By Christian Wernstedt
Sometimes we paleo folks cringe a bit in our seats when the Kitavans of Papua New Guinea are brought under discussion. The existence of the Kitavans seems to somehow challenge some of our assumptions about what aspects of a paleo diet really drive its health benefits, or even what a paleo diet is.
The Kitavans is a hunter-gatherer/semi-agriculturalist population whose members eat a diet composed of a whopping 70% of calories from carbs, only 20% from fat, and the remaining 10% from protein.
Yet with this high carb diet, the Kitavans are exhibiting the absence of obesity and disease that we associate with our standard paleo template which typically prescribes a diet much lower in carbs, much higher in fat, and higher in protein than the Kitavans' standard fare.
Further, as we tend to be fans of Gary Taubes' meticulously worked out thesis that carbohydrates drive insulin, and that insulin drives fat storage and obesity, we might be tempted to pull an Ancel Keys on the Kitavans and relegate them to some sideshow of "irrelevant" outliers.
So what's really going on with the Kitavans and how do they fit within our paleo world view?
The following discussion doesn't try to cover all the factors that may be at play, but I think that it will illustrate how the integration of observations of the Kitavans into our big picture actually deepens and strengthens our understanding of the virtues of a paleo approach to diet.
In order to proceed we need to recognize that Taubes' carbohydrate hypothesis must be interpreted within the context of additional hormonal- and environmental factors besides insulin and carbohydrates.
One of the most important factors in this regard seems to be hormone leptin, and the ways by which its function may be interfered with.
Leptin is a hormone secreted by the fat tissue which speeds up metabolism and acts as an appetite suppressant. Leptin also helps to ensure that fat is neatly stored in our fat cells, and that it is not instead packed between- or inside our organs, which is a very disruptive situation associated with Western disease.
One could say that leptin is the fat tissue's way of ensuring a peaceful relationship with other tissues in the body so that an appropriate amount of spare calories can be stowed away as fat, but no so much that the fat deposits grow dysfunctionally large.
Now, quite unfortunately, there is such a thing as leptin resistance, which makes our brains deaf to the fat tissue's leptin signal. This deafness to leptin in turn makes us hungrier than we should be, and thus allows fat deposits to expand more readily, and also into places, such as the liver, where we absolutely don't want any fat. (With a nod to Tabues, I must say that eating insulin-spiking carbs certainly doesn't help in this situation, but there is such a thing as over-eating fat and protein too, especially in the context of a slowed down metabolic rate.)
Leptin resistance can be caused through different avenues (one of them over-consumption of carbs, particularly fructose), but more to the point in our discussion of the Kitavans is that certain proteins (lectins) in grains may be a powerful causal factor in leptin resistance through blocking leptin receptors in the brain.
This potential for grains-induced leptin resistance helps to explain why Westerners eating a 60% carb diet based on a large amount of grains become leptin resistant and wind up with obesity and associated metabolic problems, whereas the Kitavans who might consume the same proportion of carbs in their diet, though from non-grain sources such as root vegetables, instead maintain leptin sensitivity, a lean body composition and the absence of disease .
The above line of reasoning is put forth and supported by two very compelling papers by Staffan Lindeberg, et al, and Tommy Jönsson, et al. (The latter is a particularly excellent example of a carefully constructed hypothesis that integrates observations from epidemiology and evolutionary biochemistry - highly recommended for the science geek.)
Another set of hormonal- and environmental factors that potentially have bearing on why the Kitavans are so healthy may be the hormone adiponectin and the nutrional factors that impact its production.
Adiponectin is a hormone that (as is the case with leptin) is secreted by the fat tissue, which quenches inflammation and increases insulin sensitivity. Adiponectin thus acts as the body's own antidote against the metabolic syndrome's hall mark symptoms which are precisely insulin resistance and inflammation. Similar to leptin, adiponectin is likely one of natures' way to make sure that we can expand our fat stores without adverse systemic side effects.
Interestingly, a dietary factor that is recognized as interfering with the absorption of magnesium, calcium, and other minerals is the lectins and phytic acid present in grains and legumes.
In other words, foods that mainstream Westerners tend to eat as dietary staples (particularly grains) block the minerals that promote adiponectin, while the Kitavans do not eat these mineral blocking foods!
I haven't seen any data on this, but I would not be surprised if it is the case that to the extent that the Kitavans actually become a little chubby (such as the individual to the right in the picture above), they probably also have higher levels of adiponectin compared to Westernes with similar body composition, and that one potential explanation for this is that the Kitavans are less prone to mal-absorption of crucial minerals. (Of course the absence of mineral deficiency also has other wide ranging positive effects.)
To wrap up this discussion, we can see that qualitative aspects of a diet (such as, in this case, the presence or absence of grains) may quite dramatically condition how our bodies deal with a particular ratio of macro nutrients, or availability of calories for that matter. (A person with his leptin receptors blocked by grain lectins tends to be a hungrier person! )
This in turn hints at a really cool epistemological aspect of the paleo dietary approach: Paleo doesn't focus on individual nutrients in a narrowly mechanistic manner. Instead it provides us with a integrative framework that isn't at all threatened by new observations or the presence of unusual cases. I don't know of any other dietary- or lifestyle approach that accomplishes this.