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Caffeine and the role of tea

Date added: 13/12/2020

Caffeine often gets a bad rap. Blamed for sleeplessness, anxiety, frequent toilet trips and worse, it is something that many people try to avoid. Yet, this is unwarranted as, in the right amounts, caffeine is a harmless and even potentially useful natural compound.

As this report prepared by the Tea Advisory Panel will show, there is another side to caffeine; one of benefit in terms of mental and physical performance. The weight of evidence is so strong that the European Food Safety Authority concluded that certain levels of caffeine in foods and drinks should be able to make a series of impressive health claims. Still awaiting sign off by the European Parliament, these include claims relating to alertness, perception of fatigue and endurance performance. This report will cover all these areas in detail and I think you will be surprised by the findings.

Caffeine is found in more than 60 different types of plants which is why we consume it in so many of our favourite foods and drinks. While the highest levels are found in strong expresso coffees, tea is also a natural source of caffeine and is ideal for people who are seeking more moderate amounts. In addition, tea contains other unique and bioactive substances, such as polyphenols, fluoride and L-theanine, and remains an important source of hydration.

So, next time you put on the kettle for a brew, consider the amazing concoction of natural plant compounds that you are about to drink, and the many benefits that science has unravelled for us.

Why the fuss about caffeine?

Alongside sugar, caffeine is one of the most polarising dietary substances around. A glance at clean eating blogs will turn up advice on how to avoid caffeine by sticking with water or herbal teas, although few convincing reasons are given. Elsewhere, particularly in sport, men’s health or workout magazines, caffeine is positively encouraged. A similar polarised view was taken by the International Olympic Committee which banned athletes having caffeine in 1984, only to relent in 2004. Now it’s claimed that three quarters of top athletes take caffeine in some form (source 1).

So what is caffeine, or 1,3,7-trimethylxanthine to give it the chemical name? Found in the leaves, nuts or seeds of more than 60 plants, caffeine is a naturally-produced central nervous system stimulant of the methylxanthine class, a relative of DNA. The role of caffeine in the plant world is to protect and defend as its bitterness helps repel predators and prevents germination of competing seeds. For us humans, however, caffeine has a very different role.

Caffeine sources and recommendations

It’s no wonder that caffeine is the most consumed psychoactive substance in the world as it is found in so many types of foods and drinks as shown below.

Sources: Crozier TWM et al. (2012) Food Funct 3: 30; European Food Safety Authority factsheet on caffeine

But how much is safe? This was examined in 2015 by the European Food Safety Authority (EFSA) (source 2) which considered the risks associated with high caffeine intakes, such as sleeplessness, anxiety, raised blood pressure, foetal growth retardation (pregnancy) and behaviour in children and adolescents.

Single doses of up to 200mg caffeine were not found to cause any health problems in the general population, as well as breastfeeding women and people participating in exercise. Around 6% of adults have more than 200mg in a single session of consuming energy drinks. Having more than 100mg of caffeine close to bedtime can delay sleep onset.

Considering daily doses, up to 400mg of caffeine daily (equating to 3mg per kilogram body weight) was found to be safe for the general healthy population. This remains the case even when energy drinks are combined with alcohol. Pregnant women are advised to keep daily caffeine intakes below 200mg since higher intakes have been associated with a risk of low birth weight in babies. Lactating women should also keep daily intakes below 200mg.

Direct data were scarce for children and adolescents so the EFSA decided that the safe adult limit of 3mg per kilogram body weight could be applied to children. Sleep onset and sleep duration may be affected at intakes of more than 1.4mg per kilogram body weight when caffeine is consumed close to bedtime. The table below summarises this information and estimates upper intakes for children and teenagers based on average population body weights (source 3).

Caffeine intakes

In 2015, the EFSA (source 4) assessed the safety of caffeine and estimated from national surveys just how much caffeine people are consuming across 16 countries in Europe. The results were surprising. In Italy, the home of the expresso, caffeine intakes were lower than in the Nordic countries, France and Ireland.

The highest caffeine intakes were in Denmark where 30% of adults consumed more than 400mg per day followed by Netherlands, Finland and Germany where 13-17% of adults exceeded 400mg per day. Most caffeine in the European diet (40-94%) came from coffee, except in the UK and Ireland where tea was the main source, providing almost 60%.

In teenage populations, the highest intakes were seen in Netherlands, Latvia and Belgium. Energy drinks were a popular source of caffeine in young people, 10% of whom typically drank more than three cans in one sitting.

