Crop geneticist Charles R. Brown has spent a decade working to make a better potato. In the beginning, he focused on beefing up the familiar white-fleshed tuber. His strategy was to recapture healthful traits from old-style spuds from the plant's native range in South America. He examined many yellow, red, and purple potatoes, none of which grows well in a U.S. climate. While cross breeding these imports with their northern cousins, Brown and his coworkers at the U.S. Department of Agriculture laboratory in Prosser, Wash., began hearing about putative health benefits from the type of pigments, called flavonoids, that give the potatoes their color.
Flavonoids include beta-carotene and related carotenoids, which are responsible for many of the yellows, oranges, reds, and greens in produce. Other reds and most of the blues, purples, and blackish tints—especially in berries and potatoes—trace to flavonoids called anthocyanins.
These chemicals are considered antioxidants because they quash free radicals, naturally forming molecular fragments that have several damaging effects. Free radicals can kill cells, transform some of the blood's cholesterol-toting lipoproteins into agents of atherosclerosis (SN: 4/21/01, p. 245: Available to subscribers at http://www.sciencenews.org/articles/20010421/fob4.asp), and induce DNA damage that might foster cancer (SN: 2/22/97, p. 126: http://www.sciencenews.org/pages/sn_arc97/2_22_97/bob1.htm).
A few years ago, Brown's group and a few others around the world began developing new lines of crops explicitly for their intense antioxidant pigments. Some early lines of red and purple potatoes are now on the market, and other colorful crops are heading that way.
Probably the most famous example is known as golden rice. It's enriched with beta-carotene, a yellow chemical from which the body fashions most of its vitamin A. Swiss and German researchers used biotechnology to design this cereal in the late 1990s to improve vitamin-poor diets in developing countries. The golden grain is still being fine-tuned for eventual commercialization.
Philipp Simon and his colleagues at a USDA lab in Madison, Wis., have been developing carotenoid-enhanced, yellow-orange cucumbers and red and anthocyanin-rich, dark-purple carrots (See A Carrot Rainbow). This team bred some of the carrots now on the market, which retain the traditional orange color but produce 75 percent more beta-carotene than carrots did 25 years ago. In another example of pigment boosting, researchers at Cornell University are breeding wheat with extra flavonoids.
There's currently evidence that, in addition to fighting inflammation, heart disease, and cancer, flavonoids can counter obesity and elevated blood sugar. Although scientists have presumed that flavonoids' benefits derive mainly from their antioxidant activity, some research has recently shown that the chemicals facilitate signaling between cells and silence genes that might otherwise foster disease.
Indeed, James A. Joseph of USDA's Human Nutrition Research Center on Aging at Tufts University in Boston says that this basic effect may contribute to flavonoids' broad range of activities.
With growing recognition of the health-promoting biological activity of plant pigments, many researchers are advocating that consumers expand the palette of colors on their dinner plates. For instance, Joseph wrote a book to guide people in choosing healthful foods by their colors (The Color Code, 2003, Hyperion Books).
Joseph acknowledges that color offers, at best, an imperfect measure of potentially beneficial antioxidant flavonoids. However, by choosing foods exhibiting a range of deep colors, he says, a person can be reasonably sure of getting a broad mix of beneficial flavonoids.
Many studies have linked anticancer benefits and protection against heart disease with diets rich in produce, especially carotenoid-rich green, leafy vegetables.
The most recent of these reports, in the Nov. 3, 2004 Journal of the National Cancer Institute, analyzed dietary and health data for almost 72,000 female nurses and 38,000 male health professionals. The study found significantly less risk of chronic illness, especially heart disease, in study participants eating the most fruits and vegetables. Of all foods analyzed, green leafy vegetables appeared most protective. In fact, for each daily serving of spinach or other greens consumed, an individual's risk of developing cardiovascular disease fell by 11 percent.
In 2003, Tiina H. Rissanen of the University of Kuopio in Finland and her colleagues reported that the more servings of vegetables and fruits that middle-aged men consumed, the lower their risk of dying from heart disease. The data pointed to berries, in particular, as being protective.
These findings were consistent with others published that year by an Australian team. For 6 weeks, nutrition scientists gave 32 men fruit extracts every morning and vegetable extracts every evening. Several potential heart-disease indicators, such as blood concentrations of homocysteine (SN: 1/4/03, p. 5: Available to subscribers at http://www.sciencenews.org/articles/20030104/fob5.asp) and susceptibility of cholesterol to oxidation (SN: 4/21/01, p. 245: Available to subscribers at http://www.sciencenews.org/articles/20010421/fob4.asp), were far lower in men taking the supplements than in volunteers who had eaten the same diet minus the supplements.
