Diet and cancer

Dietary factors are recognized as having a significant effect on the risk of cancers, with different dietary elements both increasing and reducing risk. Diet and obesity may be related to up to 30–35% of cancer deaths, while physical inactivity appears to be related to 7% risk of cancer occurrence.

While many dietary recommendations have been proposed to reduce the risk of cancer, few have significant supporting scientific evidence. Obesity and drinking alcohol have been correlated with the incidence and progression of some cancers. Lowering the consumption of sweetened beverages is recommended as a measure to address obesity.

Some specific foods are linked to specific cancers. There is strong evidence that processed meat and red meat intake increases risk of colorectal cancer. Aflatoxin B1, a frequent food contaminant, increases risk of liver cancer, while drinking coffee is associated with a reduced risk. Betel nut chewing causes oral cancer. Stomach cancer is more common in Japan due to its high-salt diet. Immigrant communities tend to develop the risk of their new country, often within one generation, suggesting a substantial link between diet and cancer.

Dietary recommendations for cancer prevention typically include weight management and eating a healthy diet, consisting mainly of "vegetables, fruit, whole grains and fish, and a reduced intake of red meat, animal fat, and refined sugar." A healthy dietary pattern may lower cancer risk by 10–20%. There is no clinical evidence that diets or specific foods can cure cancer.

Restrictive diets
A number of diets and diet-based regimes are claimed to be useful against cancer. Popular types of "anti-cancer" diets include the Breuss diet, Gerson therapy, the Budwig protocol and the macrobiotic diet. None of these diets has been found to be effective, and some of them have been found to be harmful.

Dietary patterns
Nutritional epidemiologists use multivariate statistics, such as principal components analysis and factor analysis, to measure how patterns of dietary behavior influence the risk of developing cancer. (The most well-studied dietary pattern is the Mediterranean diet.) Based on their dietary pattern score, epidemiologists categorize people into quantiles. To estimate the influence of dietary behavior on risk of cancer, they measure the association between quantiles and the distribution of cancer prevalence (in case–control studies) and cancer incidence (in longitudinal studies). They usually include other variables in their statistical model to account for the other differences between people with and without cancer (confounders). For breast cancer, there is a replicated trend for women with a more "prudent or healthy" diet, i.e. higher in fruits and vegetables, to have a lower risk of cancer.

Unhealthy dietary patterns are associated with a higher body mass index suggesting a potential mediating effect of obesity on cancer risk.

Alcohol
Alcohol is associated with an increased risk of a number of cancers. It has been reported that 3.6% of all cancer cases and 3.5% of cancer deaths worldwide are attributable to drinking of alcohol. Breast cancer in women is linked with alcohol intake. Alcohol also increases the risk of cancers of the mouth, esophagus, pharynx and larynx, colorectal cancer, liver cancer, stomach and ovaries. The International Agency for Research on Cancer (Centre International de Recherche sur le Cancer) of the World Health Organization has classified alcohol as a Group 1 carcinogen. Its evaluation states, "There is sufficient evidence for the carcinogenicity of alcoholic beverages in humans. ... Alcoholic beverages are carcinogenic to humans (Group 1)."

Processed and red meat
There is strong evidence that processed meat and red meat intake increases risk of colorectal cancer. The American Cancer Society in their "Diet and Physical Activity Guideline", stated "evidence that red and processed meats increase cancer risk has existed for decades, and many health organizations recommend limiting or avoiding these foods."

On October 26, 2015, the International Agency for Research on Cancer of the World Health Organization reported that eating processed meat (e.g., bacon, ham, hot dogs, sausages) or red meat was linked to some cancers and classed them as Group 1 (carcinogenic to humans) and Group 2a (probably carcinogenic to humans) carcinogens respectively. There is some evidence that suggests that heme and nitrite are involved in the processes linking red and processed meat intake with colorectal cancer. Heme is present in particular in red meat and nitrite is used as curing salt in many processed meats.

Processed and unprocessed red meat intake is associated with an increased risk of breast cancer.

Fiber, fruits and vegetables
There is strong evidence that consumption of dietary fiber reduces risk of colorectal cancer. Two 2020 meta-analyses found that a high fiber intake was associated with a lower risk of both premenopausal and postmenopausal breast cancers and a higher survival rate in patients with breast cancer.

A 2021 review found that there is moderate-quality evidence 200g of fruit intake per day is associated with a lower risk of breast cancer. Another review found that high total fruit and vegetable consumption are associated with reduced risk of breast cancer.

Flavonoids
Flavonoids (specifically flavonoids such as the catechins) are "the most common group of polyphenolic compounds in the human diet and are found ubiquitously in plants." While some studies have suggested flavonoids may have a role in cancer prevention, others have been inconclusive or suggested they may be harmful.

Methionine
Restriction of methionine has been suggested as a strategy in cancer growth control in cancers that depend on methionine for survival and proliferation. According to a 2012 review, the effect of methionine restriction on cancer has yet to be studied directly in humans and "there is still insufficient knowledge to give reliable nutritional advice".

Reviews of epidemiological studies have found no association between dietary methionine and breast or pancreatic cancer risk.

Mushrooms
According to Cancer Research UK, "there is currently no evidence that any type of mushroom or mushroom extract can prevent or cure cancer", although research into some species continues.

A 2020 review found that higher mushroom consumption is associated with lower risk of breast cancer.

Whole grains
There is strong evidence that consumption of whole grains decreases risk of colorectal cancer.

Soy
The American Cancer Society have stated that "there is some evidence from human and lab studies that consuming traditional soy foods such as tofu may lower the risk of breast and prostate cancer, but overall the evidence is too limited to draw firm conclusions".

A 2023 review found that soy protein lowers breast cancer risk.

Other

 * Green tea consumption has no effect on cancer risk.
 * A 2016 meta-analysis showed that women and men who drank coffee had a lower risk of liver cancer. An umbrella review of meta-analyses found that coffee was associated with a lower risk of liver and endometrial cancer.
 * A 2014 systematic review found, "no firm evidence that vitamin D supplementation affects cancer occurrence in predominantly elderly community-dwelling women."

AMPK
AMPK is thought to be a major element or mechanism in cancer-related effects of diet. It modulates the activity of cellular survival signaling such as mTOR and Akt, leading to cell growth inhibition which is relevant to cancer growth. Targeting AMPK has become a novel strategy for cancer prevention and treatment. Potential complementary or preventive options under investigation include periods of caloric restriction and AMPK agonists (typically mTOR inhibitors). However, AMPK can also promote cancer in some settings.