The Cancer Research
Flaxseed’s potential influence on breast, prostate, and other hormone-related cancers has led to an interest in its role for the prevention and survivorship of these cancers. Results have shown mixed findings, and much more human research is needed.
Interpreting the data
After a systematic review of the global scientific literature, AICR/WCRF analyzed how foods and their nutrients affect the risk of developing cancer.
“Convincing” or “probable” evidence means there is strong research showing a causal relationship to cancer—either decreasing or increasing the risk. The research must include quality human studies that meet specific criteria and biological explanations for the findings.
A convincing or probable judgement is strong enough to justify recommendations.
- There is probable evidence that foods containing dietary fiber DECREASE the risk of:
- Colorectal cancer
“Limited suggestive” evidence means results are generally consistent in overall conclusions, but it’s rarely strong enough to justify recommendations to reduce risk of cancer.
Ongoing Areas of Investigation
- Laboratory Research
Interest in flaxseed and cancer prevention often focuses on its ALA (alpha-linolenic acid) and lignans, although evidence is clearest regarding its role as a source of dietary fiber.
Alpha-linolenic acid (ALA) is a form of omega-3 fat that our bodies convert into another omega-3 fat, called EPA (eicosapentaenoic acid). EPA is a source of protective, anti-inflammatory compounds. Only a small proportion of ALA is converted into EPA, yet moderate amounts of flaxseed in limited animal and human studies have been shown to increase EPA levels.
Lignans increase antioxidant, anti-inflammatory and carcinogen-deactivating enzymes in cell studies. They also decrease growth and increase self-destruction of cancer cells. In studies with mice, lignans and flaxseed decrease cancer development and growth.
- Lignans are sometimes categorized as phytoestrogens because they have a chemical structure similar to estrogen. Earlier, this led to fears that lignans could promote cancers fueled by excess estrogen, such as estrogen receptor-positive (ER+) breast cancer. There was also concern that lignans could interfere with tamoxifen and certain other anti-estrogen cancer treatments. But in studies with mice, flaxseed and lignans isolated from flaxseed reduce development and growth of both ER+ and estrogen-negative (ER-) breast cancer. They lower levels of several growth factors that promote breast cancer and increase the expression of tumor suppressor genes.
- Limited animal studies have found that flaxseed and flaxseed oil do not interfere with actions of tamoxifen or trastuzumab (medications used for breast cancer treatment) and may even enhance their effectiveness. Most of the animal studies have focused on breast cancer. The relatively few animal studies addressing other cancers have shown some decrease in markers of inflammation and a decrease in number and size of colon cancer tumors, and in growth and spread of prostate cancer.
Phenolic acids increase cells’ antioxidant and anti-inflammatory defenses against damage that could lead to cancer in cell and animal studies. Emerging evidence in animal studies suggests they may also improve glucose metabolism, decrease insulin resistance, and alter the gut microbiota (microbes living in the colon), creating an environment in the body less likely to support cancer.
Gamma-tocopherol is one of eight compounds that make up vitamin E. Earlier research focused on the related alpha-tocopherol compound, the form listed on nutrition labels and the only form currently recognized to meet human requirements. Yet cell and animal studies suggest that gamma-tocopherol may provide even stronger anti-inflammatory protection than alpha-tocopherol. In both cell and animal studies, gamma-tocopherol decreases cancer cell growth.
- Human Studies
Overall, research in flaxseed and cancer prevention with humans show mixed results. The studies are relatively small and short term. In one month-long trial of about 30 post-menopausal women with newly diagnosed breast cancer, daily flaxseed decreased signs of cancer cell growth. In several studies of healthy women consuming flaxseed daily, estrogen levels decreased or estrogen shifted to more of a relatively inactive form. The result was less estrogen in the form that seems to promote breast cancer growth. However, studies show unexplained variability, which may partly reflect effects of individual differences in hormones, overall diet and genetics.
Population studies are not clear about the impact of ALA on prostate cancer risk, but most recent overall analyses show no significant effect. In a study of men with prostate cancer who ate flaxseed, followed a low-fat diet or both for about 30 days before surgery, each of these strategies reduced cancer cell growth compared to a control group, although the combination strategy produced best results.
Dietary Fiber: AICR/WCRF found strong evidence, including population studies, linking high dietary fiber consumption with reduced risk of colorectal cancer.
Analysis for the AICR/WCRF Third Expert Report found the evidence too limited to support a conclusion for an association of dietary fiber to several other cancers. Other analyses combining 16 to 20 prospective cohort studies have linked dietary fiber with lower risk of breast cancer.
- Viscous fiber is a type of soluble dietary fiber that forms a gel in the digestive tract, slowing digestion and thus, the rise in blood sugar after eating. Having healthier blood sugar and insulin levels could potentially help reduce cancer risk. Viscous fiber’s actions in the gut may also contribute to reducing circulating estrogen levels. Such effects could help explain why some studies link greater soluble dietary fiber to lower breast cancer risk.
- AICR/WCRF found strong evidence that foods containing dietary fiber reduce the risk of weight gain, overweight and obesity. Some studies suggest that viscous fiber like that in flaxseed may contribute to this effect by increasing a feeling of fullness. By supporting a healthy weight, foods rich in fiber indirectly contribute to a lower risk of the 12 or more cancers related to excess body fat.
- AICR-Supported Studies
- Tips for Selection, Storage and Preparation
- Whole flaxseed provides plenty of fiber, but our bodies can’t digest it enough to access its other healthful components. Ground flaxseed offers more potential health benefits, but it doesn’t stay fresh as long as whole flaxseed.
- Buy whole flaxseed to grind in a coffee or spice grinder.
- If ground flaxseed or flaxseed meal is more convenient, buy either type, refrigerated or in a vacuum-sealed package.
- Flaxseed oil is quite perishable. Buy it in refrigerated opaque bottles.
- Store whole flaxseed up to a year in an airtight container in a dry, cool cabinet.
- Store ground flaxseed in an airtight container in the refrigerator up to 30 days or in freezer up to four months.
- Add ground flaxseed to hot or cold cereal, yogurt or smoothies.
- Sprinkle ground flaxseed on salads or on top of cooked vegetables for a nutty flavor.
- Include ground flaxseed in baked muffins or other quick breads.
- Flaxseed may decrease absorption of medications, so take it one hour before or two hours after any prescription or non-prescription medicine. Talk to your physician or healthcare provider about use if taking fish oil, omega-three supplements or anticoagulant medications.
- Drizzle flaxseed oil on salads or vegetables or in smoothies. Do not cook with flaxseed oil.
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