Is charred meat bad for you?

As the end of summer approaches, you love to grill your food on the open flame.  You savor that char-grilled flavor on your meat or fish.  Perhaps you fashion yourself as a modern-day caveman, inspired by the Paleo Diet and getting back to Nature.

At the same time, you’ve probably heard that eating grilled meat is a bad idea, because compounds in the meat char can cause cancer.

According to the National Cancer Institute, grilling meat to the point of charring causes the formation of heterocyclic amines (HCAs), polycyclic aromatic hydrocarbons (PAHs), and Maillard reaction products such as acrylamide (AA) or advanced glycation end-products (AGEs).  HCAs and AGEs are formed when the amino acids, sugars and creatine in meat react at high temperatures. PAHs are formed when meat fats burn.  Maillard reaction products are those tasty brown “caramelized” substances produced by the reaction of sugars and amino acids when meats and other foods are cooked by grilling, baking, frying or toasting.

The National Cancer Institute reports that HCAs, PAHs and acrylamide have been shown to cause cancer in laboratory animals.  Added to this are a number of epidemiological studies purporting to show an association between consumption of cooked meats and cancer.

So you reluctantly curtail your inner caveman and carefully scrape the blackened parts off your meats, or grill them at a lower temperature.  Or perhaps you avoid grilling altogether, retreating indoors and lightly sautéing or boiling your meat dishes.

Relax. I’m here to make the case that charred meat is not to be feared.  It may actually be good for you, hormetically boosting your general ability to neutralize and dispose of dietary toxins. In this blog post, we will take a closer look at the animal and human studies, combined with a deeper look at the evolutionary record, aided by the perspective of modern toxicology.  I think it may change your mind.

Conventional wisdom.   It’s not just Mainstream Health that pushes the idea that charred meat causes cancer.  Even Paleo advocates like Mark Sisson warn against the dangers of grilling and cooking meats at high temperatures:

But there’s a dark side to cooking. Depending on the methods and ingredients you use and the temperature you apply, cooking can create carcinogenic and toxic compounds, and oxidized fats – and these may be involved in some of the diseases studied. It may not be the meat itself, but how we treat the meat. …The easiest way to minimize your exposure to heat-related toxins is to emphasize gentle cooking methods and de-emphasize higher heat methods.

Sisson cites the usual culprits — HCAs, AGEs, acrylamide — and oxidized lipids.  He cautions particularly againt “grilling over an open flame – the worst”, pointing to its association with higher levels of HCA.  He recommends gentle methods such as steaming, poaching, boiling, braising, simmering and baking.

We should be a bit suspicious of these claims, given the fact that humans have been cooking meat over open flames for more than millenium, inevitably creating char. If these claims were correct, we’d expect to see elevated rates of cancer among modern hunter-gathers who cook their meat — but we don’t.  Just as the demonization of meat and saturated fat as agents of disease has been challenged by reappraisals of epidemiological and biomedical evidence, I think we should take a closer look at the shibboleth that charbroiling meat or otherwise cooking it at high temperatures poses a health risk.

The studies. Let’s look more closely animal and human studies which gave rise to these concerns about the compounds in grilled meats.  Here’s what the National Cancer Institute actually says about the studies of HCAs and PAHs:

• In many experiments, rodents fed a diet supplemented with HCAs developed tumors of the breast, colon, liver, skin, lung, prostate, and other organs. Rodents fed PAHs also developed cancers, including leukemia and tumors 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 level of HCA and/or PAH exposure a person gets from cooked meats.
• Although dietary questionnaires can provide good estimates, they may not capture all the detail about cooking techniques that is necessary to determine HCA and PAH exposure levels.

What about acrylamide?  According to the American Cancer Society, “Based on the studies done so far, it’s not yet clear if acrylamide affects cancer risk in people.” They further state:

Acrylamide has been found to increase the risk of several types of cancer when given to lab animals (rats and mice) in their drinking water. The doses of acrylamide given in these studies have been as much as 1,000 to 10,000 times higher than the levels people might be exposed to in foods. It’s not clear if these results would apply to people as well, but in general it makes sense to limit human exposure to substances that cause cancer in animals.