To probe what compounds might account for such benefits, researchers have been isolating and testing the effects of various constituents of fruits and vegetables, including purified flavonoids.
John D. Folts' team at the University of Wisconsin–Madison has shown that grape skins and seeds contain flavonoids that limit blood platelet clumping. That clumping is a critical step in blood clotting, but it also contributes to the formation of clots underlying heart attacks and strokes. Although the flavonoids in either skins or in seeds have proved beneficial, the best results occurred when Folts' group added extracts of both to the diets of dogs and people.
Folts suggests that extracts of grape seeds and skins could provide all the heart-health benefits that have been linked to wine and grape juice (SN: 3/8/03, p. 155: Available to subscribers at http://www.sciencenews.org/articles/20030308/bob10.asp). His group recently began studies of one commercial extract of grape seeds and skins. Such a food supplement might prove beneficial to people who'd prefer to avoid the alcohol in wine or the sugar in juice.
Pomegranates are even richer in antioxidant flavonoids, especially anthocyanins, than purple grapes are, according to research led by Michael Aviram of Rambam Medical Center in Haifa, Israel. Over the past 5 years, his team has shown that the flavonoids in pomegranate inhibit cholesterol oxidation in human blood and slow the development of atherosclerotic disease in mice.
In one recent study, Aviram's team followed volunteers who had atherosclerosis, characterized by symptoms including a narrowing of their carotid arteries. Ten of the participants drank 50 milliliters of pomegranate juice daily for 1 to 3 years. Nine others took a flavonoid-free placebo drink. Throughout the trial, all continued to receive the heart medications that their doctors prescribed.
By the end of the study, the people drinking the pomegranate juice had experienced a 20 percent drop in systolic blood pressure, while the placebo group showed no decline. Similarly, only the pomegranate group experienced a beneficial reduction in the thickness of their carotid artery walls and a dramatic drop in the oxidation-susceptibility of their low-density-lipoprotein cholesterol, the team reported in the June 2004 Clinical Nutrition.
Related heart benefits emerged in Chinese studies of animals treated with black-rice pigments. Black rice contains flavonoids, especially pigmented relatives of anthocyanins, also found in fruits and vegetables, explains Wenhua Ling of Sun Yet-sen University of Medical Sciences in Guangzhou. He and his colleagues fed mice and rabbits cholesterol-rich diets intended to promote heart disease. The scientists supplemented the animals' food with powdered rice bran. Half the animals got black-rice bran, the rest got equivalent amounts of pigmentfree white-rice bran. The animals ingesting black-rice supplements developed far lower concentrations of oxidation by-products in their blood and in the tissues of their arteries and experienced less artery-clogging plaque, Ling's team reported in 2002 and 2003.
Ling says he was surprised by the potency of the pigments. He and his colleagues are now conducting research in which they give black rice pigments to people with atherosclerosis.
Plant flavonoids also show promise in other health areas. For instance, studies indicate that lycopene—the red carotenoid in tomatoes and watermelons—can prevent prostate cancer in animals and shrink prostate tumors in men (SN: 4/24/99, p. 271). Anthocyanins in tart red cherries seem to provide a natural inflammation-fighting alternative to aspirin (SN: 4/17/99, p. 247).
However, some of the most dramatic effects attributed to anthocyanins have emerged in studies of blue and purple foods. For instance, when Japanese scientists gave mice high-fat diets augmented with an anthocyanin-pigmented extract from purple corn, the animals maintained normal weights. Mice on the same diet but without the extract became obese. Moreover, only the latter group of animals developed excessive sugar and insulin concentrations in their blood, Takanori Tsuda of Doshisha University in Kyoto and his colleagues reported in 2003.
Last year, Tsuda's team showed that the purple-corn extract increased the activity of a gene that regulates the function of fat cells. It's the first food component to do so, the scientists reported in the March 26, 2004 Biochemical and Biophysical Research Communications. They also recommended that anthocyanins be explored as nutritional supplements for "preventing obesity and diabetes." Tsuda says, "We need to perform a human study as soon as possible."
Joseph, who is a neuroscientist, has been working with blueberries, which are rich in flavonoids. Five years ago, his team reported that adding blueberries to the diets of aging rats not only prevented further declines in memory and mental agility but also reversed those trends in the animals (SN: 9/18/99, p. 180: http://www.sciencenews.org/pages/sn_arc99/9_18_99/fob2.htm).
In 2003, his team published a follow-up study in which they examined a strain of mice that normally develops Alzheimer's disease symptoms, including brain plaques and memory loss. Beginning when the mice were young, the researchers fed them 20 grams of blueberries per kilogram of food. The animals still eventually developed Alzheimer's-like brain plaques—but no memory loss. The researchers concluded that their data "indicate for the first time that it may be possible to overcome genetic predispositions to Alzheimer's disease through diet."
