Caffeine limits blood flow to heart muscle during exercise

In healthy volunteers, the equivalent of two cups of coffee reduced the body's ability to boost blood flow to the heart muscle in response to exercise, and the effect was stronger when the participants were in a chamber simulating high altitude, according to a new study in the Jan. 17, 2006, issue of the Journal of the American College of Cardiology.

"Whenever we do a physical exercise, myocardial blood flow has to increase in order to match the increased need of oxygen. We found that caffeine may adversely affect this mechanism. It partly blunts the needed increase in flow," said Philipp A. Kaufmann, M.D., F.A.C.C., from the University Hospital Zurich and Center for Integrative Human Physiology CIHP in Zurich,.

The researchers, including lead author Mehdi Namdar, M.D., F.A.C.C., studied 18 young, healthy people who were regular coffee drinkers. The participants did not drink any coffee for 36 hours prior to the study testing. In one part of the study, PET scans that showed blood flow in the hearts of 10 participants were performed before and immediately after they rode a stationary exercise bicycle. In the second part of the study, the same type of myocardial blood-flow measurements were done in 8 participants who were in a chamber simulating the thin air at about 15,000 feet (4,500 meters) altitude. The high-altitude test was designed to mimic the way coronary artery disease deprives the heart muscle of sufficient oxygen. In both groups, the testing procedure was repeated 50 minutes after each participant swallowed a tablet containing 200 milligrams of caffeine, the equivalent of two cups of coffee.

The caffeine dose did not affect blood flow within the heart muscle while the participants were at rest. However, the blood flow measurements taken immediately after exercise were significantly lower after the participants had taken caffeine tablets. The effect was pronounced in the group in the high-altitude chamber.

Blood flow normally increases in response to exercise, and the results indicate that caffeine reduces the body's ability to boost blood flow to the muscle of the heart on demand. The ratio of exercise blood flow to resting blood flow, called the myocardial flow reserve, was 22 percent lower in the group at normal air pressure after ingesting caffeine and 39 percent lower in the group in the high-altitude chamber. Dr. Kaufmann said that caffeine may block certain receptors in the walls of blood vessels, interfering with the normal process by which adenosine signals blood vessels to dilate in response to the demands of physical activity.

"Although these findings seem not to have a clinical importance in healthy volunteers, they may raise safety questions in patients with reduced coronary flow reserve, as seen in coronary artery disease, particularly before physical exercise and at high-altitude exposure," the researchers wrote.

Although caffeine is a stimulant, these results also indicate that coffee may not necessarily boost athletic performance.

"We now have good evidence that, at the level of myocardial blood flow, caffeine is not a useful stimulant. It may be a stimulant at the cerebral level in terms of being more awake and alert, which may subjectively give the feeling of having better physical performance. But I now would not recommend that any athlete drink caffeine before sports. It may not be a physical stimulant, and may even adversely affect physical performance," Dr. Kaufmann said. "It may not be as harmless as we thought before, particularly if you suffer from coronary artery disease or if you are in the mountains."

Dr. Kaufmann noted that this study was not designed to measure athletic performance.

Although the participants were all healthy, Dr. Kaufmann said that the results raise concerns about possible effects of caffeine in people with heart disease.

"Any advice would be based on results of healthy volunteers and would be a bit speculative; nevertheless, my advice would be: do not drink coffee before doing physical activities. We hope to be able to provide data soon on the situation of patients with coronary artery disease," he said.

The researchers noted that other studies of coffee and heart disease have produced mixed results.

Although this study included only 18 participants, the researchers said that the differences they saw were large enough for them to be confident that the effect of caffeine on heart muscle blood flow is real. They pointed out that longer studies of people with heart disease will be needed in order to understand whether the blood flow effects have important health consequences.

Thomas H. Schindler, M.D. from the David Geffen School of Medicine at UCLA in Los Angeles, California, who was not connected with this study, said that if the results are confirmed, they could have important implications.

"In particular, this may play an important role in patients with obstructive coronary artery disease in the intermediate range between 50 percent and 85 percent narrowing of the epicardial luminal diameter. In this range of coronary artery disease-induced epicardial narrowing, the myocardial flow reserve (MFR) has been widely assumed to compensate for the epicardial narrowing and, thereby, to preserve the myocardial blood flow to the heart. A further reduction of the MFR, for example owing to caffeine intake, therefore could precipitate stress-induced myocardial ischemia, angina pectoris (reflecting an imbalance between myocardial oxygen supply and demand) or could also contribute to the manifestation of acute coronary syndromes. Consequently, as stated by Namdar et al., the current findings indeed raise safety questions in patients with already reduced MFR as seen in coronary artery disease, particularly before physical exercise and at high-altitude exposure," Dr. Schindler said.

Dr. Schindler said that further studies will be needed to answer the important questions raised by this study.

Source: eurekaalert.org/ amy murphy /american college of cardiology/ photo from eyecream.com

Coffee & Exercise Performance

Caffeine is part of the naturally occurring group of stimulants found in leaves, nuts and seeds of a number of plants. Common dietary sources include coffee, tea, chocolate and a variety of soft drinks and sports drinks. The caffeine content of various foodstuffs is given below.

Table 1. Caffeine content of various foodstuffs

Foodstuff	Serving size	Caffeine content (mg)
Coffee*	150 ml	50-120
Tea*	150 ml	15-50
Chocolate Drink**	250 ml	10
Milk Chocolate**	50g	40
Caffeinated Soft Drinks	330 ml	40-100

(from Maughan 1999)
* Values for coffee and tea vary widely depending on the source and
method of preparation.
** In addition to caffeine, chocolate contains theobromine, which has an insignificant effect compared to caffeine.

