Connection between cancer and meat cooked at high temperatures

High temperature cooking (particularly burning) of meat forms some cancers causing heterocyclic amines.

Some heterocyclic amines (HCAs) found in cooked and especially burnt meat are known as carcinogens.

Research has shown that heterocyclic amine formation in meat occurs at high cooking temperatures.

For example, heterocyclic amines are the carcinogenic chemicals formed from the cooking of muscle meats such as beef, pork, fowl and fish. HCAs form when amino acids and creatine (a chemical found in muscles) react at high cooking temperatures.

Researchers have identified 17 different HCAs resulting from the cooking of muscle meats that may pose human cancer risk.

NCI’s Division of Cancer Epidemiology and Genetics found a link between individuals with stomach cancer and the consumption of cooked meat and other studies for colorectal, pancreatic and breast cancer are associated with high intakes of well-done, fried, or barbecued meat.

People who eat medium-well or well done beef were more than three times more likely to suffer from stomach cancer than those who ate rare or medium-rare beef.

Other sources of protein (milk, eggs, tofu, and organ meats such as liver) have very little or no HCA content naturally or when cooked.

What are heterocyclic amines and polycyclic aromatic hydrocarbons and how are they formed in cooked meats?

Heterocyclic amines (HCAs) and polycyclic aromatic hydrocarbons (PAHs) are chemicals formed when muscle meat, including beef, pork, fish, or poultry is cooked using high-temperature methods, such as pan frying or grilling directly over an open flame.

In laboratory experiments, HCAs and PAHs have been found to be mutagenic - they cause changes in DNA that may increase the risk of cancer.

These HCAs are formed when amino acids (the building blocks of proteins), sugars, and creatine (a substance found in muscle) react at high temperatures while PAHs are formed when fat and juices from meat grilled directly over an open fire drip onto the fire, causing flames.

These flames contain PAHs that then adhere to the surface of the meat. PAHs can also be formed during other food preparation processes such as smoking of meat.

Note that HCAs are not found in significant amounts in foods other than meat cooked at high temperatures while PAHs can be found in other charred foods as well as in cigarette smoke and car exhaust fumes.

What factors influence the formation of HCA and PAH in cooked meats?

The formation of HCAs and PAHs varies by meat type, cooking methods and “doneness” level (rare, medium, or well done). Whatever the type of meat, however, meat cooked at high temperatures, especially above 300ºF (as in grilling or pan frying), or that are cooked for a long time tend to form more HCAs.

For example, well done, grilled or barbecued chicken and steak all have high concentrations of HCAs. Cooking methods that expose meat to smoke or charring contribute to PAH formation.

Both HCAs and PAHs become capable of damaging DNA only after they are metabolised by specific enzymes in the body; a process called “bio-activation”.

Studies show that the activity of these enzymes, which can differ among people, may be relevant to cancer risks associated with exposure to these compounds.

What evidence is there that HCAs and PAHs in cooked meats may increase cancer risks?

Studies have shown that exposure to HCAs and PAHs can cause cancer in animal models. In many experiments, rodents fed on a diet supplemented with HCAs developed tumours of the breasts, colons, livers, skins, lungs, prostates and other organs. Rodents fed on PAHs also developed cancers, including leukaemia and tumours of the gastrointestinal tract and lungs.

However, the doses of HCAs and PAHs used in these studies were very high; equivalent to thousands of times the doses that a person would consume in a normal diet.

Population studies have not established a definitive link between HCA and PAH exposure from cooked meats and cancer in humans. One difficulty with conducting such studies is that it can be difficult to determine the exact levels of HCA and/or PAH exposure a person gets from cooked meat.

Although dietary questionnaires can provide good estimates, they may not capture all the details about cooking techniques that are necessary to determine HCA and PAH exposure levels. In addition, individual variation in the activity of enzymes that metabolise HCAs and PAHs may result in exposure differences, even among people who ingest (take in) the same amount of these compounds. Also, people may have been exposed to PAHs from other environmental sources, such as pollution and tobacco smoke.

Nevertheless, numerous epidemiologic studies have used detailed questionnaires to examine participants’ meat consumption and meat cooking methods to estimate HCA and PAH exposures. Researchers have found that high consumption of well-done, fried, or barbecued meat is associated with increased risks of colorectal, pancreatic and prostate cancers.

Do guidelines exist for the consumption of food containing HCAs and PAHs?

Currently, no Federal guidelines address the consumption of food containing HCAs and PAHs. The World Cancer Research Fund/American Institute for Cancer Research issued a report in 2007 with dietary guidelines that recommended limiting the consumption of red and processed (including smoked) meats. However, no recommendations were provided for HCA and PAH levels in meat.

Are there ways to reduce HCA and PAH formation in cooked meats?

Even though no specific guidelines for HCA/PAH consumption exist, concerned individuals can reduce their exposure by using several cooking methods:

• avoid direct exposure of meat to an open flame or a hot metal surface and avoid prolonged cooking times (especially at high temperatures), as this can help reduce HCA and PAH formation.

• use a microwave oven to cook meat prior to exposure to high temperatures, which can also substantially reduce HCA formation by reducing the time that meat must be in contact with high heat to finish cooking.

• continuously turn meat over on a high heat source as this can substantially reduce HCA formation compared with just leaving the meat on the heat source without flipping it often.

• remove charred portions of meat and refrain from using gravy made from meat drippings, which can also reduce HCA and PAH exposure.

What research is being conducted on the relationship between the consumption of HCAs and PAHs and cancer risks in humans?

Researchers in the United States are currently investigating the association between meat intake, meat cooking methods and cancer risks. Similar research in a European population is being conducted in the European Prospective Investigation into Cancer and Nutrition (EPIC) study. PF