Nutrients
Grapes are very good sources of manganese. They are good sources of vitamin B6, thiamine, riboflavin, potassium, and vitamin C.

Glycemic Index (GI)
In a study to determine the GI of various foods, it was concluded that grapes have a low GI of 40 - 50.

Resveratrol
Resveratrol is classified as a polyphenol, a chemical substance found in plants that may have antioxidant properties. Several studies have demonstrated that resveratrol is an effective antioxidant, offering more protection than other antioxidants such as vitamins C and E. Studies on resveratrol to date have been done in vitro, and further human studies are necessary to determine its potential as a supplement.
Resveratrol is isolated from the seeds and skins of grapes, and is primarily found in red wine, but also in lesser quantities in white wine, grape juice, blueberries, cranberry juice, and peanuts. Japanese knotweed (polygonum cuspidatum) is considered to be one of the richest sources of resveratrol available.
Resveratrol content of wine is dependent on grape type and climate; red wines have higher resveratrol content, while white wine has a very low content. Wines produced in humid climates have higher resveratrol content than those produced in hot and dry climates.
Grape juice contains 10 times less resveratrol than wine, but also contains high levels of anthocyanadins, phenolic acids, and other polyphenols with antioxidant properties. Grape juice also contains piceid, a resveratrol derivative, and together with the other polyphenols, may have a beneficial health effect for those who cannot drink wine.

Cholesterol
A study conducted in Spain involving 32 participants concluded that regular ingestion of concentrated red grape juice reduces plasma concentrations of oxidized LDL cholesterol.
In another study 61 healthy subjects with LDL cholesterol were administered grape seed extract tablets (proanthocyanidin). The results suggested that grape seed extract reduced oxidized LDL.

Alzheimer's Disease
The 2006 Kame Project study published in The American Journal of Medicine concluded that fruit and vegetable juices consumption play an important role in delaying the onset of Alzheimer's disease, particularly among those who are at high risk for the disease. In a follow up study the total phenolic content of 13 fruit juices were evaluated, and it was determined that purple grape juice contained the largest number of individual phenolic compounds and also the highest concentration of total phenolics.

Heart Disease
Results of a study involving 20 healthy participants indicated that flavonoids from the juice of purple grapes decrease platelet aggregation (blood clotting) and enhance nitric oxide release, a compound in the body that inhibits the formation of clots in blood vessels.
Several studies have concluded that regular ingestion of concentrated red grape juice reduces LDL cholesterol and may contribute to a reduction in cardiovascular disease risk.

Preliminary evidence suggests that resveratrol inhibits platelet aggregation, and acts as a vasodilator (causes blood vessel dilation). Many studies suggest that consuming red wine may reduce the incidence of coronary heart disease.

Cancer
Recent studies indicate that resveratrol can block the process of cancer in the initiation, promotion and progression stages. Resveratrol appears to decrease tumor promotion activity by inhibiting cyclooxygenase-1 (COX-1), an enzyme that converts arachidonic acid to pro-inflammatory substances that stimulate tumor-cell growth.