Writing for the American Council on Science and Health, Joseph Rosen of Rutgers University criticized the rodent studies as flawed, based on the extreme levels, the mode of administration, and key differences between the endocrine organs of humans and rats, where tumors were noted.  Based on the animal studies, Rosen concluded that “There is no credible evidence that acrylamide in food poses a human cancer risk.”

What about studies of acrylamide specifically in humans?  A 2007 Dutch study linked acrylamide consumption to cancer in women, finding that women who absorbed more acrylamide were twice as likely to develop ovarian or womb cancer as those who ingested a smaller amount.  But a closer look at this study revealed that the measure of acrylamide was based upon a self-reported food questionnaire, not actual clinical measurements.  And there was no clear linkage specifically to meat consumption. As noted by Dr. Lesley Walker of Cancer Resarch UK:

In short, if you are concerned about acrylamide, there are more worrisome places to look than meats.

Evolution and dietary adaptation.  But are mice and rats a valid model for assessing the toxicity in humans of compounds from cooked foods?  Rodents may indeed be suitable animal models for many aspects of human physiology and toxicology, such as testing the safety of drugs, synthetic chemicals, and evolutionarily novel compounds. But animal models are mainly useful for understanding human physiological responses only where there are shared metabolic, immune, or detoxification pathways.  This is of particular importance in toxicology, because what is toxic to one species is often harmless or even beneficial to another. Detoxification pathways vary considerably across species.  For example, chocolate can poison dogs because the active ingredient, theobromine, is highly toxic to canines, while humans typically tolerate it well, and even find it to be beneficial. So if you relied on dog testing to determine your food choices, you would never eat another piece of chocolate.

Organisms evolve detoxification processes that respond to what they are likely to encounter in their diets and their environments. Humans have been cooking (and burning) food over hot flames since they diverged from other primates half a millennia ago, so they have had ample opportunity to adapt their detoxification responses to the compounds present in charred meat.  It’s just not novel for us.

However, to the best of my knowledge, mice and rats don’t cook their meat — with or without fire.  Meat char represents a novel toxin to rodents, so they are unlikely to have developed a strong system for detoxifying HCAs, PAHs, or acrylamide.   Thus, rodents are not a reasonable model for assessing the toxicity of compounds produced by cooking meat.

The barbecuing species.  There is compelling evidence that humans evolved to eat and thrive on a diet that includes fire-cooked foods – both meat and plants.

The evidence comes from several independent and reinforcing lines of evidence.  While the idea of “man as cook” goes back at least to the anthropologist Claude Levi-Strauss, his main interest was in how cooking changed social psychology.  The most comprehensive and convincing argument for the “cooking hypothesis” is laid out by Richard Wrangham in his 2009 masterpiece,  Catching Fire, How Cooking Made us Human.

Wrangham supports his contention that cooking shaped human biology with several independent lines of evidence — including the findings of archaeology, anthropology, nutrition, evolutionary biology and physiology.  I’ll try to summarize here the key elements and implications of Wrangham’s argument:

1. Archaeology. Based on excavation of artifacts like burnt bones at campsites, archaeologists trace the origins of cooking by fire to physical evidence dated to less than million years ago, but other evidence suggests the advent of cooking may have been between 1 and 2 million years ago.  The timing of these archaeological events are mirrored in simultaneous dramatic changes in the teeth, jaws, digestive apparatus and brain of our human ancestors, most particularly Homo erectus, between 1-2 million years ago, as elaborated in Point #4 below.
2. Anthropology.  Cooking is practiced in every human society, including by modern hunter-gatherers. Hunter-gatherers lived in a wide variety of environments including deserts, mountains, the arctic and rain forests — but cooking is universal. While foods like fruits, organ meats, grubs and fish are typically eaten raw, most hunter-gatherers prefer to cook their meat, eggs and tubers.
3. Nutrition. Cooking food dramatically increases the palatability, digestibility, tenderness and available caloric value of both meat and plants. Cooking meat and other protein denatures it, rendering it more accessible to the action of digestive enzymes.  The benefits of cooking are not restricted to meats — cooking also makes plant foods more digestible.  Given the limited availability and lean composition of meat in the tropics, equatorial hunter-gathers also need to consume plant carbohydrates, such as starchy tubers.  Cooking gelatinizes the starch, increasing its glycemic index, and making it easier to digest and absorb its sugars that one could otherwise obtain from raw starch.  Raw foodism is effective as a weight loss diet because uncooked food provides less caloric value and requires more energy and time to digest than cooked food.  Domestic animals grow faster and fatter on cooked food containing the same caloric content as the equivalent uncooked food.
4. Evolutionary biology.  More nutritive and tender cooked food “remodeled” the bodies of early humans, enabling the evolution of a weaker jaw, smaller teeth, and a smaller digestive tract. Gorillas and chimpanzee are mainly herbivores. They have large molars to chew plant foods, and voluminous stomachs and large intestines capable of digesting and fermenting fiber from plants.  By contrast, humans are less able than apes to digest, ferment and utilize fiber — largely because eating cooked food reduces the need for this capacity. The tenderness of cooked food enabled the human jaw and teeth to shrink.  Human jaws and molars are the smallest of any primate species, relative to body mass.  As a result of their more nutrient-dense cooked diet, humans eat half as much per pound of body weight as do great apes. The shift to a cooked meat diet also coincided with a doubling of the size of the brain of Homo erectus relative to that of Homo habilis and earlier hominids.  The “expensive tissue hypothesis” holds that more nutritive cooked food dramatically increased the energetic efficiency and intelligence of humans, freeing us from time spent gathering and digesting, increasing hunting and migration range and propelling reproductive success.

