Talk:Polyphenol

Merge proposal

As we deal with this complicated article, it is timely to consider merging it with tannins. They are the same compounds. Both polyphenol and tannin are in need to clean up, but that challenge should be undertaken only after we deal with the overlap. If we decide not to merge, then we can at least figure out the distinctions.--Smokefoot (talk) 22:24, 29 October 2020 (UTC)

• Oppose - although the precise chemical definitions of the two large categories should prevail, it seems safe to conclude that all tannin variants (ellagitannins, gallotannins, condensed tannins, among other subcategories) are within the general polyphenol category, whereas not all polyphenol compounds are tannins. In simple terms, tannins are a subclass of polyphenols, similar to the separate subclassification of flavonoids and other smaller polyphenols. Per WP:OVERCAT and WP:MEDMOS, I prefer to see individual topics maintained as separate to keep organization and navigation between related articles as simple as possible for the common, non-science user. My opinion is that it would be more effective to clarify the tannin article to maintain the distinction. Zefr (talk) 15:19, 30 October 2020 (UTC)
• Comment. Well I am no expert but here is my analysis: Tannin seems mostly about polyphenols, the big review (ACIE) indicates that tannins and polyphenols are the same thing or are used interchangeably, "Structure and classes of tannins 1.1 Pseudo tannins" [lousy refs, probably misleading or undue weight], History section is name dropping and vacuous, "Cellular localization" [yet a new attempt to define the topic], "Presence in soils "The convergent evolution of tannin-rich plant communities has occurred on nutrient-poor acidic soils throughout the world...." [really??]. We are doing a disservice to readers by providing this hodgepodge of misleading factoids. IMHO. --Smokefoot (talk) 14:14, 5 November 2020 (UTC)

Weak review on blood pressure effects in the journal, Nutrients

There are too many problems with this review published in the dubious MDPI journal Nutrients to include its results in the article.

First, concerning the quality of the studies reviewed, the authors themselves make the case for weak clinical research designs and evidence from the observational and prospective studies reviewed, leading to their discussion of limitations: "small number of studies or participants, involvement of manufacturers, and scarce clinical relevance of some significant results (not enough variation in blood pressure); (i) large variability of polyphenol content in plant-derived foods (depending on cultivar, preservation, temperature, sun exposure, and other factors) as well as in absorption of polyphenols among humans; (ii) dietary sources of polyphenols are typically rich in other compounds that may exert cardioprotective effects, including fiber or monounsaturated fatty acids; thus, disentangling the unique effects of polyphenols is rather difficult when studying food groups; (iii) mechanistic studies conducted on cells and animals often do not take into account the cooking processes, which may lead to a loss in total polyphenol or transformation of specific molecules into others with different biological activities; and (iv) discrepancy between in vitro and in vivo doses (in vitro doses often lead to unrealistic dietary intake in humans).

There are too many negative and doubtful factors of experimental design and interpretation to state in the article that polyphenols in food help to improve blood pressure when the overall blood pressure effect in healthy normal people and those with cardiovascular disease was only on average about 3 mmHg for systolic pressure and 1.5 mmHg for diastolic pressure (Figure 2), i.e., physiologically minimal changes which cannot be ascribed with certainty only to polyphenol consumption.

Second, Nutrients is a dubious publication for discussing physiological effects of foods because a) it has a history of publishing low-quality articles and poor editorial practices, which led to the collapse of its editorial board in 2018, as summarized in Science; b) after such a tarnished recent editorial history, what rigorous academic editor would want to be associated with the journal now, and how do we as Wikipedia medical editors view publications in Nutrients?; see the disclaimer to WP:CITEWATCH; c) predatory publishing by Nutrients may also indicate that the authors paid the publisher, MDPI, to have their article published, further creating doubt about using such a journal as a WP:SCIRS or WP:MEDRS source; and d) high-quality reviews on factors affecting blood pressure are not published in a food journal, like Nutrients, but rather would appear in a reputable clinical journal, if the quality of studies and effects reviewed were convincing, which are not the case here. Although the Nutrients impact factor has improved since the editorial board resignation, this is likely due to the MDPI practice of open access and speed of publishing, as stated in the Science article.

