Diet: primary prevention

Research conducted in recent years has identified dietary factors that may be involved in the development of certain cancers. If there are no “anti-cancer” foods, some can reduce the risk of the disease, or on the contrary increase it.

In France, the number of new cases of cancer that can be prevented by a diet in line with the recommendations amounts to 19,000 per year.

Dietary factors that increase the risk of cancer

The factors for which the links with certain cancers are proven are mainly the consumption of alcoholic beverages, overweight and obesity, and excess red meat or deli meats.

Consumption of red meats and cold cuts

It is a risk factor for colon and rectal cancer. In 2015, the International Agency for Research on Cancer (IARC) classified the consumption of processed meats (including deli meats) as carcinogenic to humans (Group 1) and the consumption of red meats as probably carcinogenic (Group 2a).

Meat defined as “red meat” in epidemiological studies corresponds to all butchered meat (excluding poultry): beef, pork, veal, lamb, mutton, and other types (horse, goat).

Meat and deli consumption contributed to nearly 5,600 new cases of colorectal cancer in 2015 (1.6% of all cancer cases).

Several mechanisms may explain the increased risk of colorectal cancer associated with meat and deli consumption: production of carcinogenic N-nitroso compounds; production of free radicals and pro-inflammatory cytokines related to excess heme iron; intake of nitrite salts by some deli meats; production of heterocyclic amines (HCAs) or polycyclic aromatic hydrocarbons (PAHs), related to high-temperature cooking.

Beta-carotene-based food supplements

The results of the meta-analyses confirm an increased risk of lung cancer associated with high-dose beta-carotene dietary supplementation (>20 mg per day of beta-carotene), particularly in smokers and ex-smokers, with a “convincing” level of evidence.

At high doses, beta carotene would have a co-carcinogenic effect by increasing the activation of pro-carcinogens from tobacco into carcinogenic molecules via the activation of phase I enzymes of xenobiotic metabolisms, such as cytochromes P450. Moreover, beta-carotene would exert a pro-oxidant effect because the activation of these enzymes is accompanied by the production of free radicals.

The consumption of dietary supplements based on beta-carotene is therefore not only useless for the prevention of cancer but it constitutes a risk factor, especially for smokers. Except in special cases and under medical supervision, it is not recommended. Nutritional needs can be met through a balanced diet.

Note: the consumption of food supplements has increased in France. It concerns 29% of adults and 19% of children.

Dietary factors that reduce the risk of cancer

Consumption of plant-based foods rich in fiber

These foods include both whole grain foods and pulses, but also fruits and vegetables.

It is associated with a decreased risk of colorectal cancer. Similarly, it is associated with a reduced risk of being overweight, a factor directly associated with the risk of 14 cancer sites. A diet rich in fiber can have various effects: reduction of hyperinsulinism, insulin resistance, circulating steroid hormone concentrations, intestinal transit time, and exposure of the colon to carcinogens present in the colonic lumen. At the colonic level, under the action of the microbiota, fibers are also at the origin of the production of short-chain fatty acids with anti-inflammatory and anti-proliferative properties.

Their consumption is below the recommendations in France for the majority of the population.

Point on the consumption of fruits and vegetables

It has a protective effect on cancers of the upper aerodigestive tract (esophagus, oral cavity, larynx and pharynx), stomach, lung (for fruits only) and colorectal cancer.

These foods also provide micronutrients (vitamins, minerals) and numerous and varied microconstituents (glucosinolates, flavonoids, sulfur molecules…) that can influence carcinogenesis by exerting antioxidant or antiproliferative activities, by modulating xenobiotic metabolism, steroid hormone concentration and hormonal metabolism, or by stimulating the immune system. Some are also a source of vitamin B9 (folate) which plays an important role in DNA synthesis and methylation, as well as in the expression of genes involved in carcinogenesis.

Update on the consumption of dairy products

A review of available studies suggests that the risk of colorectal cancer is reduced with the consumption of dairy products. Milk-based products rich in fat (butter, cream) or sugar (ice cream, desserts) are not included.

Calcium could explain in part the potentially protective effect of dairy products on colorectal cancer risk. In particular, calcium negatively regulates the production of parathyroid hormone, which may be involved in cell multiplication.

The lactic acid bacteria contained in certain dairy products could also explain the protective effect on colorectal cancer.

The links between diet and cancer remain to be clarified

The role of certain dietary factors in the increase or decrease in the occurrence of certain cancers has yet to be confirmed or refuted.

These include:

  • the risk related to the consumption of foods prepared by high temperature cooking methods (grilling, barbecue…), in particular meat and fish. Several studies point to an association with stomach cancer, but current data do not allow for a conclusion;
  • of the effect of organic food: at the end of October 2018, a French study, NutriNet-Santé, reported a significant association between regular consumption of organically grown food and decreased risk of cancer (all types of cancers combined). However, the evidence is not yet sufficient to speak of a causal link: the association between organic food and cancer risk must be confirmed by other research. Furthermore, there is no rigorous study to suggest that fruits and vegetables from conventional production processes increase the risk of cancer. Studies showing the protective effects of fruits and vegetables and cereal products have been conducted on populations eating conventional and/or organic food products;
  • the role of phytoestrogens (mainly provided by soy) in various cancers;
  • the risk linked to the consumption of fatty acids, in particular trans fatty acids;
  • the risk linked to vitamin D in the body. Two-thirds of vitamin D is provided by short exposure to the sun and one-third by dietary intake. It has been shown that in addition to its well-known role in bone mineralization, vitamin D is involved in many extra-bone cellular processes, including cell differentiation and proliferation. An association between vitamin D (based on dietary intake, supplementation, or plasma levels) and a decreased risk of colorectal cancer is suggested but remains to be confirmed. Concerning breast cancer, the data are contradictory and must be re-evaluated as a whole. It therefore appears necessary to continue research on the possible links between vitamin D and cancers;
  • the effect of coffee: although its consumption has been associated with a reduction in the risk of liver and endometrial cancers, further research is needed to understand the mechanisms of action and the ways in which it is consumed (type of coffee and preparation, volume consumed, etc.).
  • The role of health professionals: guiding your patients towards good food choices Begin to take stock of your patients’ eating habits to identify positive elements to reinforce and errors to modify, while taking into account their tastes and constraints. In addition to informing them about good practices, it is important to accompany them in their change and to do a regular follow-up. Many tools are available to health professionals.