Health Professionals Newsletter January '10
Welcome to the latest newsletter from the Tea Avisory Panel (TAP). This service will keep you up to date with current activities and events and inform you of the latest published research. We'll keep you updated on a quarterly basis with news and views from across the tea industry.
Research Update
The health effects of caffeine – sorting out fact from fiction
Caffeine is often portrayed by the media and some health professionals as undesirable, suggesting that people avoid or reduce their intake of caffeinated products. On the other hand, research suggests that some caffeinated beverages maybe beneficial to health possibly due to the presence of certain plant compounds. In a recent article published in the Nursing Standard, Ruxton summarised the current evidence relating to the health effects of caffeine consumption and provided practical advice on caffeine intakes for different population groups.
Caffeine is a natural component of the plants used to produce certain beverages like tea, coffee and cocoa. Other beverages have caffeine added for flavour or for specific stimulatory effects. It is also a popular ingredient in some medications, supplements and weight management products. The main dietary sources of caffeine in the UK are tea, coffee, cocoa products, cola and chocolate; with daily caffeine intakes of around 4mg/kg body weight (or 280mg for a 70kg individual).
In the UK, tea is one of the most popular caffeinated beverages consumed. A 2002 survey of 2,000 UK adults revealed that 77% of people drank tea (compared to 71% who drank coffee) and a more recent National Drinks Survey found that the average adult consumes 2.1 cups of tea per day (compared to 1.1 cups of coffee).
The scientific evidence examining the health effects of caffeine is varied and vast. Some simply observe and monitor the health of people who consume tea or coffee (observational studies) and attempt to link beverage consumption with effects on health. Other studies are more active and will involve people taking caffeine pills, drinking a measured amount of tea or coffee and then measure specific health outcomes, for example blood pressure or mental alertness. This creates problems for reviewers who try to summarise the evidence regarding caffeine intake because beverages like tea and coffee contain other compounds which may also have health effects. For example tea is rich in polyphenols which have been linked with a reduced risk of Cardiovascular Disease (CVD).
Heart and vascular health
Studies on the effects of caffeinated beverages and CVD are mainly observational. A review of 20 studies published between 1994 and 2004 found significant associations between regular tea drinking (4 cups per day) and heart health, for example reduced mortality, lower risk of heart attack or lower plasma cholesterol levels. Other studies encompassing data on more than 90,000 patients found that drinking around 4 cups per day of caffeinated beverages reduced the risk of CVD. A further study of 26,000 male smokers found those who drank more than 2 cups of tea per day had a 21% lower risk of having a stroke. In contrast, two observational studies, including over 200,000 subjects, found no association between coffee or caffeine intake on CVD risk.
There is concern amongst health professionals that caffeine is associated with increased blood pressure and that this may have a negative health effect. The evidence, however, is contradictory. Two large observational studies of more than 155,000 healthy individuals found no association between caffeine consumption and blood pressure. However a smaller observational study did find a link between caffeine consumption and increased systolic blood pressure, but only in young people. Intervention studies (using caffeine pills or coffee) have shown that 3-4 cups of instant coffee (around 250-300mg of caffeine) can raise systolic and diastolic blood pressure both in the short- and long-term. Nevertheless, a large statistical analysis of high quality scientific studies has suggested that coffee has a less pronounced effect on blood pressure than caffeine pills. In a similar study over 7 days, tea had a less pronounced effect on blood pressure than an equivalent amount of caffeine in powder form, suggesting that polyphenols present in tea minimize the effect of caffeine.
In another study, people with hypertension or blood pressure at the high end of normal had an exaggerated response to caffeine pills in the short-term compared to people with normal blood pressure.
There are no official recommendations for people with CVD to restrict caffeine consumption and the available evidence actually suggests a modest intake of tea or coffee may be beneficial for heart health. However, as a precautionary measure, Ruxton suggests that people at risk of or with hypertension should consider restricting their caffeine intake to a limit of 250mg per day (around 5 cups of tea).
Mental health and mood
Caffeine is believed to affect mood and performance through effects on the levels of neurotransmitters in the brain.
Ruxton has previously reviewed this topic and concluded that a caffeine intake of 37.5 – 450mg improved alertness, short-term recall and reaction time, positive reported mood and lower perceived fatigue. However some sleep disturbances were reported at intakes exceeding 400mg per day.
