White yams, a staple of the diet of the Yoruba people of southwestern Nigeria, may play a role in the society’s high incidence of fraternal twinning, according to a student’s research. “You see families consuming yams three or four times a day,” said Obinwanne Ugwonali, M.D. ’99, whose thesis on the link between yams and fertility was one of five to garner awards this year on Student Research Day. “I think this project will lead us to understanding more about the mechanisms of the human reproductive system, specifically the reason why we are typically monotocous rather than polytocous like other animals,” he said.

Ugwonali’s adviser, Frederick Naftolin, M.D., professor and chair of obstetrics and gynecology, said the research is the cornerstone of a group of related studies of yam intake and genetic predisposition to multiple births now under way at Yale and Harvard and in Nigeria. In addition to the biological and anthropological aspects, the study has medical applications, Naftolin said. “We don’t know why normally monotocous women become polytocous, but multiple pregnancy is the most common cause of prematurity, which is the major cause of perinatal morbidity and mortality,” he said.

Ugwonali’s interest in yams and fertility started when he worked in Naftolin’s lab the summer before he entered medical school. Naftolin asked him to investigate why humans are monotocous, and a Medline search led Ugwonali to information about the high rate of twinning in Nigeria. Although some Nigerian tribes have rates of fraternal twinning ranging from 20 to 30 pairs per thousand births, it peaks among the Yoruba at 41.6 per thousand. “I’m from Nigeria and I didn’t know this before,” said Ugwonali, who is a member of the Ibo tribe. For African-Americans the rate is 15 per thousand, and for Caucasians in the United States and the United Kingdom it is between 10 and 11 per thousand. Among the Yoruba, twins symbolize a duality of blessings and burdens that is celebrated in hardwood carvings.

In 1996 Ugwonali went to Nigeria on a Downs fellowship to begin his research. After analyzing age, socioeconomic factors and other variables, Ugwonali focused on diet. Demographic and scientific studies conducted in the early 1970s pointed to white yams as the culprit in the mystery of multiple births in southwestern Nigeria. Ugwonali interviewed people about their eating habits and made his own observations. “We suspected environmental factors,” he said. “The only factor that ended up being different from the ones we controlled was yams.” In laboratories at Yale and in Nigeria, he fed rats a diet of yams and saw the average size of their litters double from about four to about nine.

“Our hypothesis is that yams act as anti-estrogens,” he said, noting that he hasn’t investigated the precise chemical link between yams and fertility and has yet to isolate an anti-estrogen from yams. Anti-estrogens fool the brain into thinking there is insufficient estrogen, causing it to release more of a hormone called gonadotrophin and increase the ovulation rate, he said.

Ugwonali, who began his residency in orthopaedics at Columbia-Presbyterian Medical Center in New York, plans to continue his research in Brazil, where many Yoruba were brought as slaves in colonial times. He wants to determine if the high incidence of multiple births persists there among the Yoruba, who have maintained elements of their language, religion, cultural identity and diet, including consumption of white yams.