Deep dive: Tannins and caffeine

Tannins and caffeine, compounds found in tea and coffee as well as other foods too, are thought to impact nutrient bioavailability.

This article will explain the current scientific evidence available regarding the impact that these compounds, found in some of the world's most popular beverages, have on our health and the foods we eat. It will also aim to suggest what we can all do to ensure that we are benefitting optimally from the nutrients present in our food and drinks without nutrient bioavailability being compromised.

Editor’s note: This article has been written to provide further information and context for healthcare professionals. It is not intended to be used in commercial communications or directed at existing or potential consumers. By continuing to read, you confirm that you are a healthcare professional.

What is caffeine?

It is widely known that coffee contains caffeine, which is a natural psychostimulant, considered safe in small doses (such as in coffee). However, caffeine is actually just one of many compounds present in coffee. Interestingly, caffeine is also found in other foods, in either its natural or synthetic (human-made) form. These foods are:

• Cocoa

• Black tea

• Green tea

• Energy shots/drinks and

• Some chewing gum

The average cup of brewed coffee (250ml) contains about 100mg of caffeine, whereas some energy shots can have over 200mg per 250ml. The caffeine compound itself has a bitter taste but is odourless and white in colour.

Caffeine is also added to non-food items such as:

• Pain relievers

• Skincare products

• Makeup

• Cosmetics

For us to better understand how coffee, and other caffeine containing items, may impact the nutrients that we eat, let’s have a look at what caffeine actually does to the human body.

What does caffeine do?

Increasing perception of energy

Caffeine can cross the blood-brain barrier, and it functions by binding to adenosine receptors, which help regulate energy levels in the brain. By doing so, it can block our perception of tiredness. Like caffeine, adenosine is also a nitrogen-based compound, albeit one produced endogenously (naturally within the body).

When adenosine is bound to adenosine receptors, it causes drowsiness by slowing down nerve cell activity. This occurs naturally hour by hour each day. Caffeine blocks adenosine from binding to the receptors, inhibiting this natural response. Instead of feeling drowsy, nerve cells speed up and make us feel more alert.

The pituitary gland in the brain senses the increased activity of nerve cells and as a result promotes the body’s stress response by stimulating the adrenal glands to produce adrenaline, which can cause fight-or-flight symptoms, such as:

• Dilated pupils

• Raised heart rate

• Lack of blood flow to the stomach (decreased digestion)

• Release of sugar into the bloodstream from the liver for extra energy

Caffeine is absorbed within an average of 45 minutes following drinking, is broken down in the liver and peaks in the blood anywhere from 15 minutes to two hours. It can remain in the body for 1-10 hours. This explains why people can experience different effects from caffeine—while some feel they need caffeine to feel awake, others find it can induce feelings of anxiety. Interestingly, during the third trimester of pregnancy, caffeine can remain in the body for up to 15 hours.

The psychoactive nature of caffeine does mean that those who drink a lot of coffee and then choose to reduce or stop their consumption can experience withdrawal symptoms, such as headaches or fatigue, although these tend to only last around one week.

Laxative effects

Caffeine is often mentioned anecdotally as a natural laxative. Some medical research, albeit dated from the 1990s, suggests caffeine can act as a hyper-mobility agent, meaning that drinking caffeine may have a laxative effect for some. This is thought to be due to caffeine causing an increase in the amount of stomach acid produced. This occurs due to the release of a hormone called gastrin, resulting in increased colon movements. However, both decaffeinated coffee and hot water have been shown to have similar laxative effects too.

Due to caffeine causing gastrin release, and therefore stomach acid, caffeine can cause heartburn, which is when stomach acid is released from the stomach back through the oesophagus causing a tight and painful sensation in the chest.

Therefore, some hypothesise that the laxative effect some people experience after drinking a hot coffee is actually due to a natural ‘defecation reflex’, which occurs when we drink or eat something warm. Warm food and drink entering the digestive tract encourages blood to flow to the digestive tract and organs, increasing general activity including colon movements. More robust research, however, is required to analyse the true effect caffeine may have on bowel movements.