Tea and coffee appear to have similar effects on mood and mental health, but have different effects to caffeine pills – probably because of the presence of other compounds in tea and coffee which may also affect cognitive function independently of caffeine.
Ruxton suggests an upper limit of 400mg per day (8 cups of tea) for cognitive and mood benefits, given the potential effects on sleep disturbance noted above this level.
Physical performance
Caffeine is believed to enhance exercise performance by stimulating fat burning which spares muscle glycogen stores. It may also enhance muscle contractions and increase tolerance to fatigue. Until 2004 caffeine use was restricted by the International Olympic Committee; now there is no restriction on the use of caffeine by competitive athletes.
Eleven controlled trials were reviewed by Ruxton in 2008; only 2 found no effect. The majority reported short-term improvements in running or cycling endurance and perception of fatigue in young fit individuals consuming around 2.5-6mg/kg body weight (175-420mg per day for a 70kg individual). Other comprehensive reviews also support the conclusions of Ruxton, finding that caffeine improved physical performance by 12% and that 30% of the improvement in exercise performance was attributed to caffeine's beneficial effect on perceived exertion.
Another study reported an improvement in physical endurance and walking speed in older adults consuming 6mg/kg body weight (420mg per day for a 70kg individual), suggesting active older adults may also benefit from a moderate caffeine intake.
Pregnancy
The Food Standards Agency (FSA) has recently amended its advice to pregnant women in light of new evidence from the Committee on Toxicology. A study of 2,600 pregnant women found that those consuming more than 100mg per day were more likely to have a baby of low birth weight with the greatest effects at intakes above 300mg per day. However in a study which asked pregnant women to reduce their caffeine intake, no benefits in terms of birth weight or gestational length were observed. Nevertheless, the FSA now recommends a limit of 200mg per day (or 4 cups of tea) for pregnant women.
Hydration
It is a common perception that caffeinated drinks can lead to dehydration. In theory this is possible, because caffeine causes increased blood flow to the kidneys and inhibits the reabsorption of sodium, calcium and magnesium, thus increasing water excretion. Caffeine may also stimulate urination through effects on muscles in the bladder, but this has not been confirmed. However, in a review of 8 studies in 2008, 5 found no significant effects of a daily caffeine intake of 1.4-6mg/kg body weight (98-420mg for an average 70kg individual). In 2 studies, urine output increased significantly when caffeine intake was around 600mg per day; suggesting that caffeine intake only presents a risk to hydration at intakes of 600mg per day and higher, equivalent to 12 cups of tea.
Conclusion
The available evidence suggests that the moderate consumption of caffeinated beverages (up to 400mg of caffeine a day, equivalent to 8 cups of tea) may benefit cognitive function and heart health in adults without compromising other aspects of health. People at risk of, or with, hypertension may benefit from restricting their intake to no more than 250mg of caffeine a day (5 cups of tea) although individual response to dietary caffeine should be considered when providing advice.
Pregnant women should follow guidance issued by the FSA and limit caffeine intake to 200mg per day (or 4 cups of tea).
Athletes, vegetarians, and women who are breast feeding can follow the same advice for adults.
In the absence of official UK guidance for children, Ruxton utilises Canadian guidelines and suggests school-aged children do not need to avoid tea and coffee provided overall caffeine intake remains below 95mg per day (or no more than 2 small cups of tea).
Ruxton C (2009) Health aspects of caffeine: benefits and risks. Nursing Standard. 24, 9, 41-48.
Tea helps to reduce markers of inflammation
A recently published study found that drinking 3 cups of black tea a day for 12 weeks significantly reduced C-Reactive Protein (CRP) and uric acid (UA) (both markers of inflammation and predictors of cardiovascular disease (CVD)) in individuals who had high (and potentially more risky) baseline levels.
The pathophysiology of atherosclerosis and other cardiovascular (CV) events are associated with inflammation and the consequent instability of atherosclerotic plaques leading to cardiovascular events. The levels of CRP are known to rise during periods of inflammation and plasma levels of high sensitivity (hs)-CRP are widely suggested to predict CV events. According to the American Heart Association individuals with an hs-CRP above 3mg/L have an increased risk of developing cardiovascular disease (CVD).
