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March 4, 2024 | 10 minute read

Top Questions from Dietitians about Diet and Cancer

Key Takeaways

  • A recent well-attended webinar for dietitians covered lots of information about nutrition and cancer.
  • This blog covers the questions from dietitians that we didn’t have time to answer, or that came to our inbox after the presentation.
  • Read on to learn more about cancer and the relationship to sugar, artificial sweeteners, vitamin D, dairy, supplements and more.

The role of diet in reducing cancer risk and supporting cancer survivorship is complex. Dietitians recognize the importance of AICR resources to pull together the best evidence about the most effective strategies. However, some of the advice can benefit from additional clarification.

Along with Nigel Brockton, PhD, AICR’s Vice President of Research, I presented a webinar for dietitians called “Diet & Lifestyle for Cancer Prevention and Survivorship – Evidence over Opinion” hosted by Oldways Preservation Trust. We answered as many questions as we could, and this blog covers the additional questions that were submitted. Here are responses to the most-asked questions we received.

Sugar and Cancer

Does sugar “feed” cancer? Could high sugar consumption affect pathways that promote cancer development?

Research does not show a direct link between consumption of added sugars and cancer risk or survival.

  • Obesity is the key factor in studies that show Increased cancer risk associated with diets high in added sugars. When studies show a link statistically adjusted for body mass index (BMI), the association generally disappears.
  • Frequent consumption of sugar-sweetened beverages promotes weight gain and increases the risk of overweight and obesity. Excess body fat and weight gain increase the risk of at least 12 different types of cancer.
  • Increased levels of markers of inflammation and circulating insulin can occur with high consumption of sugar-sweetened beverages or added sugars, according to limited evidence from controlled trials. Some people may be especially sensitive to these influences. Although this could lay groundwork for cancer to develop, high consumption of added sugars can also be a marker for a diet that’s low in foods that promote health and reduce cancer risk.
  • Don’t fear fruit. AICR Recommendations advise including fruit as part of a plant-focused diet. As a valuable source of dietary fiber, antioxidant nutrients and protective phytocompounds, it’s no surprise that research shows potential benefits for reducing cancer risk. Check AICR’s Food Facts library to learn more.

Does the Recommendation to limit sugar-sweetened drinks refer only to soft drinks, or does it include all drinks with added sugar?

Studies that link consumption of sugar-sweetened beverages with weight gain and obesity ask study participants about drinks concentrated in added sugars.

  • Beverages account for more than half of added sugar consumption for a typical U.S. adult, and drinks concentrated in added sugars are by far the top sources. So strong evidence supports finding ways to reduce high consumption of all sugar-sweetened beverages.
  • Soft drinks (called soda or pop in various regions) are the number one—but not the only—type of sugar-sweetened beverage. Fruity drinks, energy drinks and sports drinks count, too. So do sweetened teas and coffee beverages flavored with many pumps of syrup. A 12-ounce sweetened coffee drink contains six to 10 teaspoons of sugar.
  • Coffee or tea sweetened with just a teaspoon of sugar is different than a sugar-sweetened beverage with 10 or more teaspoons of sugar. One teaspoon is a reasonable amount of sugar to have in a beverage a few times a day in an overall diet that limits sweet treats.

Does AICR recommend non-nutritive sweeteners?

AICR Recommendations encourage drinking mostly water and unsweetened drinks and limiting processed foods high in fat, starches or sugars.

  • Moderate consumption of beverages sweetened with low- and no-calorie sweeteners is not linked with increased cancer risk. Confusion often stems from headlines that don’t differentiate between reports that identify a sweetener as something with the potential to increase cancer risk from reports that likely amounts consumed actually pose cancer risk. As discussed in this blog about aspartame, amounts matter, and people are extremely unlikely to exceed margins of safety.
  • Decisions about low- and no-calorie sweeteners should be based on whether they help each individual maintain a healthy weight or manage other health conditions. Observational studies in which these sweeteners are associated with greater incidence of obesity, metabolic syndrome and type 2 diabetes can reflect “reverse causation.” People at risk are more likely to choose such products. Controlled intervention trials show that for people who often drink sugar-sweetened beverages, replacing them with drinks using other sweeteners may produce a small weight loss and may be helpful for people with or at risk for type 2 diabetes, at least for moderate-term use.
  • More research is needed to clarify long-term risks and benefits. We need well-conducted studies to better understand how different low- and no-calorie sweeteners affect the gut microbiome and metabolic pathways important for health. Meanwhile, evidence is clear that high consumption of added sugars is harmful to health.

Confusion about Supplements

Why shouldn’t you take nutritional supplements to help prevent cancer?

Contrary to messages you commonly see in the media, human research doesn’t support dietary supplements as an effective strategy to reduce cancer risk.

  • Misconceptions about cancer risk: Supplements can seem like the solution if you feel that the daily targets for nutrient-rich foods are unrealistic. But studies show the biggest reduction in risk is between those who eat very low amounts of recommended foods and those who eat moderate amounts.
  • False perception of “insurance”: Getting nutrients from food sources provides a broad defense against cancer development. You get not just a few specific nutrients, but a wide range of protective phytocompounds, dietary fiber and more.
  • Misunderstanding of gaps: Dietary supplements can be valuable to fill a specific nutrient gap, helping to avoid or manage some health conditions. There’s no single make-or-break food for reducing cancer risk. Focus first on foods you can enjoy as part of an overall cancer-protective dietary pattern.

During cancer treatment, some people may be unable to consume what they need. Why the emphasis on no supplements?