The article's section on health effects is adequately stated that the fate of dietary polyphenols in vivo and potential physiological effects are unknown, as there are no definitive studies or reputable reviews to show otherwise. Zefr (talk) 16:12, 16 March 2022 (UTC)

Thanks Zefr, for this comprehensive comment. I am not ready for a final verdict on Nutrients, even after reading the controversies. Some European universities demand that you publish your work as open access, which plays into MDPI's hands. And while the peer-review process is certainly not comparable to high-impact journals like Science or Nature I see it as any other entry-level journal for younger academics or those who start publishing their first papers. Anyways, in my view the study in question shows that we do know at least a little about in vivo effects and that could have added some balance to the current section that very boldly claims "we don't know nuttin". However, I am okay with stopping the discussion here because in the end the paper isn't worth spending too much time on it and I respect your assessment on it. CarlFromVienna (talk) 07:03, 17 March 2022 (UTC)

I came across the following today on Twitter. So just for the fun of it, this well-designed RCT with a very clear positiv result came out these days. I know that it cannot be used as it is a primary study. But in the light of it I find the strong language of „we don’t know nothing“ even more inappropriate. CarlFromVienna (talk) 17:39, 18 March 2022 (UTC)

There are facts we do know that reflect on the supposed health benefits of polyphenols: 1) there are no dietary reference intake recommendations by regulatory agencies (such as for vitamins and minerals, for which label decisions are based on the available literature), e.g., there are no Daily Values for any polyphenol appearing on food labels in major western countries; 2) the FDA and European Commission prevent food manufacturers from making health (or anti-disease) claims about polyphenols on food labels; 3) EFSA has acknowledged lab evidence for effects of 200 mg/day of cocoa extract and olive polyphenols having benefits, but what manufacturers are making use of such extract amounts to produce food products having common value?; and 4) "the possible functions of polyphenols in vivo remain unknown" because there are no existing methods to track and define their mechanisms and actions specifically in vivo. As with all research attempting to define a relationship between food and health, the clinical research is burdened by the numerous uncontrolled factors of people and their daily diets, and by the unknown fate and nutritional value of other food components complicating any interpretation specifically about polyphenols. Zefr (talk) 19:16, 18 March 2022 (UTC)

Thanks, Zefr, I understand. BTW, the study I linked above on cocoa flavanols, has just come back to bite me :-) CarlFromVienna (talk) 08:38, 22 March 2022 (UTC)

I hope I might interject here as well. The problem is that "Polyphenols" is - outside of research - more a marketing hype than a useful definition. Some compounds and groups have a lot of research, others don't - so any summary statement beyond "it's complicated" needs clarification. In observational studies, they're usually linked to some health benefits - but that's (in my opinion) mainly due to the fact that one can't really consume a diet with a lot of polyphenols without consuming a "healthy" diet (i.e. lots of fruits and vegetables).

I think an entry for Polyphenols is important, because of the hype around them - but I think it is important to distinguish between the group and individual compounds and classes, and not to assign any benefit (or lack thereof) to the group.

Regarding Nutrients - a lot of journals require authors now to pay for articles to be published (e.g. the massive open access online journals, but also when opting for 'gold open access'.Ggux (talk) 07:12, 16 May 2022 (UTC)

In vivo biomarkers?

This edit summary suggested that biomarkers exist for metabolized polyphenols. There is no reliable source used in the article for that measurement, which would be a significant breakthrough for assessing the fate of digested polyphenols. This seems impossible for compounds that are rapidly digested and metabolized to smaller compounds changed and distributed among body organs, then are excreted by the kidneys within minutes. Zefr (talk) 19:32, 15 May 2022 (UTC)

Polyphenol metabolites are routinely used as biomarkers of polyphenol intake - and have been for a very long time. For example isoflavnone metabolites were used as biomarker of isoflavone intake [[1]]. A more recent review expands this to a much larger range of compounds [[2]] and biomarkers of flavan-3-ol intake have been applied to a larger cohort study [[3]].

Here [[4]] is a review of ADME for flavonoids - "excreted within minutes" would be an exception. There is one study with radio-tracers (14C) which is probably the most reliable, and t1/2 for epicatechin metabolites is about 1h - for the microbial metabolites around 6h [[5]].

