01-09-2009, 08:26 PM
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Black Currants - A Clinical Perspective
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Black currant (Ribes nigrum) is a dark purple to black edible berry with a sweet sharp taste used extensively for flavoring. Presently the awareness and popularity of black currants is growing again since it represents a rich source of antioxidants and vitamins. Chemical composition is typical of berries, with a high content of vitamin C and numerous phenolic acids. Coumaric acid is the predominant phenol, although there are also four anthocyanins of which cyanidin and delphinidin are the major ones present as glucosides or rutinosides. The seeds are rich in oil with significant amounts of omega-3 and omega-6 essential fatty acids (EFAs), including gamma-linolenic acid (GLA).1
Seven small clinical studies (N<30) evaluated the various health benefits of black currant berries.2,3,4,5,6,7,8 The preparations and dosing used varied widely from concentrated extracts administered in capsules to drinks and desserts, and dosage anywhere from 50 mg to several grams. Supporting results were noted for enhancing vision acuity, increasing blood flow circulation, reducing muscle stiffness, relieving symptoms of allergic response to cedar pollen, protective effect on dental enamel from carbonated beverages, and prophylactic for certain type of kidney stones due to decrease in urine acidity. However, the effects of reducing oxidative stress evaluated in two of these clinical studies were poor even when dosing was done with gram quantities.
Human studies comparing the uptake and excretion of black currant anthocyanins show only less than 1 percent of the administered dose excreted in the urine albeit being proportional to the dose ingested.9,10 Rutinosides are generally better absorbed than glucosides.11 Further, in a human study investigating the effects of sucrose (sugar) concomitantly given with black currant anthocyanins and those of other berries, found a reduction of the anthocyanin urine excretion.12
In vitro evaluations have shown that for protein and lipid protection from oxidation, bilberry anthocyanins are superior to those of black currant.13 Such outcomes and associated laboratory experimentation suggest a poor bioavailability of anthocyanins from black currant.14 This understanding has been addressed in several review presentations where the bioavailability of anthocyanins and proanthocyanins is regarded poor and controversial.15,16,17
Nevertheless, contrary to these findings the potential anti-radical scavenging activity of black currant anthocyanins has been reported.18 Further, one human bioavailability study has shown black currant anthocyanins are well absorbed and distributed in the blood but not excreted intact in the urine but in small amounts.19 Human bioavailability studies on cyanidine glucoside, the major anthocyanin of black currant, show this glucoside is well-absorbed and is extensively metabolized to protocatechinic acid, accounting for 74 percent of the ingested cyanidine glucoside.20 This high concentration of this major metabolite may explain the short term increase in plasma and low urinary excretion of the cyanidine glucoside.
Supportive information comes from studies done with quercetin. In one study, 20 subjects ingesting daily food enriched with 100 g of a mixture of lignon berries, bilberries and black currant for eight weeks showed 30 to 50 percent higher blood levels of quercetin, indicating these berries are good source of quercetin.21 In another human study done specifically with black currant juice, containing 5 to 10mg of quercetin, there was urine excretion proportional to the intake amount of quercetin; further, glutathione peroxidase increased with dose and time.22 Other references actually recommend quercetin levels may serve as good biomarker of an adequate fruit and vegetable intake in the diet.23
The analytical methodology used to evaluate the bioavailability outcomes of anthocyanin absorption may require further refinement due to the multiplicity of the anthocyanins present and their resulting metabolites.24 Also, black currant anthocyanins and their antioxidant activity are heat labile, dropping from 45 percent to 22 percent when heated to 60 degrees C.25 This is critical since it can affect the potency of black currant's effectiveness when processed without this precaution to heat exposure, as in cooking or extract preparations.
In light to this controversial bioavailability of anthocyanins in humans, it behooves performing radiotracer studies that can follow the fate of the major anthocyanin metabolites from black currant and other berries for this matter. Such studies are yet to be performed and the question how effectively anthocyanins are absorbed from black currant remains open ended. Therefore, research on the health benefits of black currant relies mostly on clinical outcomes obtained when standardized extracts to cyanidin and delphinidin glucosides are given for specific disease states.
Based on this somewhat contradictory information regarding anthocyanin absorption from black currant, one can surmise the following corollaries:
- The metabolism of anthocyanins in animals may not necessarily relate to that in humans.
- In humans, metabolism of different anthocyanins from the same berry differs. They are extensively metabolized to compounds that are likely bioactive.
- In humans, absorption of anthocyanins from different berries differs; in part due to the various amounts of sugar present in the berries.
- Anthocyanins are mainly absorbed as glucosides.
- Sample preparation during chemical analysis can introduce bias in the subsequent evaluation.
- Likely anthocyanins are metabolized to non colored forms that escape detection.
Due to the antioxidant activity of the anthocyanins, some research has been done in humans to determine the effects of black currant on cholesterol lowering effects.26,27 These studies have shown the antioxidant effect of anthocyanins is primarily for protecting the blood vessels and affecting platelet function. Black currant had no effect on lowering cholesterol or low density lipoproteins (LDL), and markers for lipid peroxidation and lipoprotein metabolism as well as triglyceride levels also remained unaltered.
The health benefits of black currant oil have been evaluated in three small clinical studies, all showing moderate to weak support for ameliorating rheumatoid arthritis symptoms, immune enhancement and controlling blood pressure.28,29,30 Dosing in these studies was in gram quantities, requiring daily intakes of several softgel capsules.
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Black Currant—A Clinical Perspective |
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