The Cancer Research
The ever-growing body of scientific research on strawberries is focused on the potential for its compounds to influence the genes that govern cancer development. Earlier studies focused on how the fruit supports our antioxidant defenses.
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 with dietary fiber DECREASE the risk of:
- Colorectal cancer
- Weight gain, overweight and obesity*
*This is important, because there is strong evidence that excess body fat increases the risk of at least 12 different cancers.
- There is probable evidence that non-starchy vegetables and fruit combined DECREASE the risk of:
- Aerodigestive cancers overall (such as mouth, pharynx and larynx; esophageal; lung; stomach and colorectal cancers)
“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.
- Limited evidence suggests that fruit may DECREASE the risk of:
- Lung cancer (in people who smoke or used to smoke tobacco) and squamous cell esophageal cancer
- Limited evidence suggests that non-starchy vegetables and fruit combined may DECREASE the risk of:
- Bladder cancer
- Limited evidence suggests that foods containing vitamin C may DECREASE the risk of:
- Lung cancer (in people who smoke) and colon cancer
Ongoing Areas of Investigation
- Laboratory Research
Anthocyanins influence cell signaling in ways that increase antioxidant, anti-inflammatory and carcinogen-deactivating enzymes in cell and animal studies. They inhibit cancer cells’ growth and ability to spread and activate signaling that leads to the self-destruction of abnormal cells.
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 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.
Tannins such as ellagitannins and proanthocyanidins are complex compounds that are mostly unabsorbed. Microbes in the gut break them down to form other phytochemicals.
- Ellagitannins are broken down in the digestive system to ellagic acid and then to urolithins that can be absorbed into the body. In cell and animal studies, these compounds increase antioxidant enzymes, decreasing free radical damage to DNA that can lead to cancer. By influencing gene expression, they decrease growth and stimulate the self-destruction of several types of cancer cells.
- In some cell studies, urolithins inhibit the aromatase enzyme that produces estrogen and inhibits 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.
Strawberries inhibit inflammation and decrease growth of several types of cancer cells in limited animal studies. These studies often use freeze-dried strawberry powder, which is extra-concentrated in ellagic acid.
Caution is needed when interpreting laboratory studies. Cell studies often use phytochemical compounds in the form they occur in berries. But these are broken down into other compounds that circulate through the body. Moreover, laboratory studies may use phytochemicals in concentrations far beyond levels that would circulate in the body.
- Human Studies
Human studies related to strawberries and cancer risk compare groups of people who consume relatively high and low amounts of total fruit, dietary fiber, berries, or strawberries specifically.
People who eat more fruits have a lower risk of a wide range of cancers. This probably reflects combined protection from many different nutrients and compounds they contain.
Greater consumption of strawberries was associated with a lower risk of estrogen receptor-negative (ER-) breast cancer (but not ER+) in an analysis of 20 observational population studies. In a limited number of short-term clinical trials, many showed decreases in markers of inflammation and oxidative stress, increases in antioxidant defense indicators, and reductions in blood sugar rise after eating.
Dietary Fiber: Observational population studies link high dietary fiber consumption with reduced risk of colorectal cancer. One meta-analysis of 16 prospective studies also link dietary fiber with a lower risk of breast cancer. However, analysis for the AICR/WCRF Third Expert Report considered the potential for an association of dietary fiber and this and several other cancers and found the evidence too limited to support a conclusion.
Vitamin C: Population studies comparing people with higher and lower levels of vitamin C in their diets, and especially levels circulating in their blood, link higher amounts with lower overall risk of cancer. This effect is larger comparing people with very low levels to moderately increased levels than comparing people with moderate and much higher levels. Higher levels of vitamin C from foods are linked with lower risk of lung cancer among people who smoke tobacco, although not in those who used to smoke or who have never smoked. People with more vitamin C in their diet are also less likely to develop colon cancer. That’s even after adjusting for other risk factors for colon cancer, such as alcohol, red meat and tobacco. Evidence for both lung and colon cancer is rated as Limited Suggestive in the AICR/WCRF Third Expert Report, and more research is needed.
Ellagitannins and urolithins: Bacteria in the colon convert ellagitannins from strawberries and other berries to ellagic acid and then to urolithins that can be absorbed. People differ dramatically in the amount and types of urolithins they produce from ellagic acid, which could cause differences in the benefits individuals get from these foods.
Flavonoids found in strawberries: Higher levels of anthocyanins in the diet were strongly associated with lower levels of markers of inflammation, and flavonols and flavan-3-ols were linked with lower levels of oxidative stress in a cross-sectional analysis of a large population study.
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