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Tannins receive considerable attention as “anti-nutrients” alongside phytates, lectins, goitrogens, phytoestrogens, and oxalates. However, just as with other “anti-nutrients,” tannins may benefit human health.
What are Tannins?
Tannins are water-soluble polyphenols with high molecular weight (500-3000 Daltons) that are chemically reactive and form complexes with water-insoluble proteins, gelatin, and alkaloids. Tannins can be classified into two groups: hydrolysable tannins, which include gallotannins and ellagitannins, and condensed tannins, which include catechins and proanthocyanidins, the most abundant plant-derived polyphenols.
Image Credit: Gallagic acid: https://commons.wikimedia.org/wiki/File:Gallagic_acid.PNG. By Nono64 – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=9643281
Image Credit: Tannin: https://commons.wikimedia.org/wiki/Category:Tannins#/media/File:Tannin_V2.jpg. By Lisa1223S – Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=79482152
Tannins are present in a variety of plants as a form of protection and are responsible for the astringent taste of many plant foods, which produce feelings of dryness or tightness in the mouth. They are ubiquitous in foods, including cranberries, plums, blueberries, blackberries, grapes, peaches, pears, almonds, pecans, beans, lentils, buckwheat, wine, tea, and cacao beans. Of these, cacao beans have the highest amount of proanthocyanidins (6100–8100 mg/100 g).
Examples of Tannin-Containing Foods
| Food | Proanthocyanidins* | Ellagitannins* | Gallotannins* |
| Fruits | |||
| Cranberries | 194-496 | – | – |
| Plums | 32-334 | – | – |
| Blackberries | 5-46 | 150-270 | – |
| Grape seeds | 2180-6050 | – | – |
| Raspberries | 3-74 | 160-326 | |
| Mango | – | – | 30-160 |
| Nuts | |||
| Almonds | 67-257 | – | 20-34 |
| Hazelnuts | 125-645 | – | – |
| Pecans | 238-695 | 11-33 | – |
| Legumes | |||
| Beans | 5-830 | – | – |
| Peanuts | 121-141 | – | – |
| Cereal Grains | |||
| Barley | 59-153 | – | – |
| Sorghum | 413-5333 | – | – |
| Other | |||
| Wine | 1-53 | 2-5 | – |
| Tea | 1-5 | – | – |
| Cacao beans | 6100-8100 | – | – |
*Tannins (mg/100g-1 or mg/100mL-1)
Modified From: Smeriglio, A., Barreca, D., Bellocco, E., & Trombetta, D. (2017). Proanthocyanidins and hydrolysable tannins: occurrence, dietary intake and pharmacological effects. British Journal of Pharmacology, 174(11), 1244–1262. https://doi.org/10.1111/bph.13630
Are Tannins Harmful?
Despite the presence of tannins in a variety of nutritious foods, tannins are believed to contribute to iron deficiency by reducing non-heme iron absorption in plant foods. This potential decrease may be an important factor for populations at risk for iron deficiency, including pregnant women, infants and children, vegans and vegetarians, women with heavy menstrual bleeding, and those with a history of weight loss surgeries or health conditions including celiac disease or inflammatory conditions. Endurance athletes may also be at risk of iron deficiency. While these populations may need to pay special attention to iron status, the research on how tannins impact iron status is variable.
A study involving 11 non-anemic women in a double-blind crossover trial assessed the effect of three condensed tannin supplements in separate interventions. Tannin supplements were consumed three times per day for four weeks with meal challenges before and after the intervention. The results showed no significant reductions in iron bioavailability or status regardless of the dose. Furthermore, a larger study assessed the impact of black, green, and herbal tea consumption on iron status in French adults including 954 men (ages 52-68) and 1639 women (ages 42-68). Participants completed a tea consumption questionnaire and food records. Their iron status was tested via serum ferritin. Results showed that tea consumption was not related to serum ferritin concentration.
On the contrary, a recent study showed different results. Moroccans traditionally consume tea with most meals, and the country has a high prevalence of iron-deficiency anemia, prompting fortification of wheat with ferric sodium EDTA (NaFeEDTA). This study assessed the impact of polyphenols from tea on iron absorption from NaFeEDTA with Moroccan women who either had iron deficiency anemia (n=19) or were non-anemic (n=24). Test meals contained 6 mg iron with either tea or water, and fractional iron absorption was measured after 14 days. Results showed that in both groups of women, iron absorption was reduced by more than 85% when meals were consumed with tea. Thus, iron fortification could not necessarily overcome the inhibitory effect of tea and address the high prevalence of iron deficiency in Morocco.
These results suggest that general tannin consumption may not affect iron status as much as tannins consumed alongside iron-rich meals.
The Benefits of Tannin-Containing Foods
Despite variable evidence of tannins as “anti-nutrients,” there is evidence supporting their many health benefits as polyphenols and flavonoids.
