Casanova, the 18th-century lover who used to breakfast on 50 oysters, has been vindicated by a study that proves they really are aphrodisiacs.
And spring, the scientists say, is the time of year the shellfish have their greatest aphrodisiac quality.
The team of American and Italian researchers analyzed bivalve mollusks — a group of shellfish that includes oysters — and found they were rich in rare amino acids that trigger increased levels of sex hormones.
The link was announced to 15,000 scientists in San Diego at a meeting of the American Chemical Society last week.
“I am amazed,” said George Fisher, a professor of chemistry at Barry University in Miami Shores, Fla., who led the research team with his graduate student, Raul Mirza, and Antimo D’Aniello of the Laboratory of Neurobiology in Naples, Italy.
“I have been a scientist for 40 years, and my research has never generated interest like this,” Mr. Fisher said.
“For centuries, old wives’ tales have said that eating raw mollusks — oysters in particular — would stimulate the libido, but there has really been no scientific evidence as to why and if this occurs.
“We think this could be the first scientific evidence of some substance.
“Did Casanova’s 50 oysters really make him frisky? Could be.”
Previous speculation about the powers of oysters has centered on the refueling powers of their high zinc content.
Zinc is found in semen, and men lose between one and three milligrams per ejaculation.
Mr. Fisher and his team, partly funded by the National Institutes of Health in the United States, bought samples of bivalve mollusks — which also include mussels and clams — from fish markets near Mr. D’Aniello’s Naples laboratory.
They then used a process called high-performance liquid chromatography to identify which amino acids were present and in what quantities.
They found two unusual ones — D-aspartic acid and N-methyl-D-aspartate.
“They are not the normal amino acids that Mother Nature uses,” Mr. Fisher said. “You can’t just find them in a vitamin shop.”
Mr. D’Aniello had found in earlier experiments that injecting the amino acids into rats triggered a chain reaction of hormones that ended with the production of testosterone in males and progesterone in females.
“Increased levels of those hormones in the blood means you are more active sexually,” he said.
“Yes, I do think these mollusks are aphrodisiacs. If the male is having difficulties, they have to eat a lot of mussels or oysters.
“Spring, when the mollusks themselves are breeding, is best. There is the highest concentration of these two amino acids then.”
He added: “The Italians have been right about this for centuries — since before Casanova, from the time of the Romans.”
The scientists stressed that the oysters have to be eaten raw to be most effective. Cooking them reduces the quantity of the two key amino acids.