Benefits
Systolic blood pressure
A meta-analysis of 30 randomized trials found grape products lowered systolic blood pressure by about 3 mmHg, and whole grape forms (grape powder and raisins) by about 2.7 mmHg, while grape juice did not. The evidence was rated low certainty, and diastolic pressure did not change. A month of grape powder lowered systolic pressure in 24 men with metabolic syndrome, but two grape extract capsule or tablet trials found no change.
Blood vessel function (flow-mediated dilation)
Flow-mediated dilation is an ultrasound test of how well an artery widens. In 24 men with metabolic syndrome, a month of freeze-dried grape powder raised it versus placebo. Other trials did not agree: wine grape polyphenol capsules had no effect in 35 healthy men, a month of grape powder changed few vascular markers in 20 adults with obesity, and a 30-trial meta-analysis found no overall effect on blood vessel function.
Blood triglycerides and LDL cholesterol
In a crossover trial in 44 women, 36 g/day of grape powder for 4 weeks lowered triglycerides (by 15% before and 6% after menopause) and LDL cholesterol. A 60 g/day trial in 20 adults with metabolic syndrome lowered triglycerides only. Extract trials were split: 700 mg/day of a grape extract ended with lower LDL and total cholesterol than placebo in 60 adults, while red grape extract tablets did not change them in 69 adults.
Memory and attention in older adults
Two placebo-controlled trials in healthy older adults (111 people, and 96 completers out of 120) tested the grape juice extract Cognigrape at 250 mg/day for 12 weeks and reported better scores on memory, attention and language tests. Both trials involved the extract's maker, they have not been repeated by an independent group, and the results apply to that extract rather than to grape products in general.
Brain glucose use on PET scans in older adults
In a pilot trial of 10 older adults with mild memory decline, brain scans after 6 months showed falling glucose use in some brain regions on placebo but stable use with a grape formulation. Scores on the memory and thinking tests themselves did not differ between the groups. A systematic review of grape trials on cognition, half of them using grape juice, found some improvements but noted that study designs varied too much to compare.
Markers of oxidative stress and inflammation
In the 44-woman grape powder trial, a urine marker of whole-body oxidative stress and the inflammatory signal TNF-alpha fell after the grape powder period, although oxidation of LDL particles did not change. In the 24-man trial, anti-inflammatory markers rose only in men without abnormal blood fats. These are lab markers in small groups, not measures of health outcomes.
Mechanism of action
Nitric oxide and blood vessel relaxation
Nitric oxide signals artery walls to relax. In the 24-man grape powder trial, larger drops in systolic pressure went along with larger rises in blood nitrite and nitrate, a rough marker of nitric oxide, although average nitrite and nitrate levels did not differ from placebo.
Changes in how blood fats are carried
In the 44-woman trial, grape powder lowered apolipoproteins B and E, the proteins that carry LDL and other fat particles, and reduced the activity of cholesterol ester transfer protein by about 15%, an enzyme that moves cholesterol between lipoproteins.
Cell defence genes after a heavy meal
In 20 adults with obesity, taking grape powder with a high-fat, high-carbohydrate meal raised the activity of NRF2, a switch for the body's own antioxidant defence genes, in white blood cells and lowered endothelin-1, a vessel-narrowing signal, over the following hours versus placebo.
Clinical trials
Randomized, double-blind, placebo-controlled crossover trial of freeze-dried grape polyphenol powder for 30 days, with a 3-week washout. A companion report on the same men gives the dose (46 g/day, about 252 g of fresh grapes) and states that the senior author received funding from the California Table Grape Commission (Barona et al. 2012, J Nutr).
24 men aged 30 to 70 with metabolic syndrome, keeping their usual diet but avoiding polyphenol-rich foods.
Systolic blood pressure and the adhesion molecule sICAM-1 were lower, and flow-mediated dilation higher, during the grape period than the placebo period. Average nitrite and nitrate levels did not differ, and the other metabolic syndrome measures were unchanged. A small, short, grape-industry-funded trial.
Randomized, placebo-controlled crossover trial of 36 g/day lyophilized grape powder or a sugar placebo matched for energy, 4 weeks each with a 3-week washout (Zern et al. 2005, J Nutr).
44 women: 24 premenopausal and 20 postmenopausal.
Triglycerides fell 15% in premenopausal and 6% in postmenopausal women, and LDL cholesterol, apolipoproteins B and E and CETP activity were lower with grape powder. LDL oxidation did not change, while urinary F2-isoprostanes and plasma TNF-alpha fell. The powder was 92% carbohydrate.
Randomized, double-blind, placebo-controlled crossover pilot study of 60 g/day freeze-dried grape powder for 4 weeks, with a 3-week washout (Millar et al. 2018, Metab Syndr Relat Disord).
20 adults aged 32 to 70 with metabolic syndrome.
The primary outcome, an HDL antioxidant enzyme (PON1 activity), did not change, and neither did other HDL function markers. Fasting triglycerides fell versus placebo; other blood fats and metabolic measures did not change.
Randomized, double-blind, placebo-controlled crossover trial, three 2-week periods, testing capsules with 800 mg of polyphenols from a wine grape mix or from grape seed; the first author was at Unilever Research and Development (van Mierlo et al. 2010, J Nutr).
35 healthy men.
The wine grape polyphenols did not change flow-mediated dilation after a low-fat breakfast (difference versus placebo -0.4%, not significant), and showed no consistent effect after a high-fat lunch or on blood pressure, platelet function or blood fats.
Randomized, placebo-controlled, double-blind crossover trial of 60 g freeze-dried whole grape powder, first with a single high-fat, high-carbohydrate meal and then daily for 4 weeks; registered as NCT01674231 (Bardagjy et al. 2018, Arch Biochem Biophys).
20 adults with obesity.
With the meal, grape powder raised NRF2 gene activity in white blood cells at 3 hours and lowered endothelin-1 at 5 hours versus placebo. After 4 weeks of daily use there was little effect on cardiovascular markers, apart from a rise in sVCAM-1, whose starting values differed between treatments.
Randomized, parallel, four-arm trial of red wine, red grape extract tablets at a wine-equivalent dose, the same tablets at half dose, or placebo tablets, for 4 weeks (Hansen et al. 2005, Eur J Clin Nutr).
69 healthy men and women aged 38 to 74.
Red grape extract tablets showed no effect on LDL cholesterol, the HDL to LDL ratio, total cholesterol, VLDL triglycerides, clotting factor VII activity or blood pressure. The rise in HDL and fall in fibrinogen seen with red wine appeared to come from the alcohol rather than the grape compounds.
Double-blind, placebo-controlled pilot study of a grape formulation versus a polyphenol-free placebo for 6 months, with FDG-PET brain scans and memory and thinking tests (Lee et al. 2017, Exp Gerontol).
10 older adults (average age 72) with mild decline in cognition.
The placebo group showed falling glucose use in several brain regions while the grape group stayed stable. Scores on the memory and thinking tests did not differ between groups. Very small; the PET result is a marker, not a measure of everyday function.