(SCUBA. appetite control, anemia, carotene absorption)


        The most important detail in the instruction of new SCUBA divers is that they must breathe out all the way up during rapid ascent.  (SCUBA stands for Self-Contained Underwater Breathing Apparatus.)  At a 100 foot depth, the lung of a SCUBA diver contains four times as much air as it does at the surface, so three lungfuls of air are exhaled on ascent.  Our instinct on a panic ascent is to hold our breath.  There is no sensation to compel exhaling.   Failure to exhale ruptures the lungs painlessly in an immediately fatal way.  Pearl divers and diving birds and animals do not have this problem because they take only one lungful of surface air with them when they dive since they have no pressure tanks for underwater breathing.  So, for this reason, nature had no motive to develop sensations and instincts in anticipation of this problem since this need is born out of man’s invention, not the needs of nature.  If you attempt SCUBA diving, pay very close attention to this portion of the instruction. 

        Our appetite mechanism is designed to keep us from starving.  It is never necessary to tell a young farm worker to eat faster if he hasn’t finished his lunch when the boss goes back to work.  The normal appetite mechanism is triggered by the availability of glucose, but by glucose inside cells at the base of the brain, not by glucose in the blood.  In diabetes the glucose level inside the cells is low because insulin is necessary to cause glucose to cross the cell membrane from the blood and there is a deficiency of insulin.  An inappropriate increase in appetite results even though excess glucose is already present in the blood—another example of a limit to nature’s wisdom.  This is a difficult lesson for diabetic patients to learn—that for them appetite is not the best indicator of the need to eat.  Their motivation to learn is usually less than that of scuba diving students.

        The use of iron for the treatment of anemia is a more subtle example of a limitation in nature’s wisdom that nevertheless has some important lessons.  Iron deficiency is the most common cause of anemia, especially in women who need extra iron for menstruation and the requirements of pregnancy and childbirth.  Blood loss at delivery of a child approximates the sum of the losses that would have occurred with the avoided menstrual periods; but the iron in the blood of the newborn is a net loss.  Excess iron in the body is toxic.  Nature’s wisdom in this regard seems to assume that all anemia is due to blood loss.  Absorption of iron from the intestinal tract is inhibited in the absence of anemia; but is increased in all cases of anemia regardless of there being ample iron stores already in stock.  This makes it apparent that treatment of anemia with iron is actually potentially harmful when the cause of that anemia is not iron deficiency. 

         Another not so subtle point: if treatment with iron corrects anemia, then the cause of blood loss should be sought aggressively, especially in males and women after childbearing age.  When an obvious blood loss has not occurred, the loss of blood is usually from the stomach or intestines.  Only a large amount of bleeding at one time causes obvious blood in the stools or black stools (iron medication also causes black stools).  Small quantities of blood are digested to a brown color.  This will help you to understand the need for some of the irksome tests doctors like to insist on to be sure we do not miss a curable cause of anemia.

          A few years ago I concluded from a “back of an envelope” calculation that it would be easy to select a diet with a toxic excess of vitamin A assuming that all vitamin A precursors such as beta carotene were converted to active vitamin A.  Recently it has been discovered that beta carotene is not converted to active vitamin A in the presence of adequate vitamin A already on board.  This indicates that nature had already solved the problem of toxicity of excess vitamin A derived from beta carotene.  However, supplements of beta carotene beyond anything likely to be ingested from natural sources might overwhelm the mechanism limiting conversion, thus producing toxic levels in spite of  “nature’s wisdom”. 

           So, while nature’s wisdom is right on about many common, important problems that beset us, there are occasional rare events when we need to supplement nature’s wisdom with our own acquired wisdom.  This is especially true regarding diseases induced by or greatly aggravated by civilization (the topic of a future column).

John A. Frantz, M.D.

Feb. 5, 2002