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    Bill_89's Avatar
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    Lactase Persistence: A Scientific Question

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    Lactase persistence, which is the retention of the ability to digest lactose into adulthood, is a textbook example of human adaptation that occurred in the not so distant (on a geologic time scale) past. The story goes that those people who domesticated cattle and consumed their milk (most notably, northern Europeans) developed lactose persistence, while those people who did not (mostly Africans and Asians, though, importantly, NOT Africans who also happened to domesticate cattle, suggesting this is an issue of environmental context, not race) became lactose intolerant in adulthood.

    I have two big questions about this.

    First, how could have this evolved, especially so rapidly? An adaption will not evolve unless it confers increased fitness--a reproductive advantage. Was lactose intolerance so detrimental to fitness that the difference between it and lactase persistence was the difference between passing on one's genes or not? The evidence suggests yes, but I guess it's hard for me to fathom . . .

    (On an unrelated, but somewhat similar topic, I've heard some people argue that Type I diabetes is more common in northern Europeans because it conferred an advantage in a cold environment, which makes absolutely NO sense to me given what a huge fitness disadvantage Type 1 diabetes is, particularly pre-Insulin injection days.)

    Secondly, does this pose a challenge to the evolutionary rationale for avoiding grains? I am not trying to be hostile to the philosophy of grain abstinence as there seem to be plenty of nutritional/biochemical reasons for avoid them (as Mark says, there is no good reason to eat grains except as a cheap source of calories that quickly convert to glucose). This is more of a philosophic/scientific question.
    Last edited by Bill_89; 04-29-2011 at 09:54 AM.

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    Alex Good's Avatar
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    Yep.
    In all of the universe there is only one person with your exact charateristics. Just like there is only one person with everybody else's characteristics. Effectively, your uniqueness makes you pretty average.

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    Lactase persistance did not develop *because* of animal husbandry. It is a genetic mutation. It did, just by chance, confer a reproductive advantage on those who had it when living in Northern Europe where it is bloody cold and you need every last calorie you can find to make it through the winter. Being able to eat milk and preserve milk products such as cheese became an advantage *under those climate conditions*, as opposed to the African people you mentioned.

    The T1 Diabetes argument is simply rubbish.

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    Persistence into adulthood of childhood traits is a supreme boon for expedient adaptation. In any given generation you will have a degree of persistence of any particular childhood trait will persist into adulthood to a particular potency and frequency and what will will statistically add up in survival benefit over the generations. Perhaps someone has enough lactase persistence to increase their lifespan by 2 years and be somewhat stronger as a worker, then that could mean the difference between 3 children and 5, and then those genes will be passed on and will tend to flourish so long as there is selection pressure.

    Source: Richard Dawkins - The Greatest Show On Earth (wasn't about lactase in general but he used some of the physical traits of chimps to compare to humans as examples)

    There is however no such thing as childhood grain-tolerance traits. I'm not saying we don't have some adaptation, and some useful memes like cooking and soaking, but some of us are clearly better adapted to lactose than grain toxins.
    Stabbing conventional wisdom in its face.

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    Secondly, does this pose a challenge to the evolutionary rationale for avoiding grains? I am not trying to be hostile to the philosophy of grain abstinence as there seem to be plenty of nutritional/biochemical reasons for avoid them (as Mark says, there is no good reason to eat grains except as a cheap source of calories that quickly convert to glucose). This is more of a philosophic/scientific question.
    No... Mark points out in the PB that lactose persistence is probably the only genetic mutation/adaptation in the last 10,000 years that affects what some humans can eat, and differs by ancestry/genetics. Otherwise what is evolutionarily appropriate for humans to eat is pretty much the same, i.e. not grains, especially not wheat, because we haven't really faced selection pressures in recent times that would make eating grains somehow a genetic advantage.

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    Quote Originally Posted by Stabby View Post
    There is however no such thing as childhood grain-tolerance traits. I'm not saying we don't have some adaptation, and some useful memes like cooking and soaking, but some of us are clearly better adapted to lactose than grain toxins.
    While there aren't specific childhood grain-tolerance traits, I think it's worth mentioning that humans did develop an adaptation for breaking down starches:

    Starch consumption is a prominent characteristic of agricultural societies and hunter-gatherers in arid environments. In contrast, rainforest and circum-arctic hunter-gatherers and some pastoralists consume much less starch1, 2, 3. This behavioral variation raises the possibility that different selective pressures have acted on amylase, the enzyme responsible for starch hydrolysis4. We found that copy number of the salivary amylase gene (AMY1) is correlated positively with salivary amylase protein level and that individuals from populations with high-starch diets have, on average, more AMY1 copies than those with traditionally low-starch diets. Comparisons with other loci in a subset of these populations suggest that the extent of AMY1 copy number differentiation is highly unusual. This example of positive selection on a copy number–variable gene is, to our knowledge, one of the first discovered in the human genome. Higher AMY1 copy numbers and protein levels probably improve the digestion of starchy foods and may buffer against the fitness-reducing effects of intestinal disease.
    Perry, GH, et al. Diet and evolution of human amylase gene copy number variation, Nature Genetics 39:1256-1260 (2007)

    This has no impact whatsoever on lectins or gluten, so there's still a great rationale for avoiding grains, but it does suggest that humans have evolved to eat starches.
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    mysterious double post!
    Last edited by theholla; 04-29-2011 at 01:36 PM.
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    Yeah I'm not against starch in moderation. We were consuming starchy roots long ago and I can't really find evidence for tubers being unhealthy except perhaps less healthy if you base your diet on them then if you based it on fat.
    Stabbing conventional wisdom in its face.

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    Probably less healthy. Definitely less fun. Mmmmm...fatty fat fat.
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    So is lactase persistance not an example of use-it-or-lose-it? would not a person who continues to consume milk after weaning retain the ability to digest it? while a person who stops consuming milk after weaning would lose that ability?

    at least, in individuals who are not strongly genetically inclined one way or the other.

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