What does caffeine do?

Caffeine acts on the brain via several pathways. The main one involves a blocking (antagonistic effect) of adenosine receptors which results in less adenosine being taken up by brain cells leaving more in the circulation. The effect of this is to reverse drowsiness. Caffeine also boosts energy use in the brain, reduces brain blood flow, activates noradrenaline neurons, and affects the local release of dopamine which may improve mood (source 5). All of these effects lead to increased alertness, arousal, and vigilance as well as a reduced perception of fatigue and pain.

Caffeine at high doses can also increase urine output, feelings of urgency and frequency of urination, although the effects vary in individuals and depends on how much caffeine you are used to taking. One study (source 6) found negative effects on people with a diagnosed weak bladder at caffeine intakes of 4.5mg per kilogram body weight, which is actually above the EFSA safe limit of 3mg per kilogram body weight.

How caffeine wakes you up

A meta-analysis (source 7) reported that single doses of around 300mg of caffeine, typically using pills, stimulated an increased urine output of around 100ml, but the effects were significantly lessened by exercise. Another review by hydration expert, Professor Ron Maughan (source 8), concluded that the impact of caffeine on urine output was modest and transient, and lessened after habituation with caffeinated drinks. Effects on the bladder were typically seen at caffeine intakes of 250-300 mg, equivalent to 2-3 cups of coffee or 5-8 cups of tea.

Researching the benefits of caffeine

A search in PubMed using the term ‘caffeine’ reveals more than 9000 articles on this topic in the past 10 years. However, by narrowing down the search to randomised controlled trials, which are the gold standard for scientific evidence, we can see some interesting trends for caffeine.

Most research on caffeine is done using pills as these can be formulated more easily to provide a specific dose, and participants will be unaware if they are given the active pill or a placebo. However, there are a few studies on tea, coffee, cola and energy drinks as these, of course, represent a real life application of the science – very few people take caffeine pills on a daily basis! So, what does the recent science tell us about caffeine and health?

Cognitive function and performance

The table in Appendix 1 provides detail on 15 studies looking at the impact of caffeine consumption on cognitive function and performance. Studies coded green reported statistically significant results favouring caffeine, while studies coded orange found either no effects for caffeine or non-significant trends towards positive effects. Most studies fell into the green category.

Caffeine consumption (at intakes of 40mg to 800mg per day, or up to 6mg per kg body weight) improved reaction times, enhanced working memory and vigilance, improved accuracy in tests and boosted subjective feelings of energy and good mood. Only one study used black tea, while another used caffeinated beer. All of the other used pills or caffeine added to water. The overview of these results is illustrated below.

Caffeine study

Exercise performance

Turning to physical and exercise performance, it can be seen from the table in Appendix 2 that there are many more studies (n=29) than for cognitive function. Again, most of these are coded green which indicates that caffeine is an established ergogenic substance with a consistent impact on speed, accuracy and endurance capability. Given that just a few milliseconds can make all the difference in elite sport between winning and losing, it is no surprise that many athletes take caffeine in some form, whether as tea, coffee or supplements.

The results of these studies, which covered 10 different sports, are summarised below for the eight sports where caffeine improved performance.

Turning to mechanisms to explain why caffeine is having these effects, researchers suggest that caffeine may be acting on fuel use by shifting energy systems from carbohydrate to fat oxidation (burning), but also on the brain by improving accuracy and co-ordination, as well as lowering the perception of fatigue and discomfort. For example, in the study on soccer players (source 9), the reduced penalty time following caffeine consumption was thought to be due to improved passing accuracy whereas, in the study on cyclists (source 10), an increase in

nervous system activation was potentially the cause of the improved power output. The study on cross-country skiers (source 11) suggested that participants were able to exercise with a higher heart rate and work intensity due to a reduced perception of effort, or improvements in motor control. A range of acute caffeine intakes produced these effects which are summarised below. While most studies used caffeine intakes at 4-6mg per kilogram body weight, it should be remembered that the chronic safe limit is 3mg/kg. So, the use of higher intakes to deliver an ergogenic effect would be acceptable as long as this was a short term approach, and not taken daily.

Proposed EU health claims

In 2011, EFSA reviewed the evidence for caffeine and delivered an opinion (source 12) that certain levels of caffeine in foods and drinks could make five specific health claims. These were:

· For the general population – improvement of (a) concentration and (b) alertness (at an intake of >75 mg caffeine per serving)

· For adults participating in sport – support for (c) endurance performance and (d) endurance capacity (at an intake of 3mg/kg body weight one hour prior to exercise)

· For adults participating in sport – (e) reduced rated perceived exertion during exercise (at an intake of 4mg/kg body weight one hour prior to exercise).