The evolution of toxicity.   One of the corollaries of the shift from raw to cooked food by early humans was a re-tooling of their sensitivity to and tolerance for dietary toxins.  Wrangham draws out some very important, but often overlooked, consequences of this evolutionary change:

Beyond reducing the size of teeth and guts, the adoption of cooking must have had numerous effects on our digestive system because it changed the chemistry of our food. Cooking would have created some toxins, reduced others, and probably favored adjustments to our digestive enzymes….Take, for example, Maillard compounds, such as heterocyclic amines and acrylamide…They occur at low concentration in natural foods but under the influence of heat their concentration becomes much higher than what is found in nature…They can also induce a chronic state of inflammation, a process that raw-foodists invoke to explain why they feel better on raw diets. The cooking hypothesis suggests that our long evolutionary history of exposure to Maillard compounds has led humans to be more resistant to their damaging effects than other mammals are. It is an important question because many processed foods contain Maillard compounds that are known to cause cancer in other animals. Acrylamide is an example. In 2002 acrylamide was discovered to occur widely in commercially produced potato products, such as potato chips. If it is as carcinogenic to humans as it is to other animals, it is dangerous. If not, it may provide evidence of human adaptation to Maillard compounds, and hence of a long exposure to heated foods.

So while humans have adapted to better tolerate “toxins” like Maillard compounds in cooked foods, the converse is true for certain plant toxins that we expect apes to tolerate better than humans.  Wrangham observes how this shift is readily demonstrated by differences in palatability and food preference between our species and the apes:

In my experience of sampling many wild foods eaten by primates, items eaten by chimpanzees in the wild taste better than foods eaten by monkeys. Even so, some of the fruits, seeds, and leaves that chimpanzees select taste so foul that I can barely swallow them. The tastes are strong and rich, excellent indicators of the presence of non-nutritional compounds, many of which are likely to be toxic to humans—but presumably much less so to chimpanzees….The shifts in food preference between chimpanzees and humans suggest that our species has a reduced physiological tolerance for foods high in toxins or tannins. Since cooking predictably destroys many toxins, we may have evolved a relatively sensitive palate. By contrast, if we were adapted to a raw-meat diet we would expect to see evidence of resistance to the toxins produced by bacteria that live on meat. No such evidence is known. Even when we cook our meat we are vulnerable to bacterial infections….The best prevention is to cook meat, fish, and eggs beyond 140º F (60º C), and not to eat foods containing unpasteurized milk or eggs. The cooking hypothesis suggests that because our ancestors have typically been able to cook their meat, humans have remained vulnerable to bacteria that live on raw meat….We fare poorly on raw diets, no cultures rely on them, and adaptations in our bodies explain why we cannot easily utilize raw foods. Even vegetarians thrive on cooked diets. We are cooks more than carnivores.

Wrangham’s “cooking hypothesis” is certainly intriguing.  But is there any hard evidence that humans and apes (to say nothing of mice) actually have different detoxification mechanisms going on in their bodies?