AICR’s Recommendation is to not rely on supplements for cancer prevention. It’s not a recommendation to avoid supplements altogether. There are specific contexts in which supplements are appropriate, but their use for cancer prevention is not supported by the available evidence.

  • During cancer treatment or with other health conditions: If people have increased nutrient needs, limited ability to eat enough of the foods they need or trouble absorbing nutrients from foods, supplements can be a valuable tool to meet needs.
  • Starting with food is a smart strategy. When people have limited food consumption, they may fall short on needed calories, protein, fiber and a range of protective nutrients and plant compounds. Sometimes nutrient gaps can be filled by tweaking food choices. But if they can’t, it’s valuable to use supplements to fill specific gaps.

What about the role of vitamin D?

Vitamin D supports bone health and immune function, helps avoid chronic inflammation and may reduce cancer development and progression. While low blood levels of vitamin D are a concern, more is not always better.

  • Vitamin D consumption may lower risk of colorectal cancer, according to the AICR Third Expert Report. But evidence was rated as too limited to support a recommendation about it. While laboratory studies show several ways that vitamin D could be protective, observational studies and clinical trials provide mixed evidence.
  • In breast cancer survivors, inadequate or deficient blood levels of vitamin D may increase risk of poor outcomes. But the AICR CUP Global Report does not show further decrease in risk as blood levels continue to rise.
  • If blood levels of vitamin D are an indicator of cancer risk, any benefit of supplementation would be individual, depending on body production of the vitamin and consumption from food. Research on vitamin D and cancer is complex. Since we produce vitamin D in response to sun exposure, higher blood levels may reflect more time outdoors in physical activity—part of a lifestyle that’s strongly linked to lower cancer risk. And obesity’s link to increased cancer risk adds more questions. Vitamin D is stored in body fat, and people with obesity tend to have lower blood levels. It’s not clear whether people at increased cancer risk due to obesity would benefit from increased vitamin D consumption.

Clarifying the AICR Recommendations

Does the link between dietary fiber and lower risk of colorectal cancer refer only to fiber naturally occurring in foods? What about foods with added fiber and fiber supplements?

All types and sources of fiber may help lower the risk of colorectal cancer, according to analyses in the AICR Third Expert Report. However, recommended levels of dietary fiber should be achieved in the context of an overall dietary pattern consistent with the Recommendations.

  • AICR Recommendations call for a dietary pattern that supplies at least 30 grams of dietary fiber per day. That level can be reached by giving whole grains, vegetables, fruits and pulses (dried beans and peas) the largest portion of the plate. This dietary pattern supplies dietary fiber, antioxidant nutrients and protective plant compounds that protect against cancer.
  • Human studies of isolated fiber demonstrate benefits for a more diverse gut microbiome or slower rise in blood sugar after eating, which could help reduce unhealthy elevations in insulin levels. These isolated fibers could be added to foods or consumed as supplements.
  • Foods with added fiber or fiber supplements should not be seen as an equal replacement for foods in a healthy dietary pattern, but they can be incorporated within a healthy diet to meet individual needs. But don’t let a “health halo” from high fiber content overshadow concerns about high calories, added sugars or unhealthy fats.

What about dairy and cancer risk? Does dairy have inflammatory effects and increase breast cancer risk?

Despite claims you may hear about dairy products acting through hormones or growth factors to increase cancer risk, overall research does not support claims that even moderate dairy consumption increases breast cancer risk. People may have legitimate reasons to avoid dairy, but research does not support doing so for cancer prevention.

  • Overall human research does not link dairy with increased risk of breast cancer. If anything, the AICR/WCRF Third Expert Report identifies limited evidence that diets high in calcium may be associated with reduced risk of pre- and post-menopausal breast cancer. Another analysis pooling results of studies following over a million women found adult dairy consumption was not associated with any increase in ER+ or ER- breast cancer.
  • Laboratory studies need to be interpreted with caution. For example, certain growth factors can trigger signaling pathways that enhance breast cancer cell growth. But normal blood levels vary widely, and small increases related to dairy or other protein consumption are not necessarily clinically meaningful. Likewise, human intervention trials show no effect, or sometimes a small decrease, in biomarkers of inflammation with consumption of dairy products.
  • Among breast cancer survivors, dairy product consumption shows no association with all-cause mortality or breast cancer mortality, according to the AICR/WCRF CUP Global Report.

Do AICR Recommendations apply to reducing risk of blood cancers as well as for solid tumors, like breast, colorectal and lung cancers?

It’s not clear how much the AICR Cancer Prevention Recommendations can help reduce risk of cancers such as leukemia and lymphoma. In large population studies, as people’s lifestyle habits align more closely with the Recommendations, their overall cancer risk decreases.

  • Some biologic mechanisms driving cancer development seem to be similar across types. Following the AICR Recommendations can support antioxidant defenses, reduce chronic inflammation and avoid elevated insulin levels.
  • Causes of hematologic cancers such as leukemia, lymphoma and myeloma are not well understood. Studies don’t provide a consistent answer on the role of lifestyle factors such as diet, alcohol and physical activity.
  • Emerging research suggests that obesity may increase risk of blood cancers. The AICR Recommendations provide strategies that support reaching and maintaining a healthy weight.

With over 1,000 webinar attendees and countless more viewing the recording, it’s clear that today’s abundance of research and conflicting stories based on individual studies make it challenging to keep up. If you’d like to view the Oldways webinar recording, it’s available here.

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