Some of the benefit to memory appears to trace to enhanced brain-cell signaling in animals eating the blue fruit, Joseph says. Old neurons and those in individuals with Alzheimer's disease "are like old married couples: They don't talk to each other," he says. Blueberry's flavonoids "make them [communicate] like young lovers again."
Joseph speculates that, by fostering communication between neurons, the anthocyanins or related chemicals in blueberries affect learning and memory and also spur the growth of new nerve cells, a process called neurogenesis.
His group's latest studies indicate that in blueberry-fed animals, both anthocyanin concentrations and the rates of neurogenesis correlate with performance on memory tests.
Most flavonoid researchers argue that people should get healthy doses of these chemicals from colorful foods, not dietary supplements. Indeed, several studies have shown that megadoses of a flavonoid can trigger harmful effects, probably by boosting oxidation.
That's what biochemist Homer S. Black of Baylor College of Medicine in Houston witnessed in studies of mice. His group showed that a diet overly enriched with beta-carotene increased skin cancer resulting from ultraviolet light's oxidative damage rather than protecting against it. Black is therefore "leery of supplementing with single antioxidants," but he heartily recommends consuming a broad mix of them every day in whole foods.
Another reason for consuming antioxidants in foods rather than as supplements is that the chemistry of these agents can be influenced by what's eaten along with them. For instance, the carotenoids tinting most vegetables preferentially dissolve in lipids. Unless these pigments are eaten along with fat—such as the oil in salad dressing—the body absorbs only a small fraction of them, notes Steven J. Schwartz of Ohio State University in Columbus.
He recently showed that avocado, a high-fat fruit, facilitates carotenoid uptake by the body. In tests with 11 men and women, his team showed that adding about half an avocado to an undressed salad dramatically increased carotenoid absorption. For instance, lutein absorption more than quadrupled, and beta-carotene absorption rose 12-fold. Schwartz now recommends mashed avocado as a salad dressing.
Anthocyanins, in contrast to carotenoids, are extremely water soluble, so they don't need fat to be absorbed, reports chemist Ronald L. Prior of the USDA lab at the Arkansas Children's Nutrition Center in Little Rock.
However, adds Joseph, because these flavonoids appear to be useful to the brain and "something has to be [fat] soluble to get across the blood-brain barrier," even anthocyanins may benefit from a fat chaser. Toward that end, he suggested that the California Avocado Commission consider teaming its favorite fruit with his. The result: a recipe for avocado-blueberry smoothies (http://www.avocado.org/recipes/view.php/Avocado/Blueberry/Smoothie?recipe_id=902).
Now, doesn't that make a colorful image?
This article shouldn't have categorized as flavonoids the plant pigments in the carotenoid family. And although frequently found in green vegetables, carotenoids don't contribute to the green color but rather hide behind it, emerging only when—as in the yellow tips of old broccoli—the green chlorophyll is lost.
I envision a beautifully colorful potato salad utilizing multiple colors of potatoes. But would a cooked mixture be like carrots with potatoes (minimal bleed) or like beets with anything else (maximum bleed)?
The red and blue pigments in the new potato lines are "water soluble and will leach," notes USDA's Charles R. Brown. However, unpeeled potatoes bleed minimally. Potatoes tinted yellow to orange shouldn't bleed at all because their carotenoid pigments aren't water soluble.—J. Raloff
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Department of Biochemistry
Technion Faculty of Medicine
Rappaport Institute for Research in the Medical Sciences
Department of Dermatology
Baylor College of Medicine
One Baylor Plaza
Houston, TX 77030
24106 North Bunn Road
Prosser, WA 99350
John D. Folts
Department of Cardiovascular Medicine
University of Wisconsin, Madison
Madison, WI 53792
James A. Joseph
U.S. Department of Agriculture
Human Nutrition Research Center on Aging
7111 Washington Street, Room 919
Boston, MA 02111
Wen Hua Ling
Department of Clinical Nutrition
Sun Yet-sen University
People's Republic of China
Ronald L. Prior
Arkansas Children's Nutrition Center
1120 Marshall Street
Little Rock, AR 72202
Tiina H. Rissanen
University of Kuopio
Steven J. Schwartz
Interdisciplinary Graduate Program in Nutrition
235 Parker Food Science Building
2015 Fyffe Court
Ohio State University
Columbus, OH 43210
Philipp W. Simon
Department of Horticulture
University of Wisconsin, Madison
Madison, WI 53706
Center for Biomarkers of Preventive Medicine
From Science News, Volume 167, No. 2, January 8, 2005, p. 27.