Lynton, the body's use of fats is quite complex; the simpler proverb
goes, "Fats burn in the fire of carbohydrates.," but we get into
ketogenic reactions, fasting or unfed states, reserves of glycogen,
and so on.
   Having serum cholesterol circulating doesn't mean you'll make
vitamin D3, for example, or androsterone or progesterone. It simply
means the substrate is there, and if the associated minerals, vitamins,
etc. are also there, you're in luck. No sun, no vitamin D, as Iranian
women forced into the chodor and living indoors are finding again.