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← Meet Bill Durham: Lactase Persistence Expert

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Showing Revision 1 created 10/10/2013 by Cogi-Admin.

  1. My name is Bill Durham. I'm Bing Professor in

  2. Human Biology, in human biology and anthropology at Stanford University.
  3. >> Well, we're really happy you're joining us, especially to talk about a
  4. very important concept. Now, we've just
  5. been learning about lactose intolerance, and about
  6. the idea that as all infants, we had the ability as mammals to
  7. digest these lactose containing products. And as
  8. we get older, our lactase expression diminishes.
  9. But in some people, the lactase gene sort of stays
  10. on, and we continue to make lactase. And so I guess
  11. we'll start out by asking, why, in human evolution, would
  12. something like this happen? How could something like this even happen?
  13. >> Well from a pan-mammalian background, where the
  14. lactase activity peaks around birth and then tails
  15. off quickly into the weaning period and into
  16. adulthood, we assume, we know that that's pan-mammalian.
  17. Even your house cat is lactose malabsorbed, a malabsorber
  18. or lactose intolerant. At least every house cat that's
  19. been tested is, even though people will give milk
  20. to their house cats. And we don't know is
  21. whether the people have found the threshold, or the
  22. cat's found the threshold, to avoid diearaha and other
  23. consequences. What's interesting, then, is from that background of
  24. a peak, some human populations show persistant lactase activity into
  25. adulthood, while the majority continue to decline.
  26. So it's that, minority of lactose-tolerant adults
  27. that are really fascinating. it's a wonderful
  28. question. Because here's a case where you have
  29. an intraspecific polymorphism. You have the existence
  30. in, if the ongoing persistence of a genetic
  31. difference between human populations, with regard to
  32. some relatively simple trait. And a trait that
  33. has to be relatively recent in human history.
  34. >> Sure.
  35. >> There were no sources of milk prior to dairy animals.
  36. >>
  37. [LAUGH].
  38. >> So, what we have here is, I mean,
  39. of the mother's milk, there were no mammalian sources of
  40. milk for human populations before agriculture. So, we assume
  41. that the panamalian background, means that at the dawn of
  42. history, human populations were all lactose intolerant. And that
  43. there would be at best, a mutationly low frequency of
  44. individuals that varied on that characteristic. And because it was
  45. a very low variation and there was no milk available,
  46. we would have no way of knowing how much
  47. there was or how often it recurred and so on.
  48. But what we have then, in the meantime, is
  49. this great invention called agriculture. The domestication of plants and
  50. animals at the service of the human food supply.
  51. We have every reason to believe that, the main animals
  52. that are used for milk today were originally domesticated for
  53. meat. They were valued first for meat, and that milk
  54. was, as the expression goes, was part of
  55. a secondary products revolution where they realize you
  56. could use more then the meat from these
  57. animals. Goat, sheep, you know, cattle and so on.
  58. And so they already probably had them in
  59. close association with human communities. They were probably already
  60. harvesting the meat regularly. They probably had mastered,
  61. over winter, water supply, hearding activities, and that sort
  62. of thing. And then suddenly there's evidence for using the,
  63. the milk product of these animals. some of the earliest evidence comes from
  64. Sahara rock paintings that are dated to 4,500 B.P., that show,
  65. in fact, bringing the calf up near the
  66. female, the adult female, and a human being underneath.
  67. Now we infer that the human being underneath
  68. is doing this. What else is a human being
  69. doing under a cow?
  70. [LAUGH]
  71. and the question really is what was the added value of drinking fresh milk?
  72. It's complicated on the one hand, because
  73. you had the meat value of these animals
  74. you domesticated. So, it seems unlikely that
  75. it would be just the protein or food
  76. supply. Yet, that was important to our
  77. argument. The question, and it's also complicated because,
  78. there are ways to get some food value from milk without drinking it fresh. You
  79. can all societies that I know of, have ways of processing fresh milk into low
  80. lactose forms. Sour milk is a low lactose form. All the things like yogurt and
  81. kefir, drinks that are widely known in some
  82. places, and most cheeses, break down the lactose
  83. externally using lactobacillus bacteria, using a separation of curds and whey.
