Sunday, January 27, 2013

The Carnivore Connection Hypothesis: Part 1

The carnivore connection hypothesis (CCH) proposes that insulin resistance is an evolutionary adaptation to a low carbohydrate diet, which is based on four lines of evidence [1] [2] [3]. 

1.      “That during the last two million years of evolution, humans were primarily carnivorous, i.e., flesh-eating hunters consuming a low-carbohydrate, high-protein diet”
2.      “That a low-carbohydrate, high-protein diet requires profound insulin resistance to maintain glucose homeostasis, particularly during reproduction”
3.      “That genetic differences in insulin resistance and predisposition to NIDDM can be explained by differences in exposure to carbohydrate during the past 10,000 years”
4.      “That changes in the quality of carbohydrate can explain the recent epidemic of NIDDM in susceptible populations.” 

Carbohydrates in Traditional Diets 

The third line of evidence is an attempt to explain the differences in IR and T2D between populations - how the Pima and Nauruans have some of the highest rates of T2D and how Australian Aborigines and Eskimos have higher rates of T2D than Australians/Canadians of European descent. 

The differences are based on the first line of evidence, which suggests that hunter-gatherers are mostly carnivorous.  The CCH papers (1994, 2002 and 2011) cite 1977 book by Eaton and Konner to suggest hunter-gatherers ate between 10-125 grams of carbohydrate a day, but in their Paleolithic Nutrition paper (1985) they estimated an average macronutrient ratio of 34:21:45 (P:F:C), which they later (1997) updated to 37:22:41 [4]. 

Cordain, et al (2000) found a problem with previous estimates and instead suggests on average hunter-gatherers got 19-35% of their calories from protein, 28-58% from fat and 22-40% from carbohydrate [4].  So although on average, hunter-gatherers consumed less carbohydrate that we currently do, the average wasn’t as low as 10-125 grams (at 2,000 calories 22-40% carbohydrate is 110-200 grams, at 3,000 calories it’s 165-300 grams) 

Jenny Brand-Miller*, one of the co-authors of the CCH papers should be aware of this as she is also a co-author of [4].  But that’s ok, it doesn’t necessarily refute the CCH, though one could speculate at the possibility of any selection bias.  What may somewhat refute the CCH are cultures that have historically eaten high carb diets, yet are vulnerable to IR/T2D 

The CCH uses the Pima, Nauruans, Australian Aborigines and Eskimos as examples of cultures who have historically eaten low carb diets and are vulnerable to IR/T2D.  The traditional Nauruan diet seems to be mainly fish and coconut, the traditional Aboriginal diet may have been high carb [5], but would have depended on location and season, and the traditional Eskimo diet was likely ketogenic.  But the Pima traditionally ate a very high carb diet** [6]. 

Due of their high carb diet, the CCH would predict the Pima to be quite carbohydrate tolerant and resistant to IR/T2D, yet they have among the highest rates of IR/T2D in the world. 

The CCH also can’t explain how other high carb eating HGs/TCs are vulnerable to IR/T2D.  For example, native Hawaiians have higher rates of T2D than Caucasians living in Hawaii (14% vs 3%) [7] and the traditional Hawaiian diet was likely very high in carbohydrates [8].  There are bound to be other examples. 

The alternative explanation, which I mentioned in the thrifty gene hypothesis (which is actually kind of similar to the CCH) is that HGs/TCs have had less adaptation to western foods whether it’s alcohol, milk, fruit, seeds, refined sugar, nutrient poor foods, etc. 

* For those who like gossip: Jennie Brand-Miller’s professional career is devoted to GI, many of her published works are on the benefits of low GI diets.  She is a co-author of “The Australian Paradox: A Substantial Decline in Sugars Intake over the Same Timeframe that Overweight and Obesity Have Increased[9] (there’s an entire website to rebut the Australian Paradox paper), the Paleo related paper I mentioned above, and has argued that it’s high GI foods, but not sugar, that has caused increases in obesity and T2D 

** Gary Taubes tells us the Pima got fat and diabetic because they increased their carbs and Jennie Brand-Miller tells us the Pima got diabetic because they were previously eating low carb.

This blog post is in two parts.  Read part 2 here

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