Grape skin, the outer layer of the grape (Vitis vinifera), is either green, red, or purplish-black in color. The skin, stem, seeds, and juice of the grape are used in making wine. Although the skin, stem, and seeds are often used in making the nutritional supplement, grape skin extract, the extract sometimes contains grape skin only. Generally, the skin of red grapes is used in making nutritional supplements.
In 1535, sailors on Jacques Cartier's expedition to Canada became seriously ill with scurvy, a vitamin deficiency. This degenerative disease of connective tissues was caused by the lack of vitamins in the typical seafarer's diet—a menu of dried meat and biscuits. The crew was saved by the advice of a Native American, who recommended drinking tea made from the bark of a particular species of pine tree. In the 1930s, researchers discovered that the ascorbic acid (vitamin C) in fruits and vegetables prevented scurvy.
The pine extract, however, contained very little vitamin C. For more than 50 years, European biochemists have been researching the seafarers' more likely rescuer—a family of antioxidant polyphenols (acid compounds) called pycnogenols, whose primary active compounds are pigments called oligomeric proanthocyanidins (OPCs). French chemist Jack Masquelier isolated OPCs from peanut skins in 1947 and coined the term "pycnogenols" to describe the unique class of polyphenols to which OPCs belong.
Although people have been drinking wine for centuries, scientific research into the health benefits of products derived from red grapes began in Europe in the mid to late twentieth century. Supplemental OPCs have been used in Europe since 1950 to treat weak blood capillaries, postsurgical edema (swelling), cirrhosis (liver disease), varicose veins, and diabetic retinopathy (eye disease resulting from diabetes). Early identification of OPCs as useful for treating capillary fragility gave researchers some indication of their potential value in connective tissue disorders. However, this limited focus tended to overlook the additional therapeutic possibilities of OPCs and, until the latter part of the twentieth century, distracted scientists from investigating broader uses for OPCs.
Aside from pine bark, OPCs are concentrated in grape seeds and skins, wine, green and black teas, beans, and the skins of many fruits. Generally, the more intense the color, the more OPCs in the food, which explains why red wine has a greater health benefit than white wine. When red wine is made, the "must" is used—the skins, seeds, and stems. The must is left in the mixture for a long period of time as the wine ferments and the OPCs emerge, giving red wine its characteristic flavor and color.
In the case of white wine, however, the must is taken out early, so the wine neither darkens nor absorbs as many OPCs. Grape juice also contains OPCs. However,
researchers have found that grape juice may not confer the same health benefits as red wine.
Red grape skins contain an array of bioflavonoids (quercetin, catechins, flavonols, and anthocyanidins) and nonbioflavonoid polyphenols (acid derivatives). One important nonbioflavonoid in grape skin is called resveratrol. Resveratrol is a plant-specific enzyme that exists in 72 plant species, such as grapes, peanuts, and pine trees. Grapes are the most abundant source of this health-promoting enzyme.
Resveratrol's presence in the plant is induced by stress, injury, infection or ultraviolet irradiation. It is thought that the injury to the grape skin, produced during the wine-making process, significantly increases resveratrol levels. The relatively high quantities of the enzyme in the grape skins are thought to help the plant resist fungal infections, diseases, adverse weather, and insect or animal attack.
There are many possible therapeutic applications of the resveratrol in red grape skin. In clinical studies, resveratrol demonstrated equivalent or better anti-inflammatory effects compared to the well-established anti-inflammatory drugs phenylbutazone and indomethacin. In animal studies, resveratrol inhibited both the acute and chronic phases of inflammation.
In humans, some researchers have found that resveratrol thins the blood more effectively than aspirin, which is often used to decrease the risk of a heart attack. In fact, the phrase "French paradox" refers to the idea that although French men consume a high-fat diet, they have one-third as many heart attacks as American men. Moreover, French men have high cholesterol and blood pressure
Preliminary tests in animals also indicate that resveratrol may interfere with the development of cancer in three ways: by blocking the action of cancer-causing agents, by inhibiting the development and growth of tumors, and by causing precancerous cells to revert to normal.
Although researchers are uncertain about how much resveratrol is needed to produce beneficial effects in humans, supplementation with red grape skin extract or consuming a glass or two of red wine may prevent or alleviate the following conditions:
- bruising (capillary fragility)
- cancer (cancer-inhibiting effects)
- fungal infection
- heart disease (hardening of the arteries and high cholesterol)
- inflammation (including bursitis and tendonitis)
- Raynaud's syndrome (a blood vessel disorder)
- varicose veins
- vision problems (including cataracts and glaucoma)
- wound healing
Red grape skin extract is prepared in capsule form as a nutritional supplement. For adult maintenance, the therapeutic range is thought to be 200–600 mg at 30% anthocyanins (OPCs), although guidelines have not been definitively established.
The resveratrol found in red grape skin and its extract is also found in red wine and concord grape juice. However, grape juice has been found to have fewer benefits than red wine, due to the technique for processing the grapes. For example, grape juice has only one-third the anti-clotting properties of red wine.
Although research is limited, scientific investigators have not issued any precautions regarding the use of grape skin or grape skin extract. However, people should be aware of the known side effects of red wine and resveratrol.
There are many potential side effects to consuming excessive quantities of red wine (such as allergic reactions to sulfites, intoxication, and liver damage) in order to obtain the health benefits of resveratrol. Each individual must weigh the risks versus the benefits of consuming alcohol.
However, resveratrol itself is also a phytoestrogen (plant estrogen). The estrogenic properties of this chemical may play a role in the beneficial cardiovascular effects in red wine. These positive effects include increasing high-density lipoprotein (HDL), the "good cholesterol." On the other hand, it has been noted that drinking red wine may support the proliferation of certain breast cancer cells that require estrogen for growth. Thus, resveratrol may have undesirable side effects in some people, including those women with a history of breast cancer or postmenopausal women taking hormone replacement therapy.
Scientific investigation on the interactions of grape skin or grape skin extract with drugs, foods, or diseases is very limited and inconclusive. However, if the resveratrol in grape skin is consumed in red wine, a wide range of adverse interactions with drugs and foods may result. It is advisable to consult a physician before consuming alcohol in combination with any type of prescription or over-the-counter medication.
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American Heart Association, National Center. 7272 Greenville Avenue, Dallas, TX 75231. http://www.americanheart.org.
National Cancer Institute. Public Inquiries Office, Building 31, Room 10A03, 31 Center Drive, MSC 2580, Bethesda, MD 20892. http://www.nci.nih.gov.