Below is an approximation of this video’s audio content. To see any graphs, charts, graphics, images, and quotes to which Dr. Greger may be referring, watch the above video.

In muscle meat, there’s a heme protein that contributes to the meaty taste of meat. Well, there’s a heme protein in the roots of soybean plants too, that can be churned out to provide a similar flavor and aroma in plant-based meat, which is used to make the Impossible Burger possible. The question is: are there any downsides?

When the European Food Safety Authority was considering the safety of adding heme iron to foods, their main concern was a potential increased risk of colon cancer. We know meat causes cancer. Processed meat—bacon, ham, hot dogs, lunch meat, and sausage—is considered a Group 1 carcinogen, meaning we know it causes cancer in people, with the same level of certainty that something like smoking causes cancer. Whereas something like a burger just probably causes cancer in people, kind of like DDT. But what’s the role of heme iron?

I mean, there are all sorts of potential mechanisms to explain the cancer risk. Meat’s got the pro-inflammatory long-chain omega-6 arachidonic acid, more of the aging- and cancer-associated methionine, trans fat, endogenous hormones like IGF-1, not to mention the ones that are implanted in animals as hormonal growth-promoters. Then there are all the toxic pollutants that build up the food chain, like pesticides—I didn’t even know about the formaldehyde.

According to the prestigious IARC, the International Agency for Research on Cancer, there is strong evidence that heterocyclic aromatic amines contribute to the cancer-causing mechanism. These DNA-damaging compounds are formed when muscle tissue is exposed to high, dry heat like grilling, roasting, baking, and broiling—basically anything above steaming or stewing. There is also strong evidence that the formation of so-called N-nitroso compounds contribute to the cancer-causing mechanism. These are carcinogens that can form inside our gut when we eat the meat. But there is also strong evidence, according to the IARC, that heme iron contributes to the cancer-causing mechanism. Normally I might leave it there, but other authoritative bodies I respect, like the American Institute for Cancer Research and the World Cancer Research Fund, are more tentative. While they agree there is some evidence that the consumption of foods containing heme iron might increase the risk of colorectal cancer, they consider the evidence suggesting such a connection to be limited.

Much of the available evidence is based on lab animal data such as this, in which dietary heme was found to disrupt the gut flora, aggravate inflammation, and potentiate the development of intestinal tumors in mice. But it’s critical to note that in all the laboratory animal models that have been used, the rodents ingested meat or heme equivalent to people eating up to 40,000 pounds of meat a day. Even the smallest dose would be like a dozen Impossible Burgers a day.

It’s easy to see how casual readers could get confused, though. In this study, ascribing a central role for heme iron in the colon cancer development associated with meat, the authors claimed they were aimed at determining at nutritional doses, which was the main factor involved in cancer promotion. So, doses of heme were chosen to mimic red meat consumption and… boom! A significant increase in tumor load. The researchers conclude that their findings strongly “suggest that at concentrations that are in line with human red meat consumption, heme iron is associated with the promotion of colon [cancer development].” But if you look at the actual diet they were given and do the math, that’s 500 times the level of heme found in people’s diets, in excess of like 70 pounds of meat a day. Of course, even if they really did use the right doses, they’re still going to end up with data on the wrong species, which brings us to clinical studies, which we’ll explore next.

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