Cancer and diet: Risks, prevention, and more

Researchers have reported associations between nutrition and cancer for a while. However, it is challenging to determine the precise link between dietary factors and cancer risk.

Certain factors can raise the risk of specific cancers. These factors include a poor diet and a sedentary lifestyle.

A 2019 study examined the link between 15 cancer types and specific dietary factors. Researchers found that each year, more than 5% of new cancer cases could be associated with a poor diet.

This article examines the connection between cancer and diet. It discusses whether a healthy diet can reduce cancer risk, the foods that may cause and prevent cancer, and the food-related myths surrounding cancer. It also identifies the foods to eat and avoid during cancer treatment.

According to the Dietary Guidelines for Americans 2020–2025, a healthy diet across the life span may reduce a person’s risk of certain cancers, such as breast, colon, and rectal cancers.

Healthy eating may reduce the risk of developing cancer by sustaining energy for physical activity and supporting the immune system.

According to the American Cancer Society, a healthy diet includes:

  • foods high in vitamins, minerals, and other nutrients
  • vegetables in a variety of colors
  • fruits, especially whole fruits, in a variety of colors
  • fiber-rich legumes, such as peas and beans
  • whole grains

A healthy dietary pattern limits or does not include:

  • processed meats, such as sausage, bacon, and lunch meats
  • red meats, such as beef, pork, and lamb
  • highly processed foods
  • refined grain products
  • sugar-sweetened beverages, including soft drinks, fruit drinks, and sports drinks
  • alcohol

Dietary patterns with the most benefits tend to be those based primarily on plant foods, healthy protein sources, and unsaturated fats.

There is no evidence that any one food can prevent cancer.

However, many studies have examined dietary components and nutrients that may help increase or decrease cancer risk.

Additionally, laboratory and animal studies sometimes indicate that certain compounds may be carcinogenic or demonstrate anticancer activity.

Human studies have yet to confirm that any food can cause or prevent cancer.

The following foods, nutrients, additives, and other dietary components may have associations with reduced cancer risk.

Antioxidants

Antioxidants block the activity of unstable molecules, known as free radicals, that can damage cells. Dietary antioxidants include vitamins C and E, carotenoids, and other food components.

According to the National Cancer Institute, laboratory and animal studies suggest that antioxidants prevent the free radical damage associated with cancer development. However, human research has yielded mixed results.

Consuming more fruits and vegetables, which are high in antioxidants, may decrease the risk of some cancer types, notes the American Cancer Society. However, this does not necessarily mean the benefits are from antioxidants.

Additionally, clinical trials of antioxidant supplements do not show a reduction in cancer risk. In fact, some studies indicate an increased risk of cancer among people taking antioxidant supplements.

Ideally, people should aim to get their antioxidant intake from whole foods rather than supplements.

Coffee

Researchers have conducted many studies to investigate whether coffee reduces or raises the risk of various types of cancer.

Drinking coffee may lower the risk of:

  • endometrial cancer
  • liver cancer
  • cancers of the mouth, throat, and voice box
  • basal cell skin cancer
  • melanoma

It is unclear how coffee may reduce cancer risk. However, coffee contains hundreds of biologically active compounds that may protect against cell damage, contribute to DNA repair, and inhibit the spread of cancer.

Conversely, a 2019 study found an association between drinking very hot beverages and an increased risk of esophageal cancer.

Hot drinks above 149°F (65°C) are also included on the International Agency for Research on Cancer (IARC) list of probable carcinogens.

Soy

Soy is a nutrient-dense source of protein that contains many bioactive food components, including isoflavones.

Some human and laboratory studies suggest that consuming soy may decrease the risk of prostate and breast cancers, according to the American Cancer Society. However, more studies are necessary before researchers can draw firm conclusions.

No evidence supports the use of supplements containing soy for decreasing cancer risk.

A 2019 study found that soy supplements may lower the risk of estrogen receptor-positive breast cancer but increase the risk of estrogen receptor-negative breast cancer.

While soy from food sources may be beneficial, people should use soy supplements cautiously.

Fruits and vegetables

Fruits and vegetables contain many vitamins, minerals, fiber, and other nutrients that may help lower cancer risk.

Research is ongoing to determine whether properties in the following fruits and vegetables may prevent cancer:

  • dark green vegetables
  • cruciferous vegetables, such as broccoli, cabbage, Brussels sprouts, and cauliflower
  • soy products
  • allium vegetables, such as garlic and onions
  • legumes, such as kidney beans, chickpeas, and lentils
  • tomato products

According to the National Cancer Institute, diets high in fruits and vegetables may lower the risk of the following cancers:

  • mouth
  • pharynx
  • larynx
  • esophagus
  • stomach
  • lung
  • colon
  • pancreas
  • prostate

Experts recommend eating 1–2.5 cups of fruits and 1–4 cups of vegetables daily to help prevent the above cancers.

Whole grains

A whole grain is a grain that contains all the parts of the original kernel, including the bran, germ, and endosperm.

Examples of whole grains include brown rice, quinoa, and oatmeal. Whole grains are richer in fiber and other nutrients than refined grains. Refined grains include white rice, white pasta, and white bread.

A series of systemic reviews and meta-analyses published in 2019 found that eating fiber-rich foods decreased the occurrence of chronic diseases, including colorectal cancer, by 16–24%.

The Dietary Guidelines for Americans 2020–2025 recommend that half of the grains people consume are whole grains.

Dairy products and calcium

According to the American Cancer Society, some evidence suggests that dairy products and calcium may reduce the risk of colorectal and breast cancer.

