What are caffeine's effects on the body?

Short-Term Effects

Caffeine is used socially in drinks such as coffee, tea, and colas by a majority of the adult population in the United States; it also is popular worldwide. Although most use in this manner is moderate (such as one to two cups of coffee per day) and likely within safe health boundaries, higher intake and continual moderate use are thought to have distinct adverse effects on the health of numerous body systems, including cardiac, gastrointestinal, and renal organs.

Central nervous system (CNS) stimulation by caffeine increases levels of dopamine, epinephrine, and adrenaline in the body to increase alertness, concentration, and mood; insomnia or anxiety can occur at higher doses. Caffeine blocks the reception of adenosine in the brain, a process thought to regulate natural sleep patterns, making patients feel more awake. With acute use, caffeine constricts blood vessels to ease symptoms of vasodilating headaches, such as migraines, and increases gastric secretions and smooth muscle relaxation in the stomach to cause heartburn pain. Caffeine relaxes the renal vasculature to cause increased urination. Intake during evening hours may result in low energy levels and excessive fatigue the next day.

Caffeine also is an ingredient in many over-the-counter and prescription headache medications because of its own symptom relief and its amplification of other pain-relieving drugs. Some studies have suggested that caffeine can improve certain types of memory and attention span, although higher doses are generally observed to have detrimental effects. Dry mouth, poor appetite, and dizziness are possible directly after large caffeine intake, and caffeine acutely worsens existing ulcers and anxiety disorders. Within one hour of ingesting caffeine, some people may feel edgy and have increased heart rate and blood pressure because of caffeine’s effects on heart muscle and rhythm. A nursing infant may become jittery or may experience sleep disturbances from caffeine present in the mother’s breast milk.

Side effects, particularly mild effects like stomach upset and insomnia, can begin with moderate caffeine doses as low as 50 milligrams (mg). Caffeine circulates in the body within five to thirty minutes and may cause acute effects for up to twelve hours. Its half-life in adults ranges from three to six hours but is shorter in smokers because of enhanced liver metabolism. The half-life increases to five to ten hours in women taking oral contraceptives, nine to eleven hours in pregnant women, and thirty hours in newborns.

Caffeine is metabolized by CYP 450 demethylation twice, and both metabolites are active in the body as well. Several cups of coffee may provide a serum level of 5 to 10 micrograms per milliliter (microg/mL).

Long-Term Effects from Chronic Use

Some chronic effects of caffeine are simply extensions of short-term effects of use. The Diagnostic and Statistical Manual of Mental Disorders recognizes four caffeine-related disorders: caffeine intoxication, caffeine-induced anxiety disorder, caffeine-induced sleep disorder, and caffeine-related disorder not otherwise specified.

Caffeinism, a diagnosis similar in part to panic disorder or generalized anxiety disorder
, acknowledges the dangers of caffeine in high amounts, particularly with repeated use. High levels of caffeine intake leading to this diagnosis cause anxiety, rapid heart rate and breathing, diarrhea and excess urination, tremors, and increased blood pressure. Irritability and agitation from caffeinism may be indistinguishable from anxiety disorders in the physician’s office, so the disorder is best identified by discussing caffeine use during symptom review. Prescribed antianxiety medications are unlikely to improve symptoms of caffeinism if caffeine use continues.

Any amount of chronic caffeine use can lead to abdominal pain, insomnia, irritability, and anxiety. Supposedly beneficial effects of caffeine, such as improved alertness and performance, also become less pronounced with chronic use. Heavy caffeine use has been linked to the development of osteoporosis from bone density loss and to peptic ulcer or gastroesophageal reflux diseases from smooth muscle relaxation and heartburn. Emotional lability (instability), prolonged hypertension, and cardiac disease are possible, and dehydration can occur with continual use.

Caffeine that remains in the body leads to adrenal exhaustion and, hence, tolerance. With tolerance, the body requires even more caffeine to obtain the same mood- and performance-heightening effects. Thus, caffeine is reinforcing, and users will ingest increasing amounts to experience alertness and concentration.

Abrupt caffeine discontinuation causes physiologic withdrawal even with only moderate chronic use (for example, two cups per day). Headache is the primary withdrawal symptom and may be throbbing at first; poor performance and depression over time may occur as a result of the sudden changes in dopamine and adrenaline levels. Other reported symptoms include reduced concentration and lethargy. Renewing caffeine intake relieves withdrawal symptoms temporarily but continues the habitual cycle of caffeine-induced symptoms, tolerance, and withdrawal symptoms. Although caffeine is rarely taken to get high, its narrow therapeutic index of 8 to 20 microg/mL and its variable effects can lead to chronic misuse.

Acute Intoxication

Overdose, or acute intoxication, is rare from social use. At doses of 5 to 50 grams or serum levels of 100 to 200 mg per kilogram of body weight, caffeine is potentially lethal, although highly sensitive persons may experience severe overstimulation with individual doses as low as 250 mg. Several cases have been recorded of fatal cardiac arrhythmia caused by caffeine toxicity, usually in combination with a preexisting medical condition exacerbated by particularly high caffeine intake. Caffeine intake greater than 300 mg per day may cause miscarriage or poor neonatal growth during pregnancy, and intake greater than 600 mg per day by pregnant women may induce premature or aborted birth.

Moderate toxicity from overdose (for example, the use of large quantities of energy drinks) causes vomiting, muscle stiffening, and heart muscle irritation. When caffeine levels in the blood are extremely high, intoxication causes metabolic changes in the body, including low potassium levels, high sugar levels, and ketosis. The CNS, cardiac, and musculoskeletal effects can include repeated seizures, muscle posturing and hypertonicity, and ventricular fibrillation or tachycardia.

More severe acute overdose effects involve breathing problems. Pulmonary edema occurs when blood vessels around the lungs dilate, leading to life-threatening blocked airways and hypoxia. Rhabdomyolysis, or muscle cell breakdown, and metabolic acidosis contribute to acute renal failure.

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