The Physiological Effects of Caffeine

The Physiological Effects of Caffeine

Introduction: 

Caffeine is the most consumed psychoactive compound in the world. It is present in many foods, beverages, medicines and supplements. Caffeine containing foods and beverages have been consumed for a perhaps as long as 700,000 years (Snyder 20). Caffeine is a highly effective stimulant that has been shown to enhance mood, cognitive capability, alertness, and enhance athletic performance. When ingested, caffeine produces a number of effects on the central nervous, muscular, digestive, respiratory, and cardiovascular systems of the body.

Metabolism of caffeine: 

Caffeine is a water soluble compound and therefore spreads through the body rapidly to anywhere there is water. Additionally, caffeine easily passes through cell membranes. These properties allow caffeine to rapidly enter the blood stream where it is then cycled through the liver (60 Snyder). In the liver the P45 enzyme system produces a number of metabolites (Ruxton 16). This process happens over and over until all the caffeine has been metabolized (60 Snyder). Less than six percent of caffeine is expelled in urine and most of the drug is removed from the body in about 12 hours following consumption (Ruxton, 15). The rate caffeine metabolism can be altered by a number of other factors including liver disease or use of other drugs (Snyder 61).

Caffeine and the central nervous system:

The caffeine molecule is structurally similar to adenosine a chemical that is present throughout the body. Adenosine can produce lethargy and inhibits the release of neurotransmitters. When caffeine is ingested it binds to receptor sites and allows nerve cells to fire neurotransmitters more rapidly speeding up the communication between cells. Additionally, when caffeine is metabolized by the liver the chemicals axanthine and 1-methylxanthine are created. These metabolites are found to be more effective than caffeine alone in taking the place of adenosine at receptor sites and may enhance the drug’s effect (Snyder 54-55).

Caffeine’s impact on mood, sleep, and cognitive function: 

The stimulant effects of coffee have been shown to have an impact on mood, reduce drowsiness, increase performance on tasks requiring alertness, decrease short term memory, and reduce performance on fine motor tasks. While the effects of caffeine on sleep are well established, it generally delays the onset of sleep and heavy use is associated with insomnia, its effect on mood is less clear. It is known that caffeine consumption affects a number of neurotransmitters other than adenosine including noradrenalin, dopamine, serotonin, acetylcholine, glutamate, and gamma-amino butyric acid. It is unclear whether these changes to neurotransmitters result in measurable changes to mood or cognition (Ruxon 2008). Some studies report increased anxiety, tension, depression, and anger while others do not. Other studies reported increased vigor and alertness while others do not. It seems that the effects on mood may vary depending on the individual (Snyder 80-83).

The British Nutrition Foundation analyzed 23 studies involving the effect of caffeine on mood and cognitive performance. These studies involved varying doses of caffeine ranging from 37.5mg to 450mg given to users along with placebo. The subjects were given cognitive and mood tests before and after receiving their dose of caffeine. 17 of the studies produced improved results in cognitive function. However, the studies also showed mixed results relating to mood and most showed a decrease in fine motor skills such as marksmanship (Ruxton 17-18).

Caffeine’s effect on the cardiovascular, respiratory and digestive systems: 

Given that caffeine spreads throughout the body easily once ingested, it affects many bodily functions. In the cardiovascular system caffeine can raise blood pressure and heart rate. However, studies vary in exactly how long this lasts. Most studies point to a short-lived increase in blood pressure after caffeine ingestion and other studies point to higher blood pressure among heavy coffee users. Variations in the results of studies are likely due to a number of factors including tolerance to the drug and individual sensitivity to caffeine (Snyder 86).

Caffeine also affects the digestive system. It stimulates secretion of stomach acid, slows passage of material through the small intestine and speeds passage of the large intestine. The secretion of stomach acid may be more pronounced in individuals with ulcers. One study showed that normal people when given a 250mg dose of caffeine have increased stomach acid production from 200mg per hour to 2000mg per hour while patients with ulcers had increased production from 300mg to 47000mg per hour (Snyder 89).