The compounds are not rapidly digested - a large proportion ends up in the large intestine and is metabolised by the gut microbiome [[6]] and enter (and leave) the body much later. But this is not new - that has been known since the early 2000s (see literature by Crozier or Rice-Evans). Ggux (talk) 22:10, 15 May 2022 (UTC)

All those sources explain the limitations which leave the issue of polyphenol biomarkers dicey at best. I believe the only thing we can say is that measurement attempts have been made, with little evidence of accuracy for fate, biological activity, and quantitative evidence of effect. The first paragraph of the health effects section seems to accurately explain what is a relatively poor "state of the art". Zefr (talk) 22:35, 15 May 2022 (UTC)

There are very good data for some of them - but obviously not for all of them. There is very good evidence of the fate of those that have been investigated. It depends on whether polyphenols are seen as a single entity or whether they are seen as a group with many different members. I would distinguish between different parts and state that for some, there are very good data whereas for others there are very little. Ggux (talk) 05:33, 16 May 2022 (UTC)

iron absorption and erythropoiesis

Could you elaborate zefr ? thanks.Atchoum (talk) 17:07, 8 December 2022 (UTC)

Cancer

As of 2019, there is little evidence that dietary flavonoids lower the risk of cancer.

The reference was retrieved in 2019, but the Linus Pauling Institute article cited is from 2016 and is a survey of even older publications. Is the really the best and most current citation to put here? ThreeRocks (talk) 23:04, 9 March 2023 (UTC)

I have raised that issue before but it is the best overview to date even it was last reviewed in 2016. Unfortunately if you go looking for systematic reviews on polyphenols it is just garbage published in the unreliable MDPI journals such as [7], [8]. There are no good reviews to date and much more research needs to be done. Psychologist Guy (talk) 00:04, 10 March 2023 (UTC)

I think to balance this section one must not search for "polyphenols" in general, because that would really be a tough claim to make, that all polyphenols or most of them or whatever subset (like flavonoids) reduce cancer risk. If you look for specific polyphenols like soy isoflavones there are studies on PubMed to be found and I just added one of them. As soy is a staple in Asia there are also prospective cohort studies and meta analysis of these. CarlFromVienna (talk) 13:50, 10 March 2023 (UTC)

I have also changed the summary of the LPI that was clearly wrong. Reading the actual text, it states: "...found an overall 11% reduced risk of breast cancer with the highest versus lowest intake of soy isoflavones..." for Asian woman and "...results suggested that intakes of flavonols and flavones may also be inversely associated with the risk of breast cancer" and "a pooled analysis of four case-control studies that stratified by menopausal status showed inverse associations between breast cancer and intakes of flavonols, flavones, or flavan-3-ols in postmenopausal women only" and "Finally, a meta-analysis of four prospective cohort studies found an overall 16% reduced risk of breast cancer recurrence in women with high versus low isoflavone intakes" and "A meta-analysis of 13 observational studies also suggested an inverse relationship between prostate cancer risk and consumption of soy products, especially tofu" and "suggesting that isoflavones may reduce prostate cancer incidence" whatever you make of this, claming that the LPI sees "no evidence" is wrong. They do say that "whether supplementation with specific flavonoids could benefit cancer prevention or treatment" needs research, but now we're talking supplements! CarlFromVienna (talk) 13:57, 10 March 2023 (UTC)

Iron chelation and accumulation

I recently cited an article which states:

The iron-chelating effects of EGCG, curcumin, myricetin, ginsenosides, and ginkgetin are thought to be an underlying mechanism through which polyphenols prevent neurotoxicity, leading to a neuroprotective effect against neurodegenerative diseases like Parkinson's Disease, Alzheimer's Disease, and Huntington's Disease.

and

the iron-chelating and inhibitory effects on absorption of iron associated with polyphenols may lead to poor iron status

Both of these are referenced to review articles, for instance. This review article they cite states:

In fact, these compounds have been known to have neuroprotective effects due to their iron-chelating properties [154, 156, 269–271].

Additionally, a review of 34 randomized controlled trials "showed that dietary polyphenol supplementation restricts iron absorption".

Based on this I edited in the following:

While the chelation of iron by polyphenols may help prevent the buildup of iron in the brain, protecting against some neurodegenerative diseases, this chelation also inhibits the absorption of iron in the small intestines, which can contribute to iron deficiency.

In hindsight, I would change "can contribute" to "may contribute". However, I intentionally qualified the language of the sources to "the chelation of iron ... may help prevent" to make the language less certain than what was used in the sources cited. @User:Zefr, please explain what you believe was an "exaggerated interpretation" and please cite the text from WP:MEDRS that you believe disqualifies the usage of this source. Photos of Japan (talk) 08:12, 31 August 2024 (UTC)

The reverted text was "While the chelation of iron by polyphenols may help prevent the buildup of iron in the brain, protecting against some neurodegenerative diseases, this chelation also inhibits the absorption of iron in the small intestines, which can contribute to iron deficiency", cited by this report mostly of lab research.

