Red meat, human vulnerability, and.. mammal pets?

12993520_545709718944195_6669954929821747069_nExciting day! Another diet-related talk at ASU’s Center for Evolution & Medicine. This was a nice break from the horror that is the last 2 weeks of the semester..

It’s taking me awhile to “digest” all the information (hehe), but I found the seminar fascinating and wanted to summarize some main points. Lots of open questions remain, but John Pepper of National Cancer Institute really shows how examination of any health problem needs to focus not only on proximate causes, but the ultimate or evolutionary causes.

So.. Pepper asks- why is mammal meat bad for humans, specifically?

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Meet Dr. Pepper!
In humans, red meat (he refers to it just as mammal meat) is linked to inflammatory diseases (cardiovascular, alzheimer’s, arthritis). What’s the mechanism behind this?
The inflammation from mammal meat has to do with our antibodies attacking something coming from other species.. When we eat mammal meat, we in fact incorporate something non-human from the diet- sialic acid.
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Both human and other mammals have sialic acid in their tissues, actually, but humans have a unique mutation that replaces the form found in other mammals (ancestral form- Neu5Gc) with a different one- uniquely human (Neu5Ac).
So.. if we eat meat we get the new aquired ancestral sialic acid, it becomes part of our cells, and the small structural differences in the two get recognized by the immune system.. which responds with a defense- inflammation!
Chimpanzees are humans’ closest evolutionary relatives, sharing a common ancestor 6–7 million years ago..
WHY does human sialic acid differ uniquely? The “Malaria hypothesis” (see Martin&Rayner, 2005) proposes that in Africa, early humans escaped from the ancestral pathogen they shared with chimpanzees. They managed to do so by replacing the pathogen’s binding target (ancestral sialic acid Neu5Gc) with novel Neu5Ac. With time, a population of that old evaded pathogen evolved to infect humans again by recognizing the new Neu5Ac..leading to the origin of malaria.
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The longer an animal has been domesticated, the more humans share parasites and diseases with them

If the Malaria Hypothesis explains why the initial change in humans happened.. why has it remained the same to this day? I mean, it’s been some several million years now- has this mutation been advantageous this whole time? It’s an important question because this sialic acid mutation poses a COST on our health: this trait causes chronic inflammation in people who eat mammal-derived foods + it also now causes vulnerability to malaria.

The hypothesis for why the human sialic acid modification is still around is that it
provides benefits- specifically, protection from parasites and pathogens via increased inflammation. This is relevant because of what humans have been doing for the last ~15,000 years. Animal domestication!
Humans are more vulnerable to shared pathogens from other mammals (than from non-mammals). So being around cattle, for example, carries a risk of catching pathogens from which that cattle suffers. Such animal pathogens impose a strong selective pressures on humans.. Pepper suggests that the uniquely human sialic acid (Neu5Ac) allows our diet to adapt us to the issue of animal pathogens by adjusting our inflammatory tone (how much inflammation we are experiencing): “those human populations that are exposed to domesticated food-mammals and their pathogens are also eating mammal-derived foods that are pro-inflammatory (both meat and dairy).”
Inflammation is a great example of a trade-off. It both has benefits (protection from parasites & infections) and costs (chronic disease, metabolic expense of mounting an immune response). The optimal balance for this trade-off would depend on how strong of a pathogen pressure you’re experiencing.
This increases inflammatory PROTECTION only where it’s most needed (like around animals). So this auto-immune inflammation from mammal foods in the diet not only increases likelihood of chronic disease, but protects against shared mammalian pathogens.
…..    ……    ……
It got me thinking about human culture and our ability to modify our environment in all sorts of ways- an example of “maladaptation” to modern times! Living in cities, not exposed to higher pathogen load from being around domesticated animals..yet having access to all the mammal meat we can buy = all put you in a situation where the good old sialic acid mutation might do more harm than good. Should people go vegan? Should they simply cut down on red meat? There was no discussion on the effect size of mammal meat eating and chronic disease, so I wouldn’t necessarily jump onto any lifestyle changes based on this talk. Yet the process of understanding this health concern through the lens of evolutionary medicine is quite fascinating!
 P.S. I’m not an expert on this topic. If you have something to correct or add, please comment 🙂
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Very cool use of evolutionary medicine principles in this case & a glimpse into why it’s important to use them if we want to understand disease.


No, we are not carnivores. But we’re no apes either.

I often read on vegan health groups that humans are not meant to eat meat because our guts are long like those of the apes (who are predominantly plant-eating) vs. the short guts of carnivores (meat-eating animals). In fact, I used to argue this way myself in my vegan days. 🙂

However, once you understand a topic a bit better, the simplified and incorrect statements about it simply irritate you.