Diuretic effects

Generally, it is understood that caffeine has a very mild diuretic effect. This means that in large doses, caffeine can mean we pee more and as a result, lose more water than usual. The reason for this is largely unknown and more research is required to provide greater insight. One understanding is that the antagonistic binding of caffeine to adenosine receptors in the brain causes reduced sodium reabsorption in the kidneys, which reduces the reabsorption of water too (this always naturally follows where sodium moves in the body), and therefore we lose more by excreting it in our urine.

Gut bacteria effects

An interesting, though small-scale, study (with only men as participants) in 2019 concluded that those drinking a large amount of caffeine each day (82.9 mg or more) resulted in a higher relative abundance of certain beneficial bacteria in their intestines, such as Faecalibacterium and Roseburia , and lower levels of the potentially harmful bacterial genus Erysipelatoclostridium.

Again, more research is required to confirm these findings, but this may be a promising beginning in exploring caffeine’s wider health benefits.

Tannins

Now that we have a clearer picture of the effect caffeine has on the body, let’s turn to tannins. These differ from caffeine in that they are not considered a stimulant, but rather provide important antiseptic and antibacterial properties, alongside other benefits. They also contain a taste, which is why they have different uses such as for the production of wine. Much like caffeine, they are naturally occurring compounds, also often found in tea and coffee too.

What are tannins?

Tannins are naturally occurring, water-soluble, phenolic (mildly acidic) compounds found in plants. The compound can also be referred to as tannic acid. They exist in plants as a natural defence mechanism against pests and animals.

In food and drinks, tannins are responsible for a texture, or mouthfeel, rather than just a taste; a dry sensation or a soft, velvety feel. For this reason, tannins are an essential component of wine. Tannins are also found in:

• Cocoa

• Unripe fruit

• Olives

• The skin of some nuts

• Black tea

• Herbal teas (such as chamomile, cinnamon, peppermint, and hibiscus tea. This is because the herbs themselves are sources of tannins).

• Coffee (tannins are what can give coffee its bitter taste).

Anti-nutritional properties

Tannins are known as having anti-nutritional properties, reducing the bioavailability of certain nutrients. Anti-nutrients impair the digestion and absorption of nutrients by binding to them, meaning enzymes cannot recognise molecules and break them down ready for absorption.

Protein

Tannins can affect protein digestibility due to their activity alongside dietary protein. Tannins either interact with protein-digesting enzymes or bind to form tannin-protein complexes with dietary protein. This subsequently reduces its absorption, and as a result, the amount of essential amino acids we acquire from the diet may be reduced.

Amino acids are the building blocks of all proteins. There are nine essential amino acids that must be obtained via the diet, as the body cannot produce certain proteins—such as hormones, neurotransmitters and DNA—required for the body to function.

The impact tannins have on dietary protein may be of concern to some populations and those following certain diets, such as vegetarians and vegans. From a health perspective, proteins from plant sources (nuts, seeds, beans, and lentils for example), have been shown to be preferable to animal sources. A 2020 analysis by the BMJ, for example, concluded that unlike animal proteins, plant proteins are associated with lower risk of all-cause mortality. Environmentally, there is more to debate here. While livestock production has caused well-documented, widespread deforestation and pollution, the production of processed plant-based protein may be causing its own unique environmental implications, which is also worth considering.

Iron

Iron is an essential component of the diet as it is required for the transport of oxygen around the body in our red blood cells (RBCs).There are 2 types of iron—non-heme iron and heme iron. Tannins bind with iron in the diet, similarly to protein, forming indigestible complexes. They have a higher affinity for non-heme iron found in plant foods such as beans, peas, leafy green vegetables, and nuts than animal-derived non-heme iron.

When tannins bind with iron, the bioavailability of the micronutrient may be reduced. Therefore, food and drink rich in tannins possibly predisposes one to iron-deficient anaemia. However, a 2017 review, found some dissonance in the previously accepted view that iron absorption was generally inhibited by tannins. The current understanding is that tannins specifically within tea are shown to inhibit iron absorption, whereas there is less evidence for the effect of ‘condensed’ tannins, found in food, impacting iron bioavailability.

