"Peter Webb"  wrote in
news:cbc9qd$270d$1{at}darwin.ediacara.org: 


>> Calories were not readily available to most of our ancestors - they
>> took care to conserve the ones they had, and avoided wasting them on
>> unused functions.
 
 
> Yes, but I don't see that the adaptations that occur in response to
> cardio training involve any additional energy consumption. (Though the
> training itself obviously does). Its not tissue mass - cardio training
> doesn't increase your body weight (usually the opposite). Nor have I
> have ever heard a theory that fit people need more food than unfit
> people at the same level of activity.
 
> So what is the evolutionary advantage of reversing the adaptations to
> exercise, which compensates for the huge disadvantage that unfit
> people have when confronted by hungry tigers? What is the hidden (to
> me) cost of being fit?


First of all you should be aware that not every aspect of our physiology 
is an adaption that is the result of selection - some are due to drift, 
especially if there was no real selective pressure on the trait for our 
ancestors. For most of human history, if you didn't get exercise you 
didn't eat, so the loss of cardio fitness in the absence of exercise 
would never be selected against. Meanwhile, the general loss of unused 
muscle mass was (as you have noted previously) selected for, so the same 
mechanism that caused that would also cause the loss of cardio fitness 
unless our ancestors routinely were exposed to conditions in which they 
were inactive but well fed - and I would guess those conditions were 
rare.

Our current problem is that we have two different trends going on that 
were both originally adaptive. One is the tendency to put on additional 
weight when starvation is not occurring. For most of human evolution, 
humans underwent periods of starvation on a regular basis. So it was 
advantageous, during periods of excess food, to add fat.The second is the 
tendency to lose  muscle and  fitness in response to inactivity, which we 
have discussed above. Now people can get lots of calories without 
expending any energy, and the result is readily apparent.

Yours,

Bill Morse
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