GI Modifiers: Tannins

Summary

Gastrointestinal Modifiers: Tannins

definition: Tannins have long been defined as compounds present in water soluble plant extracts that had the property of converting (tanning) animal hide to leather. Tannins are a large, diverse group of complex polyphenolic compounds of medium to large molecular weight that are widely distributed among plants, often in bark of roots and stems, or outer layers of plant tissues where they are ascribed a protective function. The principal chemical property of tannins is the ability to form strong complexes with proteins, starches and other macromolecules. Precipitation of complexes will occur under appropriate chemical conditions; tannins may also form precipitates with alkaloids.

tannins - chemistry:
There are two main classes of tannins: hydrolyzable and condensed:

• Hydrolyzable tannins: compounds formed from gallic acid or epigallic acid units condensed to a central sugar molecule, usually glucose. On hydrolysis, they yield gallic or epigallic acids and sugar. For example:
Gallitannins: Rheum spp. (Rhubarb root); Arctostaphylos uva-ursi (Bearberry leaf); Quercus infectoria (Oak galls)
Ellagitannins: Eucalyptus spp. (Eucalyptus leaf); Punica granatum (Pomegranate rind/bark); Quercus albus (White Oak bark).
Mixed: Hamamelis virginiana (Witch hazel)

• Condensed tannins: also known as procyanidins, are condensed dimers or oligomers of catechin, epicatechin or similar units. Mixtures of these oligomers are powerful antioxidants known as OPC's (oligomeric proanthocyandins), and are available as extracts of grape seed or maritime pine bark known commercially as Pycnogenol.
Examples: Krameria triandra (Rhatany Root); Vitis vinifera (Wine grape seed); Pinus sylvestris (Scotch Pine Bark).

bioavailability of tannins and tannin decomposition products:
• Tannins, especially hydrolyzable tannins, have very low bioavailability following oral ingestion. This is due both to their poor lipid solubility and ability to complex proteins.

• Since gallic and epigallic acids are hepatotoxic, low bioavailability of tannins is important in minimizing potential toxicity of these compounds. Hydrolysis of tannins occurs mostly in the large bowel at neutral to alkaline pH, rather than acid pH, and hydrolysis is largely dependent upon bowel flora.

• Radiocarbon studies have shown OPC's distribute across compartments following oral ingestion, including across the blood-brain barrier. These studies do not necessarily distinguish between the OPC molecules and their degradation by-products.

overview of pharmacokinetic interactions:
• mechanisms: Tannins act topically to astringe mucosal surfaces, and following oral ingestion and consequent hydrolysis alter the fluidity of the bowel contents (hence their use as anti-diarrheal remedies). They are also attributed with antihemorrhagic, anti-inflammatory and antacid properties. The possibility of tannins forming precipitates with alkaloids, proteins or other large molecules simultaneously present in the digestive tract should be considered as a possible pharmacokinetic interaction that may reduce bioavailability of the involved substrates.

• research: There is little research available on the pharmacokinetics of the phenolic degradation products of tannins and their effects on absorption of drugs. Serafini et al. showed that the antioxidant effects in plasma of both green and black tea phenolics following oral ingestion in humans was completely abolished by adding milk to the tea.
(Serafini M, et al. Eur J Clin Nutr 1996 Jan;50(1):28-32.)

• dietary concern: Beverages with high tannin content such as green or black tea without milk are best avoided during consumption of alkaloidal medications.

• herbal concern: Herbs with high tannin content should not be prescribed in combination with herbs of high alkaloid content, particularly in mixed liquid extract form due to the possibility of precipitate formation.
(Brinker F. 1998, 169; Moore M. Bad Form, 1995.)



