The Cancer Research
All nuts support a health-promoting diet, but they differ in the nutrients and plant compounds they provide. Walnuts are unique in several ways and have been studied more extensively than other nuts regarding cancer prevention.
Emerging research shows potential for walnuts to contribute to a cancer-preventive diet through several compounds possibly working together. Ellagitannins, melatonin and gamma-tocopherol may each work through different paths to reduce oxidative stress, inflammation, and gene expression that can lead to cancer.
AICR’s Third Expert Report and Continuous Update Project (CUP) found evidence too limited to draw any conclusions about walnuts – or any nuts – and cancer risk.
Interpreting the data
After a systematic review of the global scientific literature, AICR/WCRF analyzed how vegetables and its nutrients affect the risk of developing cancer.
- Evidence categorized as “probable” 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 probable judgment is strong enough to justify recommendations.
- Evidence categorized as “limited suggestive” 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
In mice, several studies found that consuming walnuts decreased breast and colorectal tumor growth compared to the animals eating a standard diet. Limited studies in mice also show reduced growth of prostate cancer. Evidence from studies like these suggests that consuming walnuts can influence the expression of genes that regulate cell growth and the types of gut bacteria that potentially influence colon health.
Laboratory studies on compounds found in walnuts are much more abundant than studies on whole walnuts:
- In cell and animal studies, both Ellagitannins are broken down in the digestive system to ellagic acid and then to urolithins, which are the compounds that can be absorbed into the body. Ellagic acid and urolithins increase antioxidant enzymes, decreasing free radical damage to DNA that can lead to cancer. By influencing gene expression, they decrease growth and stimulate self-destruction of mouth, esophagus, breast, cervix, colon and prostate cancer cells.
- In some cell studies, urolithins inhibit both the aromatase enzyme that produces estrogen and the growth of estrogen-responsive breast cancer cells.
- Animal studies show decreased inflammation in the colon with ellagitannins and the compounds that form from them at levels that could be reached by people eating a healthful diet.
Emerging evidence in animal studies suggests that phenolic acids may also improve glucose metabolism and 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 tocopherol and tocotrienol compounds that make up vitamin E. Earlier research focused on alpha-tocopherol, 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.
Melatonin is a hormone people produce in response to darkness, and it’s also found in a few foods. Melatonin is best known for helping to regulate the body’s circadian rhythms. Decades of cell and animal studies show that melatonin also supports antioxidant defenses by scavenging free radicals that could damage DNA, as well as stimulating the body’s antioxidant enzymes and DNA repair. Melatonin has been especially studied for the ability to decrease growth of both estrogen-receptor-positive and -negative breast cancer in isolated cells and in animals. Such studies also show decreased growth of prostate and other cancers, with effects seen all across stages of cancer development.
- Human Studies
Human studies on walnuts and cancer risk compare groups of people who consume relatively high and low amounts of walnuts or all nuts combined.
The AICR/WCRF Continuous Update Project reports do not include analysis of nuts and cancer risk. However, a combined analysis of nine prospective cohort studies found 15% lower overall cancer risk associated with eating an ounce of nuts daily. Several specific types of cancer show trends for reduced risk with higher nut consumption (usually an ounce a day), but only links with colorectal and endometrial cancers were statistically significant.
A few small, short-term intervention studies show that eating walnuts may raise antioxidant levels in the body and decrease markers of DNA damage. But study results are inconsistent. More human studies are needed to clarify walnuts’ influence on inflammation, damage from free radicals and cancer risk.
Gut Microbiome: Bacteria in the colon convert ellagitannins and ellagic acid to highly active compounds called urolithins. Urolithins can be absorbed out of the digestive tract and may act within the colon, where they can nurture bacteria that make up the gut microbiome. Emerging evidence from human intervention trials shows that a few weeks of daily walnut consumption enhanced populations of gut bacteria that seem to have health-promoting effects.
Weight: Observational cohort studies show nuts are compatible with efforts for weight loss, and trends link nuts with less weight gain. Since nuts contain concentrated calories in relatively small portions, some people wonder if nuts might promote weight gain. This is an important question, since excess body fat and weight gain increase risk of at least 12 cancers. Limited research shows that walnuts don’t provide as many calories to the body as would be predicted based on standard calculations. Analysis for the AICR Continuous Update Project (CUP) found evidence on nuts and weight gain, overweight and obesity too limited for any conclusions. However, as noted in the CUP review, intervention trials and observational cohort studies show trends linking nuts with less weight gain and compatibility with efforts for weight loss.
- AICR-Supported Studies
- Tips for Selection, Storage and Preparation
- Choose shelled walnuts that are not shriveled or discolored.
- Walnuts should smell fresh and not rancid.
- For the same size shelled walnuts, the heavier the better.
- Select walnuts with shells unstained, intact and with no signs of mold.
- For freshest flavor, store shelled walnuts in the refrigerator for up to one month.
- Store in an airtight container. If you buy shelled walnuts in sealed packaging, transfer to an airtight container once open.
- Shelled walnuts will keep up to a year in a sealed container in the freezer.
- It’s best to keep unshelled walnuts in the refrigerator, but they will stay fresh up to six months if you store them in a cool, dry and dark place.
- Unshelled walnuts remain freshest if you crack them just before eating.
- Add walnuts to hot or cold cereal.
- Enjoy them in salads as a healthy way to add crunch.
- Toss with stir-fried vegetables, pasta or cooked grains like brown rice or quinoa.
- Coat fish or poultry with chopped walnuts and herbs.
- Combine walnuts with dried fruit for a portable trail mix snack.
- Make a walnut-apple-dried fruit chutney to serve with poultry or seafood.
- Toast walnuts to releases their rich, sweet flavor. For small batches, toast walnut halves in a dry, heavy skillet over medium heat for 2-3 minutes or until golden brown. To toast larger amounts, place walnut halves in a single layer in a shallow pan and bake in a 350 degree F oven for 5-10 minutes. Stir the walnuts a couple of times during toasting for even browning.
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- Rock CL, Flatt SW, Barkai H-S, Pakiz B, Heath DD. A walnut-containing meal had similar effects on early satiety, CCK, and PYY, but attenuated the postprandial GLP-1 and insulin response compared to a nut-free control meal. Appetite. 2017;117:51-57.