Purple corn is fast approaching classification as a functional food—an integral component of the diet that provides energy and essential nutrients. Researchers in the fields of food and nutrition are intensely searching for functional foods in almost every corner of the world and from a diversity of plants. Examples include purple corn (Zea mays L., Poaceae), green tea (Camellia sinensis [L.] Kuntze, Theaceae), soy isoflavones (Glycine max [L.] Merr., Fabaceae), various nuts, plus various other natural substances in the human diet containing antioxidant and other substances with alleged or proven potential disease-preventive properties.
Purple corn (frequently referred to as blue corn) is botanically the same species as regular table corn. Yet by a twist of nature, this corn produces kernels with one of the deepest shades of purple found anywhere in the plant “kingdom.” Research has shown that purple corn contains cell-protecting antioxidants with the ability to inhibit carcinogen-induced tumors in rats. Many plant-derived substances are believed to show these properties, but few have also demonstrated anti-inflammatory capabilities and the potential to help prevent obesity.
The kernels of purple corn (maiz morado in Spanish) have long been used by the people of the Peruvian Andes to color foods and beverages, a practice just beginning to become popularized in the industrialized world. They also make a fermented/alcoholic drink from the kernels which they call chicha morada.
Rich in Anthocyanins
The source of this natural alternative to synthetic food dyes is the largest group of natural, water-soluble pigments in the plant world, known as “anthocyanins.”2(The word anthocyanin is derived from the Greek terms, anthos, meaning flower, and kyanos, meaning blue.3) Anthocyanins are responsible for the purple, violet, and red colors attending many plants. Anthocyanins belong to an even larger class of plant chemicals known as flavonoids and are found in diverse plants, including many food plants.
Researchers at the Horticultural Sciences Department of Texas A&M University in College Station, Texas, recently determined that the mean anthocyanin content of whole, fresh purple corn from Peru was 16.4 mg/g, which was much higher than fresh blueberries (1.3-3.8 mg/g). On a dry weight basis, the mean content of purple corn was comparable to blueberries (17.7 and 9.2-24.0 mg/g, respectively). The kernel pericarp held by far the greatest concentration of anthocyanins, contributing 45% of the total content. More intriguing, the in vitro antiradical capacity of purple corn extract against the DPPH (2,2-diphenyl-1-picrylhydrazyl) radical was greater than that of blueberries (Vaccinium corymbosum L., Ericaceae), which have shown higher antioxidant values than many other commercial food plants.
Powerful Antioxidant
Digging deeper, the most abundant anthocyanin found in purple corn, called “C3G” (3-O-? -D-glucoside6,7), also known as cyanidin-3-O-?-glucopyranoside,8 has been keeping researchers very busy lately. In a number of tests designed to assess the potential health benefits of this anthocyanin, one study after another has proven its antioxidant strength. Like other anthocyanins, C3G is found in a wide variety of food plants and is actually the most common anthocyanin found in nature. C3G is the most abundant anthocyanin in some foods, such as the juice of ruby oranges (Citrus sinensis [L.] Osbeck “Blood orange,” Rutaceae)8 and blackberry (Rubusallegheniensis [L.] Bailey, Rosaceae) extract.9 Red wine also contains appreciable amounts, but other anthocyanins predominate.
C3G displays significant in vitro antioxidant activity. In one study C3G came out on top when compared to 13 other anthocyanins in the ORAC (oxygen radical absorbance capacity) assay, which tests for antioxidant activity. The strength of C3G was 3.5 times that of Trolox® (a synthetic and potent antioxidant analogue of vitamin E).13 To date, the radical scavenging/antioxidant capacity of C3G has been demonstrated in at least a dozen different assays. In one in vitro study, C3G was tested for the potential to prevent damage caused by ultraviolet (UV) light. Its ability to inhibit the oxidation of fat cells induced by UVB (280-315 nm) light was at least 40 times that of vitamin E; however, vitamin E is a weak inhibitor of UVB-induced lipid oxidation because it rapidly breaks down under UV light.
Oxidative stress and immune suppression caused by UV light are well-known for their role in the induction of skin cancers.
Oxidative stress is described as a state in which there is an excess of oxygen-based free radicals. To avoid the damage they can cause to cells, the body produces antioxidants to inactivate these free radicals. If they prove insufficient, however, the body suffers from oxidation of lipids, proteins, and nucleotide bases. In models of oxidative stress using oxidative injury to the liver, male rats fed a diet containing 0.2% C3G (2 g/kg of feed) for 2 weeks beforehand showed significantly less liver injury compared to the control group. A similar study in rats fed C3G in liquid form (0.9 mmol/kg) also found significant hepatoprotective effects.