Furthermore, there is some evidence that long-term consumption of tannins alongside iron-rich foods gradually present less of a problem for iron bioavailability suggesting the body is able to adapt to tannin-rich diets and ensure iron is still absorbed. More research will be necessary to determine how the body is able to acclimatise to tannin-rich diets. However, we know that nearly 90% of iron reserves in the body are retained through senescent RBC recycling, whereas nutritional intake accounts for the remaining 10%. It could be that the body is able to better retain and recycle iron when someone has a diet particularly rich in tannins.

The benefits—the good news!

Tannins are a natural part of our diets and come with health benefits too. For example, they are known as having antimutagenic properties, which means they are protective compounds against unwanted changes (mutations) in our DNA.

Tannins also function as antioxidants, protecting cells against oxidative damage. Oxidative damage can be harmful by causing damage to fatty tissue, DNA, and proteins in the body. Tannins also have antimicrobial functions, limiting the growth of fungi, yeast, bacteria and viruses. Therefore, tannins have important uses in prolonging the shelf life of certain foods. Lastly, tannins have been reported to contribute to the function of the cardiovascular system—helping blood clot well and maintaining blood pressure.

So when should we enjoy tea and coffee?

Caffeine, although it may have various impacts on digestion and excretion, is not known to readily impact the bioavailability of the food we eat, or nutrients obtained through supplementation. It is possible that caffeine may possess some important health benefits too such as encouraging the growth of beneficial gut bacteria. Therefore, the current scientific research doesn’t propose any concerns with consuming caffeine alongside food or supplements. What is important to consider with caffeine is its lasting effects on our perception of tiredness, which can impact our sleep, or induce feelings of anxiousness.

On the other hand, tannins present in many plant foods, as well as tea, coffee, and wine, may have an effect on the bioavailability of some nutrients such as iron and protein. While there is more research required to determine how tannins affect a wider range of nutrients, a safe precaution would be to enjoy tea and coffee an hour or two apart from mealtimes and nutritional supplements for the time being.

Cooking methods

Interestingly, the way in which plant foods are prepared may also be key in rendering their anti-nutritive properties less effective, therefore increasing the bioavailability of nutrients in our diets. The processes to consider when preparing plant foods are:

• Soaking

• Boiling

• Sprouting

• Preventing germination

• Pressure cooking

• Fermentation (tannin levels may be reduced as a result of lactic acid fermentation, leading to increased absorption of iron)

• Milling cereals (this removes tannins and other anti-nutrients present in the bran of grains)

Complementary nutrients

Lastly, iron bioavailability is improved when taken alongside vitamin C, as this functions to neutralise tannins. Therefore taking the two together, or in foods that contain both, along with the above cooking suggestions, should ensure that the body has access to the iron it requires to keep our blood, and us, healthy.

Most importantly, this knowledge should not deter anyone away from tannin-rich foods, as there are evidently important health benefits associated with eating tannins, and incorporating more plants into the diet generally ensures a multitude of vitamins, minerals, and fibre is available for the body to use.

Conclusion

Caffeine and tannins are both naturally occurring compounds—although synthetic versions are available of both—that may impact our ability to absorb nutrients.

Consumption of caffeine is responsible for the feeling of greater alertness, due to the suppression of adenosine receptors. Tannins do not act on the brain in the same way, but rather have a greater impact on the digestion and absorption of nutrients. Therefore, the latter compound seems of greater interest when considering nutrient bioavailability. This therefore relates to both coffee and tea—drinks both containing tannins, which can impact the successful absorption of specific nutrients in our diets.

Therefore, while more research is needed to conclude on the anti-nutritive properties as well as health benefits of caffeine and tannins, it may be advisable to enjoy both drinks at different times of the day to mealtimes, optimising nutrient bioavailability. We recommend leaving one hour before or after a mealtime to enjoy a good old cup of tea.

About the author:

Ruby Dalziel