Humans are biologically designed to be carnivorous, so how did we get to red meat causing cancer?
Paul Stapleton says the apparent carcinogenic properties of preserved meats may also have something to do with unwise and relatively recent changes in the way we fatten up livestock
The recent news that red meat, especially preserved meats such as bacon and ham, causes cancer may come as a surprise. Putting bacon in the same category as tobacco seems extreme given that, unlike cigarettes, it is a good source of protein and other nutrients.
Our species, homo sapiens, are clearly meant to eat meat. Inside our mouths we have canine teeth, albeit not so sharp; we have the binocular vision of a predator; and we have only one stomach to process our food, unlike the four stomachs of cows and some other grass eaters. Thus, how is it that a fundamental and natural food product such as red meat could be carcinogenic?
The answer to this question, like many related to human health and disease, is complex, but a brief tour into our evolutionary past can shed some light.
The latest research based on fossils and sophisticated dating techniques suggests that homo sapiens emerged in eastern or southern Africa about 200,000 years ago. Before the advent of agriculture, about 10,000 years ago, the fossil record indicates that we did eat meat, but from a very wide variety of animal sources. However, with the start of agriculture, our ancestors found that only a few animals were easy to domesticate - cows, pigs, goats and sheep being the most notable.
The big resulting change brought by agriculture meant that meat was much more available, but the animals it came from much more limited. Back then, with all the extra meat on hand, but without refrigerators to keep it edible, we learned to preserve meat using salt and smoke. In sum, during about 95 per cent of our evolution, our DNA adapted to a certain diet, but this suddenly changed about 10,000 years ago. By comparison, our closest cousin, the chimpanzee, who remained in the forest from the time it split from our common ancestor six million years ago, has continued eating its mostly vegetarian diet until the present. For chimps, meat is a rare treat.
But another big change has taken place much more recently that may be further contributing to an unhealthy diet. And this change has occurred within just the last couple of generations. A preponderance of the red meat we now eat comes from livestock that spend the last few months of their lives being fattened up on corn, rather than their natural food, grass. And they spend their lives motionless in crowded pens. This results in fatty and tender beef and pork, a type of meat never eaten by our ancestors.
Could this new super-tasty meat that has arrived in the last blink of an eye in the history of our species be one of the causes of disease? Answering this will take more research, but big changes in diet and behaviour can often have negative effects on health.
In the meantime, there is another more banal reason for why researchers are just now making this connection between red meat and cancer. Simply stated, through our long evolution, we seldom lived long lives. Those that reached their fifth decade were exceptional. What this means is that no matter what we ate through most of our history, it is unlikely that we would have got cancer from including it in our diet because generally speaking, cancer is a disease of the aged. During our long evolution, as long as we reached sexual maturity and had a chance to reproduce, nature had succeeded in propagating our species. In other words, diseases like cancer did not have much of an impact on our survival as a species because they hit us long after we had children. As far as evolution is concerned, reproduction is all that matters.
And finally, as for canine teeth being a good indicator that we should eat meat, look at the hippopotamus. Its canines are often a foot long, yet it is a vegetarian.
Paul Stapleton is an associate professor at the Hong Kong Institute of Education