Discussion of each of the three concepts is just conjecture: a) iron chelation by polyphenols prevents buildup in the brain, b) such chelation protects against neurodegenerative diseases, and c) chelation inhibits intestinal iron absorption. None of this is established fact, and none of this is supported by a WP:MEDRS review or a clinical guideline proposing such use of polyphenols as an approved therapy, WP:MEDORG. Zefr (talk) 16:16, 31 August 2024 (UTC)

Comment: phenols, especially catechol and pyrogallol derivatives, (which includes polyphenols) will no doubt bind Fe (hence the evolution of catechol-based siderophores such as enterobactin). Binding Fe(III) is just what they do regardless of the location of polyphenols. I guess sequestering Fe could be relevant or possibly the Fe-polyphenol complex has the correct shape for some binding. --Smokefoot (talk) 17:55, 31 August 2024 (UTC)

"such use of polyphenols as an approved therapy"

Please note that the edit in question was made to the "Toxicity and adverse effects" section. Polyphenols have been substantially looked at for potential neuroprotective benefits, and many people take them for that reason. I wanted to point out that, one of the proposed mechanisms for this effect, their iron-chelating ability, can itself have deleterious effects. The purported health benefits of polyphenols are often touted, but there is inadequate discussion of their potential harm.

Zefr has stated that the statment "chelation inhibits intestinal iron absorption" is not "supported by a WP:MEDRS review". This is a systematic review that was published in perhaps the most influential nutritional journal of the last 100 years. I'm not even mentioning the effects of erythropoiesis that that review found (which Zefr previously reverted), just the mundane claim that polyphenols chelate iron and that this can inhibit intestinal absorption (which can contribute to iron deficiency).

With regards to protection against neurodegenerative diseases, I am not stating that "It is an established fact that polyphenols prevent neurodegenerative disorders", just that they may help prevent some of them (or they may not). We don't even know if the iron accumulation we see in many neurodegenerative disorders is a cause or effect of them. The sources I quoted use much stronger language:

"In fact, these compounds have been known to have neuroprotective effects due to their iron-chelating properties"

I do not know that there is sufficient evidence to make such a strong assertion, but the sources provided demonstrate that, at the least, their chelating effect may help prevent neurodegenerative diseases.

I didn't spend a long time mulling over my original wording, and I am open to changes, for instance:

While the chelation of iron by some polyphenols may help prevent the buildup of iron in the brain, which may help protect against some neurodegenerative diseases, this chelation also inhibits the absorption of iron in the small intestines, which may contribute to iron deficiency.(citing above sources)

But without really understanding what part of WP:MEDRS Zefr believes these sources do not meet, I cannot suggest wording changes to address their concerns. Photos of Japan (talk) 20:31, 31 August 2024 (UTC)

The Xu review is 3 years old, and has not since been substantiated by any further quality review, regulatory authority (such as EFSA or FDA) or clinical organization, which leads the concept of in vivo chelation by polyphenols to WP:UNDUE as a non-mainstream issue. WP:MEDSCI encourages following the prevailing scientific consensus, for which there is no evidence of widely-held endorsement of this presumed effect as an issue of concern or benefit to the public.

A critical point is the presumption that polyphenol parent compounds are intact after ingestion when the chelating capability seen in vitro could apply. They are not intact, but rather rapid metabolism breaks them into multiple small units with unknown fates in vivo and rapid excretion (i.e., negligible bioavailability), making the whole story of potential physiological benefit or adverse effect moot, as explained in the article.

To consider citing the Xu review under Research in the article, it's difficult to propose useful wording. The authors discuss extensive limitations of the studies reviewed, giving pause about the certainty of presumed effects, and conclude that polyphenol ingestion only "may" affect erythropoiesis or serum iron.

For a minor topic receiving no regulatory or clinical attention, I don't feel it would be useful to say "A 2021 review of preliminary research found that ingesting polyphenols may affect erythropoiesis or serum iron." That doesn't impress as conclusive enough for the encyclopedia, MEDMOS, Writing for the wrong audience. Zefr (talk) 22:07, 31 August 2024 (UTC)

Nothing in that article substantiates the claim that polyphenols broadly are metabolized in the small intestines to the extent that they cannot chelate iron ions or otherwise inhibit intestinal absorption of iron to a significant degree. The fact that polyphenols do inhibit iron absorption in vivo to a significant degree is already established in the literature, as cited by the sources I've already cited

"the intake of polyphenol-rich foods or beverages restricts dietary nonheme iron absorption"

(2, 3, 4, 5).

