Benefits
Memory test scores in older adults (Concord grape juice)
In 12 older adults with early memory decline, 12 weeks of Concord grape juice improved a verbal learning score versus a matched placebo drink, while delayed verbal and spatial recall did not differ significantly. In a 10-person grape powder study, scores on memory and thinking tests did not differ from placebo after 6 months. Both were very small; Welch Foods funded the juice trial and the California Table Grape Commission the powder trial.
Spatial memory and simulated driving in working mothers (Concord juice)
In a crossover trial in 25 healthy working mothers aged 40 to 50, 355 mL a day of Concord grape juice for 12 weeks improved immediate spatial memory and, in 17 of them, simulator driving performance versus a matched placebo drink. Other measured tests were not reported as improved. Welch's co-sponsored the trial and a Welch's employee was a coauthor.
Blood sugar after meals with raisin snacks
In a 12-week trial in 46 adults with overweight and fasting glucose of 90 to 150 mg/dL, raisin snacks three times a day lowered blood sugar after meals and HbA1c by more than processed snacks did, while fasting glucose did not change. A similar 51-person trial in people with type 2 diabetes also found lower after-meal glucose. Both came from one center, and both list the California Raisin Marketing Board as a collaborator.
Blood pressure with raisins and grape juice
In the raisin snack trials, diastolic pressure in one and systolic pressure in the other fell more with raisins than with processed snacks. In 64 adults with raised blood pressure, 8 weeks of Concord grape juice did not change 24-hour blood pressure, the main outcome. A meta-analysis found a small systolic drop with whole grape products but not with grape juice, rated low certainty.
Artery function in smokers (Concord grape juice)
In a crossover trial in 26 healthy smokers, 2 weeks of Concord grape juice improved flow-mediated dilation, an ultrasound test of artery widening, and lowered pulse wave velocity, a measure of arterial stiffness, versus placebo. It also blunted the short-term drop in artery function after smoking a cigarette.
Large LDL particles and immune cell responses (grape powder)
In a 24-person crossover trial in adults with obesity, freeze-dried table grape powder (46 g twice daily for 3 weeks) lowered large LDL particles versus a sugar-matched placebo powder, but plasma inflammatory markers such as CRP did not change. Stimulated blood cells released more of two inflammatory signaling proteins, a finding whose meaning is unclear.
Macular pigment in older adults (table grape powder)
In 34 older adults in Singapore, 46 g a day of freeze-dried table grape powder for 16 weeks was followed by a rise in macular pigment density, a measure of the eye's light-filtering pigment, within the grape group, in a trial funded by the California Table Grape Commission. The abstract reports a time effect and within-group changes rather than a clear difference from the placebo group.
Mechanism of action
Polyphenols in the skin and flesh
Analysis of a freeze-dried California table grape powder found anthocyanins (mainly cyanidin and malvidin types), flavonols such as quercetin and kaempferol, and the flavanols catechin and epicatechin. Concord grape juice supplied by Welch Foods for trials contained about 2,100 mg of total phenolics per liter in one study and 777 mg of polyphenols per 355 mL serving in another.
Very little resveratrol
The same grape powder supplied 0.04 mg of resveratrol per 23 g serving, about one serving of fresh grapes, so the trial dose of four servings a day gave roughly 0.16 mg. Resveratrol supplement trials commonly use 150 mg a day or more, so effects of isolated resveratrol cannot be assumed for grapes.
Sugar, fiber and food form
Fresh grapes are about 15% sugar, split roughly evenly between glucose and fructose, with under 1% fiber. Drying concentrates both: raisins are about 65% sugar and 4.5% fiber. In a meta-analysis, whole grape forms and grape juice did not give the same blood pressure results.
Lab and animal findings
A grape-industry research summary lists animal studies in which grape-enriched diets reduced anxiety-like behavior, memory impairment and blood pressure rises in rat models of oxidative stress, and limited nerve cell damage after reduced brain blood flow in gerbils. These are animal results that have not been confirmed in people.
Clinical trials
Randomized, double-blind, placebo-controlled trial of 100% Concord grape juice (Vitis labrusca) at 6 to 9 mL/kg/day (444 to 621 mL/day) versus an energy- and carbohydrate-matched placebo drink for 12 weeks; funding and the drinks came from Welch Foods, Inc. (Krikorian et al. 2010, Br J Nutr)
12 older adults with memory decline but not dementia (5 juice, 7 placebo).
Verbal list learning improved more with juice than placebo (P = 0.04); delayed verbal recall (P = 0.10) and spatial recall (P = 0.12) did not differ significantly. Depressive symptoms, weight and waist size did not change. Fasting insulin ended higher in the juice group (13.7 vs 10.0 uU/mL, P = 0.03), which the authors called unexpected.
