Rui Hai Liu on Studying the Health Benefits of Whole Foods

Scientist Interview: January 2012

Rui Hai Liu

According to Essential Science IndicatorsSM from Thomson Reuters, the work of Dr. Rui Hai Liu ranks at #4 among the 2,902 researchers that currently make up the top 1% in Agricultural Sciences. His record in this field includes 53 papers cited a total of 2,700 times between January 1, 2001 and August 31, 2011.

Liu is a Professor in the Department of Food Science at Cornell University in Ithaca, New York. He is a Fellow of the International Academy of Food Science and Technology, a Fellow of the Division of Agriculture and Food Chemistry of the American Chemical Society, a Fellow of the Institute of Food Technologists, and Fellow of American Association for the Advancement of Science. He is also the Associate Editor of the Journal of Food Science, and serves on the advisory and editorial boards of several other journals.

In the interview below, correspondent Gary Taubes talks with Liu about his highly cited work.

SW: Tell us how you got started studying antioxidants, the subject of your most-cited research papers?

We began with work that was eventually published in Nature in 2000—"Nutrition—Antioxidant Activity of Fresh Apples," (Eberhart MV, Lee CY, Liu RH, 405[6789]: 903-4, 22 June 2000). In that paper, we proposed a new concept in the area of antioxidant research; we proposed that the combination of phytochemicals in fruits and vegetables is critical to their potent antioxidant functions and anti-cancer activities.

SW: What was the conventional wisdom with antioxidants prior to your Nature paper, and what prompted you to suggest a change?

We all knew that the consumption of fruit and vegetables lowers the risk of cancer and cardiovascular disease. People thought at the time that maybe it was the bioactive compounds, the phytochemicals, but they were trying to isolate a specific compound—vitamin A, vitamin E, vitamin C, and then other bioactive compounds like beta carotene, alpha carotene , and then maybe lutein, and lycopene from tomatoes.

The idea was that you could lower the risk of cancer and cardiovascular disease by consuming, for instance, beta carotene-rich fruits and vegetables. Since beta carotene is present in abundance in these vegetables and fruit, it has been extensively investigated as a possible cancer-preventive agent. Then researchers did a clinical trial testing beta carotene as a means to prevent lung cancer. The results were unexpected. The beta carotene group in the clinical trial had almost 20% more lung cancer than the placebo group. Other clinical trials, such as using vitamin E, found similar effects for individual compounds.

"We eat whole foods, and there are thousands of phytochemicals in these whole foods, in fruits and vegetables and whole grains. We suggested that the combination of phytochemicals in fruits and vegetables is critical to their potential antioxidant functions and anticancer properties."

Those results made me think, considering the evidence that fruits and vegetables lower the risk of cancer and cardiovascular disease, maybe we should study whole food and not individual compounds. In that 2000 Nature paper, we reported that fresh apples have significant antioxidant activity and anti-proliferative activity. That was probably the first paper to suggest that a combination of phytochemicals are responsible for the health benefits of fruits and vegetables, not one compound alone, that whole foods or food patterns are necessary to get a benefit.

We don't eat vitamin C alone, after all. We don't eat beta carotene alone either. We eat whole foods, and there are thousands of phytochemicals in these whole foods, in fruits and vegetables and whole grains. We suggested that the combination of phytochemicals in fruits and vegetables is critical to their potential antioxidant functions and anticancer properties. This research provided evidence that consumption of fruits and vegetables may play a significant role in reducing the risk of chronic diseases, such as cancer. These findings also suggested that to improve their nutrition and health, consumers should consume antioxidants from fruits and vegetables, instead of dietary supplements.

SW: Before we go on, we should probably ask you: what exactly is a phytochemical?