In addition to many other sources (one going all the way back to 1975)

Naively, I assumed that inhibiting iron absorption would diminish iron stores in the body. This has not been demonstrated, and is the reason for the lack of regulatory or clinical attention.

"...polyphenols inhibit iron absorption. However; the effects of these dietary factors on iron absorption do not necessarily translate into an association with iron status and iron stores"

"Absorption of non-heme iron is inhibited by ... polyphenols in some vegetables, coffee, tea, and wine. These inhibitors chelate non-heme iron so it is not available for uptake."

"The majority of studies have found no relationship between tea and coffee intake [45,48,60,66,77] or polyphenol intake and iron status [54,67]. Three studies have found a negative relationship between serum ferritin concentration and tea intake [46,47,49]. A review by Temme and van Hoydonck (2002) concluded tea consumption does not appear to affect iron status in populations who mostly have adequate iron status. However, in populations with marginal iron status, there appears to be a negative association between tea consumption and iron status [78]."

Looking at one of the sources they cited I was surprised to read that in addition to polyphenols,

"fish, fortified breakfast cereals, beans and pulses, iron-rich food, and consumption of polyphenol-rich beverages with meals were not associated with ferritin."

and more broadly

"There is no clear picture from the literature of the relation between dietary iron intake and body iron stores."

and

"A previous study ... concluded that altering the availability of nonheme dietary iron has little effect on iron status when the diet contains substantial amounts of meat."

I would therefore agree that it is undue weight to mention the fact that polyphenols inhibit the absorption of iron in the intestines given that it has not been established that inhibiting the absorption of non-heme iron, in general, affects iron status (except perhaps exacerbating it in individuals with already low iron stores). Photos of Japan (talk) 19:53, 3 September 2024 (UTC)

It is well established in the literature that vitamin c enhances iron absorption. It is less clear for polyphenols because the evidence is currently limited. There has only been one review published on trials. Health organizations who put out evidence-based guidelines and position statements mention vitamin c and iron absorption but they do not mention polyphenols because the evidence doesn't currently meet the Bradford Hill criteria. Psychologist Guy (talk) 20:29, 3 September 2024 (UTC)

There seems to be confusion about the 2021 review. That polyphenols inhibit absorption of iron is already established in the literature, what isn't established is what effect that has on iron status. The 2021 review considers "polyphenol supplementation on iron status and erythropoiesis", and takes as already established (as all of the literature that I have seen) that polyphenols inhibit iron absorption.

"Health organizations who put out evidence-based guidelines and position statements mention vitamin c and iron absorption but they do not mention polyphenols"

I don't know whether this claim is true or not. Our own article on Vitamin C doesn't reference any such guidelines by health organizations or position statements. A Google search of "world health organization iron deficiency" brought up the WHO's program for Iron Deficiency Anaemia and it appears to treat ascorbic acid and polyphenols largely the same (page 66-67):

Enhancers of iron absorption include:

• ascorbic acid or vitamin C, present in fruits, juices, potatoes and some other tubers, and other vegetables such as green leaves, cauliflower, and cabbage;

Inhibitors of iron absorption include:

• iron-binding phenolic compounds (tannins); foods that contain the most potent inhibitors resistant to the influence of enhancers include tea, coffee, cocoa, herbal infusions in general, certain spices (e.g. oregano), and some vegetables;

Examples of simple but effective alterations in meal patterns that enhance iron absorption might include:

• separate tea drinking from mealtime - one or two hours later, the tea will not inhibit iron absorption because most of the food will have left the stomach;
• include in the meal fruit juices such as orange juice, or another source of ascorbic acid such as tubers, cabbage, carrots, or cauliflower;

Photos of Japan (talk) 21:42, 3 September 2024 (UTC)

Recommendations from the CDC on preventing iron deficiency also treat ascorbic acid and polyphenols similarly:

"Enhancers of iron absorption are heme iron (in meat, poultry, and fish) and vitamin C; inhibitors of iron absorption include polyphenols (in certain vegetables), tannins (in tea), phytates (in bran), and calcium (in dairy products)"

