Magnesium-Rich Foods Reduce Diabetes Risk, Study Says

Eating foods rich in magnesium can reduce the risk of type two diabetes onset, according to two studies by Harvard researchers.

Of the 18.2 million cases of diabetes in the United States, 90 to 95 percent are classified as type two—a rate which is increasing. Diabetes was the sixth-leading cause of death in the U.S. in 2000, contributing to 213,062 deaths, according to the American Diabetes Association.

Two separate research teams comprised of researchers at the Harvard School of Public Health and Harvard Medical School (HMS) published their findings on magnesium and reduced type II diabetes risk in the January 2004 issue of the journal Diabetes Care.

One team used data compiled by the Women’s Health Study (WHS) to track 38,025 women from 1993 through 1999. The other team focused on 85,060 women identified by the Nurses’ Health Study (NHS), who were tracked for 18 years, and 42,872 men chosen from the Health Professionals Follow-Up Study, who were tracked for 12 years.

In both studies, the participants were adults with no personal history of diabetes, cardiovascular disease or cancer.

Data was collected by regularly mailing participants questionnaires asking about their daily routines—from eating habits to exercise levels to cigarette and alcohol use—and whether they had acquired documented cases of diabetes since first entering the study.

While magnesium played a positive role in reducing type II diabetes risk in both studies, the results of the two studies were not identical. The WHS team concluded that only overweight and obese women would have a reduced risk of type II diabetes onset with increased magnesium intake, whereas the NHS study found that both men and women of all weight groups would have decreased type II diabetes risk.

HMS Assistant Professor of Medicine Simin Liu, the senior author in the WHS findings, said he believed that the studies differed because of varying definitions of what it means to be overweight. For the WHS study, a person was labeled overweight if her Body Mass Index (BMI), a measure of total body fat, was above 25—the lower limit of the overweight category as defined by the National Institutes of Health.

The NHS study, conversely, used a cutoff BMI of 27 to define an overweight person.

Liu said he believed that the results of the NHS study would mirror the results of the WHS study—indicating that only overweight and obese people would benefit from magnesium intake—if the two studies had both used the standard 25 BMI measure.

The NHS team decided to use 27 as the minimum BMI cutoff to define overweight because 27 is the median BMI for overweight people, said HMS Assistant Professor of Medicine Frank B. Hu, the senior author of the NHS study.

He added that his findings on dietary magnesium intake were independent of BMI and thus would not have changed with a different BMI index. He defended his study as possibly more accurate than the WHS study because of the larger pool of participants in the NHS group.

Despite the disagreements, both studies indicated that increased dietary magnesium intake was good for the general population. According to both reports, Americans do not ingest the recommended levels of magnesium, which has recently become a speculative factor contributing to the increasing rate of type II diabetes onset.

A December 2003 study in the Journal of the American College of Nutrition, also performed by Harvard scientists, speculated that “higher dietary magnesium intake may reduce the risk of developing type II diabetes” because women with higher magnesium consumption tended to have greater insulin sensitivity. Decreased insulin sensitivity, also known as insulin fasting, is the immediate cause of type II diabetes.

“The primary concern here is not which group is affected the most,” Hu said. “Whether you are overweight or obese or not, you need enough magnesium.”

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.