Phyto is a Greek word, meaning plant. So "phytochemical" means "plant chemical" and, in this case, it means the compounds responsible for reducing the risk of major chronic diseases. It is estimated that more than 5000 individual phytochemicals have been identified in fruits, vegetables and whole grains, but a large percentage still remain unknown and need to be identified before we can fully understand the health benefits of phytochemicals in whole foods. The 2002 article in the Journal of Agriculture and Food Chemistry—"Antioxidant and antiproliferative activities of common fruits" (see accompanying table, paper #1)—extended the concept to other fruits and whole foods.

SW: How do you measure the antiproliferative activity of an apple or any other fruit?

We extracted the phytochemicals from fresh apples and other fruits and then used them to treat tumor cells in cell culture and compared the results to controls. We found that the phytochemical extract of the whole food—apples—strongly inhibited tumor cell proliferation.

SW: And what kind of tumor cells did you use?

We tested the extract in colon cancer cells and liver cancer cells, and we later looked at breast cancer cells in culture. We also did animal studies. The reason this work is so highly cited is because of the focus on whole foods.

SW: Your second most-cited paper in the past decade is on thermal processing of tomatoes and how that affects these health benefits (see accompanying table, paper #2). What was the context of this work, what did you find, and why was that paper so influential?

We all tend to think that the processed fruits and vegetables have lower nutritional value than their fresh equivalent. This is the conventional wisdom and this is what you'll read in the textbooks. The idea is that these foods lose vitamin C during the processing and so they were considered less nutritive.

What we reported in that first Nature paper is that the vitamin C in apples contributed less than 0.4% of the total antioxidant activity indicating that most of antioxidant activity of a fresh apple comes mainly from the combination of phytochemicals. That means that even if you lose vitamin C during the processing, you won't see any significant decrease in antioxidant activity.

This suggested that we should look at the effect of processing on phytochemical content and antioxidant activity, which we did first in tomatoes. What we found was that the processed tomatoes actually had higher antioxidant activity than the unprocessed ones. The same was true for bio-accessible phenolic compounds. We also did another study on the processing of sweet corn, and it, too, increased antioxidant activity and bio-accessible phenolic compounds, confirming what we saw in tomatoes and contradicting the conventional wisdom.

Highly Cited Papers by Rui Hai Liu and Colleagues,
Published Since 2001

(Listed by citations)
Rank Paper Citations
1 Sun J, et al., "Antioxidant and anti proliferative activities of common fruits," J. Agr. Food Chem., 50(25): 7449-54, 2002. 337
2 Dewanto V, et al., "Thermal processing enhances the nutritional value of tomatoes by increasing total antioxidant activity," J. Agr. Food Chem., 50(10): 310-4, 2002 320
3 Wolfe K, Wu XZ, Liu RH, "Antioxidant activity of apple peels," J. Agr. Food Chem., 51(3): 609-14, 2003. 232
4 Adom KK, Liu RH, "Antioxidant activity of grains," J. Agr. Food Chem., 50(21): 6182-7, 2002. 227
5 Chu YF, et al., "Antioxidant and anti proliferative activity of common vegetables," J. Agr. Food Chem. 50(23): 6910-6, 2002. 191

These two studies opened up a new area looking at the effect of processing on the nutritional quality of foods. This information may have a significant impact on consumers' food selection by increasing their awareness of the health benefits of processed fruits and vegetables, eliminating some food safety concerns, and by encouraging them to obtain their phytochemicals or bioactive compounds from a variety of sources including fruits, vegetables, and grains.

SW: Do you think this is true then for all fruits and vegetables?

I think it probably depends. In some fruits and vegetables, processing increases activity and in others it may even decrease it.

SW: How do you translate this to nutritional advice?

I encourage consumers to eat more fruits, vegetables, and whole grains on a daily basis. The key is to increase the total amount up to 9 to 13 servings of fruits and vegetables a day in all forms, including fresh, cooked, processed, and maybe even refrigerated or frozen and canned, 100% fruit juices, 100% vegetable juices and dried fruits, which are all considered as servings of fruits and vegetables a day. You should have all kinds of fruits and vegetables to provide a wide variety of bioactive compounds and nutrients for optimum nutrition.

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