Peter Webb  wrote or quoted:

> People who exercise at the gym primarily do so for one of two reasons - to
> build muscle mass or improve cardio fitness.
> 
> Its easy to see why the body only puts on muscle in response to weight
> training. If there is excess food available, your body is better storing
> this as fat rather than as muscle, as fat provides 9 kCal of energy per Gram
> you carry around, whereas muscle (protein) provides only 4 KCal/gm. Carrying
> fat around is more energy efficient than carrying muscle, unless you have
> some other need for the muscle. Weight training simply tricks your body into
> thinking you need muscle for other purposes, so it alters the balance
> towards the less efficient storing of food as muscle.
> 
> The adaptations that occur in response to cardio training are harder to
> explain. These include increased capillaries, more mitochondria, and
> numerous others. However, none of these seem to have a downside.

They all have energetic costs to maintain.  If you are not using muscles,
they are probably not needed.  Maintining them would be a significant
energy drain.  That's basically why muscles that are not used eventually 
atrophy.

> Anybody know why not exercising makes you unfit, or has a plausible theory?

Tissues that are not used represent a drain in terms of the enegry 
needed to heat them - and the cost of lugging them around with you 
wherever you go.  Also the more tissue blood has to flow through the 
harder the heart has to work.

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.
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