How Tea And Coffee Can Reduce Mercury Absorption From Fish

Apr 3, 2023 | General, Nutrition | 0 comments

A photo of a seafood meal of smoked salmon, with a cup of coffee in the background

Can Tea And Coffee Reduce Mercury Absorption From Fish And Seafood?

Yes, tea and coffee have both shown the ability to reduce mercury absorption from meals. For the drinks to have this effect, it is best to drink them at the same time of the meal. Only true teas from Camellia sinensis such as black tea, green tea, white tea, and oolong tea are able to produce this effect due to their polyphenol content. Herbal teas generally do not have the required chemical composition to reduce mercury absorption from meals.

This amazing effect of tea and coffee is important because the many health benefits of fish and seafood are mitigated by several factors. Pollutants such as microplastics and heavy metals accumulate in many commonly eaten seafood species. Due to the process of biomagnification through aquatic food webs, some seafood species can contain significant levels of these pollutants. In particular, the heavy metal mercury (Hg) is one of the most notorious.

A wooden table with a mug of milky black tea on the left and a small cup of espresso coffee on the right.
Tea And Coffee Both Have The Ability To Reduce Mercury Absorption From Meals

Whilst elemental mercury is not particularly well absorbed by the body when ingested from food, in ocean food chains, around 50% of the mercury is in the form of a complex called methylmercury (MeHg) – a highly toxic compound that is readily absorbed by the body. Methylmercury is known to enter the bloodstream in significantly higher levels than elemental mercury, and can cross the blood-brain barrier. [1]

Significant exposure to methylmercury can cause widespread damage in the body, including to the brain, heart, kidneys, lungs, and immune system. Therefore, taking steps to reduce mercury absorption from meals is a vital part of being able to safely consume fish and seafood.

A photo of liquid elemental mercury being poured in to a glass dish.
The Heavy Metal Mercury Is One Of The Most Common And Toxic Pollutants Of Fish And Seafood

Thankfully, research has revealed that the total concentration of Hg and MeHg in seafood does not always reflect the amount that will become available for absorption during the digestion process. Once a compound is in the intestinal tract, there are a variety of factors that determine how much of it will actually be absorbed through the intestinal epithelium. The amount of the compound that will be absorbed by the body is defined as it’s bioaccessibility.

“Recent research suggests that assuming all of the ingested Methylmercury is absorbed into systemic circulation may be in error. This may result in inaccurate estimations of exposure and risk, and may hamper our ability to properly balance the potential risks and benefits of seafood consumption.”

Mark A. Bradley, School of Dietetics and Human Nutrition, McGill University. (2017) [2]

There are various factors that can reduce mercury absorption from seafood or fish based meals, including cooking the meal thoroughly, and drinking tea or coffee at the same time as the meal. Despite their very different origins, these two drinks share this remarkable ability due to the fact they are both filled with flavanols and compounds that result from their plant origins. Indeed, of all the beverages that mankind drinks, Coffee is the most rich in polyphenols of them all, with tea following along behind in second place. In this article, we will take a closer look at the chemistry of these two drinks, and why they may reduce mercury absorption the next time you eat sushi or smoked salmon.

For a quick video summary of this topic, you can see our quick video on the subject here:

Tea And Mercury Absorption

When consumed at the same time as a meal, black and green teas have both have the ability to reduce the amount of mercury absorption from foods. The compounds in teas show the ability to significantly reduce bioaccessibility of food bound methylmercury. Research has found that teas can reduce the bioaccessibility of methylmercury in a meal to 14.7% of the original value. These results are supported by a 2009 study, in which an extract of green tea polyphenols showed the ability to reduce absorption of mercury from a meal by 82–92%. In this same study, a black tea extract showed a reduction of 88–91%. [3] [4]

A cup of black tea next to a plate of sushii
Drinking Tea With A Seafood Meal Can Reduce Mercury Absorption By Around 90%

To understand why teas have this effect, we need to first look at the compounds responsible. Teas derived from Camellia sinensis leaves such as black tea, green tea, white tea, and oolong tea contain a rich variety of polyphenols. These polyphenols are the compounds more commonly referred to as tannins, though this name is misleading as tea contains no tannic acid.

The ability of these polyphenols to interact with certain metals in the is known to be the reason that tea can reduce the absorption of iron when consumed at the same time as a meal. Whilst tea’s effects on iron absorption are often regarded as a negative aspect, it is perhaps a silver lining that the polyphenols in tea can play a similar role in reducing the absorption of mercury and methylmercury.

