just beginning to understand, and studies have shown that people who eat more fruits and vegetables have a lower risk of heart disease, stroke, diabetes -- and some kinds of cancer.
Since its inception in 1991, the 5 A Day campaign, led by the National Cancer Institute and Produce for Better Health Foundation, has upped its daily recommendation to as many as 13 servings under a new campaign name.
And in bestselling health books and the popular press, the talk of fruits and vegetables is sometimes breathless. Pomegranate juice is a "miracle medicine"! Blueberries are "the super berry"! Kale can keep you alive! Tomatoes for life everlasting!
Eat or drink this produce, we are told, and the powerful clout of super-antioxidants and tumor-fighting chemicals they contain will bash that cancer before it gets going.
In fact, the anti-cancer clout of fruits and vegetables is nuanced and complex, and a story still evolving in labs across the country. At times the science has proven to be murky. Small studies that rely on what people remember of their diets from years past often find a strong preventive effect of eating lots of fresh produce.
But recently, some large population studies -- which follow tens of thousands or hundreds of thousands of people for years -- have reported weak or nonexistent connections between produce and cancer. Scientists are still trying to understand these seeming contradictions.
This much seems to be true: When it comes to fighting cancer, the power of produce will depend on who -- genetically -- you are, the variety of vegetables or fruits you select and even whether you cook them or not.
You also have to eat the whole thing. Sorry, no shortcuts with pills containing lycopene, beta carotene or vitamin C.
Not all fruit and vegetables, it seems, are equal when it comes to their cancer-fighting capacity. Some of the strongest evidence is for cruciferous vegetables, such as broccoli, cauliflower and kale. Dark, leafy greens such as spinach and chard, cooked tomatoes, apples, pears and citrus all show cancer-protective effects in some large studies.
And preliminary clinical trials are finding that various berries -- black raspberries, blueberries and even pomegranates -- may be cancer-fighters, shrinking tumors and even staving off cancer recurrence in some patients.
Much of the research on fruits, vegetables and cancer is founded on the "antioxidant hypothesis." Antioxidants, including vitamins A, C and E, and carotenoid pigments including lycopene (the red in tomatoes) and beta carotene (the orange in carrots), act to mop up or neutralize free radicals that can cause DNA damage. Accumulation of such damage can cause normal cells to turn cancerous. So foods high in antioxidants should -- in theory -- help prevent cancer from initiating.
Handfuls of studies involving a few hundred people tested this idea in the 1980s and early 1990s by comparing the diet history of subjects diagnosed with cancer to those of similar people without cancer.
The consistency of these studies led the World Cancer Research Fund, a nonprofit based in the United Kingdom that funds and promotes research on cancer and lifestyle, to conclude in a massive 1997 report that there was "convincing or probable" evidence that fruits and vegetables have a preventive effect against cancers of the mouth, pharynx, esophagus, lung, stomach, colon, rectum, larynx pancreas, breast and bladder.
The report even suggested that 20% of cancers worldwide could be prevented if everyone just ate 14 to 28 ounces of varied fruits and vegetables every day.
The catch, however, is that this type of research, which relies on people remembering their diet from many years past, is open to bias. People diagnosed with disease are especially likely to misjudge what they used to eat, partly because they're looking for a reason for their illness.
What's more, recent larger and better-designed studies have appeared to be washouts.
In 2004, for example, researchers at the Harvard School of Public Health found that among a total of more than 109,000 men and women tracked from 1984 to 1998 in two long-term studies, there were zero cancer protective effects of eating five-a-day.
And another large survey of more than 521,000 people from 10 countries in Europe found no relationship between overall fruit and vegetable consumption and lymphoma, breast, prostate cancer or gastric cancer.
Unlike the previous studies, which were open to the bias of a tainted memory, such long-term studies enroll and track populations before they get sick, giving a better picture of the relationship between diet and health.