But while magnesium-rich foods have had a positive effect in preventing diabetes, the studies found that multivitamins and other magnesium supplements have not had a similar effect.

“The NHS study doesn’t show any supplemental effect of magnesium, only of magnesium-rich foods,” said Liu, who also contributed to the NHS study. “This suggests that there may be something else in those foods that works with magnesium to reduce diabetes risk. For now, I can only recommend foods that are rich in magnesium.”

Those foods include “whole grains, nuts, and green leafy vegetables,” according to the NHS study.

Hu and Liu both said that there should be more experimental research performed to better assess the effects of magnesium and magnesium supplements on type II diabetes development.

Liu said that magnesium has not been the only substance found to lower the risk of type II diabetes among overweight women.

“For several other factors we have looked at, like dietary fibers, we have also seen this type of phenomenon,” he said. “There’s a theory going around that these people are more susceptible to dietary factors.”

—Staff writer Alan J. Tabak can be reached at tabak@fas.harvard.edu.