However, due to research suggesting dairy products and calcium may increase prostate cancer risk, experts do not make recommendations about the consumption of dairy products for cancer prevention.

Vitamin D

Observational studies and randomized controlled trials have explored the association between vitamin D and cancer risk.

According to the National Cancer Institute, observational studies suggest a link between higher vitamin D levels and a lower risk of colorectal and possibly bladder cancer.

By contrast, studies indicate an association between higher vitamin D levels and prostate cancer and perhaps pancreatic cancer.

However, evidence from randomized controlled trials suggests that vitamin D supplements do not lower the risk of cancer development overall or of developing specific cancers.

Although vitamin D levels and cancer risk remain a topic for debate, most people in the United States do not get enough vitamin D.

The National Institutes of Health’s Office of Dietary Supplements recommends that most adults get 15 micrograms (mcg) of vitamin D daily.

The following table provides insight into the link between diet and specific cancers, according to the World Cancer Research Fund and the American Cancer Society:

Researchers need to conduct more studies to confirm the links between specific foods and certain cancer types.

Common myths concerning diet and cancer include the following:

Myth: Sugar fuels cancer growth

Evidence indicates that cancer cells metabolize glucose faster than normal cells. People often misinterpret this and mistakenly assume that sugars in foods and drinks “feed” cancer cells and fuel tumor growth.

No studies show that eating sugar worsens cancer or that stopping eating sugar shrinks cancer.

However, diets high in added sugars may indirectly increase the risk of developing cancer by affecting insulin levels and contributing to obesity.

Myth: Juice cleanses and detoxes reduce cancer risk

No scientific evidence supports that consuming only juices for one or several days reduces cancer risk.

Advocates promote juice cleansing or detoxification to remove “toxins” from the body. However, scientific evidence does not support these claims.

Regardless of whether a person consumes liquid or solid foods, the kidneys and liver constantly work to remove the toxins that enter the body. Limiting the diet to only juice may prevent the body from getting enough nutrients to function correctly.

Myth: Breast cancer survivors should avoid soy

Soy products do not increase cancer risk, including the risk of breast cancer.

Soy products contain chemicals called isoflavones. Isoflavones are similar in chemical structure to the hormone estrogen.

Estrogen promotes the growth of some breast cancers, so some people believe that isoflavone-containing foods may have the same effect.

However, neither isoflavones nor soy products increase the risk of breast cancer.

Myth: Some superfoods prevent cancer

People use the term “superfoods” to describe nutrient-rich foods, such as kale, spinach, and blueberries, that may significantly benefit health and well-being.

While experts agree that a healthy dietary pattern containing plenty of plant foods may reduce cancer risk, there is not enough evidence to suggest any one food can prevent the development or progression of cancer.

Myth: A daily multivitamin prevents cancer

Research indicates that multivitamins or other supplements do not lower cancer risk.

Some research shows a slightly higher risk of certain cancers with multivitamin use and an increased risk of cancer with high dose supplements containing beta-carotene and vitamins A and E.

The American Cancer Society does not recommend using dietary supplements for cancer prevention.

There is no specific diet for people going through cancer treatment.

However, some cancer treatments can make people more prone to foodborne illnesses. With this in mind, people can be mindful of how they prepare and handle food.

Chemotherapy, radiation therapy, and immunotherapy can cause several side effects, such as diarrhea, dry mouth, and sore mouth and throat. People may find it helpful to avoid certain foods and beverages to relieve some of these issues.

Foods and drinks that may worsen diarrhea include:

  • high fiber foods, such as whole wheat breads and pasta
  • drinks that are extremely hot or cold
  • sugar-sweetened beverages, such as fruit punch or regular soda
  • fried, fatty, or greasy foods, such as french fries and hot dogs
  • gas-causing foods, such as beans and raw fruits and vegetables
  • regular milk products
  • spicy foods, such as chili, hot sauce, and salsa
  • alcohol
  • sugar-free products sweetened with sorbitol or xylitol
  • caffeinated foods or drinks

People with dry mouth should avoid the following:

  • alcohol
  • spicy, salty, hard, sour, or crunchy foods
  • tobacco products
  • secondhand smoke
  • mouthwash containing alcohol

People with a sore mouth and throat can try avoiding foods and drinks that may aggravate symptoms, such as:

  • spicy foods, including chili peppers, curry dishes, and hot sauces
  • citrus fruits, such as oranges and lemons
  • salty foods
  • raw vegetables
  • tomatoes and ketchup
  • sharp or crunchy foods, such as potato and tortilla chips and crackers
  • alcohol

The foods people eat during and after cancer treatment can affect their recovery.

According to the National Cancer Institute, people with cancer often need extra calories and protein in their diet.

The correct intake of calories and protein is crucial for energy, immunity, and healing. If people have difficulty chewing and swallowing, adding sauces and gravies to meals may help make them more palatable.

People can discuss their diet with a qualified healthcare professional, such as a dietitian, if they are unsure what to eat when undergoing cancer treatment.

Experts continue to find many associations between diet and cancer, but it is difficult to find a direct link between dietary factors and cancer risk.

A healthy eating pattern over a lifetime may decrease a person’s risk of several cancer types.

Several studies have examined the connection between certain foods or dietary components and lower cancer risk. Although no one food can prevent cancer, evidence suggests the following foods may be associated with a reduced risk of cancer:

  • antioxidants
  • coffee
  • soy
  • fruits and vegetables
  • whole grains
  • dairy products and calcium
  • vitamin D

During cancer treatment, people often need to increase their intake of calories and protein to manage treatment side effects.

A qualified healthcare professional, such as a dietitian, can provide people with strategies for maintaining a healthy diet during and after cancer treatment.