Does Burnt or Charred Food Cause Cancer?

The worry is that burnt toast, over-browned potatoes, or charred grilled meat causes cancer. There is real chemistry underneath it, which is why the claim deserves an answer rather than a shrug.

Two separate stories are usually mixed together. Starchy foods make one compound. Meat makes different ones. The evidence is not the same in each case.

NCI explains how acrylamide forms in food. Some vegetables contain an amino acid called asparagine. Potatoes are one. When they are heated to high temperatures with certain sugars present, acrylamide forms.

NCI names the major food sources. French fries and potato chips. Crackers, bread, and cookies. Breakfast cereals. Canned black olives. Prune juice. And coffee. It notes levels vary widely by maker, cooking time, and cooking method and temperature.

Then it gives the sentence that changes the picture. People take in substantially more acrylamide from tobacco smoke than from food. NCI reports that smokers carry three to five times higher levels of acrylamide markers in their blood than non-smokers.

NCI is precise about the split between laboratory findings and human findings.

In rodent models, acrylamide exposure increases the risk of several types of cancer. In the body, acrylamide is converted to a compound called glycidamide, which causes mutations in and damage to DNA.

But NCI reports the human side differently. A large number of studies in people have looked at this. They include both case-control and cohort designs. NCI says they found no consistent evidence that dietary acrylamide is linked to the risk of any type of cancer.

The National Toxicology Program's Report on Carcinogens has its own label for acrylamide. It is reasonably anticipated to be a human carcinogen. That rests on studies in lab animals given acrylamide in drinking water. NCI adds the caveat that matters. Toxicology studies show humans and rodents absorb acrylamide at different rates. They also break it down differently.

Workplace data point the same way. NCI says high acrylamide exposure at work, which happens by breathing it in, causes nerve damage. But it adds that studies of workplace exposure have not suggested higher cancer risks.

This detail is easy to get wrong, so it is worth stating exactly.

EPA regulates acrylamide in drinking water. NCI says the acceptable exposure level was set low on purpose. It allows for uncertainty in the data on cancer and nerve effects.

FDA regulates the amount of residual acrylamide in materials that contact food. NCI states there are currently no guidelines governing the presence of acrylamide in food itself.

The meat side involves two different compound families, formed by two different mechanisms.

Heterocyclic amines, or HCAs, form when amino acids, sugars, and creatine or creatinine react at high heat. Creatine and creatinine sit in muscle. That is why HCAs are not found in meaningful amounts outside meat cooked at high temperatures.

The second family is polycyclic aromatic hydrocarbons, or PAHs. They form a different way. Fat and juices drip onto a hot surface or open fire, causing flames and smoke. The smoke carries PAHs that stick to the meat surface. PAHs also turn up in smoked foods, cigarette smoke, and car exhaust.

NCI gives a temperature marker. Meats cooked above 300 ºF, as in grilling or pan frying, or cooked for a long time, tend to form more HCAs. It names well-done grilled or barbecued chicken and steak as having high HCA concentrations.

One step in the biology explains why people differ. NCI says HCAs and PAHs can damage DNA only after certain enzymes process them. That step is called bioactivation. How active those enzymes are varies from person to person.

NCI's summary here has two halves, and the second half is usually dropped.

In animal experiments, rodents fed HCAs developed tumors in several organs. NCI lists breast, colon, liver, skin, lung, and prostate among them. Rodents fed PAHs developed leukemia and tumors of the gut and lungs. But NCI states the doses were very high. They were equal to thousands of times what a person eats in a normal diet.

In people, the picture is unsettled. NCI says population studies have not established a definitive link between HCA and PAH exposure from cooked meat and cancer. Some studies did find a link. Eating a lot of well-done, fried, or barbecued meat went with higher risks of colorectal, pancreatic, and prostate cancer. Other studies found no link with colorectal or prostate cancer.

One classification is worth reading carefully. In 2015 an IARC panel found that eating red meat is probably carcinogenic to humans, Group 2A. But NCI notes what IARC did not say. It did not conclude that HCAs and PAHs were linked to cancer rates. The classification was about red meat, not about the char.

NCI states there are currently no federal guidelines addressing consumption of foods containing HCAs and PAHs.

It cites one 2007 report, from the World Cancer Research Fund and the American Institute for Cancer Research. That report recommended limiting red and processed meat, including smoked meat. But NCI notes it gave no recommendations for HCA and PAH levels.

So there is no threshold to hit, and any page that supplies one has invented it.

For anyone who wants to reduce exposure, NCI names specific methods rather than general advice.

• Avoid direct exposure of meat to an open flame or hot metal surface, and avoid prolonged cooking, especially at high temperatures.
• Use a microwave to cook meat before high-heat finishing, which cuts the time in contact with high heat.
• Turn meat over continuously on a high heat source, which NCI says substantially reduces HCA formation compared with leaving it in place.
• Remove charred portions, and skip gravy made from meat drippings.

For acrylamide, NCI lists four methods shown to lower content in some foods. Shorter cooking, to avoid heavy crisping or browning. Blanching potatoes before frying. Not storing potatoes in a refrigerator. And post-drying in a hot air oven after frying.

Both compound families damage DNA in the laboratory. Neither has been shown, in human studies, to raise cancer risk at the levels found in ordinary food. For acrylamide specifically, smoking delivers far more than any meal does.

Reducing heavy charring is reasonable. Treating a piece of burnt toast as a cancer exposure is not supported by what NCI publishes.

The meat question in more depth is covered in cooked meat and cancer. The processed meat classification is handled in red and processed meat. The wider dietary picture is in diet and cancer risk.

• NCI: acrylamide and cancer risk
• NCI: chemicals in meat cooked at high temperatures