These polyphenols in tea include rutin, theogallin, chlorogenic acid, and caffeic acid. Fresh tea leaf is also unusually rich in the flavanol group of polyphenols known as catechins. Of these, tea contains (bear with me here) Epicatechin, Catechin gallate (CG), Gallocatechin gallate (GCG), and Epigallocatechin gallate (EGCG). [5]

Whilst listing them in this way may sound like one is reciting an ancient dialect, the important thing to note is that these polyphenols can individually decrease Methylmercury bioaccessibility by around 55% when ingested alongside a meal of grilled fish, with values of 57% noted for Atlantic wreckfish and 54% for yellowfin tuna. [6]

Smoked Salmon With Black Tea
Tea Has The Ability To Reduce Mercury Absorption From Seafood Such As Smoked Salmon

Tea contains significant levels of the catechins GCG and GC, with these two compounds accounting for nearly half (44%) of the total catechin content of green tea. These two compounds are chemical scavengers, meaning they are able to bind and react with certain molecules at a chemical level, thereby neutralising or deactivating them. Indeed, it is not only heavy metals that these compounds react with, as they also show the ability to reduce the amount of of cholesterol absorbed from meals. [7]

The scavenging ability of these two catechins may be a key factor in teas’ ability to reduce mercury absorption from foods.

Different forms of tea and their respective colours, with the least oxidised forms being more pale, and a darker colour for the longer oxidised tea leaves.
Longer Oxidation Of Tea Leaves Leads To The Polymerisation of Catechins Into Larger Polyphenols And Leads To A Darker Tea Colour

Due to the way different types of tea are oxidised, teas lighter in colour such as green tea and white tea contain a higher volume of catechins than black tea. This is because the leaves for these teas are processed very quickly after they are picked, meaning they retain much of the chemical profile of a fresh tea leaf. Black tea still contains significant numbers of catechins, but black tea is made from tea leaves that have been withered and rolled. The withering and rolling process stimulates the resynthesis of many catechins in the leaves into new, larger compounds – Theaflavins, Thearubigins, and Theasinensins. [8] [9]

Due to the vast array of polyphenols in teas, it is a far better option to drink tea with a seafood meal to reduce mercury absorption than taking supplements of the compounds individually. There is likely a significant degree of synergy between the various compounds present that an individual supplement would not be able to replicate. But if tea is not to your taste, you may be pleased to learn that coffee too has the ability to reduce mercury absorption, and provides a very similar protection in this regard.

Coffee And Mercury Absorption

Coffee is made from the dried beans of Coffea plants, which contain a multitude of polyphenols that can reduce the bioaccessiblity of methylmercury in the intestinal tract. Like tea, coffee is rich in polyphenols, and has also been shown to reduce mercury absorption by 50-60% when consumed alongside fish or seafood. [10]

Coffee Reduces Mercury Bioaccessibility
Research Has Found That Coffee Can Reduce Mercury Absorption From Fish And Seafood Meals By Around 50-60%

Made from the beans of the Coffea plant, coffee contains approximately 2000 different chemicals, including quercetin, cinnamic acid, ferulic acid, hydroxycinnamic acid, and melanoidin. Unfiltered coffees also contain diterpenes, such as cafestol and kahweol, which make up around 1% of the coffee beans weight. Diterpenes, however, are often removed by the filtration process as they are not particularly water soluble. [11] [12] [13]

Despite their very different origins and sources, coffee and tea do share some of the same polyphenols, including quinic acid and caffeic acid. Quinic acid has an astringent taste and is partly responsible for the perceived acidic taste of coffee. Caffeic acid, which is unrelated to caffeine despite it’s name, is present at levels of around 0.13 mg per 100 ml of brewed coffee. Whilst we mentioned it in our discussion on it’s role in tea, it is worth reminding ourelves that that caffeic acid is one of the polyphenols that has shown the ability to reduce mercury absorption by up to 55%. [14] [6]

Importantly, quinic acid and caffeic acid are able to form an ester together, to create Chlorogenic acid (CGA), which constitutes a major polyphenol in coffee. Chlorogenic acid and its derivatives account for approximately 3% of the roasted coffee powder, and a single cup of coffee contains 20–675 mg of CGAs. [15] [16] [17]

Chlorogenic acid reacts with saliva in the mouth and many of the chemicals in the stomach acid to produce a variety of secondary compounds. CGAs have been linked with numerous health benefits, and may also play a role in coffee’s ability to reduce mercury absorption from meals. Previous studies have revealed that CGAs show protective effects against various toxins and metals. [18] [19]

Coffee And Smoked Salmon
Coffee Contains Over 2000 Compounds, Many of Which Are Active In The Digestive System

When considering how and why these compounds may reduce mercury absorption, it is important to note that caffeic acid and chlorogenic acid work together in the body with the endogenous antioxidant Glutathione. Glutathione is a potent antioxidant used throughout the body. Notably, it is also secreted in the gastric mucosa, where it offers protection and protects against injury to the stomach. [20]

