By Harriet Sugar Miller, for Zester Daily
July 24, 2014
Garlic, broccoli, green tea and turmeric: Health experts keep telling us to consume these foods to fight cancer.
But articles from a New York Times science journalist are challenging the view that diet can prevent cancer. The evidence on the influence of specific foods is weak, George Johnson wrote in a series of recent columns and in a book just released in paperback. Johnson reported that the results of studies connecting diet and cancer were conflicting and the numbers of preventable cases, small.
Ergo, diet doesn’t matter?
Diet certainly matters, says Stephanie Maxson, senior clinical dietitian at the University of Texas’ MD Anderson Cancer Center, the country’s leading cancer treatment hospital, according to U.S. News & World Report’s most recent rankings. Eating a plant-based diet, as opposed to a diet high in fat or animal protein, is important for preventing cancer as well as all chronic disease, she says.
Faced with these competing points of view, we must make many decisions each day about what to feed ourselves and our families. How do we evaluate the research when it’s not perfect? How do we make the best decisions for our health? Here are five starting points:
1. Why is the evidence conflicting?
The evidence is often conflicting because the science is complicated, says Maxson. “It’s true that the research findings regarding individual foods and nutrients are often inconclusive. But this is not because diet has no effect on cancer risk. It’s because the study of food and nutrients is very complex,” she says.
The nutrients contained in a head of broccoli, for example, will depend on the cultivar, where and how it was grown and how it’s prepared, she says. “How the nutrients in a food behave in the body depends on all the other foods it’s consumed with, the genetics of the person consuming it, and the microorganisms in the colon of the person consuming the food.”
Variables like these can explain why one study might find broccoli protective and another not, and as scientists unravel the nuances, they’re able to design better research. “We’re only beginning to understand the complexities of diet, nutrition and cancer relationships,” says Dr. Stephen Clinton, a researcher and physician who, along with a small group of scientists, is currently revising the U.S. government’s dietary guidelines.
2. Are there research-based guidelines on diet and cancer?
If you do a PubMed search, you’ll come up with more than 50,000 studies on diet and cancer. How are we to make sense of them?
The American Institute of Cancer Research and its partner, the World Cancer Research Fund (AICR⁄WCRF), can serve as guides. These two nonprofit organizations study the studies.
Since the 1990s, a panel of AICR⁄WCRF experts — researchers and physicians from around the world– have been reviewing the evidence and producing periodic reports as well as broad dietary guidelines, which are used by hundreds of cancer treatment centers, including MD Anderson. “AICR⁄WCRF is the world’s preeminent organization working to define evidence-based recommendations regarding how diet and nutrition impacts cancer risk and survivorship,” says Clinton.
While the recommendations are for preventing cancer because that’s where most of the research is focused, cancer survivors, they say, should also follow these guidelines once treatment is completed. The suggestions include:
- Be lean. As the columnist points out, obesity increases the risk of many common cancers; body fat produces hormones that drive cancer growth.
- Consume plant foods primarily.
- Eat a variety of them.
Despite the inherent flaws in scientific research, “the evidence that whole diets involving a wide variety of plant foods provides real protection remains as strong as ever,” said AICR’s director of research in response to the columns.
3. What about specific foods and nutrients?
As part of its review process, AICR⁄WCRF judges the strength of the evidence on various foods and nutrients.
AICR⁄WCRF reports that there’s “probable” evidence that foods containing lycopene (tomato sauce, for example) or selenium (Brazil nuts, broccoli, garlic) decrease risk of prostate cancer and that diets high in calcium increase risk.
There’s “convincing” evidence that foods with fiber decrease risk of colorectal cancer and that red and processed meat increase it, they say.
Most often, however, the organizations deem the evidence “limited” or “suggestive,” meaning that more study is needed.
We know of many food constituents that have anti-cancer properties,” says Dr. Steven Zeisel, director of the University of North Carolina’s Nutrition Research Institute and a member of AICR⁄WCRF’s expert panel that reviews all the studies. Garlic, broccoli, green tea and turmeric, for example, have been shown to fight cancer through extensive good research, he says. “But we do not know precisely which mixture of these constituents works best.”
Guidelines thus focus on patterns that decrease risk, such as plant-based diets, rather than individual nutrients or foods, he says. “What you eat certainly matters.”
4. How much cancer may be preventable?
According to AICR⁄WCRF estimates, approximately one-third of the most common cancers in the U.S. are preventable by a healthy diet and weight along with physical activity. For some types, the estimates for preventability are particularly high: colorectal, 50%; endometrial and esophageal, almost 60% and 70%, respectively.
In the U.S. alone, that’s nearly 375,000 cases of preventable cancer each year, said AICR’s director of research in a letter to the New York Times.
You can’t just look at each food individually and calculate risk, says its director of communications. “You have to look at the total diet and how all the foods you choose to eat and avoid together impact your cancer risk. You owe it to yourself to play the odds.”
5. What might your plate look like?
Playing the odds according to AICR‘s New American Plate design is fairly straightforward:
Fill at least two-thirds of your plate with plant foods, at most one-third with animal foods. Control portion sizes. Limit red meat to no more than 18 ounces weekly. As for dairy products, however, the evidence is still uncertain.
Meanwhile, as we continue the inevitably long wait for sufficient research-based evidence to warrant public policy on cancer and diet, I’ll be sticking to healthy plant-heavy patterns, lunching on this Mediterranean salad and not worrying about an occasional dollop of tzatziki. A walloping portion? Now that’s another story.
Zester Daily contributor Harriet Sugar Miller has been an independent health journalist and cancer survivor for two decades. She blogs about the nutrition-cancer connection at www.eatandbeatcancer.com and is writing a book, with practical guidelines and easy recipes.