Uric acid (UA) is predicted to have antioxidant functions in-vivo, however epidemiological studies link elevated UA with increased risk of CVD. In addition, hyperuricaemia is considered to be a risk factor for many clinical conditions including hypertension, CV events, diabetes mellitus, stroke and the metabolic syndrome. Hyperuricaemia may also be associated with changes in the levels of CRP.
The objective of this current, randomized, controlled study, carried out in Maritius, was to determine the effects of black tea on fasting UA and CRP in humans at risk of CVD. Two hundred and sixty three men and women between 25 and 60 years were recruited into the study; 80 were waiting for cardiac surgery, 90 had undergone cardiac surgery in the past 6 months and 93 were healthy volunteers.
The intervention group consisted of 70% of the study population. They were required to consume 3 cups of black tea per day (3 x 200ml of hot water, each containing 3g of black tea infused for 5 minutes, without milk or sugar) for 12 weeks, followed by a 3 week washout period consuming the same volume of hot water. The control group consumed the equivalent volume of hot water for the duration of the study.
Fasting blood samples were taken at baseline, after the intervention period and after the washout period. Subjects were maintained on their usual diet and asked to record their intake at main meals for the duration of the study period. A dietary questionnaire was completed after each of the blood tests and assessed food items that were eaten daily during the 3 main meals (breakfast, lunch and dinner) over the course of the study period.
Two hundred and thirty two people completed the study; 137 male (59%) and 95 females (41%) representing a 12% dropout rate. There were no reported adverse health events during the intervention period. According to the dietary questionnaires the diet remained relatively consistent for both the tea and control group from the baseline to the washout period.
Results were stratified according to baseline UA levels:
| Males | Females | ||
|---|---|---|---|
| Group 1 N=28 |
2.5-5 mg/L | Group 1 N=23 |
2.4-4 mg/L |
| Group 2 N=69 |
5.1-7 mg/L | Group 2 N=36 |
4.1-6 mg/L |
| Group 3 N=44 |
7 mg/L | Group 3 N=32 |
>6 mg/L |
After 12 weeks, UA levels in the intervention group with the highest baseline levels (Group 3) significantly decreased by 9.4% in males (p<0.01) and by 7.1% in females (p<0.001). After the 3 week wash out period, UA levels rose (although they still remained lower than baseline) in both men and women in Group 3, however they were no longer statistically significant. Changes in UA levels in other baseline groups and in control groups did not reach statistical significance.
Results were also stratified according to baseline CRP levels corresponding to data from the American Heart Association:
| Group 1 Low risk |
0-1.5 mg/L |
| Group 2 Average risk |
1.6-3 mg/L |
| Group 3 High risk |
>3 mg/L |
After 12 weeks, CRP levels in the high risk intervention group (Group 3) decreased by 52% in males (p<0.01) and by 41.1% in females (p<0.01); after the 3 week wash out period changes in CRP levels were no longer statistically significant. Those in the average risk intervention group also saw significant falls of 43.4% (p<0.001) and 21% (p<0.05), for men and women respectively after 12 weeks. Changes in CRP levels in other baseline groups and in control groups did not reach statistical significance.
There was no significant correlation between CRP and UA levels at baseline, 12 weeks and washout for either the intervention or control group.
This study has demonstrated that a daily intake of 9g of black tea (3 cups) per day can reduce the levels of CRP and UA in individuals who have the highest levels of these inflammatory markers. When tea drinking ceases the levels of these markers appear to rise, thus regular, daily tea drinking is probably required in order to have lasting changes. The authors suggested that it is likely that the polyphenols present in tea are playing a role in this process. Longer study periods are required to confirm these study's results, as well as investigate the exact mechanisms.
Bahorun, T., Luximon-Ramma, A., Gunness, T.K., Sookar, D., Bhoyroo, S., Jugessur, R., Reebye, D., Googoolye, Ka., Crozier, A., Aruoma, O.I., Black tea reduces uric acid and C-reactive protein levels in humans susceptible to cardiovascular diseases, Toxicology (2009), doi:10.1016/j.tox.2009.11.024
Further research supports the role of tea in reducing the risk of diabetes
According to a systematic review and meta-analysis conducted by Huxley and colleagues, drinking 3 to 4 cups of tea per day reduces the risk of diabetes.