Herbs

Common herbs with significantly high tannin content:
• Acacia catechu (Catechu wood extract ) 220-500,000ppm
• Agrimonia eupatoria (Agrimony plant) 50,000-80,000 ppm
• Alnus glutinosa (Black Alder Bark) 200,000 ppm
• Arctostaphylos uva-ursi (Bearberry Plant) 60,000-200,000 ppm
• Areca catechu (Betel Nut Seed) 150,000-250,000 ppm
• Camellia sinensis (Tea leaf ) 90,000-130,000ppm
• Coffea arabica (Coffee Seed) 90,000 ppm
• Geranium maculatum (Cranesbill ) 100,00-125,000ppm
• Hammamelis virginiana (Witch Hazel) 150,000 ppm
• Heuchera americana (Alum root) 90,000-200,000ppm
• Ilex paraguariensis (Mate Leaf) 40,000-160,000 ppm
• Juglans nigra (Black Walnut Fruit) 147,000 ppm
• Krameria triandra (Rhatany Root) 80,000-200,000ppm
• Myrica cerifera (Bayberry bark)
• Polygonum bistorta (Bistort rhizome) 115,000-120,000ppm
• Potentilla erecta (Tormentil)
• Pinus sylvestris (Scotch Pine Bark) 169,000 ppm
• Punica granatum (Pomegranate Rind, Root Bark) <280,000 ppm
• Quercus alba (White Oak Bark) 78,500-200,000 ppm
• Quercus infectoria (Oak galls) 400,000-700,000ppm
• Rubus idaeus (Red Raspberry Leaf) 100,000-120,000 ppm
• Rubus villosus (Blackberry root bark) 100,000-130,000ppm
• Rumex acetosella (Sheep Sorrel Root) 80,000-140,000 ppm
• Rumex crispus (Curly Dock Root) 30,000-60,000 ppm
• Salix alba (White Willow Bark) 50,000-70,000 ppm
• Thymus vulgaris (Common Thyme Plant) 80,000-100,000 ppm
• Tsuga canadensis (Eastern Hemlock Bark) 100,000-150,500 ppm
• Vaccinium myrtillus (Bilberry Leaf, Fruit) 60,000-200,000 ppm

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Do not rely solely on the information in this article.

References

Bone K. Herbal Pharmacokinetics - The Behaviour of Large Molecules: In: Proceedings NHAA International Conference, Sydney 1998.

Brinker F. Herb Contraindications and Drug Interactions. Second edition., Sandy, OR: Eclectic Institute Inc, 1998.

Duke JA. Handbook of Phytochemical Constituents of GRAS Herbs and Other Economical Plants. CRC Press, 1994.

Hoffmann, D. Phytochemistry . (Forthcoming title, in press 1999, publisher to be confirmed).

Moore M. Herbal/Medical Contraindications. Albuquerque, NM: Southwest School of Botanical Medicine, 1995.

Moore M. Bad Form. Tannin and alkaloidal herbs that should not be combined in pharmacy. Albuquerque, NM: Southwest School of Botanical Medicine, 1995.

Serafini M, Ghiselli A, Ferro-Luzzi A. In vivo antioxidant effect of green and black tea in man. Eur J Clin Nutr 1996 Jan;50(1):28-32.
Abstract: OBJECTIVE: Evaluation of the vitro antioxidant activity of green and black tea, their in vivo effect on plasma antioxidant potential in man and the effect of milk addition. DESIGN: The antioxidant activity of the tea, with and without milk, was tested in vitro by measuring the length of the peroxyl radical induced lag-phase. The in vivo activity was tested on two groups of five healthy adults. Each group ingested 300 ml of either black or green tea, after overnight fast. The experiment was repeated on a separate day, adding 100 ml whole milk to the tea (ratio 1:4 ). Five subjects acted as controls. The human plasma antioxidant capacity (TRAP) was measured before and 30, 50 and 80 min from the ingestion of tea. RESULTS: Both teas inhibited the in vitro peroxidation in a dose-dependent manner. Green tea was sixfold more potent than black tea. The addition of milk to either tea did not appreciably modify their in vitro antioxidant potential. In vivo, the ingestion of tea produced a significant increase of TRAP (P <0.05), similar in both teas, which peaked at 30-50 min. When tea was consumed with milk, their in vivo activity was totally inhibited. CONCLUSIONS: The paper shows that tea possesses a strong antioxidant activity in vitro which is believed to be exerted by its polyphenols moiety. It also provides compelling evidence that tea has also a potent in vivo activity in man. The promptness of the in vivo response suggests that the absorption of the bioactive components of tea takes place in the upper part of the gastrointestinal system. The inhibition of this effect by milk is thought to be due to the complexation of tea polyphenols by milk proteins. These findings might help to clarify the putative role of dietary poly- phenols in modulating oxidative stress in vivo.