Chronic Disease. Polyphenol intake is associated with a reduced risk for chronic diseases and is demonstrated in several studies on topics such as cardiovascular disease, kidney disease, diabetes, and cancer. Polyphenol consumption is associated with decreases in inflammatory markers associated with chronic disease, such as interleukin-6, which may be a marker of atherosclerosis, and c-reactive protein (CRP), which may predict cardiac events. The Takayama study included 29,079 adults and found that polyphenol intake, which was primarily from beverages including coffee and tea, was inversely associated with cardiovascular as well as digestive disease mortality.
Proanthocyanidins can improve vascular health through mechanisms including improvements in oxidative stress and endothelial function, which can benefit both cardiovascular disease and chronic kidney disease. In a study of 948 women (ages 70 years and older) assessed over 5 years, proanthocyanidin consumption was associated with reduced risk for moderate kidney insufficiency and renal disease events.
A recent observational study of 676 Danish participants found that a higher total polyphenol and phenolic acid intake was associated with a lower likelihood of having metabolic syndrome, lower HDL cholesterol concentrations, and a lower risk for high systolic blood pressure. Phenolic acids from coffee and flavonoids from tea, cocoa products, and fruits were the largest contributors to polyphenols in the diet.
Additionally, a recent article reviewed epigallocatechin-3-gallate (EGCG), a type of condensed tannin found in green tea, and its many health benefits. Health benefits include its ability to scavenge free radicals, chelate transition metals, activate other antioxidants, inhibit prooxidants, regulate cholesterol levels, and possibly play a role in the treatment of reproductive cancers, including ovarian, endometrial, vaginal, and cervical cancers. EGCG may also be useful in the management of diabetes.
Detoxification and antioxidant systems. Epicatechin can upregulate nuclear factor erythroid 2 (NF-E2) p45-related factor 2 (Nrf2), which is a transcription factor that regulates the detoxification and antioxidant systems in the body. Through their effect on upregulating Nrf2, epicatechins may be associated with neuroprotection against cerebral ischemic stroke and reduced risk of cardiovascular disease. Further, in mice with obesity induced by a high fat diet, epigallocatechin 3-gallate (EGCG) from green tea activated Nrf2 compared to mice on a high fat diet without EGCG and reduced plasma glucose, insulin levels, and advanced glycation end products. According to animal models, ellagic acid, found in berries, pomegranate, grapes, blackcurrants, and walnuts, may modulate phase I and phase II detoxification enzymes.
Chelation. Tannic acid in tea can chelate harmful heavy metals. In a recent animal study investigating the effect of tannic acid and tea on the hearts, lungs, and brains of rats exposed to lead and cadmium, tea infusions were more effective than isolated tannic acid in inhibiting the absorption of heavy metals, possibly because of the various compounds found in tea, such as catechins. White and green tea, which have a high catechin content, were most effective in reducing cadmium and lead absorption while black tea was the least effective. Teas were the least effective in reducing cadmium and lead in the lungs compared to the brain and hearts. However, tannins may also inhibit the absorption of essential minerals, including iron, copper, and zinc, though the level of inhibition varies by nut. For example, a study on the chelation abilities of tannins in almonds, walnuts, and hazelnuts found that almonds bound to as much as 84% of zinc ions, walnuts bound to only 8.7% of zinc ions, and hazelnuts showed no chelation of zinc ions.
Cognitive Function. Tannin-rich foods, such as cocoa, may improve cognitive function. Two randomized controlled trials investigated changes in brain-derived neurotrophic factor (BDNF) following flavonoid intake. Twelve weeks of consuming a high-flavanol cocoa drink led to higher serum BDNF levels compared to a low-flavanol control. BDNF regulates the development and maintenance of normal brain function and is implicated in mild cognitive impairment and neurodegenerative diseases, including Alzheimer’s and Parkinson’s disease.
Muscle Health. According to a recent review, polyphenols from cocoa may attenuate cell damage and inflammation during and after exercise. Cocoa contains catechins, which confer antioxidant potential and may help improve performance and adaptation to exercise by reducing oxidative stress and inflammation. Importantly, there may be a benefit to food sources of polyphenols over supplemental sources. A recent study investigated the effects of flavanol-rich cocoa supplementation on exercise adaptations in endurance athletes and found a decrease in mitochondrial biogenesis with flavanol supplementation compared to the control group, though it did not impact exercise performance. The study used 5 g of cocoa containing 425 mg of flavanols in its experiment, whereas 100 g of semisweet dark chocolate has only 170 mg of flavanols and procyanidins plus a number of other bioactive compounds, which may be more representative of human consumption.
Ellagitannins may support muscle and healthy aging. Urolithins are a class of compounds metabolized in the gut from ellagitannins, a type of tannin found in foods including pomegranate, strawberries, almonds, and walnuts. Urolithin A is a mitophagy activator, which means it helps recycle damaged and dysfunctional mitochondria. A recent 4-month intervention involving untrained adults (ages 40-64 years) explored the long-term use of oral supplementation with Urolithin A in doses of 500 mg and 1000 mg. Both doses resulted in improved leg muscle strength, which may support healthy aging. Additionally, the 1000 mg dose resulted in improvements in aerobic endurance and physical performance as well as statistically significant reductions in plasma CRP levels. The 500 mg group experienced reductions in acylcarnitines, which is associated with improved mitochondrial efficiency. Evidence from another recent study supported the potential use of Urolithin A in counteracting age-related muscle decline during a randomized clinical trial involving 66 older adults (ages 65-90 years) receiving either a placebo or 1000 mg supplement of Urolithin A.
