Rui Hai Liu on Studying the Health Benefits of Whole Foods
Scientist Interview: January 2012
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You've also done highly cited work on the antioxidant activity of grains. Can you tell us about that?
I think that this is one of major contributions from our lab. Until we got into this, everyone said to eat your antioxidant from fruits and vegetables, and nobody was talking about the antioxidants from whole grains. Why not? Because the methodology to analyze the phytochemicals and antioxidant activity of whole grains was similar to the method used for fruits and vegetables, and the grain phytochemical content and antioxidant activity were found using this method was very low.
However, when we studied the effect of processing on the nutritional quality of sweet corn, we discovered that processed sweet corn had a higher antioxidant activity and higher phenolic content than unprocessed sweet corn because some of the antioxidants or phytochemicals came in the bound form. They were connected to the cell wall, to fiber, and they were not easily extracted by the traditional method used for fruits and vegetables. Phytochemicals and phenolics, as we discovered, exist mainly in the free form in fruits and vegetables. There's only a very small portion in the bound form.
Because the sweet corn results turned out to be different after processing, it prompted us to look at other whole grains. We looked at both the free form and bound form of antioxidants, which is something previous researchers hadn't done. As a result, they dramatically under-estimated the antioxidant content of whole grains—roughly by 85% in corn, 76% in wheat and 75% in oats – when not including bound phytochemicals.
We were the first to analyze the entire profile of phytochemicals or antioxidants in whole grains. Nobody had ever done that before. We suggest that whole grains contain unique phytochemicals that complement those found in fruits and vegetables when consumed together. That's why our papers were so highly cited and why they're so important. They started a whole new area of antioxidant research—the antioxidants in whole grains.
How has your research evolved in the last decade, since these early papers were published?
We continue to work on the health benefits of the phytochemicals in fruits, vegetables, and whole grains. We did animal studies, for instance, showing that apple phytochemicals can lower mammary cancer rates in rats, and we studied the mechanism of action. We publish array of papers on that.
Can you tell us how you did these studies?
We have a rat model of induced mammary cancer that is very similar to human breast cancer. We feed the rats apple extract, equivalent to consumption by humans of one, three, or six apples a day. And we found that the apple extract inhibited tumor formation in a dose-dependent manner. We used the whole apple, extracted all the compounds, the phytochemicals, concentrated it, and then that's what we fed the rats on top of their usual diet.
"I encourage consumers to eat more fruits, vegetables, and whole grains on a daily basis."
What research are you doing now that really excites you?
Well, this is something we've been working on for a few years and many people already consider it a revolution in antioxidant research methodology—it's called the cellular antioxidant activity assay, or CAA assay. The point is that virtually all previous research on antioxidant activity has been looking at the compounds effect in a test tube using chemical antioxidant activity assays. It's very difficult to interpret those data and relate it to human health, because a compound in the test tube may act differently than it does in physiological conditions, in cells, or in the human body.
If you have an antioxidant that supposedly protects against Alzheimer's disease, you want to know that it gets into the brain cell and that it works in a brain cell. If you have a compound that prevents prostate cancer, you should be able to detect that compound in prostate tissue. Test tube assays don't reflect cellular uptake and real bioactivity. Some compounds may have activity only after they're metabolized. Test tube assays won't cover those.
So we developed a new cellular-based assay for detecting antioxidant activity within cells (Wolfe KA, Liu RH, "Cellular antioxidant activity [CAA] assay for assessing antioxidants, foods, and dietary supplements," J. Agr. Food Chem. 55[22]: 8896-907, 31 October 2007, DOI: 10.1021/jf0715166). Many people consider this a milestone in antioxidant research. Many labs around the world are using this assay to measure antioxidant activity.
Which one of your accomplishments gives you the greatest sense of satisfaction?
I'd say it's this whole food research, the suggestion that that it's this combination of phytochemicals in whole foods that are responsible for their health benefits. Second would be this discovery that the processing of fruits and vegetables can actually increase their antioxidant activity.
What do you think is the most challenging part of studying antioxidant activity?
The most challenging part is that food is very complicated. Studying the health benefits of fruits and vegetables and whole grain is not as simple as isolating a single key component and simply studying that.
Rui Hai Liu, Ph.D.
Department of Food Science
Cornell University
Ithaca, NY, USA
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RUI HAI LIU'S MOST CURRENT MOST-CITED PAPER IN ESSENTIAL SCIENCE INDICATORS:
Sun J, et al., “Antioxidant and anti proliferative activities of common fruits,” J. Agr. Food Chem. 50(25): 7449-54, 4 December 2002 with 337 cites. Source: Essential Science Indicators from Clarivate.
KEYWORDS: ANTIOXIDANTS, FRUIT, VEGETABLES, APPLES, PHYTOCHEMICALS, ANTI-CANCER ACTIVITIES, CARDIOVASCULAR DISEASE RISK, ANTI-PROLIFERATIVE ACTIVITY, WHOLE FOODS, CELL CULTURES, COLON CANCER, LIVER CANCER, BREAST CANCER, ANIMAL STUDIES, THERMAL PROCESSING, TOMATOES, FLAVONOIDS, VITAMIN C, POLYPHENOLS, SWEET CORN, WHOLE GRAINS, BIOACTIVE COMPOUNDS, FREE FORM, BOUND FORM, WHEAT, OATS, CELLULAT ANTIOXIDANT ACTIVITY ASSAY.
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