Just a muse.
Endogenous substances are those that are already contained within our bodies. They are part of basic human equipment.
Exogenous substances are those that come from outside of our bodies. This includes various substances in foods, drugs (most, though not all), and pesticides.
I often mull over all of the tools we use in the Track Your Plaque program to achieve control over this thing called coronary plaque. It struck me that just about all the supplements we use that seem to provide outsized benefits are all endogenous substances themselves:
--Omega-3 fatty acids from fish oil
--Vitamin D
--l-arginine
--Niacin (vitamin B3)
Many of the other substances, though not directly relevant to our plaque-control efforts, but are among the most effective nutritional supplements, also supplement endogenous levels: calcium pyruvate, creatine, acetylcarnitine, DHEA, testosterone, progesterone, growth hormone, pregnenolone, phenylalanine, tyrosine, melatonin, etc.
Curiously, most drugs are not meant to directly supplement endogenous levels, but are designed either to enhance or block an enzyme (e.g., acetylcholinesterase inhibitors that block breakdown of acetylcholine; HMG CoA reductase inhibitors to block cholesterol synthesis; angiotensin converting enzyme inhibitors to reduce blood pressure), to exert toxic effects on an organism (antibiotics, antivirals), or to exert an entirely unique effect that does not ordinarily occur in the human body (some anti-cancer drugs, for instance). (This is an admitted, vast over-simplification.)
That's not to say that any endogenous substance is desirable or safe when supplemented. Cortisol, thyroid hormone, and estrogens are three examples of endogenous substances that have downsides when administered at slightly more than physiologic concentrations.
Nonetheless, it makes me wonder if the world of endogenous substance supplementation has not been fully explored. Are there other endogenous substances that are as potent and wonderful, for instance, as vitamin D but not yet fully appreciated? I'm sure there are.