A Queen's-led team has discovered the reason
why garlic is so good for us.
Researchers have widely believed that the organic compound,
allicin – which gives garlic its aroma and flavour –
acts as the world's most powerful antioxidant. But until now
it hasn't been clear how allicin works, or how it stacks up
compared to more common antioxidants such as Vitamin E and coenzyme
Q10, which stop the damaging effects of radicals.
"We didn't understand how garlic could contain such an
efficient antioxidant, since it didn't have a substantial amount
of the types of compounds usually responsible for high antioxidant
activity in plants, such as the flavanoids found in green tea
or grapes," says Chemistry professor Derek Pratt, who led
the study. "If allicin was indeed responsible for this
activity in garlic, we wanted to find out how it worked."
The research team questioned the ability of allicin to trap
damaging radicals so effectively, and considered the possibility
that a decomposition product of allicin may instead be responsible.
Through experiments with synthetically-produced allicin, they
found that an acid produced when the compound decomposes rapidly
reacts with radicals.
Their findings are published in the January 2009 issue of the
international chemistry journal Angewandte Chemie.
"Basically the allicin compound has to decompose in order
to generate a potent antioxidant," explains Dr. Pratt,
who is Canada Research Chair in Free Radical Chemistry. "The
reaction between the sulfenic acid and radicals is as fast as
it can get, limited only by the time it takes for the two molecules
to come into contact. No one has ever seen compounds, natural
or synthetic, react this quickly as antioxidants."
The researcher is confident that a link exists between the
reactivity of the sulfenic acid and the medicinal benefits of
garlic. "While garlic has been used as a herbal medicine
for centuries and there are many garlic supplements on the market,
until now there has been no convincing explanation as to why
garlic is beneficial," says Dr. Pratt. "I think we
have taken the first step in uncovering a fundamental chemical
mechanism which may explain garlic's medicinal benefits."
Along with onions, leeks and shallots, garlic is a species
in the family Alliaceae. All of these other plants contain a
compound that is very similar to allicin, but they do not have
the same medicinal properties. Dr. Pratt and his colleagues
believe that this is due to a slower rate of decomposition of
the allicin analogs in the onions, leaks and shallots, which
leads to a lower level of sulfenic acid available to react as
antioxidants with radicals.
The study was funded by the Natural Sciences and Engineering
Research Council of Canada (NSERC) and the Ontario Ministry
of Innovation. Other members of the research team are Queen's
Chemistry post-doctoral researcher Vipraja Vaidya and Keith
Ingold, from the National Research Council of Canada.