It is often asserted that caffeine contributes to dehydration because caffeine supposedly has diuretic properties. One study conducted by professors at the School of Sport and Exercise Science at Loughborough University took a look at past studies and literature on the diuretic effects of caffeine. They found that most of the studies had been conducted with caffeine itself rather than caffeine containing beverages such as coffee, tea, or cola. Additionally, these studies were often conducted in young, healthy, males and were inconsistent in regards to habitual caffeine use (R.J. Maughan & J. Griffin 411). The Loughborough University literature study reached three broad conclusions:

1. Large doses of caffeine exceeding 250mg can have an acute diuretic reaction.

2. Single caffeine doses equal to levels in most beverages have little or no diuretic action.

3. Habitual users experience little to no diuretic action (R.J. Maughan & J. Griffin 416).

The British Nutrition Foundation conducted a similar study in which they reviewed eight studies of caffeine’s impact on hydration between 1990 and 2006. These studies were conducted in healthy adults with double blind placebo methodology. The author states that in theory caffeine may have a negative effect on hydration because it increases blood flow to the kidneys and inhibits re-absorption of sodium, calcium and magnesium, thus expelling more water. However, Ruxton goes on to say that this theory may be flawed because much of the research was done with caffeine capsules and preformed on rats rather than humans (Ruxton 20).

Recent studies cited by Ruxton produced almost uniform results. There were some small differences in urine output and body weight in users who consumed large quantities of caffeine and no difference in those who consumed caffeine and performed an exercise test. An example of caffeine’s effect on hydration is the most recent study cited by Ruxton where 59 active male students abstained from caffeine for six days. Over five days the students were given three or six mg per kg bodyweight of caffeine. No impact on fluid electrolyte balance was found (Ruxton 22).

Caffeine and physical activity:

Caffeine has often been associated with improved exercise performance. Studies have varied but many show improvement in sub maximal aerobic exercise (distance running, cycling, ect.) while studies in anaerobic events (sprinting, weightlifting, ect.) are less conclusive. The reasons behind improvement shown in these studies are unclear. At rest caffeine has shown to increase fat oxidation and body temperature causing more fat to be used as energy (Snyder 88). It is speculated that during aerobic exercise the increased fat oxidation will allow for preservation of stored glycogen allowing one to exercise at higher level of exertion for longer. Another theory is that caffeine may reduce the perception of pain by releasing endorphins and thus allow participants to exercise harder without realizing it (Goldstein et al, 5).

One study looked at low doses of caffeine on nine male college students ages 19-25. The students were young, healthy, aerobically trained, and indicated non-habitual caffeine use. After abstaining from heavy exercise a day before the trial students were given either a placebo, 1.5mg/kg caffeine (equal to about 1.2 cups of coffee), or 3mg/kg of caffeine 30 minutes before beginning to warm up. Exercise tests were then performed at three different levels of exertion. Heart rate was taken every 15 seconds. The study concluded that during low to moderate intensities, caffeine contributed to a lower heart rate but no significant difference was found at higher intensities (McClaran & Wetter 11).

Another study took eight male distance runners who competed regularly for over two years and compared their results in an eight km race. The runners were on a controlled diet 24 hours before the race and refrained from alcohol, caffeine, and kept training intensity light before competition. The runners either ingested a placebo, 3mg per kg (of body mass) of caffeine, or no supplement. The subjects than competed in an eight km race at maximum effort (Bridge & Jones 434).

Throughout the race heart rate was collected and following the race blood lactate and a perceived exertion test was administered. Runners who ingested caffeine showed an average improvement of 23.8 seconds. These results varied from between ten to 61 seconds. Heart rates and blood lactate concentration were significantly higher in the caffeinated runners. These findings suggest that caffeine may change the perception of effort by the runners (Bridge & Jones 435-436).