  84. Lactose is water soluble, it drains off with the whey, and the
  85. curds are free of lactose. So most cheeses have very little
  86. lactose concentration, something like yogurt is
  87. full of lactobacillus, that's already digesting.
  88. >> Sure.
  89. >> The lactose for human consumption. So here's the real challenge. In a world
  90. where most populations have ways to process milk
  91. and to get everything that's in the milk.
  92. >> Mm-hm.
  93. >> Why would some populations still evolve this persistent lactase activity?
  94. >> Sure.
  95. >> Because there are external sources of
  96. lactase, why does it have, what, what was
  97. the advantage of producing it internally to the
  98. human organism? The leading argument of the day
  99. is that calcium, that the advantage of drinking fresh milk, is the
  100. advantage of an additional calcium supply especially in areas where there is,
  101. a calcium deficiency caused either by inadequate calcium in the
  102. diet. Well, that's not too likely if you're drinking milk.
  103. >> Sure.
  104. >> but a vitamin D scarcity. And then
  105. people say, well, what's the advantage? Milk is obviously
  106. rich in calcium but what's the advantage of having
  107. why, why can't you again just eat the yogurt?
  108. >> Mm-hm.
  109. >> And why can't you again just eat the milk product, the cheese,
  110. which is going to have the calcium in it? Except for the water soluble calcium
  111. which you might lose some of. What's
  112. interesting with some research what's called the lactose
  113. effect. If you feed human infants from birth,
  114. in a compare, a controlled comparison, cow's milk
  115. >> Mm-hm.
  116. >> from birth, or mother's milk, versus soy milk, you see, beginning in a
  117. matter of, of a few days, you see a big differential in the amount of
  118. bone absorbed calcium, in other words, the calcification of bone
  119. >> Mm-hm.
  120. >> Is enhanced, you can, this is
  121. controlling for the amount of calcium, you take
  122. soymilk, and you supplement the calcium in it, so that it matches the cows milk.
  123. They have the same calcium, and the
  124. presence of the lactose enhances the absorption of
  125. the calcium according to this, these data. it's
  126. a magnificent curve and it, what it shows
  127. is that the ability to drink fresh milk
  128. has the advantage of making the calcium that's
  129. there in equal proportion in the two milks.
  130. It makes it more available to the infants
  131. who can absorb, who can digest the lactose.
  132. So what's particularly interesting about that is it's
  133. saying that a lactase persistence compensates for the
  134. vitamin D deficiency that's built into the solar
  135. radiation pattern on the surface of our Earth. So
  136. that milk, like epidermal demelanization, they follow similar curves.
  137. >> Mm-hm.
  138. >> And that you get the persistent lactase
  139. activity. And the lactose tolerance at high latitudes.
  140. >> Right. And just to sort of reword what you say here to make
  141. sure you know we're clear on this, in the case for skin pigmentation,
  142. as you've sort of alluded to, the idea
  143. is as human migration moved away from the equator
  144. >> Yeah.
  145. >> to higher and I guess we should also say lower latitudes
  146. >> Yes.
  147. >> They had less sunlight and so they
  148. >> Well, you can say higher latitudes north and south.
  149. >> Sure higher latitudes north and south. They lost pigmentation in the
  150. skin, because they needed to let more sunlight in for vitamin D.
  151. >> There was selection for the entrance of more sunlight.
  152. It wasn't that they recognized the need.
  153. >> Sure. Good, yes. Exactly
  154. >> The same with milk, it wasn't that
  155. we were sitting around here, wow, hoping and wishing
  156. we could have another source of calcium. What's
  157. happening is that those that happened to have this
  158. >> Mm hm.
  159. >> Out-reproduced by, you can even test whether it's a 3%, a 2%, a 5%. How
  160. much does the difference have to be in
  161. the long run, in terms of differential survival and
  162. reproduction, in order for this to work out in
  163. the time frame that we have from 4,500 years B.P.,
  164. or whatever. And I'm one of those who thinks
  165. that it's actually a pretty small advantage. A pretty small
  166. advantage, but persistent generation after generation, and no fail,
  167. it's there. If you have the lactose mechanism for absorbing
  168. calcium and making strong bones, less likely to have
  169. rickets, less likely to have Harrison Sulcus, less likely to
  170. have broken bones in young adulthood and
  171. all those things. You'll have that rigid calcification
  172. of bone, which you can see starting
  173. right out in infancy with the lactose affect.
  174. >> Thank you so much.
  175. >> Of course,
  176. thank you.