Despite prior evidence that the herb might help treat problems processing blood sugar, the researchers were unable to even detect ginseng compounds in the participants' bloodstream after they took it. They also saw no differences in the subjects' blood sugar.
"We think you don't absorb enough of it to have an effect," said Dr Samuel Klein, a professor at the Washington University School of Medicine and the study's senior author.
He and his colleagues gave 15 people Korean ginseng root extract, or the active chemical in ginseng, called ginsenoside Re, or a fake ginseng pill for 4 weeks.
All the people who participated were overweight or obese and had recently received a diagnosis of type II diabetes or had developed a precursor to diabetes called impaired glucose tolerance.
Those who ingested the ginseng extract took 3 grams (about a teaspoon) each day for 2 weeks, then 8 grams each day for another 2 weeks.
The ginsenoside Re group took 250 mg (roughly the same weight as a vitamin pill) each day for 2 weeks, then 500 mg for the next 2 weeks.
High end doses
Klein said these doses were on the high end of what people would ordinarily take.
Before and after the 4-week treatment, the researchers gathered information on how well the participants regulated their blood sugar and how sensitive they were to insulin, the hormone that helps the body use sugar in the blood.
None of the measurements were different after taking ginseng root, ginsenoside Re or the placebo for 4 weeks.
And when Klein's team looked for evidence of ginseng in the bloodstream, they couldn't detect it.
"We don't think we got a bad batch of ginseng," Klein said.
That's because the researchers also used the ginseng to repeat the experiments that had led others to believe the root might help diabetics.
They injected obese mice and bathed human muscle tissue (left over from operations) with the same ginseng they gave to the people in the study; in both cases, the ginseng extract and the ginsenoside Re did indeed improve sugar processing.
Dr Chun-Su Yuan, director of the University of Chicago's Tang Centre for Herbal Medicine, said that he believes ginsenoside is broken down into different compounds when it's eaten.
That might explain why he found injecting ginseng into animals has anti-diabetic effects, while the new research found that taking it orally doesn't work in humans, said Yuan, who was not involved in the current study.
"My suggestion is to consider different routes of administration," he said.
Korean ginseng is inexpensive (about $10 - R 68.45 for a bottle of 50 pills). It is not available in injectable form.
Several companies that market Korean ginseng supplements, including Nature's Way and Swanson Vitamins, did not respond to phone or email requests for comment.
Ginseng root extract is a popular supplement, taken by an estimated 6 million Americans every year.
Klein said that he doesn't want the study, which is published in the journal Diabetes Care, to dictate anyone's medical care.
He advises patients to talk with their doctors before stopping or starting any herbal supplements. (Reuters Health/ April 2011)