Why check basal body temperature? Part 2

Wait, can’t you just estimate your metabolic rate (energy production) by looking at your caloric intake, exercise program and weight?  Don’t all skinny non-exercisers have high metabolisms and all obese dieters have slow ones? Not necessarily (though the second example is more often true).  I frequently see thin patients with very low energy production.  This can happen when they don’t digest, absorb and/or assimilate the nutrients from their food well, due to factors like inadequate stomach acid production.  Another cause is inflammation, when nutrients are burned without creating fuel for one's cells (ATP).  A dramatic example of this is cachexia, or wasting syndrome.