Saturday, February 11, 2012

Oysters as aphrodisiacs: less science-based evidence

                                                        the bivalve Mimachlamys varia

This post continues the January "Oysters as Aphrodisiac" series on good and bad science. Three different hypotheses are made for the claims that eating oysters increases first, sexual desire,  and second, performance, or some mix of both. The dopamine hypothesis states that "Eating oysters increases the desire/motivational aspect of sexual activity". The D-aspartic acid and NMDA hypothesis is that "Eating molluscs increases the release of sexual hormones in humans" and increases desire and subsequent sexual activity. The third is that "Eating oysters high in zinc increases male performance."

The dopamine hypothesis, addressed in my January 27 post, postulated that eating oysters, which at specific times show elevated dopamine production, specifically in the larval stage of settling, would increase dopamine in humans. Research does not support this hypothesis. Today's post examines scientific evidence for the second hypothesis. The zinc hypothesis will be examined next and posted on February 13, just in time for food preparations for Valentine's Day.

The second hypothesis that eating oysters increases/heightens sexual activity went viral after reports were published in 2005 based on a lecture given at the American Chemical Society in San Diego. This lecture, describing an experiment conducted by  Italian and American scientists on bivalves, was given by Dr. George Fisher, a professor of chemistry from Barry University, in Miami Shores, Florida.  Dr. Fisher's results indicated that rats that had been fed bivalves demonstrated the release of the sexual hormones estrogen and progesterone, both involved in heightened sexual activity. Dr. Fisher was later interviewed by reporters who generalized the experimental results to all bivalves, explicitly oysters, which of course are bivalves, just not the ones tested.

The Precautionary Principle

While all oysters are bivalves, not all bivalves are oysters. Experiments on one bivalve cannot necessarily be generalized to all bivalves. The charts below illustrate  that 1) while oysters, mussels, clams, rats, and humans are all animals,  not all animals are either rats or humans; and 2) the differences between bivalves and humans is huge. Bivalves are invertebrates, like sponges and worms, while humans are mammalian vertebrates, like elephants and rats. On top of this difference, humans and rats are found in different orders: humans are in the order Primate, suborder Anthropoid, family Hominid while rats are in a totally different order: Rodent, the most prolific of the mammals.
  • Kingdom
    • Phylum
      • Class
        • Order
At first blush, all bivalves might seem more alike than not. The class Bivalvia includes clams, oysters, mussels, scallops, and many other families. Bivalves are comprised of about 12000 species, one as different from others in the Mollusc class as Homo sapiens is from other primates such as elephants.

The bivalves tested  by Dr. Fisher and associates were mussels and clams: Mytilus galloprovincialis, Tapes decussates, Chamalea gallina and Donax trunculus) Mytilus is a mussel, Donax is a small clam, as is Chamelea gallina is a species of small saltwater clam, a marine bivalve mollusc in the family Veneridae, the venus clams. Tapes decussates is also a clam. So from the research I conducted on the internet, no oyster was tested.  The clams and mussels contained D-aspartic acid and NMDA (N-methyl-D-aspartate), which reportedly released testosterone and estrogen in rats.

The principal published critic of this exiting news was Dr. Robert H. Shmerling, an associate professor of medicine from Harvard Medical School, who commented  "The findings are certainly interesting, but we still have a ways to go before saying that there is scientific evidence that clams, oysters and scallops boost libido." Shmerling noted that no evidence existed confirming that ingesting D-aspartic acid and NMDA led to sex hormone release in humans, and further noted that there is a questions of how much estrogen or progesterone was effectlvely increased (that is, with notable positive sexual results).

We know that considerable research on diseases such as cancer is first conducted on rats. Since both humans and rats are mammals, rats are stage one in testing, which then suggests further testing based on similarities among mammals. But there are  important differences among mammals. The relationship between rats and humans and cancer is an a better researched example.

Even though rats are frequently used to test the link between certain household chemicals and cancer, rats are considered relatively poor predictors of human cancer risk. Colon tumors are the second leading cause of death from cancer in humans, yet spontaneous colon tumors rarely occur in rats. (  

No human was tested in the bivalve-libido experiment, which did not involve oysters. As any culinary connoisseur well knows, not all bivalves are alike. Another precaution: lab animals can be as significantly unlike wild animals as farmed Atlantic salmon is different from wild Chinook. Both are salmon, one is fatty and naturally beige, while the other is muscular and red from eating krill. Seafood lovers cannot mistake one for the other. So more relevant, further research is suggested before conclusions are drawn regarding the two chemicals studied in the Fisher bivalve experiment.  Beware of extrapolating the chemical processes of one organism to those of another. The way chemicals function in oysters or rats may be radically different from how they function in humans. Or may not.

The next post addresses the link between eating oysters, high in zinc, to increased sexual performance in males. The benefits of eating foods high in specific nutrients is the cornerstone of most dietary prescription. Conclusions on the folk theory that oysters are aphrodisiacs will be posted on February 13.

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