So what is the effect of caffeine on different types of exercise?

Although the mechanism whereby caffeine may aid performance is not fully understood, there is substantial research that concludes that caffeine does improve physical performance. Its effect also appears to be widespread across a diverse variety of sports and exercises. Studies have also been wide ranging and have included well-trained athletes and relatively sedentary individuals of both sexes and different age groups.

One of the most comprehensive and recent reviews (Doherty and Smith 2004) looked at 39 published studies. Of these, 21 involved endurance exercise, 12 used short duration and high-intensity exercise and the remaining 6 used a graded exercise test. Including all these data, caffeine improved performance by 12.4%, relative to the placebo trials and this was shown to greatest effect in those who undertook exercise for a longer duration at any one time.

There are also a number of studies that show the beneficial effects of drinking coffee and/or caffeine ingestion on high intensity exercise. These include improved performance on a 1500 m run (Wiles 1992) and Anselme (1992) showed that anaerobic power in a cycling test was improved by the ingestion of the equivalent of two cups (250mg) of caffeinated coffee. Cycling was also the sport studied more recently by Bell (2001) when caffeine (5mg/kg) was shown to improve performance in a high intensity cycling test.

The beneficial effect has also been demonstrated in swimming trials. Collomp (1992) showed that in a swimming test (2x100m) there was improved performance after ingestion of 250 mg of caffeine.

What is the effective amount of caffeine?

Recent studies used small amounts of caffeine (1-2mg/kg). In many studies, coffee was used whilst others have used caffeine. However, they all show that small amounts of caffeine are effective in improving exercise performance significantly and these smaller amounts, as little as 90mg caffeine, are not associated with any unwanted side effects.

Caffeine and fluid requirements

The diuretic effect of caffeine is often over stressed, particularly in situations where dehydration is a major issue. This affects particularly competitions held in hot, humid climates where the risk of dehydration is high and is more important for endurance athletes where dehydration has a greater effect on performance.

Athletes competing in these conditions are often advised to increase their intake of fluid but also advised to avoid tea and coffee because of their mild diuretic effect. Current research, however, shows that, not only is this mild diuretic effect insignificant during exercise (Armstrong, 2002), but the negative effects caused by cutting such drinks from the diet may be more damaging (Maughan and Griffin, 2003). Conclusions from published studies show that intakes of less than 300mg caffeine a day will not affect levels of body's fluids.

Conclusion

In many of these studies, where performance was improved by the ingestion of caffeine or drinking coffee, there was the also the additional benefit of an associated reduction in the sensation of fatigue.

Given the various initiatives aimed at promoting physical activity to improve health, anything that encourages participation by reducing the discomfort and fatigue most people feel when exercising, has enormous potential implications for improving public health. Caffeine, in the form of coffee or as a pure ingredient, has that ability.

Source: positively coffee

Caffeine

Caffeine is a legal stimulant which can be an endurance aid for activities of > 30 minutes duration. It is a member of a group of compounds called methylxanthines found naturally in coffee beans, tea leaves, chocolate, cocoa beans, guarana, and cola (kola) nuts and similar to the asthma medication theophylline.

During prolonged exercise, the onset of fatigue correlates closely with the depletion of muscle glycogen stores (and is delayed if glycogen is spared). The metabolism of free fatty acids (FFA) as an alternative energy source can lead to decreased use of muscle glycogen. Caffeine can increase blood FFAs, and it is felt that this is its major method of action. In one study, caffeine produced a 50% increase in FFA at 3 to 4 hours. This effect was seen after 300 mg of caffeine (an average 6 ounce cup of brewed coffee contains 100 - 150 mg of caffeine but a Starbuck's 8 oz cup contrains 250 mg!).

There is speculation that some of its benefits may also be related to its central nervous system effect as a stimulant, and a recent study has demonstrated a direct positive effect on the muscle fiber itself via a mobilization of calcium from the sarcoplasmic reticulum with a reported 7% increase in power output over a 6 second cycle exercise task.

In one controlled study, subjects were able to perform for 90 minutes to fatigue as compared to 75 minutes in controls (a 20% increase) after the drinking the equivalent of 3 cups of coffee or 6 caffeinated colas 1 hour before, even though values for heart rate and oxygen uptake were similar in both groups. Another study, looking at performance with acute altitude change (4300 meters), demonstrated a 50% increase in performance with caffeine supplements. How this would help at lesser elevations, riding in the Rocky Mountains for example, is not clear.

The suggested dose of caffeine for the recreational rider is 5 mg per kg of body weight (range 3 - 9 mg/kg) taken 1 hour before the ride although some riders take smaller doses periodically throughout the ride itself.

But there are potential side effects. Caffeine can cause headaches, insomnia, and nervous irritability. In addition it is a diuretic (can cause an increase in urinary water loss) and can lead to dehydration. However the biggest negative is that in high concentrations it is considered a drug and is banned by the US Olympic Committee and US Cycling Federation (to exceed the US cycling Federation's legal limit for caffeine - urine concentration of 12 micrograms/ml - one would have to ingest 600 mg of caffeine (6 cups of coffee) and have a urine test within 2 to 3 hours). Fortunately the ergogenic effects can be achieved well below these limits.

Habitual use will induce tolerance so a period of abstinence is recommended for several weeks before the event. The bottom line is that most endurance athletes consider caffeine useful if used correctly.

Source: cptips.com- Cycling Performance Tips/ photo courtesy of uncp.edu