It is well understood that the global burden of Type 2 diabetes (DM) is set to rise in the coming decade. The condition causes considerable morbidity and mortality in those affected and is associated with economic, health and societal costs. Those with Type 2 DM are at greatly increased risk of other chronic illnesses including cardiovascular disease.
Identifying modifiable risk factors for the prevention of type 2 DM is of considerable public health importance. Whilst obesity and physical activity have been identified as two such modifiable lifestyle risk factors, more specific dietary and lifestyle factors remain uncertain. Observation studies have suggested diets high in fat, trans-fats and red meats are independently associated with an increased risk of Type 2 DM and conversely a high intake of whole grains may be protective.
A number of studies have highlighted the potential role of tea and coffee in reducing the risk of developing Type 2 DM. An earlier meta-analysis has suggested that individuals who have the highest coffee consumption have approximately one-third the risk of Type 2 DM compared to those with the lowest levels of consumption. Such a review has never been carried out for tea and decaffeinated coffee consumption. The purpose of the current review was to update the previous meta-analysis of the association between coffee consumption and risk of DM and to conduct a supplementary overview of the evidence for decaffeinated coffee and tea consumption on subsequent risk.
The systematic review included studies published between 1966 and July 2009. Criteria for study inclusion were:
• Published quantitative estimates of the association between intakes of total coffee, decaffeinated coffee, total tea (including green and black tea) with newly diagnosed Type 2 DM.
• Findings in these studies should be adjusted for age and body mass index
Criteria for exclusion were:
• Studies involving animals and human studies of Type I DM
• All cross-sectional studies and any case-control studies with no information on incident Type 2 DM
• Studies that did not include a quantitative measure of intake
The literature search yielded 2,435 articles (847 were identified as duplicates and discounted). Following a review of 1,588 abstracts, 120 papers were reviewed in full and 20 of these were included in the present review. The sample size totaled 517,325 with 21,897 cases of newly diagnosed Type 2 DM. Cohorts were predominantly white populations but 21% of data were derived from Asian cohorts (n=110,147). Age at the start of the studies ranged from 20 to 98 years and the median duration of follow-up ranged from 2 to 20 years. All but one of the studies used self reported food frequency or self administered questionnaire to estimate beverage consumption.
A total of 7 studies (n=286,701 participants) reported on the associated between tea consumption and the subsequent risk of Type 2 DM. The authors made a decision to pool the estimates of relative risk (RR) that corresponded as closely as possible to 3 to 4 cups of beverage per day, compared with none. The consumption of black, green and oolong tea was grouped together to give an estimate of total tea consumption.
Pooled estimates indicated that individuals who consumed more than 3 to 4 cups of tea per day had an approximate one-fifth lower risk of Type 2 DM than those consuming no tea (RR, 0.82 [95% CI, 0.73-0.94]). Unfortunately there was insufficient data to examine whether a dose-response relation was present. Nor was it possible to examine the potential effect of confounding as relevant data was not reported.
The authors don't believe the caffeine content of tea is solely responsible for the protective effects, rather, the broad range of other compounds are also likely to play a part. For example, tea catechins have been shown to inhibit the carbohydrate digestive enzymes, suggesting glucose release from dietary carbohydrates may be delayed resulting in lower levels of glucose and insulin. All forms of tea have also shown to increase insulin sensitivity and there is a suggestion that green tea may protect pancreatic beta cells.
According to this substantial review, a moderate intake of tea is associated with a reduction in the risk of developing Type 2 DM. The authors suggested that further intervention trials would help support these findings as well as help understand the mechanism behind this effect.
Rachel Huxley, Crystal Man Ying Lee, Federica Barzi, Leif Timmermeister, Sebastien Czernichow, Vlado Perkovic, Diederick E. Grobbee, David Batty and Mark Woodward. (2009). Coffee, Decaffeinated Coffee, and Tea Consumption in Relation to Incident Type 2 Diabetes Mellitus - A Systematic Review With Meta-analysis. Arch. Intern. Med. 169(22):2053-2063.