Randomized, placebo-controlled crossover trial of 355 mL/day Concord grape juice (777 mg total polyphenols) versus an energy-, taste- and appearance-matched placebo for 12 weeks each with a 4-week washout; registered as NCT01411631 with Welch's as collaborator, and one author was at Welch Foods Inc. (Lamport et al. 2016, Am J Clin Nutr)
25 healthy, employed mothers aged 40 to 50 with preteen children; 17 also did a driving simulator test.
Immediate spatial memory and driving performance improved after juice relative to placebo. The abstract reports significant gains only for these two of the outcomes measured, which also included verbal memory, executive function, attention, blood pressure and mood. People who had juice first kept better scores during the placebo period, which complicates the crossover comparison.
Double-blind crossover trial of 100% Concord grape juice at 7 mL/kg/day versus a matched placebo drink, 8 weeks each with a 4-week break; supported by a grant from Welch Foods Inc. and the NIH (Dohadwala et al. 2010, Am J Clin Nutr)
64 otherwise healthy adults with prehypertension or stage 1 hypertension taking no blood pressure medicine (average age 43).
No effect on the primary outcome, 24-hour average systolic and diastolic blood pressure, or on blood pressure responses to stress. In secondary analyses the nighttime dip in systolic pressure grew slightly with juice (P = 0.005) and fasting glucose fell 2 mg/dL versus a 1 mg/dL rise on placebo (P = 0.03).
Randomized, double-blind, placebo-controlled crossover trial of Concord grape juice for 2 weeks, with artery tests before and after smoking a cigarette; the abstract does not state a funder (Siasos et al. 2014, Am J Hypertens)
26 healthy adult smokers in Greece.
Compared with placebo, juice improved pre-smoking flow-mediated dilation (P = 0.02) and pulse wave velocity (P = 0.04), and it prevented the acute fall in flow-mediated dilation and rise in arterial stiffness after smoking at days 7 and 14. Short-term marker results in smokers.
Randomized, unblinded, single-center trial of raisins versus prepackaged processed snacks (such as crackers and cookies) three times a day for 12 weeks; ClinicalTrials.gov NCT01260272 lists the California Raisin Marketing Board as collaborator (Anderson et al. 2014, Postgrad Med)
46 generally healthy adults with overweight (BMI 25 to 34.9), fasting glucose of 90 to 150 mg/dL and systolic pressure above 120 or diastolic pressure above 80 mmHg (31 raisins, 15 snacks).
Fasting glucose did not change in either group. After-meal glucose fell 13.1 mg/dL with raisins (P = 0.03 vs snacks) and HbA1c fell 0.12% (P = 0.036 vs snacks). Systolic pressure fell 6.0 to 10.2 mmHg from baseline with raisins; diastolic changes were significantly greater than with snacks. Body weight did not change.
Randomized, double-blind crossover trial of freeze-dried whole California table grape powder, 46 g twice daily (about four servings of grapes a day), versus a placebo powder matched for sugars, acids and fiber, 3 weeks each; supported by the USDA and a grant from the California Table Grape Commission, which also supplied the powders (Zunino et al. 2014, Br J Nutr; dietary follow-up analysis Zunino et al. 2017, Nutr Res)
24 adults aged 20 to 60 with obesity (BMI 30 to 45) who completed, of 33 enrolled.
Grape powder lowered large LDL particles and large LDL cholesterol versus placebo (P < 0.05). Plasma inflammatory markers including CRP did not change, and T-cell cytokines did not differ. Stimulated monocytes made more IL-1 beta and IL-6; a follow-up analysis found this was not explained by differences in diet.
Double-blind, placebo-controlled pilot study of a whole grape powder (described by the California Table Grape Commission, which lists it among its brain health studies, as equal to 2 1/4 cups of grapes a day) versus a polyphenol-free placebo powder for 6 months; the publisher funding record lists a California Table Grape Commission grant (11-002966) (Lee et al. 2017, Exp Gerontol)
10 older adults (average age 72) with mild decline in cognition.
PET scans showed falling glucose use in several brain regions in the placebo group and stable use in the grape group. Scores on memory and thinking tests did not differ between groups. Very small, and the scan result is a marker rather than a measure of daily function.
Double-blind, randomized, placebo-controlled trial of 46 g/day freeze-dried table grape powder or the same amount of placebo powder for 16 weeks; registered as NCT05064865 with the National University of Singapore as sponsor; the publisher funding record names the California Table Grape Commission as funder (Hu et al. 2023, Food Funct)
34 older adults in Singapore without known eye disease.
Macular pigment optical density showed a significant time effect, rising from 0.56 to 0.61 within the grape group only; plasma antioxidant capacity and total phenolics also rose only in that group. Dietary and plasma lutein did not differ between groups. The abstract does not report a significant group-by-time difference for the main outcome.