Human guts, when compared to those of existing apes, have similarities AND differences. Humans and apes show the same gut anatomy- simple acid stomach, a small intestine, etc.. However, humans stand apart from all apes: more than half of human gut volume is found in the small intestine while all apes have by far the greatest total gut volume in the colon; also the overall size of the human gut in relation to body size is small in comparison to that of apes.


What this means is that humans have adapted to a “high quality” diet. Here high quality means a calorie and nutritionally DENSE diet, which includes animal foods and tubers. A high quality diet for humans means that we need to eat a smaller volume of food to obtain the nutrients and energy we need.

The way our gut differs from apes says a lot. Most large primates have expanded colons, which is an adaptation to fibrous low-quality diets [“low-quality” here = highly fibrous foods such as leaves and bark]. The large colon allows fermentation of low-quality plant fibers (which allows extraction of additional energy in the form of volatile fatty acids). Our relatively enlarged small intestine (the principal site of nutrient digestion & absorption) and smaller colon reflects an adaptation to an easily digested diet that is nutrient-rich.

There is a general consensus that current hominoids (apes and humans) come from a strongly plant-eating ancestry. Apes, however, evolved into larger bodies that allowed to sustain themselves on lower quality diets. By eating both animal matter [to satisfy requirements for essential nutrients] and plant sources [primarily for energy] humans were able to avoid the constraints imposed by body size increases in apes (such as lower mobility and sociality in apes). Simply speaking: the bulkier they get and the more time they spend eating, the less they move around and the less social they are, which is a disadvantage compared to humans.uuuu2


This dietary change in humans (adding animal and other dense foods), which departed from known plant-dominated diets of the apes, was eventually reflected in our brain size (much larger), overall form of our guts (shift in gut proportions, overall gut size) as well as dentition (smaller teeth, jaws). Concerning our particularly large brains: our brains are particularly energetically “expensive” as we expend a larger proportion of our daily energy on brain metabolism than other primates (in comparison to other primates, our brains are 3 times the size). Paleontologists believe that fast brain evolution happened about 1.8 mln. years ago and was associated with important changes in diet and foraging behavior (some argue it is specifically the addition of meat that allowed for such large brains to evolve). Apart from switching to high quality nutrition, humans show other adaptations to having a larger brain- compared to other primates we are “undermuscled” (less skeletal muscle) and fattier. Greater level of body fatness in human infants in fact helps grow a large brain by having stored energy and reducing energy requirements of the rest of the body (that has less muscle mass).


The point is: no, we’re definitely not “meant” to be vegetarian. Also, the point is not to say that vegetarianism doesn’t make sense for many of us. There are plenty of great reasons to avoid animal foods (ethics, environment, etc.), plus it’s easy to have an adequate veg. diet for adults with availability of supplements (vegan diets are not recommended for small children, though, considering brain development; the several vegan PhDs I know did not raise their children vegan specifically because of this) … but stating that we are not meant to thrive on both animal and plant sources is incorrect. Contrasting us with true carnivores [like cats] to show how very different we are (e.g. hey we don’t have claws and sharp teeth… um.. we however do have large brains to allow for sophisticated tool creation that replaces those) is also a terrible idea- we are not true carnivores either and have a dual dietary strategy [plants + animal sources].

Lastly, all this material should not support the notion that we ought to eat bacon 10 times a day.. meat clearly has a place in our diet, but this shouldn’t be used to justify a purposeful meat overload (I’m not sure what the benefit is for advocating heavily animal-based diets, considering modern animals are fattier and less packed with phytochemicals than the wild ones + there’s the whole issue of antibiotic and hormone use at the minimum. Unless, of course, you go on the “carbs are evil” side, but I am not in favor of that view… or you imagine our ancestors ate predominantly meat, which does not look to be the case since the diets varied dramatically depending on environmental circumstances). 

(Note: I’m not pretending to actually be an expert on any of these topics; I simply read peer-reviewed articles and hang out with evolutionary anthropologists 🙂 ).


Milton, K. (2003). The critical role played by animal source foods in human (Homo) evolution. The Journal of nutrition, 133(11), 3886S-3892S.

Leonard, W. R., Snodgrass, J. J., & Robertson, M. L. (2007). Effects of brain evolution on human nutrition and metabolism. Annu. Rev. Nutr., 27, 311-327.

P.S. The bottom line is: these t-shirt designs are both incredibly dumb (disclaimer- i laughed a bit..but immediately felt guilty :D)

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