Upon ingestion, both caffeic acid and chlorogenic acid form “an intricate antioxidant network” with glutathione, and the compounds work together scavenging free radicals and toxins. Interestingly, research also suggests that coffee consumption can increase the levels of glutathione present in the body. [21] [22]

Much research has been conducted on the role of glutathione in both the mitigation of heavy metal toxicity, and it’s ability to chelate heavy metals already in the body. Glutathione is known to bind readily with methylmercury, and is noted in for it’s role in the detoxification of mercuric chloride. This ability is the result of glutathione’s structure. Glutathione contains a sulfhydryl group, which allows it to react with other thiol groups and bind to metals. In the case of mercury, this results in the formation of a mercuric-glutathione complex. [23] [24]

Smoked Salmon And A Milky Coffee On A Table
The Compounds In Coffee Work With Glutathione In The Stomach To Form An Intricate Antioxidant Network

Due to this ability, glutathione can both neutralise and transport mercury, and thus is able to protect cells from oxidative stress damage caused by ingested methylmercury. Whilst many of the polyphenols in coffee show the ability to reduce mercury absorption independently, the way these various polyphenols work together with glutathione may also be a mechanism at play in neutralising mercury and methylmercury in the gastric environment. [25] [26]

Whilst glutathione can play a key role in reducing mercury toxicity, it is important that individuals do not take the glutathione precursor N-Acetyl-L-Cysteine with a fish or seafood meal. This is because the amino acid cysteine is a key transporter of methylmercury, and is actually responsible for moving methylmercury into cells and tissues. Therefore, those seeking to reduce mercury absorption from a meal may find much better results from sticking to a coffee instead.

Different Varieties Of Coffee Contain Different Levels Of Polyphenols
The Amount Of Polyphenols In A Cup Of Coffee Depends On Factors Such As Roasting And The Presence Of Milk

As a result of the rich chemical profile of the coffee bean, a cup of coffee can offer an effective and pleasant way to reduce mercury absorption from a meal. Whilst research suggests that coffee is less effective than tea for reducing the bioaccessbility of methylmercury, it nonetheless still provides a fantastic option to accompany a seafood or fish meal.

Light and medium roast coffees have been found to have higher polyphenol content than dark roasts, which is likely due to the increased roasting time denaturing the compounds within the coffee beans. Finally, milk contains proteins that can strongly interact with the phenolic compounds in coffee, especially the CGAs. Milk therefore can exert a negative influence on the antioxidant properties of many of the compounds in coffee, which may be food for thought for those planning to ordering a cappuccino with their sea bass. [27]

Conclusion

When eating fish and seafood, a cup of tea or coffee may not be the first thought for many people when choosing what to drink. Thanks to their own respective blends of polyphenols, scientific evidence strongly indicates that this unlikely combination may be more synergistic than one would think. To reduce mercury absorption from fish and seafood meals, it may be best practice to get the kettle boiling beforehand.

Sushi With Agari Green Tea (Konacha)
Sushi Is Traditionally Served In Japan With A Type of Green Tea Called Agari or Konacha

It is interesting to note that it is traditional for a type of green tea called agari (あがり) or Konacha (粉茶) to be served with sushi in many Japanese restaurants. These teas are usually made from surplus tea leaf material, yet can still be full of beneficial polyphenols.

Whilst it is unlikely that the tradition of drinking tea with sushi was chosen to deliberately reduce mercury absorption from the meal, it is nonetheless fascinating and rather serendipitous that this traditional combination may be ideal from the standpoint of modern nutrition. Perhaps Professor George Nicholas said it best when he wrote, “It’s taken thousands of years, but Western science is finally catching up to Traditional Knowledge”.

On the topic of sushi however, it is important to note that raw fish provides the highest concentrations of mercury when compared to cooked samples of the same species. Whilst tea and coffee can reduce mercury absorption from the meal, cooking the fish or seafood well can also play an important role, as cooking leads to drastically lower mercury bioaccessibility than raw fish. Thorough cooking combined with drinking a cup of tea or coffee with the meal can reduce the absorption of methylmercury from the meal by up to 99%. [6]

Methylmercury remains one of the most common and harmful pollutants of fish and seafood. When both cooking and polyphenol-rich beverage treatments were combined, only 1% of MeHg remained bioaccessible. These results suggest that dietary practices should be considered when setting consumer guidelines around methylmercury ingestion.


If you would like to learn more about seafood, fish and mercury, then you may want to look at our long form review of the subject here.


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Written by Keymer Health

3rd April, 2023

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