What I'd like to do is zoom in and examine some of these amazing lifeforms in closer detail. Parasites are widely reviled, and for many good reasons. But people tend to overlook the fact that parasites also often grant their host a small benefit, thus maximizing their own fitness.
There is a strong tendency among fancy-pants Global Northers, sheltered as we are in the most hygienic environment that has ever existed for our species, to unconsciously project our relatively parasite-free daily existence backwards into the past, by failing to account at all for the role of parasites in our evolution. It's this blindness to parasitic co-evolution, I'd argue, that lies at the root of the grain-blamers' obsession with grass-fed animal flesh, cholesterol denialism, and other expressions of low-carber paleofantasy. After all, they see the evidence of low LDL in extant hunter-gatherers who eat lots of meat, and can't make sense of the apparent paradox in light of what current health science tells us about the effects of saturated fat and cholesterol. How can these kinds of foods be bad for us, they wonder, when they're obviously not bad for the !Kung? Maybe the secret lies, they speculate, in the nutrient profile of the prey animal. Maybe there's some esoteric "optimal" macronutrient ratio in hunter-gatherer diets that modern man has lost knowledge of. Or maybe cholesterol and saturated fat aren't really bad for anyone, and the truth is being suppressed by a powerful grain lobby.
I submit there's a simpler answer to resolve this paradox: lipid-eating parasites have been hitching rides inside hominid bodies since before we were hominids in the first place.
That's what this little half-tribute series will be about. I'm calling it the Paleo Parasite Profiles, and each installment will focus on a particular clade of the hidden passengers who helped give our paleo ancestors good heart health by keeping their serum lipids low. And I say "half-tribute" because I don't want you to forget that many of these pugnacious little buggers impose a high cost on us for their fat-munching services. Nor do I want you to get the impression that there's anything altruistic or mutualistic in the relationship; it's almost purely exploitative (in the parasite's favor) from our point of view. There's no spiritual kindness motivating the parasites; they're entirely self-serving. From their point of view, we are just walking bags of nutrients. They sneak their way inside us, take root somewhere dark and safe, and gorge on fat and cholesterol to their little organelle's content.
They're nature's ultimate low-carbers.
So, let's start with the spooky-cutest parasite of the bunch.
Genus & species: Giardia lamblia
|Giardia lamblia, spooky-cutest low-carber of them all.|
Sources of infection: water or food contaminated by fecal matter of carriers; unprotected anal sex. G. lamblia is one of the leading causes of water-borne diarrhea in the U.S. He's a particular problem for hikers and campers taking their water from rivers, and to children in under-developed countries. Giardia is the most common water-borne disease in humans. In the ancestral environment, he'd have run rampant through a small band of hunter-gatherers.
Manner of infection: Giardia has two life-stages: the cyst, which exists in a protective shell outside of a host; and the trophozoite (pictured at left), which lives inside a host. Giardia cysts most commonly enter a host by getting ingested in contaminated food or water. They weather the acid storm of the stomach and make their way to the small intestine where, in the duodenum, the cysts open up and each release two bi-nucleated trophozoites (this process is called excystation). Each trophozoite is equipped with a large adhesive disc made of cytoskeletal proteins on its ventral side, which it uses to attach to the epithelial walls of the duodenum and the jejunum. They reproduce asexually through binary fission, then eventually grow a new cyst, detach from the intestine wall and move out of the host's body through its feces.
Symptoms of infection: Giardia infection can be asymptomatic and/or chronic, but often runs its course in a matter of weeks. When symptomatic, it usually presents through either chronic or intermittent foul-smelling diarrhea that's light colored, greasy and mixed with mucous. Other common symptoms are weight loss (especially in the context of good appetite and adequate food intake), listlessness, vomiting, nutrient malabsorption, and retarded growth in immature animals.
Lipid service: G. lamblia trophozoites do not have the capacity of de novo fatty acid synthesis, and have to obtain their cholesterol and phosphatidylcholine from an external medium. In layman's terms, that means they can't make their own cholesterol, and have to get it from their environment. And their environment is us.
When a G. lamblia cyst enters an environment that's rich in LDL or VLDL cholesterol (like, say, the duodenum of a hunter-gather who eats a lot of meat), it releases its trophozoites. The trophozoites then start to absorb various kinds of lipids from the host medium, and use them to synthesize the lipids they need for their own purposes (chiefly, the building and maintenance of cell membranes). When and if they pass into a lipid-poor environment (like, say, the ilium or large intestine of a hunter-gatherer who eats a lot of meat), cyst formation is stimulated, and Giardia gets the heck out of there. The drop in environmental lipids acts like a signal telling him, "no more food here, time to move on."
Humans infected with G. lamblia have been found to possess lower lipid parameters (LDLc, HDLc, and total cholesterol) than uninfected control groups.
All of this, taken together, means that a Giardia-infected hunter-gatherer population that eats a lot of meat can be expected to have a comparatively low lipid profile compared to the fancy-pants (and largely parasite-free) Global North low-carbers who are trying to mimic them. And remember that since Giardia infection can be both chronic and asymptomatic, it wouldn't necessarily be obvious to an outside observer that parasitic infection was playing a role here.
Obviously, we fancy-pants Global Northers would like to avoid parasitic infections, even by spooky-cute Giardia here. But, we'd also like to maintain a "primitive" lipid profile below about 150 or so. We can't do that by eating lots of fat and cholesterol, unless we're infected with Giardia or some other lipid-eating parasite. But we can do it by eating a whole-foods, plant-based diet.
Or by using statins.
Which do you prefer?
More on Giardia lamblia:
Mechanisms of Giardia lamblia Differentiation into Cysts (Lujan, Mowat, & Nash, 1997)
Lipid metabolism in Giardia: a post-genomic perspective (Yichoy, et al., 2011)