H

erceptin, Gleevec, Keytruda and … cashews?

Surprising new research scheduled to be unveiled at a major cancer meeting next month suggests that cashews and other tree nuts might be as effective as some of oncology’s most effective treatments at keeping colon cancer from recurring after surgery — and even keeping patients from dying.

If the benefit is real, a daily handful or two of cashews (cost: less than $1) could work as well as standard chemotherapy (cost: thousands of dollars).

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But the reported benefit comes with a big “if.”

Although the study is from a respected clinical trial, this finding was not from the original research but, instead, an add-on. The original trial randomly assigned colon cancer patients to either of two drug treatments. Later, researchers looked at cancer survivors who simply went about their lives, doing as they pleased, and tried to evaluate whether eating nuts was associated with better outcomes.

In that kind of observational study, it’s not possible to say whether a given behavior — in this case, eating nuts — caused an outcome, or was instead simply associated with the true cause.

“This is the challenge with observational studies,” said Dr. Richard Schilsky, chief medical officer of the American Society of Clinical Oncology. “All you can say is that these patients [who ate nuts] had better outcomes. But is eating nuts a proxy for other things that have an effect, including generally healthy behaviors?”

Given that uncertainty, some doctors said they were worried about what message cancer patients might draw.

“The fear I have is that patients will say, ‘I’ll eat nuts instead of having chemo’” after surgery, said Dr. Manish Shah, director of gastrointestinal oncology at Weill Cornell Medicine and NewYork-Presbyterian Hospital, who was not involved in the study.

Even the study’s lead author was cautious. “This is the first study to show an association between nut consumption and cancer outcomes,” said Dr. Temidayo Fadelu of Dana-Farber Cancer Institute. “When you see an association that is more dramatic than you expect, you have to repeat [the study] in another [group of patients]. … There could be underlying confounders that we didn’t control for.”

In other words, people who eat tree nuts, such as pecans, almonds, and walnuts, might be different from people who do not, something called “healthy patient bias.” They “might be more health-conscious,” said Shah. They might be wealthier, or better connected to the health care system, or have healthier habits in general. Any of those attributes might help patients survive colon cancer.

The researchers did not find an association with eating peanuts. Only tree nuts seemed to matter, to an eye-popping degree.

Of 826 patients, those who reported having two or more servings per week (as 19 percent did) had a 46 percent lower risk of their cancer returning and a 53 percent lower risk of dying than those who said they did not eat tree nuts.

The study is an outgrowth of a famous clinical trial that began in 1999, testing chemotherapies in 1,264 patients with stage 3 colon cancer (meaning it had spread to lymph nodes but not to distant sites like the liver and lungs). Some patients filled out questionnaires about diet and lifestyle, once while receiving chemotherapy and again six months after treatment ended. Ever since, researchers have mined the data for associations — such as that smoking early in life is linked to a recurrence of colon cancer and that drinking lots of sugary beverages or being inactive is linked to a higher risk of recurrence and death.

Fadelu and his colleagues studied nut consumption because that has been associated with lower mortality, mostly because of a reduced risk of cardiovascular disease.

“We definitely think something is going on,” with tree nuts providing a biological benefit, Fadelu said. For one thing, they decrease insulin resistance, “a potential mechanism” by which they might keep colon cancer from recurring, he said.

On the other hand, some unknown characteristic of people who can afford to pop a handful of cashews rather than a candy bar might be the true driver of lower cancer recurrence and mortality, he acknowledged: “Nut intake might be part of additional [healthy] behaviors.”

Experts not associated with the study echoed that caveat and added others. For instance, what people remember and report about their diets is notoriously unreliable. It could be that those who are more health-conscious to begin with simply said they ate lots of tree nuts, while others didn’t, regardless of their true diets. Also, it’s not clear how any component of tree nuts might exert a strong anti-cancer effect. “There’s nothing I know that could be a clear causal mechanism,” ASCO’s Schilsky said.

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Still, the nut result fits with another study to be presented at ASCO, also mining the classic colon cancer trial.

It found that patients who follow most American Cancer Society guidelines for survivors — maintaining a healthy weight, being physically active, and eating a diet high in vegetables, fruits, and whole grains with minimal red or processed meat, as 9 percent did — had a 42 percent lower risk of dying over seven years and 31 percent lower risk of their cancer returning compared to people who did not.

Although it’s not clear what the magic bullet is, “this suggests that things patients do probably do make a difference,” Shah said.

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