November 8, 2023 | 7 minute read

Does diet after breast cancer make a difference? It’s a hot topic and trying to keep up with new research can feel overwhelming. If your solution is to save time by scanning headlines, you may get steered off track. Headlines about a recent study provide an example of just such a situation.

A recently published study tested whether breast cancer recurrence rates could be reduced by improving lifestyle behaviors. Unfortunately, in the main analysis, the rates of recurrence were not significantly different for women assigned to the five-year diet and physical activity intervention compared to the women who were simply provided with an educational brochure about healthy lifestyle choices.

The headline in newsletter summaries of the study proclaimed, “Research: Diet not linked to reduced cancer recurrence.” So, if you only read the headline, you’d miss the important secondary finding and the possible explanation for why the primary analysis failed to show a benefit.

The aha moment for people who read the whole study: Regardless of group assignment, women who made the most improvements in diet had lower breast cancer recurrence than those who made the least improvement in diet.

Let’s dig further into the study results for key take-home points on diet and lifestyle after –cancer—and equally important points on the challenges of this type of research and over-simplified media stories about cancer survivorship.

Breast Cancer Research: Findings from the Study

The DIANA-5 study, conducted in Italy, recruited women who were diagnosed with Stage I to III breast cancer within the last five years and had completed surgery and chemotherapy. All women were at increased risk of recurrence due to:

• The subtype of tumor (estrogen receptor-negative) or
• Abnormal blood levels of hormones (testosterone or insulin) or
• Presence of metabolic syndrome

These women were randomly assigned to one of two groups. One group was enrolled in an intervention aimed at increasing physical activity, avoiding alcohol, eating a healthy plant-focused diet and managing weight. The other group was a control group. The researchers considered it unethical to not inform the control group about the potential role of lifestyle. Women in the control group received one presentation and a brochure about the AICR Cancer Prevention Recommendations.

The women in the intervention group received:

• Dietary recommendations and the AICR Cancer Prevention Recommendations, including emphasis on whole grains, vegetables, fruits and legumes; and limiting sugar-sweetened beverages, sweets, refined grains and red and processed meats.
• Cooking classes and community meals, once a month during the first year, and every few months over the next four years.
• Monthly physical activity sessions to encourage women to participate in 30 minutes of activity each week. Note: the sessions were canceled due to poor participation, so physical activity was simply encouraged by providing a pedometer and encouraging activity-tracking.

After five years, the number of breast cancer events—which included recurrence, metastasis or breast cancer death—was essentially the same in women assigned to the intervention group as in the control group. This is why the headline declared that diet was ineffective at reducing breast cancer recurrence. However, a closer look reveals the likely reasons why recurrence rates did not differ between the groups: diet in the two groups did not differ very much.

Changes in Diet Make a Difference

Demonstrating the impact of a dietary intervention requires that diet in the intervention group differs from the control group. In the DIANA-5 trial, women in the control group also made some improvements in their diets. That’s good for them, but bad for being able to detect a difference between the groups.

Researchers in this study created scores based on the women’s reports of their diets and gave them a dietary index score (made by evaluating their intake of recommended foods versus discouraged foods). After the first year, the average total dietary index score improved by 4.5 points in the intervention group. This was a bigger improvement, on average, than in the control group.

Individuals varied in the number of changes they made. Among women in the study, those in the top third of improvement raised their scores more than 6.8 points. Women in the bottom third raised their scores by no more than 2 points—and some slightly reduced their dietary index score.

What if women actually improved their diet? When all women in the study were considered –together—regardless of their group assignment—the women who improved their dietary score most were 41 percent less likely to have a breast cancer recurrence than women who made the least improvement. Diet matters, and even small dietary improvements (not perfection) are associated with better outcomes.

Motivation to Make Dietary Changes

People vary in their response to nutrition programs and materials. Some people start with a bit of information, learn more as needed and make changes. Others may be challenged by emotions, physical health, lack of cooking and meal planning skills or other family/social influences that make change difficult.

If the goal is to make healthy eating habits achievable for as many people as possible, we need to find out how we can help each individual take a few steps forward. In previous versions of DIANA trials, the intervention group received more frequent cooking classes and group meals to taste new foods. Such programs are expensive. So for cost reasons, their use was decreased in this DIANA-5 trial. But for people who find new eating behaviors difficult, finding ways to provide more intensive support in an affordable way may be crucial to achieving new habits.

Implications as You Read About Studies Like This

The job of a headline is to catch your attention. You might assume that scanning headlines can help you keep up-to-date on the latest research. But as you can see from the example here, the headline only captured part of the findings.

As you read media accounts about research on diet and cancer, here are three tips to keep in mind:

1. Results of intervention trials reflect both the effectiveness of the recommended strategies and the effectiveness of the intervention. If an intervention doesn’t produce enough change, or isn’t set up to adequately measure change, don’t make assumptions about the effectiveness of the strategies themselves.
2. Although physical activity was encouraged as part of the intervention in this trial, the current report does not provide information about changes in physical activity. Other studies suggest that the best results come from both nutrition and physical activity.
3. Consider results of a single study in the context of overall research. For example, in the LEANer study, conducted among women during chemotherapy, an intervention that included frequent sessions with registered dietitian nutritionists to encourage healthy eating and physical activity more than doubled the rate of pathological response to treatment. And some observational studies link closer adherence to the AICR Recommendations with better outcomes after cancer.

With the pace of research today, it’s hard for anyone to keep up with it, much less separate preliminary findings from evidence strong enough to support choices about a healthy lifestyle. Rather than getting frustrated—and even misled—by only reading the headline, turn to trusted sources like AICR to keep you up-to-date.