TUESDAY, Oct. 16, 2007 (HealthDay News) -- Garlic lovers, take heart: The pungent root may promote healthier responses in blood vessels.
So say researchers who found that compounds in garlic cause tissues or blood vessels to release a chemical called hydrogen sulfide. In large quantities, this compound can be deadly, but it's also an essential molecule within the body, causing blood vessels to relax and reducing dangerous inflammation.
But how you take your garlic matters, the research showed. "If you prepare it in certain ways, you can lose the compounds that cause it to release hydrogen sulfide, so that helps explain why there has been such great variability in studies," noted senior researcher David Kraus, an associate professor of environmental health sciences at the University of Alabama, Birmingham.
His team published its findings in this week's online edition of the Proceedings of the National Academy of Sciences.
In the array of garlic health studies, more than half have shown some positive effect, but that effect has tended to be small, and some trials have even shown negative health effects, Kraus noted. Some of his team's experiments used juice extracted from supermarket garlic. Human red blood cells exposed to tiny amounts of the juice began emitting hydrogen sulfide. Most hydrogen sulfide production took place at the membrane of the red blood cells, although a fraction was made within the cells.
When working with garlic, Kraus first crushes the clove. He then waits for the reaction that produces the compounds that trigger hydrogen sulfide release. "We usually let the garlic crush for 15 minutes," he said.
Other studies of garlic's health effects have failed, Kraus said, because they look for activity that is impossible -- a reduction in blood cholesterol levels, for example. One such trial was done by Christopher D. Gardner, a nutrition scientist and assistant professor at Stanford University's Prevention Research Center in California.
"We used real raw garlic and two commercial supplements in doses higher than people are advised to take," Gardner noted. "We assumed that at least one of the three would work. These were people whose LDL ['bad'] cholesterol was elevated. We worked with 192 people for six months, and they [LDL cholesterol levels] didn't budge, not even a bit, month after month."
According to Gardner, Kraus' work now offers a reason for that failure, because hydrogen sulfide has no effect on cholesterol.
Another expert urged caution in interpreting the Birmingham team's results.
Eric Block, professor of chemistry at the State University of New York, Albany, has also done extensive work on garlic. He called the paper "provocative" but expressed some concerns.
For example, he said, "the benefits of garlic on cardiovascular disease remain controversial, because they have not been established by the gold standard method of placebo-controlled, double-blind clinical studies," he said.
It's also uncertain that garlic's purported beneficial effects are due to the mechanism described in the new report, Block added. Clinical trials are needed to help prove that point, he said.
According to Block, Kraus' team, "should be more conservative in over-extending some of their conclusions in the absence of additional work." However, "their work does represent a significant advance in the science of this amazing, ancient, ever-popular herb," he said.
Kraus stressed that his study only looked at the effect of fresh garlic, not garlic supplements. "What we are proposing is that you eat a garlic-rich diet," he said. "We haven't really tried to look at supplements yet."
"Garlic-rich" has different meanings, depending on the part of the world being studied, Kraus added. "In the Middle East, that would be 5 to 10 cloves of garlic a day," he said. "If you go to the Far East, it would be even higher."
There's more on garlic and health at the U.S. National Center for Complementary and Alternative Medicine.