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The Unclear Relationship Between Diet and Cancer

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While most of us are stocking our kitchens with milk, bread, and the usual pantry staples, the two items that Carol Martin of Alexandria, Virginia, simply cannot be without are blueberries and pomegranate juice. Since being diagnosed with carcinoid cancer of the lung in 2004, Carol has faithfully followed a healthy diet in hopes that it may help halt the spread of cancer.

“My gut feeling is that the pomegranate juice or the blueberries, they’re helping somewhat,” says Carol. “Maybe not a lot, but psychologically, they’re some help.”

She is not alone in her mission. In a new study out of the University of Oklahoma Health Sciences Center, women diagnosed with breast cancer frequently altered their diet to include less fat and alcohol and more fruits, vegetables, and whole grains.[1] Despite this only one-third of the women in the study reported receiving nutritional information from their doctor, with more than half reporting that they would like more nutrition-related information.

But Does Diet Really Make a Difference?

The American Cancer Society (ACS) is among the organizations leading the way to ensure that people are informed about the role nutrition plays in cancer prevention. In 2006 its advisory panel on nutrition, exercise, and cancer survivorship updated its dietary recommendations, which mirror those of other well-known institutions, such as the National Cancer Institute (NCI), American Heart Association, and American Diabetes Association.[2] In general, the ACS recommends a diet that centers on plant-based foods, emphasizes whole grains, and limits red meat. More specifically, it calls for the following:

Protein from predominantly lean sources, such as beans and skinless fish and poultry. Red meat should be eaten only sparingly, if at all.

Carbohydrates in the form of fruits, vegetables, and whole grains.

High-fiber foods, including oats, brown rice, and whole-grain breads and pasta, rather than processed foods and refined starches like pastries and sweets.

Five or more servings of fruits and vegetables per day. This might seem like a lot, but keep in mind that these can easily be spread out across meals and snacks. Also, the actual size of a “serving” might be smaller than you think (see sidebar “Size Really Does Matter!”).

Limited servings of alcohol (12 ounces of beer, 5 ounces of wine, or 1.5 ounces of distilled spirits per day).

Making Sense of the Science

There is no lack of data relating diet to cancer. Studies suggesting that eating a well-balanced diet is related to better overall health are easy to find.[3] Other research linking diet and cancer has shown relationships between an increased risk of cancer and an increased intake of alcohol (breast, liver, and mouth/esophageal cancers), soy (breast and endometrial cancers), and processed meats (colorectal and stomach cancers) as well as a decreased intake of fruits and vegetables (lung, colorectal, stomach, and mouth/esophageal cancers).[4] The most recent scientific research, however, does not conclusively identify diet as a direct cause of or prevention for cancer.

Recent Studies

A 2007 study that reanalyzed data from several previous studies found no causal relationship between diet and breast cancer among pre- and postmenopausal women.[5]

A recent study that looked at carbohydrates specifically also failed to find a link to breast cancer.[6] Although there is some data that eating fruits and vegetables may slightly decrease the risk of colon cancer, the evidence connecting fiber to colon cancer is not strong.[7]

Harvard University’s Nurses Health Study,[8] which examined the eating habits of more than 100,000 women, indicated that a relationship may exist between colon cancer and red meat, but the authors found no relationship between colon cancer and fiber.

The role of fat in particular has been heavily scrutinized, and, again, the results are unclear. The Women’s Health Initiative dietary modification trial attracted a large amount of attention in 2006 when its results failed to find a link between fat intake and breast and colorectal cancers. In the study nearly 49,000 postmenopausal women age 50 to 79 were randomly assigned to either a low-fat diet group or a control group that received nutrition-related education. After eight years there was no statistical difference among the two groups in terms of cancer diagnoses.[9][10] It is important to note, however, that the women in the low-fat group did not reduce their fat to the goal level of 20 percent; in fact, most lowered their intake to only about 30 percent. Other critics have argued that the participants’ age may have been a factor and that the study failed to differentiate between specific types of fat consumed. The Nurses Health Study also initially failed to find a relationship between overall fat intake and breast cancer and has found no association between types of fat and breast cancer in postmenopausal women.[11] When the researchers examined types of fat consumed by premenopausal women, however, they found that animal fat was associated with an increased risk of breast cancer.[12]

So why doesn’t the science solidly support a relationship between food and cancer? Part of the problem is that there are more than 100 different forms of cancer. Isolating diet to cancer in the same way researchers can to more consistent diseases, such as heart disease, is difficult. Also, the way in which a research study is conducted can affect the results. For example, some studies ask women to discuss food they ate weeks or months or even years earlier, which may produce inaccuracies. Other studies fail to follow women’s diets over long periods of time. The size of a study also makes a difference, as larger samples of participants (for example, several thousand as opposed to several hundred) can help researchers correctly determine whether their findings are true or are merely the result of chance.

Despite a lack of clear consensus among the data, organizations such as the ACS and the NCI continue to urge women to follow their guidelines. Wendy Demark-Wahnefried, PhD, RD, of the University of Texas M. D. Anderson Cancer Center, says women should not be alarmed by the apparent contradiction. “Why are those recommendations in there? They’re in there in large part because heart disease is the leading killer in the United States. If we make recommendations for cancer survivors, we know they’re also at risk for heart disease, so the recommendations might be cardio-protective,” she says. “For the lay person that’s out there, it’s very confusing.”

