CORVALLIS, Ore. – Flavonoids, a group of compounds found in fruits and vegetables that had been thought to be nutritionally important for their antioxidant activity, actually have little or no value in that role, according to an analysis by scientists in the Linus Pauling Institute at Oregon State University.
However, these same compounds may indeed benefit human health, but for reasons that are quite different – the body sees them as foreign compounds, researchers say, and through different mechanisms, they could play a role in preventing cancer or heart disease.
Bioflavonoids join an existing known list of plant compounds that deliver health benefits because they alter metabolism in ways that reduce damage caused by toxins. For example, cabbage and other cruciferous vegetables contain compounds called isothiocyanates such as sulforaphane and indole-3-carbinol. The liver reacts to these isothiocyanates as if they are toxins and the liver therefore makes more enzymes that break down toxins. As a result any real toxins get broken down more quickly and therefore do less harm to the body.
The body metabolises flavonoids into forms that have little antioxidant activity.
“What we now know is that flavonoids are highly metabolized, which alters their chemical structure and diminishes their ability to function as an antioxidant,” said Balz Frei, professor and director of the Linus Pauling Institute. “The body sees them as foreign compounds and modifies them for rapid excretion in the urine and bile.”
Attempts to turn up big health benefits of antioxidant vitamins have been somewhat of a disappointment. The foods that appear to be associated with longer and healthier lives probably deliver most of their benefit due to compounds in them that alter human metabolism in a large assortment of ways.
Flavonoids alter metabolism in several health-promoting ways.
The large increase in total antioxidant capacity of blood observed after the consumption of flavonoid-rich foods is not caused by the flavonoids themselves, Frei said, but most likely is the result of increased uric acid levels.
But just because flavonoids have been found to be ineffectual as antioxidants in the human body does not mean they are without value, Frei said. They appear to strongly influence cell signaling pathways and gene expression, with relevance to both cancer and heart disease.
“We can now follow the activity of flavonoids in the body, and one thing that is clear is that the body sees them as foreign compounds and is trying to get rid of them,” Frei said. “But this process of gearing up to get rid of unwanted compounds is inducing so-called Phase II enzymes that also help eliminate mutagens and carcinogens, and therefore may be of value in cancer prevention.
“Flavonoids could also induce mechanisms that help kill cancer cells and inhibit tumor invasion,” Frei added.
It also appears that flavonoids increase the activation of existing nitric oxide synthase, which has the effect of keeping blood vessels healthy and relaxed, preventing inflammation, and lowering blood pressure – all key goals in prevention of heart disease.
If we could optimally turn up every enzyme pathway that breaks down toxins we could reduce our rate of aging and delay the onset of cancer and other disorders of old age.
Biogerontologist Aubrey de Grey makes what to my mind is a persuasive argument against the expectation that antioxidants will deliver large health benefits: The metabolic cost of making and retaining antioxidants in the body is pretty low. If antioxidants could deliver benefits as large as some of their advocates claim then natural selection would long ago have selected for mutations that boost body antioxidant levels. So why expect consumed antioxidants to deliver a big benefit?
So (I hear you asking) why wouldn't the body make more detoxifying enzymes even in the absence of foods consumed that up-regulate detoxifying enzymes? My guess is that those enzymes are more metabolically expensive to keep around.
|Share |||Randall Parker, 2007 March 09 02:03 PM Aging Diet Studies|