Anti-inflammatory Capabilities
In a study on the anti-inflammatory potential of C3G, male rats administered the anthocyanin orally in liquid form (0.9 mmol/kg) prior to chemically-induced acute inflammation showed significantly less inflammation and significantly attenuated levels of pro-inflammatory cytokines (interleukin-6, interleukin-? , and tumor necrosis factor-? , and inducible nitric oxide [iNOS] expression) and nitric oxide (a free radical).23 Based on these results, it is possible that this plant pigment may also suppress the inflammatory response in diseases marked with inflammation.
Preventing Cancer
Could the anthocyanin pigment also help prevent some types of cancer? That question was put to the test in rats first treated with a carcinogen (1,2-dimethylhydrazine) and then fed a diet containing a known environmental carcinogen (PhIP or 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine) that also targets the mammary gland, prostate, and large intestine in rats and causes colorectal cancer. Incidentally, the carcinogen used in the study, known as a heterocyclic amine, is the most abundant of around 20 other types found in cooked meats and fish. Both the early signs of colorectal cancers and the numbers of malignant and benign tumors that formed in the colons of rats that had the purple pigment in their diet (5% of feed for 32 weeks; a nontoxic dose based on previous carcinogenicity studies of PCC) were significantly reduced, and there were no adverse effects. The authors of the study note that extract or juices of plants that contain high amounts of anthocyanins have previously been reported to inhibit mutagenesis induced by heterocyclic amines.
The oxidation of fats or lipids in blood serum contributes to the condition known as atherosclerosis. When male rats were fed a diet containing a high amount of C3G (0.2% of feed for 2 weeks) in place of sucrose content in the control diet, their blood serum showed a significantly lower level of oxidation along with a significant decrease in the susceptibility of their serum lipids to undergo oxidation, yet their body’s natural antioxidants (serum levels of vitamins C and E, glutathione, and uric acid) remained unaffected. Another intriguing discovery in this study was that the rats with C3G in their feed also showed significant decreases in levels of total cholesterol—about 16% less.
Anti-obesity Potential
What would happen if rats were fed C3G as part of a high-fat diet? To find out, researchers in Japan compared the body weights of male mice fed a high-fat (HF) diet with another group fed the same HF diet but with the addition of purple corn color (PCC) which provided C3G (0.2% or 2 g/kg of feed). Results were also compared to 2 control groups: one fed a normal diet and one fed a normal diet with C3G. After 12 weeks, the results were obvious: mice in the PCC-HF group showed significantly less signs of developing obesity, yet exhibited no significant difference in food consumption compared to the control groups with or without the PCC in their feed.
When related to the primary control group (no HF diet or PCC), the adipose tissue weights of the PCC-HF group were not significantly different. In addition, fatty tissue in HF-diet group was found to be growing in size but showed no increase in the PCC-HF group. The HF-diet group also developed a state of hyperglycemia along with an over-production of insulin. Interestingly, this was not observed in the PCC-HF group in which both pathologies were completely normalized. In conclusion, the researchers stated that their tests of PCC provide a nutritional and biochemical basis for the use of the pigment or anthocyanins as a “functional food factor”—one that may be beneficial for helping to prevent diabetes and obesity.
It now remains for future studies to determine the possible contributing effects of other substances from purple corn which are extracted along with PCC.
More recent efforts to determine the potential anti-obesity mechanisms of purple corn pigment have focused on the effect of C3G on fat cell dysfunction, fat cell-specific gene expression, and the regulation of chemical messengers (adipocytokines) secreted by fat cells, such as the fat-derived hormone adiponectin. After feeding male mice a diet containing PCC to provide C3G (2 g/kg of feed for 12 weeks), gene expression levels of adiponectin in white fatty tissue was upregulated 1.7-fold compared to the control group not fed the food colorant.
Plasma and gene expression levels of adiponectin are decreased in obese humans and mice and in insulin resistant states. When adiponectin was administered intravenously to mice fed high-fat/sucrose diets, weight gain was significantly inhibited. Adiponectin (i.v.) also lowered plasma glucose levels in lean mice fed a high-fat meal.
Rich in C3G (approximately 70 mg/g), about 50,000 kg of PCC is used in Japan as a food color for confections and soft drinks annually. So far, PCC remains to be officially approved for use as a food colorant by the U.S. Food and Drug Administration. However, approval seems likely because “grape skin color” and “grape skin extract” (“enocianini” or “enocyanin”) made from Concord grapes (Vitis vinifera L., Vitaceae) are also rich in anthocyanins2 and both are FDA-approved for use in beverages and non-beverage foods.
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