Progress on approving these claims for use on products has been very slow. The claim relating to perceived exertion failed the ‘safety test’ as the required dose didn’t comply with EFSA safe limits but the remaining four claims were scientifically supported and recommended by the European Commission (source 13). However, they remain in limbo due to persistent concerns by MEPs that energy drink products, often high in sugar, will misuse the claims. Even up to January 2017, no final decision had been made about the future of the claims.

Tea as a source of caffeine

We have already established that tea is a source of caffeine – around 40-50mg per serving, which is around half the amount found in coffee. However, tea also contains a rich array of natural bioactive compounds such as polyphenols, fluoride and L-theanine which work together to influence body processes. Tea also provides a source of healthy hydration as it is naturally sugar-free. These aspects of tea will now be briefly described.

Polyphenols

These are anti-oxidant plant compounds found in tea as well as apples, onions, berries, cocoa, red wine and green leafy vegetables. Tea, onions and apples provide 80% of the polyphenols in the UK diet.

The polyphenols in tea are catechins, part of the flavonoid family. As green and black teas come from the same plant source, they have similar levels of flavonoids but different types due to the longer oxidation process in black tea. Research suggests that tea polyphenols have important vascular effects, such as relaxing blood vessels leading to blood pressure control (source 14), and protecting ‘bad’ LDL cholesterol from being oxidised (which makes it less harmful) (source 15). This probably explains why tea drinking, at intakes of around four cups a day, is associated with reduced risk of heart disease and stroke (source 16).

Fluoride

The tea plant takes up fluoride from the soil with the amount depending where in the world it is grown. Tea provides the majority of fluoride in the UK diet and most types are an official ‘source’ of fluoride under EU regulations (source 17). A mug of regular black tea, made with a tea bag, contains around 1.2mg of fluoride, so drinking four servings achieves the recommended fluoride intake of 3.5mg daily and is within safe limits (see chart below). Fluoride is recognised for its role in promoting oral health since incorporation of fluoride into the enamel matrix of teeth improves resistance to decay (source 18). These benefits can be maximised by drinking the tea unsweetened.

L-theanine

This highly bioavailable amino acid is virtually unique to tea and is believed to have mood-enhancing effects. The limited research so far suggests that L-theanine interacts with caffeine in tea to enhance attention switching and the ability to ignore distractions (source 19), as well as improving concentration and learning (source 20). In a recent study, L-theanine was found to reduce stress after performing a stress computing task (source 21).

Hydration

Five studies have looked at the hydrating properties of tea including black (regular), rooibos and lemon herbal tea (source 22). All found that tea provided similar hydration to plain water, except the study on lemon tea which found that it was slightly superior to water.

In the randomised crossover trial commissioned by the Tea Advisory Panel (source 23), 21 healthy men drank either tea or water at 4 or six mugs daily. There were no significant differences afterwards when measures of hydration in blood or urine were compared. The results for the daily urine output are shown below. This and other studies confirm that tea isn’t dehydrating and can, in fact, make a contribution to our daily fluid requirements of around 2-2.5 litres a day. This will be reassuring to the seven in ten adults who drink tea regularly.

Myths and tips

So far, we’ve seen that caffeine in moderate amounts provides benefits in terms of mental and physical function, and that a key source, regular black tea, offers moderate caffeine levels as well as several other bioactive compounds. Now, we’ll look at who can drink tea and how much is the optimal amount.

Children and young people

Weak, milky tea was a traditional drink for children in much of the UK but fell out of fashion thanks to heavy marketing of sugary juices, cordials and fizzy drinks. This could be changing as a result of stricter advertising rules and the sugar tax – indeed sales of soft drinks in the UK have already declined.

As reported in a review in the Journal of Human Nutrition and Dietetics (source 24), children from the age of four years can enjoy 1-2 cups of unsweetened, milky tea daily while older school age children can increase this to 2-3 cups daily. For dental health and to help prevent obesity, water, unsweetened tea and milk are all ideal drinks. It is a myth that young children should avoid tea.

Pregnancy and lactation

EFSA guidelines set an upper intake level of 200mg caffeine for this group, which equates to 4 servings of tea daily. There is no need for pregnant women to completely avoid caffeine. The natural fluoride content of tea could be helpful in supporting oral health in pregnancy, something that can be affected by pregnancy hormones, leading to an increased risk of tooth decay and gum disease. Herbal teas, such as rooibos, camomile or mint, are all caffeine-free and can be drunk in unlimited amounts.