And Then There’s the “O” Word

In addition to differences in study design and difficulties with interpreting unclear results, there is a third factor lurking in the diet/cancer equation: obesity. “The most important aspect of nutrition in the cause of cancer is through overweight and obesity,” says Walter Willett, MD, DrPH, chair of the Department of Nutrition at Harvard School of Public Health and a leading researcher on the Nurses Health Study. “Excess fatness contributes to cancers of the colon, breast (after menopause), pancreas, kidney, endometrium, esophagus, and probably leukemia and lymphoma. There is also some evidence that overweight may increase the recurrence of breast cancer.”

A well-known example of how weight loss can muddy research results comes from the Women’s Intervention Nutrition Study.[13] This seven-year study of postmenopausal women has been praised for being the largest of its kind to show that reducing fat intake reduces risk of cancer. In the study women were told randomly to either reduce their fat to less than 15 percent of their total daily calories or to follow their normal diet. Among the women instructed to eat a low-fat diet, recurrence of cancer was reduced by 24 percent. Further, women who were estrogen receptor–negative had a 42 percent reduction on the low-fat diet. Though these findings seem to imply that fat restriction was the key factor, Dr. Demark-Wahnefried offers a warning about the low-fat group: “The kicker was that those women actually lost 6 pounds of weight, and that might be driving the results,” she says. “Diet probably plays a very big role [in cancer]. However, what seems to be popping out as probably the most important factor that you may be able to modify is weight status. It’s not necessarily about just eating fruits and vegetables or limiting fat but more about the impact that those factors have on weight.”

To Supplement or Not to Supplement?

If the jury is still out on overall diet, do we at least have a verdict on nutritional supplements? Unfortunately, the answer here is, again, no.

Although the percentage of cancer survivors who report taking some sort of supplement, including vitamins, herbs, and minerals, has been reported to be as high as 80 percent, there is little scientific evidence to show that supplements reduce risk of cancer.[14][15] In fact, in some cases—such as with beta-carotene and lung cancer—high doses of supplements may actually increase cancer risk.[14]

Even vitamin D, which has recently enjoyed a lot of news coverage related to cancer, is not such an easy call: a recent report in the American Journal of Clinical Nutrition notes that the effects of vitamin D are influenced by outside factors, such as type of cancer, overall diet, and lifestyle factors like exercise, which can have an impact on the metabolism of vitamin D.[16] Separating these out can make understanding its ultimate impact on cancer somewhat difficult.

“If you look at both sets of recommendations from the American Cancer Society and the World Cancer Research Fund, they are against the use of supplements until we learn more,” says Dr. Demark-Wahnefried. “Most of the supplement trials have been dismal failures.” Further, women can generally obtain the necessary amount of vitamins and minerals by following ACS dietary guidelines. Whether additional nutrients are needed should be discussed with your healthcare provider.

Messages from the Media

Television, radio, and newspaper reports often highlight findings from a particular study concerning diet and cancer, leaving the impression that women should focus on that specific food or supplement. Dr. Demark-Wahnefried says this isn’t necessarily the case. Also, rather than focus on one or two single foods, she notes that women should concentrate on their overall dietary intake. “You can’t distill everything into one magic food,” she says. “It’s more of a diet pattern. You’re not only at risk for cancer, but you’re at risk for other diseases.”

This is exactly the perspective that Tricia Beach of Clarksburg, Massachusetts, takes. Tricia was diagnosed with breast cancer in 1988 and has spent the past 20 years constructing her diet around fresh fruits and vegetables while avoiding processed and prepackaged foods. She says her dietary decisions are driven by her concerns about cholesterol as much as by her concerns about cancer recurrence. Tricia notes, however, that she generally relies on the media for information, and she has never received nutrition advice from any of her oncology physicians or nurses. “I’m surprised no one has ever talked to me about being a cancer survivor and nutrition and diet,” she says. “I guess maybe they figure, You’ve survived; why do we need to talk about nutrition? You’re living proof that what you’re doing is okay.”

Carol Martin also reports that she has never received specific advice from her healthcare providers about diet and that trying to piece together the information herself can be tiresome. “At times you get kind of saturated with information,” she states. “It would be nice to have a hotline where you can call in and say, ‘I have carcinoid cancer. Can you give me the latest info?’”

And the most accurate, latest information, Dr. Denmark-Wahnefried warns, is probably not coming from the media. These reports, she says, can be confusing because they are often specific to one type of cancer, and the recommendations from one study may not apply equally across all types of cancers. She also points out that when one particular study seems to attract the spotlight, it’s important to keep in mind that it’s only one study. “You need repeated findings,” she explains. “When you’re looking to make healthy lifestyle changes, you should go to sources where the experts have really weighed the evidence and made recommendations,” which, she states, include the World Cancer Research Fund, American Institute for Cancer Research, and American Cancer Society.

Future Directions

Clearly, scientists still have plenty of work facing them before we know conclusively how diet relates to cancer. In the meantime women are encouraged to maintain a diet consistent with ACS guidelines. What you eat likely does play a role in your cancer health—whether directly through food choices or indirectly through weight. More importantly, a well-balanced diet is one of the best ways you can make a difference in your overall health. And in terms of helping drive future research, this is precisely where women can help make a difference for one another. “The one thing I would encourage people to do is to participate in clinical trials,” says Dr. Demark-Wahnefried. “We do need a lot more evidence. This area of research is really in its infancy.”

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(Cancer Consultants, Oncology Resource Center, Article by Emily Kuhl, PhD, March 10, 2009)


The information contained in these articles may or may not be in agreement with my own opinions. They are not being posted with the intention of being medical advice of any kind.

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