Caffeine is completely absorbed by the stomach and small intestine within 45 minutes of ingestion. After ingestion it is distributed throughout all tissues of the body and is eliminated by first-order kinetics.

The half-life of caffeine — the time required for the body to eliminate one-half of the total amount of caffeine consumed at a given time — varies widely among individuals according to such factors as age, liver function, pregnancy, some concurrent medications, and the level of enzymes in the liver needed for caffeine metabolism. In healthy adults, caffeine's half-life is approximately 3-4 hours. In women taking oral contraceptives this is increased to 5-10 hours, and in pregnant women the half-life is roughly 9-11 hours. Caffeine can accumulate in individuals with severe liver disease when its half-life can increase to 96 hours. In infants and young children, the half-life may be longer than in adults; half-life in a newborn baby may be as long as 30 hours. Other factors such as smoking can shorten caffeine's half-life.

Like alcohol, nicotine, and antidepressants, caffeine readily crosses the blood brain barrier. Acute usage of caffeine also increases levels of serotonin, causing positive changes in mood.

The precise amount of caffeine necessary to produce effects varies from person to person depending on body size and degree of tolerance to caffeine. It takes less than an hour for caffeine to begin affecting the body and a mild dose wears off in three to four hours. Consumption of caffeine does not eliminate the need for sleep: it only temporarily reduces the sensation of being tired.

With these effects, caffeine increases the capacity for mental or physical labor. A study conducted in 1979 showed a 7% increase in distance cycled over a period of two hours in subjects who consumed caffeine compared to control tests. Other studies attained much more dramatic results; one particular study of trained runners showed a 44% increase in "race-pace" endurance, as well as a 51% increase in cycling endurance, after a dosage of 9 milligrams of caffeine per kilogram of body weight. The extensive boost shown in the runners is not an isolated case; additional studies have reported similar effects. Another study found 5.5 milligrams of caffeine per kilogram of body mass resulted in subjects cycling 29% longer during high intensity circuits.

While relatively safe for humans, caffeine is considerably more toxic to some other animals such as dogs, horses and parrots due to a much poorer ability to metabolize this compound.

To learn more about caffeine visit Overcaffeinated.org

Caffeine Tolerance and Withdrawal

Because caffeine is primarily an antagonist of the central nervous system's receptors for the neurotransmitter adenosine, the bodies of individuals who regularly consume caffeine adapt to the continual presence of the drug.

Caffeine tolerance develops very quickly, especially among heavy coffee drinkers. Complete tolerance to sleep disruption effects of caffeine develops after consuming 400 mg of caffeine 3 times a day for 7 days. Complete tolerance to subjective effects of caffeine was observed to develop after consuming 300 mg per day for 18 days, and possibly even earlier. Partial tolerance to caffeine has been observed in all other areas, studies with mice indicate that after a long period of caffeine exposure the learning benefits of caffeine observed earlier cannot be found to any significant level. Considering that 80% to 90% of American adults consume caffeine daily, and their mean daily caffeine intake exceeds 200 mg/day, it can be surmised that a large fraction of the U.S. adult population is completely tolerant to most of the effects of caffeine.

 

Caffeine Addiction Symptoms

A reduction in caffeine use can cause anxiety, irritability, inability to concentrate and diminished motivation to initiate or to complete daily tasks; in extreme cases it may cause mild depression. These effects have come to be known as a "crash".

Learn more about the caffeine levels in soda and caffeine in food.

 

Caffeine Withdrawl Symptoms

Withdrawl symptoms can include headaches, irritability, an inability to concentrate, and stomach aches. These symptoms may appear within 12 to 24 hours after discontinuation of caffeine intake. Most withdrawl symptoms peak at roughly 48 hours, and usually last from one to five days