Photos of Japan (talk) 21:58, 3 September 2024 (UTC)

Yes that is a good source, I was wrong, it appears a few health authorities do mention it. Polyphenols do inhibit absorption of non-haem iron in a single meal, this is mentioned in the SACN Iron and Health Report as we do have data on this going back a few decades [9]. However there is no good evidence they inhibit heme iron and it is important to note there is still only limited research in this area. There isn't much research on this topic as the most widely researched enhancers of non-haem iron absorption and its bioavailability are vitamin c and meat. Anything to do with polyphenols is unlikely to have clinical support at this stage due to the limited amount of studies we have. I have no objection to citing SACN or WHO but I wouldn't cite that 2021 review, as there has been no replication. What's interesting from reading the SACN report is 1. haem iron in fish and red meat is well absorbed but also enhancers non-haem iron from foods consumed in the same meal. 2. vitamin c enhances non-haem iron absorption and is most effective with meals containing high levels of polyphenols, which are inhibitors of non-haem iron absorption. The latter point sounds contradictory but that is what they have found. Interesting stuff but if any of this is relevant it is probably best on the iron article. Psychologist Guy (talk) 22:22, 3 September 2024 (UTC)

Thank you for your reply. I went ahead and added the following text per this discussion. I am open to any changes or addendums to the wording. Actually, the Oregon State website which is used for much of this article also has a section on iron that corroborates this, but I had somehow missed this despite looking at that site several times, so I cited it as well. The point about vitamin C is interesting. Of interest is also the finding from the 1975 research that observed polyphenols inhibiting the absorption of uncooked heme iron (but not cooked hemoglobin). I don't think anyone else has researched its effect on uncooked hemoglobin due to the presumption of it being nutritionally irrelevant (sushi?). The authors conjectured that perhaps the polyphenols were crosslinking the globins, inhibiting access of proteases towards breaking them down and making the heme more available. Not relevant to the article though. Photos of Japan (talk) 00:33, 4 September 2024 (UTC)

Ok thanks for adding that content, it seems to be well sourced. I need to do a bit more research into this topic myself. I have not eaten meat in 20 years, I take an non-haem iron supplement that has both vitamin c and polyphenols in it. My iron levels are good. I have never thought about the polyphenols before. When I have more time I would like to look into some of theory behind iron absorption enhancers and inhibitors. If I come across any other sources I will put them here. Psychologist Guy (talk) 01:01, 4 September 2024 (UTC)

I'm not convinced of the value of the addition or the hurry to put it in the article. All the sources are 13-20 years out of date (including in the LPI and Frontiers reviews), with no current WP:MEDASSESS review, clinical or government authority discussing it as an adverse effect or toxicity concern. It's the definition of low-to-absent research interest with negligible practical significance to the general reader, WP:UNDUE.

It's also preferable to let the talk page discussion play out to establish consensus, WP:CON. Only 3 editors have participated, which does not produce consensus - the article has plenty of experienced editors who may wish to comment. Zefr (talk) 02:15, 4 September 2024 (UTC)

"They are not intact, but rather rapid metabolism breaks them into multiple small units with unknown fates in vivo and rapid excretion (i.e., negligible bioavailability), making the whole story of potential physiological benefit or adverse effect moot"

This is what you wrote earlier, but the source you cite has an entire section on how polyphenols inhibit iron absorption, which begins with "Flavonoids can bind nonheme iron, inhibiting its intestinal absorption". Every source that I have seen, and I will link several more, states matter-of-factly that polyphenols inhibit iron absorption:

Iron: Fact Sheet for Health Professionals National Institute of Health, Office of Dietary Supplements:

"In addition to ascorbic acid, meat, poultry, and seafood can enhance nonheme iron absorption, whereas phytate (present in grains and beans) and certain polyphenols in some nonanimal foods (such as cereals and legumes) have the opposite effect [4]."

"Some plant-based foods that are good sources of iron, such as spinach, have low iron bioavailability because they contain iron-absorption inhibitors, such as polyphenols [23,24]."

Fact sheet on anemia, World Health Organization

"avoid foods that slow down iron absorption when consuming iron-rich foods, such as bran in cereals (wholewheat flour, oats), tea, coffee, cocoa and calcium."

Preventing and Controlling Iron Deficiency Anaemia Through Primary Health Care, A Guide for Health Aministrators and Programme Managers, WHO

"Many compounds are known to inhibit the absorption of iron, among them phytates, polyphenols (including tannins), and soy protein."