Adults

Within the EFSA guidelines of 400mg per day, there is plenty of scope to enjoy regular caffeinated drinks as this equates to 8 servings of tea or 3-4 coffees, depending on the type. Tea is most popular in the over 65s who consume, on average 3-4 servings daily at present. The myth that caffeinated drinks leads to dehydration, especially in older people, has thankfully been busted by studies showing that tea is similar to water in supporting normal hydration (source 25), (source 26). Benefits in relation to tea and heart health have been seen at intakes of around 4 servings per day (source 27).

Conclusions

This report has explained how caffeine, in moderate amounts, can enhance mental and physical function. Caffeine may get a bad rap in some areas of the media but this isn’t deserved when we look at the evidence. Intakes of up to 400mg of caffeine daily are perfectly safe and can provide benefit. This equates to around eight servings of tea, or four servings of instant coffee.

Drinking at least four cups of tea daily is associated with a reduced risk of heart disease and provides enough fluoride to protect our teeth from decay. Unique to tea are the cognitive and mood benefits linked with L-theanine, a powerful amino acid.

Tea is a suitable drink for anyone over the age of 4 years, thanks to its moderate caffeine content and benefits arising from natural fluoride, polyphenols and L-theanine, as well as the provision of sugar-free hydration. New mothers and pregnant women can also feel reassured that the modest caffeine levels in tea will not cause any issues as they are well within international guidelines.

Hopefully, this report will provide surprising, yet reassuring, information on our most consumed psychoactive substance. Sitting down to a relaxing, yet stimulating, cup of tea may just be the best way to get the most from caffeine.

About the Tea Advisory Panel

Founded in 2007, the Tea Advisory Panel (TAP) provides independent scientific advice to the UK Tea and Infusions Association. TAP regularly commissions research and publishes scientific articles on all aspects relating to black, green and herbal teas. TAP members include dietitians Dr Carrie Ruxton and Lynn Garton, nutritionist Dr Emma Derbyshire, media medic Dr Catherine Hood, and industry tea expert, Dr Tim Bond. For more information, please see TeaAdvisoryPanel.com

Appendix

1) menshealth.com/health/caffeine-and-olympics

2) EFSA (2015) EFSA Journal 13: 4102.

3) evidence.nhs.uk/formulary/bnf/current/approximate-conversions-and-units/prescribing-for-children/weight-height-and-gender

4) EFSA (2015) EFSA Journal 13: 4102.

5) Nehlig et al. (1992) Brain Res Brain Res Rev 17: 139-70.

6) Lohsiriwat S et al. (2011) Urol Ann 3: 14-8.

7) Zhang Y et al. (2015) J Sci Med Sport 18: 569-74.

8) Maughan RJ & Griffin J (2003) J Hum Nutr Diet 16: 411-20.

9) Foskett, et al. (2009) Int J Sport Nutr Exerc Metab 19: 410-423.

10) Paton et al. (2015) J Sports Sci 33: 1076-1083.

11) Stadheim et al. (2013) Med Sci Sports Exerc 45: 2175-2183.

12) EFSA (2011) EFSA Journal 9: 2053 and 2054.

13) European Commission (2015) Draft amending Regulation (EU) No 432/2012

14) Liu G et al. (2014) Br J Nutr 112: 1048-1054

15) Ruxton C (2016) Prim Health Care 26: 34-42

16) Zhang C et al. (2014) Euro J Epidemiol 30: 103-13.

17) Ruxton CHS (2014) Nurs Standard 28: 52-59.

18) European Food Safety Agency (2013) EFSA Journal 11: 3332.

19) Bryan J (2008) Nutr Rev 66: 82-90.

20) Vuong QV et al. (2011) J Sci Food Agri 91: 1931-9.

21) White DJ et al (2016) Nutrients 8: 53.

22) Ruxton C (2016) Prim Health Care 26: 34-42.

23) Ruxton CH, Hart VA (2011) Br J Nutr 106: 588-95.

24) Ruxton CHS (2013) J Hum Nutr Diet 27: 342-357.

25) Ruxton CH, Hart VA (2011) Br J Nutr 106: 588-95.

26) Maughan RJ et al. (2016) Am J Clin Nutr 103: 717-23.

27) Gardner EJ et al. (2007) Eur J Clin Nutr 61: 3-18.