A collection of ten studies between 1997 and 2006 by the British Nutrition Foundation indicated similar results as well as some evidence that caffeine may improve performance in anaerobic activities. Three of the studies indicated that caffeine had no impact on performance while the two studies that specifically tested for anaerobic qualities noted significant performance improvement (Ruxton 21). Another study of fatigue in tennis (primarily an anaerobic sport) noted some improvement among both men’s and woman’s performance after consuming caffeine but inconclusive because of the difficulty of testing tennis matches in laboratory conditions (Hornery, et al. 208-209).

Medicinal use of caffeine:

Over-the-counter pain medications such as aspirin often contain caffeine. The addition of caffeine increases the effectiveness and reduces the time it takes for the medication to take effect. Caffeine also increases the rate of breathing and has been used in small amounts to help restore breathing in newborn babies with sleep apnea. Studies have shown that caffeine may aid in fertility by increasing mobility of sperm. Injections of caffeine have been used to counteract opiate poisoning on the brain and restore breathing of the user (Snyder 89-92).

Dangers associated with caffeine:

Few negative side effects have been reported when caffeine consumed in moderate doses (up to 300mg or 2-3 cups of coffee) among a healthy population. People with existing digestive problems may find these problems get worse after consumption of coffee as coffee stimulates stomach acid production (Snyder 89). Chronic use of caffeine in large doses has been associated with insomnia, dependence, and in some cases heart arrhythmias. Studies linking caffeine consumption to heart disease and cancer are inconclusive (Snyder 105-108). Dependence is psychological and also quite possibly physical. Chronic caffeine users often report headache, lethargy, and anxiety when abstaining from use (Snyder 99-100).

Studies have shown negative effects when embryos of rats and mice were exposed to doses of caffeine. The side effects included low birthweight, still birth, premature birth, and birth defects. It is unclear if caffeine causes the same problems in humans but caffeine will pass easily from mother to fetus. Many doctors recommended that pregnant women reduce or eliminate caffeine from their diet (Snyder 112).

Overdose from caffeine is possible but extremely rare. The lowest known fatal overdose occurred at a level of 3200mg administered intravenously. Fatal overdose from oral consumption requires at least 5000mg (approximately 40 cups of strong coffee consumed rapidly). Large amounts of caffeine result in vomiting so the risk of fatal overdose from coffee or other beverages is unlikely (Snyder 92-93).



Large doses of caffeine (>1000mg) have resulted in:
Hyperventilation
Rapid heart beat
Involuntary muscle contractions
Twitching of the heart
Low levels of potassium
High levels of blood sugar (Snyder 92-93)

Conclusion:

Caffeine is a widely used mild central nervous system stimulant that works primarily by countering the effects of the chemical adenosine. To a lesser extent caffeine influences a number of other physiological processes in the body. For most people caffeine is safe if used in moderation and is effective in combating fatigue, increasing cognitive, and physical performance.





Sources cited:

Bridge, C.A., & Jones, M.A. “The Effect of Caffeine Ingestion on 8km Run in a Field setting”. Journal of Sports Sciences, 24(4). (April 2006): 433-439.

Goldstein, Erica, et al. “International Society of Sports Nutrition Position Stand: Caffeine and Performance”. Journal of the International Society of Sports Nutrition. ( Jan 27, 2010): P.5.

Hornery, Daniel J., Farrow, Damian. , Mujika, Inigo. ,& Young Warren (2007). “Fatigue in Tennis”. Sports Med 37(3). (2007): 199-212.

Maughan, R.J., & Griffin, J. “Caffeine Ingestion and Fluid Balance: a Review”. J Hum Nutr Dietet 16. (2003): 411-420.

McClaran, R. & Wetter, Thomas, J. “Low Doses of Caffeine Reduce Heart Rate During Submaximal Cycle Ergometry”. Journal of the International Society of Sports Nutrition 4-11. (October 9, 2007): P11.

Ruxton, C.H.S. “The Impact of Caffeine on Mood, Cognitive Function, Performance, and Hydration: a Review of Benefits and Risks”. Nutrition Bulletin 33. (2008): 15-25.

Snyder, Solomon M.D. Caffeine: The Most Popular Stimulant. New York: Chelsea House Publishers, 1992.