Caffeine: Physiological effects

Last modified: Thursday, 25. December 2008 - 9:10 am

Caffeine, by blocking the action of the body’s adenosine, affects a wide variety of organs, as well as the brain, the gut, and basic metabolism. Theophylline works more actively on respiration and the heart. Caffeine is more active in the gut and in the central nervous system. Theobromine has very weak, if any, effect on the brain, but it retains the methylxanthine effect on the kidneys, increasing urination.

Caffeine dissolves easily in fats, so it encounters no barrier as it spreads in the body when taken by mouth. It rapidly crosses the mucosa of the stomach and soaks through the blood-brain barrier. In the bloodstream, the peak level of caffeine is achieved within half an hour. It takes four hours for the body to clear half a dose of the drug. This rate of metabolism of caffeine is slower in newborns and in women late in pregnancy. Smokers, though, rid caffeine more rapidly. Children also rid their bodies of caffeine more readily than adults.

Metabolic processes speed up appreciably under the influence of caffeine. Fatty acids are released into the blood, and a general increase in metabolism is evident as there is increased muscle activity, raised temperature, or both. More calcium is made available through caffeine’s action in the muscles for contraction, but this effect is evident only at caffeine doses higher than people commonly use. Gut motility and secretion increase with a release of stomach acid and digestive enzymes. Urination is also stimulated; caffeine directly affects the kidneys, cutting into their ability to reabsorb electrolytes and water. For every cup of coffee or two to three cans of caffeinated soft drink consumed, about 5 mg of calcium is lost in the urine.

Breathing rate increases in response to caffeine. The effect on respiration occurs at the level of the brain stem’s respiration control center. Theophylline has the most potent action of all the methylxanthines, affecting the smooth muscle of the bronchial tree in the lungs. This is why theophylline is a treatment for asthma. Doctors may recommend weak tea for their asthmatic patients with colds; this bronchodilating action of the theophylline in the tea will aid in clearing mucus.

Caffeine temporarily increases blood pressure, but the body readily compensates and adjusts back to its normal blood pressure. However, people with hypertension may have a more sensitive response to the drug, as caffeine may raise the blood pressure to a higher level in those with chronic high blood pressure.

For migraine patients, the effect of caffeine on the blood vessels around the brain is beneficial; it constricts both the inner and outer vessels, relieving pain. Also, because caffeine increases the acidity in the stomach, it speeds the absorption of pain medications.

Sports competitors recognize that caffeine can boost performance. Large amounts of caffeine release free fatty acids into the bloodstream, reserving the stores of glycogen in muscle for later use. Two to four cups of coffee contain the amount of caffeine that can enhance exercise performance in the average man weighing about 165 lb (75 kg), and it is two to three cups for a 130-lb (59 kg) woman. A down side to caffeine use during a sporting event is the need to urinate, caused by caffeine’s diuretic effect on the kidneys.

Caffeine content of common dietary and medicinal sources

Source Standard amounl (in milligrams)

Bottled beverages (12 oz)


Red bull




Mountain Dew


Diet Coke


Dr. Pepper


Coca-Cola Classic


Coffee (8 oz)






Decaf brew


Tea (8 oz)


Ice tea






Green tea




Hot cocoa (8 oz)


Chocolate milk (6 oz)


Chocolate bar (1 oz)


Medications (per tablet)






Midol, Maximum Strength






Harmful side effects

Some people find that caffeine irritates their gastrointestinal tract. It is still unclear if the effect is from caffeine itself, or from another as-yet-undetermined substance that could be in coffee. Regardless, people with stomach ulcers or irritation may not find relief by switching from caffeinated to decaffeinated coffee.

Effects of caffeine on the heart can be considerable. Rapid or irregular heartbeats can result from ingesting large amounts of caffeine. People at risk for heart attacks might be ill advised to drink coffee or indulge in other sources of caffeine. Caffeine intake exceeding the amount found in five or more cups of drip coffee a day results in a statistically increased risk of cardiac arrest.

Frankly toxic effects, such as persistent insomnia and anxiety, only become evident when people drink more than eight or nine cups of coffee or tea a day. Convulsions and delirium can follow enormous doses, and a near-fatal dose can induce a state similar to that of a diabetic lacking insulin. Blood sugar surges, and ketones appear in the urine. The lowest recorded fatal dose of caffeine was 3200 mg, which was given by accident directly into the bloodstream. It takes the equivalent of 40 cups of coffee consumed by mouth in a short interval for caffeine to kill a person.

Long-term health effects

When studies in the 1980s raised concern about possible adverse effects, including miscarriage, birth defects, and infertility, doctors advised pregnant women to cut out caffeine entirely. Most of these studies have not been confirmed. Certainly, the amounts of caffeine used to cause birth defects in rodents exceed the usual amounts consumed by people. By the year 2000, doctors simply told women who were pregnant or planning pregnancy, to keep caffeine consumption within the bounds of a cup or two of coffee a day.

Most studies find that moderate use of caffeine does not impair fertility, risk miscarriage, or increase the chance of having a baby with birth defects. The March of Dimes has concluded that moderate caffeine is of low risk to pregnant and nursing women.

A study in the early 1990s compared more than 2,800 women who had recently given birth to 1,800 women diagnosed as infertile. Caffeine habits had little or no impact on the reported time it took to conceive in those who had given birth, and was not a risk factor in the infertile. A similar study in Denmark only pointed to smoking as a factor in delayed conception.

Babies born to women who consumed large amounts of caffeine during pregnancy might demonstrate delayed growth or delayed mental or physical development. However, caffeine consumption equivalent to about a cup and a half of coffee a day had no effect on a child’s birth weight, length, and head circumference; nor did it have an effect on follow-up exams of the children at eight months of age and at seven years. In a study of 1,500 women, neither the motor skills nor the intelligence of these children was affected by their mother’s caffeine consumption. Review of more than 20 studies since the 1980s show no evidence that caffeine causes either low birth weight babies, or early birth.

One study carried out between 1959 and 1966 found that very high levels of a caffeine metabolite is a marker for spontaneous abortion. Measured at 11 weeks gestation, the amount of the metabolite, paraxanthine, in blood serum was higher in women who had lost a pregnancy than in women in the control group. The risk of spontaneous abortion in women with the very highest level of paraxanthine was twice that than for the women with the lowest recorded levels. The levels were measured more than 30 years later, and the findings reported in 2000.

Caffeine can enter the milk of breastfeeding mothers. Babies younger than six months cannot metabolize caffeine as well as do adults. Mothers are advised, however, that up to three cups of coffee, or several cans of soda, can be consumed without passing caffeine on to their nursing infants.

The American Cancer Society states that there does not seem to be any relationship between caffeine and cancer. However, other adverse effects for women remain a concern, such as the possibility that large amounts of caffeine could contribute to osteoporosis (thinned and fragile bones), particularly in elderly women. As caffeine is a diuretic, which increases loss of fluids and electrolytes in the urine, it could rob the body of calcium. Nevertheless, a study published in 2001 concluded that the net effect of carbonated sodas on the body’s calcium is negligible, and that the loss of calcium in urine due to carbonated drinks is too small to affect calcium balance.

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