Peaceful Interactions between Plants and Animals

 

       This is a sequel to Chemical Warfare between Plants and Animals.

       In 1992 my wife and I noticed a spectacular tall tree in the botanical gardens of Christchurch, New Zealand.  It turned out to be a “dodo tree” indigenous to the island of Mauritius far from land in the Indian Ocean where only 13 mature dodo trees survive.  The tree’s fruits were tasty to dodos but scarcely utilized for food by other animals.  Apparently the seeds seldom germinate without passage through a dodo gizzard and dodos have been extinct for 300 years.  They were large flightless birds adapted to these islands which had no predators until Portuguese  sailors arrived.  In New Zealand the trees have been propagated by subjecting the hard-coated seeds to an hour or so in a gem stone polisher.  In Mauritius they now persuade turkeys to swallow the seeds (separated from the fruit—unattractive to turkeys).  The gizzard is a muscular organ of birds between the gullet and stomach into which the birds introduce pebbles to do the work of teeth—hence the expression “rare as hen’s teeth”.  If we had gizzards, dentists wouldn’t have happened.   Dodo trees are an unusual example of peaceful interactions between plants and animals.

        Here are some more familiar examples of such peaceful interactions.  The sweet nectar of flowers motivates pollinators, bees, humming birds and bats.  Sweet fruits similarly motivate eating of seeds and subsequent dispersal with fine tuning of the requirements to promote germination and more fine tuning to the tastes of the animals that do the dispersing.  This results in many tasty and very sweet fruits carried to such extremes that mammals probably have evolved to avoid having a “sweet tooth”.

       Why is lactose (milk sugar) scarcely sweet?  If milk contained any other ordinary sugar, it would be almost as sweet as watermelon.  Lactose is created in all milk glands at the expense of some sophisticated biochemical manipulations and also requires special enzymes for digestion in all mammalian young.  This biochemical complexity would require some selective advantage in order to have occurred at all.  My take: it would be harder to wean the young from sweet milk at the appropriate time resulting in fewer offspring per such mother during her reproductive life—a substantial selective advantage for mothers producing milk with lactose instead of a sweeter sugar.  Perhaps sweeter  milk of early mammalian progenitors without the lactose chemistry resulted in the entire lineage having a sweet tooth throughout life. The evolution of the lactose biochemistry thus may have resulted in return to a more varied and nutritious diet with enhanced survival of all mammalian species.  This kind of thinking may help future physicians steer our communities toward appropriate policies to cooperate with nature’s long-term effort to limit the damages of having a sweet tooth. Getting “coke” machines out of schools becomes a “no-brainer” with this kind of insight.

        Sweets are the most habit-forming food especially when taken on an empty stomach.  The mechanism is a more sudden and greater increase in blood sugar than when sweets are eaten with other foods.  An unnecessarily large secretion of insulin causes a low blood sugar about one half hour later producing a large and prompt increase in hunger.  Artificial sweeteners yield the same result because the insulin secretion is triggered by the mere taste of sweetness in the absence of any trace of carbohydrate.  Repeated inappropriate episodes of insulin secretion blunt but do not extinguish this response, justifying the statement about sweets being the most habit-forming food.

Saccharin’s sweetness was discovered accidentally and for a long time promoted only to make the lives of diabetic patients more comfortable with some success.  In recent years hundreds of artificial sweeteners have been discovered at considerable expense and a few of the best ones over-promoted to the general public without a trace of impact on the accelerating obesity epidemic. Because none of the artificial sweeteners produce the sweet sensation nearly as quickly as sugar, the latest trend in commercially sponsored such research looks for substances which enhance the sweetness of sugar itself, permitting the taste sensations of a larger amount of sugar when the additive is present even though it has no sweet taste of its own.  The first of these products (currently known as Product 951) is in the pipeline for marketing next year (2007).  Somewhat alarmingly this product will not have to be identified on food labels because it is active in such a tiny amount that its presence can hide behind innocuous phrases like “natural and artificial flavors”.  How did such a labeling exception for rather frivolous addition of new and potentially subtly toxic chemicals creep into the regulations?  Their extreme potency does not qualify as a valid reason.

        Terse summary: Don’t look to the food industry to solve the obesity epidemic.  Incidentally, it is OK to eat sweets while doing hard labor, but in this case our instincts give the right answer too emphatically to need emphasis.

 

John A. Frantz, MD 

June 19, 2006