"Tannins, which are present in tea and to a lesser extent in coffee, are also iron absorption inhibitors (Fig. 4). Other polyphenols are found in nuts and legumes. "

"For example, the inhibitory effect of tannins could be avoided by encouraging people to wait until after the meal before drinking tea or coffee. In general, however, it may be culturally more acceptable to encourage the addition of an absorption enhancer to the meal than to discourage consumption of an inhibitor."

Iron deficiency anaemia: pathophysiology, assessment, practical management, review published in BMJ Open

"Compounds such as phytate, oxalate, polyphenols and tannin, which are found in plants, diminish the uptake of non-haem iron"

Iron bioavailability and dietary reference values, published in The American Journal of Clinical Nutrition

"The inhibiting effect of polyphenols on iron absorption has been shown with black tea and herb teas (14–16). At comparable amounts, the polyphenols from black tea were shown to be more inhibiting than the polyphenols from herb teas and wine (16, 17). The fact that polyphenol quantity, as well as type, influences iron absorption was also shown in a study with spices. Chili, but not turmeric, inhibited iron absorption in Thai women, although turmeric contained more polyphenols than chili (18). In cereals and legumes, polyphenols add to the inhibitory effect of phytate, as was shown in a study that compared high and low polyphenol sorghum. After complete phytate degradation, iron absorption from low-polyphenol sorghum increased significantly, whereas iron absorption from high-polyphenol sorghum was not improved (19)."

Iron Absorption: Factors, Limitations, and Improvement Methods, review published in an ACS journal

"Phytate and polyphenols are the major iron absorption inhibitors in plant-based foods because they make a complex with dietary iron in the gastrointestinal tract."

"Polyphenols are known iron bioavailability inhibitors and are assumed to work similarly to phytate by forming a complex with iron."

Inhibition of iron absorption by polyphenols as an anti-cancer mechanism, published in QJM

"all major types of food polyphenols can strongly inhibit dietary non-haem iron absorption, and a dose-dependent inhibitory effect of polyphenol compounds on iron absorption has been demonstrated."

I added the text to the article because I did not think anybody would object to it and thought this discussion was finished. The CDC, the WHO, the NIH, the SACN, and numerous review articles published in credible journals all matter-of-factly state that polyphenols inhibit iron absorption. The fact that polyphenols inhibit iron is routinely mentioned in general resources on iron deficiency. This has been described by the literature for decades and I see no indication that anything has changed. The fact that I don't see any more modern guidelines written on anemia or iron deficiency by these groups is no indication that their views on polyphenols have changed. I am unaware of the existence of any source that casts doubt on polyphenols inhibiting the absorption of iron, although it is not known what effect the inhibition of non-heme iron has on iron status in general since most people often get adequate iron from heme iron in their diets. Photos of Japan (talk) 05:38, 4 September 2024 (UTC)

I'm not trying to revive a discussion or anything. I just happened to notice that a few hours ago the WHO posted a message on their bluesky account advising the public to "Avoid coffee & tea during meals, as they contain some elements that can inhibit iron absorption in foods." Obviously a social media account is not a reliable source, but it does further support the idea that the WHO hasn't changed its stance on this from its previous publications. Photos of Japan (talk) 18:55, 27 November 2024 (UTC)

What you are saying is true, it seems well established in the literature that polyphenols do inhibit absorption of non-haem iron. There seems to be a strong consensus on this because it is mentioned by authorities such as WHO, SACN, NIH. The best source I have seen is the SACN who are a governmental authority but the source is 14 years old and we do not have an updated review but that is because of lack of new research. What Zefr has said about the poor bioavailability of polyphenols is also true. For example the Oregon State University Micronutient information center cited above says "in general, the bioavailability of flavonoids is low due to limited absorption, extensive metabolism, and rapid excretion". An issue in this topic area as stated is that the data is coming from mostly older research. There are a lot of bogus health claims being thrown around about polyphenols but this content about polyphenols as iron bioavailability inhibitors is well established in the literature. However, it is a valid point raised by Zefr that there isn't a current WP:MEDASSESS review discussing all this. In conclusion I would support including anything by WHO, SACN OR NIH because these are reliable sources but remove any weaker sources such as anything by Frontiers or MDPI. Psychologist Guy (talk) 09:01, 4 September 2024 (UTC)