Benefits
Insulin response to a meal during a high-fat diet
In a randomized crossover trial in 27 overweight or obese men on a controlled high-fat diet, a week of 600 g of blackberries a day lowered the insulin response to a test meal by about a fifth versus a gelatin control, and an insulin resistance score improved. Blood sugar after the meal did not differ. It was a single 7-day trial and could not be blinded.
Fat oxidation during a high-fat diet
In the same 7-day trial, men measured for 24 hours in a room calorimeter burned about 141 g of fat a day after blackberries versus 132 g after the gelatin control, a 7% difference, with a lower respiratory quotient. Total energy burned did not differ, and the trial did not measure body weight change; weight was tracked only to check diet adherence. Only 17 men had usable calorimeter data after equipment failures.
Source of anthocyanins, ellagitannins and fiber
Blackberries are rich in anthocyanins, chiefly cyanidin 3-glucoside, along with flavonols and ellagitannins such as sanguiin H-6 and lambertianin C. A 148 g serving carries roughly 7.4 g of fiber, mostly insoluble. Levels vary with variety, growing conditions and ripeness.
How blackberry pigments are absorbed
After a single serving of blackberries or blackberry puree, the anthocyanins show up in blood and urine mostly as methylated and glucuronidated forms rather than as the original pigment. In one study of 5 volunteers, only about 0.16% of the anthocyanins eaten were recovered in urine over 24 hours, so very little of the pigment leaves the body intact in urine.
Plasma vitamin C after a serving of blackberry juice
In a Brazilian study whose abstract does not give the number of participants, blackberry juice made with water or with skim milk raised plasma vitamin C, while plasma antioxidant capacity rose only after the water-based juice and not significantly. There was no placebo drink, so the result shows what a serving delivers, not a health effect.
Balance and short-term memory in aged rats (animal work)
Aged rats fed a diet with 2% blackberry for 8 weeks did better than control rats on balance and coordination tasks and on a short-term memory maze. This is animal work at a fixed diet level; it has not been tested in people.
Mechanism of action
Pigments are heavily transformed after eating
Blackberry anthocyanins are absorbed in small amounts and are quickly converted to methylated, glucuronidated and sulfated forms. Any effect in the body is therefore more likely to come from these metabolites than from the original pigment.
Ellagitannins calm inflammatory signaling in lab tests
Ellagitannin-enriched blackberry extracts blocked NF-kB signaling and IL-8 release in stomach-lining cells and reduced ethanol-induced stomach damage in rats, with sanguiin H-6 and lambertianin C partly responsible. These are cell and animal findings using concentrated fractions, not the whole fruit in people.
Fiber and seeds may shape the after-meal response
In a crossover study of blackberries eaten with apple, blending the fruit gave a lower blood sugar peak than eating it whole; the authors suggested that grinding the seeds releases fiber and other components. Both versions contained the same fruit, so the study compares forms, not blackberries versus none.
Clinical trials
Randomized, controlled, unblinded crossover pilot feeding trial at the USDA Beltsville Human Nutrition Research Center (registered NCT01932879), funded by USDA ARS and a USDA NIFA award; no berry industry funding was declared. Each man ate 600 g/day of frozen blackberries or a calorie and carbohydrate matched gelatin dessert for 7 days, with a 7-day washout. (Solverson et al. 2018, Nutrients)
27 overweight or obese men (BMI over 25, average age about 58) on a fully controlled diet with 40% of energy from fat; 24 were analyzed for the meal test and 17 for the calorimeter after weather-related power outages.
Versus gelatin, blackberries lowered the 24-hour respiratory quotient (0.810 vs 0.817, p = 0.04) and raised fat oxidation from 132 to 141 g/day (p = 0.04). The insulin response to a test meal was lower (6485 vs 8245 units, p = 0.0002) and HOMA-IR improved about 9%, but the glucose response did not differ (p = 0.12) and energy expenditure was unchanged. The gelatin test meal carried about 11 g more sugar, and the control was not matched for fiber.
Randomized crossover pharmacokinetic study at the University of Porto, Portugal, comparing 250 mL of blackberry puree (650 mg anthocyanins) alone or with 12% ethanol. (Marques et al. 2016, Mol Nutr Food Res)
18 adults of normal weight or with overweight or obesity; blood drawn over 2 hours and urine at 2 hours.
The main circulating forms were methyl-cyanidin-glucuronide and 3'-methyl-cyanidin-3-glucoside, and urine levels tracked blood levels. Alcohol raised the peak levels of both. Peak levels tended to be lower in participants with overweight or obesity. No health outcome was measured; this is an absorption study, not a reason to pair the fruit with alcohol.
Single-arm absorption study in France: one meal with 200 g of blackberries, with urine collected for 24 hours. (Felgines et al. 2005, J Agric Food Chem)
5 healthy volunteers; no control group.
Urinary excretion of blackberry anthocyanins and their metabolites totaled about 0.16% of the amount eaten, peaking 2 to 4 hours after the meal and continuing through 24 hours. Monoglucuronides made up more than 60% of what was recovered, alongside methylated and sulfated forms. No health outcome was measured.
Single-serving study in Brazil comparing blackberry juice prepared with water or with defatted milk; no placebo drink. (Hassimotto et al. 2008, J Agric Food Chem)
Human volunteers in Brazil; the abstract does not state how many took part or their health status.
Plasma vitamin C rose significantly after both juices. Plasma antioxidant capacity (ORAC) rose only after the water-based juice and the rise was not significant; plasma urate and vitamin E did not change. Urinary antioxidant capacity fell 1 to 4 hours after both juices. Without a control drink, no health effect can be inferred.
Rat study by a USDA ARS group in Boston: aged (19-month-old) Fischer 344 rats fed a control diet or a 2% blackberry diet for 8 weeks. No people took part. (Shukitt-Hale et al. 2009, Nutr Neurosci)
Aged Fischer 344 rats; not a human study.
Blackberry-fed rats did better on three balance and coordination tasks (accelerating rotarod, wire suspension and small plank walk) and had better working memory in a water maze than control rats. Effects in aged rats on a fixed diet may not carry over to people.
Mouse screening study in Sweden: C57BL/6J mice fed a high-fat diet with 20% freeze-dried lingonberry, blackcurrant, bilberry, raspberry, acai, crowberry, prune or blackberry (Rubus fruticosus) for 13 weeks. No people took part. (Heyman et al. 2014, J Nutr Metab)
C57BL/6J mice, 12 per group; not a human study.
Lingonberry, and to a lesser degree blackcurrant, bilberry and raspberry, curbed weight gain and improved liver and blood measures. Blackberry did not significantly improve any of the measured parameters, although it raised the amount of fat and cholesterol excreted in feces.