Gut Microbiome. Polyphenols may act as prebiotics, which influence the gut microbiota and may alleviate neuroinflammation implicated in depression. Polyphenol biotransformation by the gut microbiota creates bioactive phenolic acids that may promote resilience to depression and anxiety.
A randomized controlled trial investigated the prebiotic effects of cocoa in 22 individuals who consumed either a high-cocoa flavanol drink (494 mg cocoa flavanols per day) or a low-cocoa flavanol drink (23 mg cocoa flavanols per day) for four weeks. Results from fecal samples showed a significant increase in Bifidobacteria and Lactobacilli populations and a significant decrease in Clostridia counts in the high-cocoa flavanol group compared to the low-cocoa flavanol group.
A double-blind trial investigated the dose effect of proanthocyanidin-standardized cranberry juice, cranberry powder, or placebo on H. pylori suppression in 522 adults. Results showed that 44 mg of proanthocyanidin-standardized cranberry juice administered twice daily for 8 weeks resulted in the highest H. pylori suppression rate compared to previous trials using non-standardized cranberry juices or single doses. Cranberries, being naturally rich in phenolic acids and proanthocyanidins, may reshape the gut microbiome by improving Akkermansia muciniphila growth and stimulating gut mucus production, which reduces gut permeability.
Identifying urolithin-based metabolic phenotypes (metabotypes) may improve personalized nutrition strategies by addressing inter-individual variation in response to diet. A recent study involving older adults with “leaky gut” evaluated the difference between urolithin metabotypes, intestinal permeability, and cardio-metabolic risk factors while following a polyphenol-rich diet. Urolithin metabotypes (UM) are classified based on distinctive urolithins in the urine and feces after consuming foods with ellagic acid or ellagitannin. “UMA” individuals produce only Urolithin A, and “UMB” individuals produce Urolithin A as well as Isourolithin A and/or Urolithin B. After following a polyphenol-rich diet, UMB participants had significantly reduced levels of zonulin, which is a peptide that regulates intestinal tight junctions. High levels of zonulin may indicate an impaired intestinal barrier. This study demonstrates that urolithin-based metabotypes may respond differently to polyphenol-rich diets.
Reducing Tannins & Effects of Tannins
While not everyone is sensitive to the effects of tannins, those with iron-absorption concerns can take steps to reduce the impact of tannins. Soaking, boiling, and steaming can reduce the tannin content of legumes. Additionally, the catechin content of rhubarb and pears can be reduced via cooking by 28% and 26%, respectively. Removing the brown skin of nuts may reduce tannins as they contain the majority of phenolic compounds. For example, total phenolic content is reduced by 90% after removing skin from walnuts. However, because the skin contains the majority of phenolic compounds, removing the skin of nuts may also impact health benefits associated with tannin consumption, such as antioxidant activity. When preparing tea, catechin and total phenolic content increase with brewing time, though most phenolics are extracted in the first two minutes.
Ascorbic acid, or vitamin C, may offset decreased non-heme iron absorption caused by polyphenolic compounds like tannins. Therefore, consuming vitamin C-rich foods, such as citrus fruits, berries, and leafy green vegetables, alongside plant sources of iron may improve iron status.
While tannin-rich foods may reduce iron bioavailability at meals, timing of tannins may improve absorption. A study involving 12 iron-replete women assessed the effect of tea consumption one hour after an iron-containing meal. Participants were given a porridge meal containing iron in three separate interventions during which water was given with the meal, tea was given with the meal, or tea was given one hour after the meal. Results showed that iron absorption was significantly higher when tea was administered one hour after the meal compared to tea administered with the meal.
Final Remarks:
Tannins are ubiquitously present in the diet and would likely be difficult to avoid entirely. Here’s a recap of what you should know about this “anti-nutrient:”
- Tannins are polyphenols that are responsible for the astringent taste of a variety of foods, including tea, wine, cocoa, almonds, pears, and grapes.
- Tannins may reduce iron absorption at meals, especially when consumed alongside iron-rich foods. Consuming plant sources of iron alongside vitamin C-rich foods may improve absorption.
- Polyphenols, including tannins, confer a wide range of health benefits, including improvements in metabolic health, detoxification, heavy metal chelation, cognitive function, depression, diabetes, and exercise performance and adaptation.
- Tannins can be reduced via soaking, boiling, steaming, and removing skin from nuts.
- If iron deficient, it is best to space iron-rich meals 1 hour apart from tea consumption to improve iron absorption.
If you have concerns about how tannins impact your health, talk to your doctor, nutritionist, or another health practitioner about your concerns and situation to determine whether limiting tannin-rich foods in your diet makes sense for you.
