The Acute Effects of Caffeine

Cardiovascular Effects
"Two important measures of cardiovascular function are the pressure of the blood as it flows through the arteries (blood pressure) and the heart rate. Blood pressure is of special concern because high blood pressure is an indication of strain on the heart and blood vessels and of possible obstruction somewhere in the circulatory system. Anything that causes or adds to high blood pressure could be dangerous.
A person's blood pressure at any given time depends on two things: the output of blood from the heart and the resistance of the circulatory system to the flow of blood. The output from the heart is determined in part by the rate at which the heart beats. WHen both the resistance to blood flow and the volume of blood pumped through the system at each heart beat remain constant, blood pressure and heart rate rise and fall together.
Caffeine significantly increases the blood pressure in subjects who have been without the drug for some days. Complete tolerance to this effect develops quickly...
Increased heart rate usually accompanies the use of caffeine, although the change is generally small and not statistically significant. In some studies, including the one mentioned above on slightly hypertensive subjects, reduced heart rate was found after caffeine administration. Other researchers have reported that caffeine causes an initial decrease and then an increase in heart rate...
A recent concern has been the possible role of caffeine in the occurrence of arrhythmias-irregularities in heart beat, sometimes known as palpitations. Forms of arrhythmias are thought to be involved in some cases of death from heart failure. A well-publicized experiment, reported in 1983, demonstrated that caffeine given orally or intravenously will reliably produce arrhythmias in subjects who had previous arrhythmic symptoms.

Respiratory Effects
Caffeine has been shown to increase the rate of breathing by heightening the sensitivity of the part of the brain that responds to the level of carbon dioxide in the blood. Caffeine can improve the depth of breathing by strengthening the action of the diaphragm, which is the main muscle concerned with inhaling and exhaling. One study has found that caffeine could be useful for people with lung disease who suffer from breathlessness...

Energy Expenditure and Weight Loss
Caffeine's short-term effects on the body's use of energy might be of interest to people who wish to lose weight. When ingested with a meal, caffeine increases the rate at which the food is converted into usable energy. When caffeine is taken between meals, it causes fats to be transferred from deposits in the cells to the blood stream. Here, as free fatty acids they can be used as energy by most of the organs of the body.
Caffeine also raises the activity level of the body, which can mean that the energy derived from food is used up in exercise rather than being stored as fat. In addition, caffeine stimulates the temperature regulating centres of the body, which in turn produces an increase in body temperature. To sustain this change, energy that might have otherwise been deposited as fat is used. Thus, even when the body is at rest a greater amount of food is burned...

Digestion and Excretion
Drinking coffee increases the secretion of acid into the stomach, but it may be that, in addition to caffeine, other coffee components produce this effect. Although caffeine stimulates acid secretion, it also reduces the peristaltic action of the stomach, the action that causes the emptying of the stomach's contents into the small intestine. Caffeine also slows down the passage of material through the small intestine, yet speeds its passage through the large intestine.
All of the above-mentioned effects can contribute to digestive upset, and even to ulcers of the stomach and small intestine. People who already suffer from digestive upset are usually advised to give up caffeine-containing drinks. In fact, there is evidence that these people are more strongly affected by caffeine than are healthy people...
As well as these effects on the digestive system, coffee and tea also reduce the body's absorption of specific nutrients, particularly iron, an essential mineral. The specific chemical or chemicals that cause the inhibition of iron uptake are not known, but caffeine and the tannins and other components of tea are the most likely agents. In addition, to this, caffeine's ability to increase urination- by 30% for up to three hours following ingestion-can cause significant increases in the excretion in urine of calcium, magnesium and sodium. Though some tolerance does develop to this effect, it could contribute to a deficiency in these minerals.

Caffeine as Medicine
Along with caffeine's beneficial effects on breathing, the drug is also successful in inducing breathing in newborn babies who experience breathing failure and continue to have spells of apnea, or cessation of breathing for more than 20 seconds. Caffeine drug also is effective in reducing the amount of apnea.
Caffeine is frequently included in both prescription and nonprescription headache preparations and other pain relievers. The amount is small-much less per tablet than in an average cup of coffee. Exactly why caffeine was first included in these products along with analgesic drugs (pain relievers) such as aspirin and paracetamol is not known, though it may have been added to counter possible depressant effects of these drugs. Caffeine may also have been included because it is especially effective as a remedy for headaches caused by caffeine withdrawal...
[Research] has shown that the addition of caffeine does indeed increase an analgesic's effectiveness and reduces the time needed for the drug to take effect. When combined with caffeine, 30% less analgesic is needed. This characteristic of caffeine is true not only as regards headaches but with a wide variety of pains, including those from oral surgery and childbirth. How caffeine enhances analgesia is not known.
A small amount of research has suggested that caffeine may also enhance the much more potent analgesic effects of the opiate drugs, including morphine and heroin. However, to date, doctors have used caffeine only as an antidote for opiate overdoses. Injections of caffeine into an opiate user's muscles counteract the effects of opiate poisoning on the brain, and restore the user's breathing if it has failed.
Caffeine has also been used as an aid to fertility. A major cause of human infertility is sperm that aren't mobile enough to reach and fertilize the egg. Studies of nonhuman mammals have shown that when caffeine is added to semen it can increase the mobility of their sperm and enhance fertilization...[Studies find] women are twice as likely to become pregnant if prior to artificial insemination caffeine is added to the semen of their infertile mates.

Caffeine As Poison
Death from caffeine overdose has usually involved accidentally administration by hospital personnel of caffeine by injection or by tablet, or suicide using caffeine-containing tablets... Children have died from caffeine overdose after eating many wake-up, weight-control, or other caffeine-containing pills.
The acute fatal dose of caffeine taken by mouth is at least 5,000 mg- the equivalent of about 40 strong cups of coffee consumed in a very short period of time. Thus, death from a coffee "binge" is unlikely. Moreover, caffeine in high doses causes vomiting, which would add to the difficulty of consuming enough of the drink to cause death.
The actual cause of death from caffeine poisoning is not known, though in general the toxic (poisonous) effects of drug are related to the drug's effects at lower doses. A wide variety of effects have been observed in patients who have received about 1,000 mg of caffeine, including the following:

  1. abnormally fast or deep breathing (hyperventilation)
  2. rapid heart beat (tachycardia)
  3. involuntary, uncoordinated muscle contractions (convulsions)
  4. rapid, uncoordinated twitching of the heart (ventricular fibrillation)
  5. low levels of potassium in blood (hypokalemia)
  6. high levels of blood sugar and ketone bodies in urine, as in diabetes (glycosuria and ketonuria)

The prolongation of any of these effects of large doses of caffeine can lead to death.

-pp. 87-95, Caffeine: The Most Popular Stimulant by Richard J. Gilbert, Ph.D. (1986)

Coffee Tree in Office


Great article, I have been trying to quite my addiction to caffeine lately, but I didn't realize that it could potentially have some heatlh benefits if taken in moderation. I find it hard to moderate my caffeine intake though, as one cup leads to another, and next thing I know I'm high on 4 cups of coffee and drinking water to detoxify my body from the excess caffeine in the system. I wish I could control it and be smarter about it :)

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