Caffeine

Caffeine (C₈H₁₀N₄O₂) is a chemical stimulant of the central nervous system. It belongs to a class of organic compounds called alkaloids, which also include morphine, codeine, LSD, cocaine and nicotine. Caffeine is found in a variety of products, including certain supplements, headache medications, coffee, tea, most sodas, sports drinks and chocolate. Varying amounts of it can be found in more than 60 plants, including the following:

• Cocoa beans
• Coffee beans
• Cola (kola) nuts
• Guarana
• Yerba mate leaves
• Tea leaves

People have been consuming caffeine since the first pot of tea was brewed in China around 2700 B.C. It is widely used for its ability to help people wake up, stay alert, maintain energy levels and respond quickly. Some estimates suggest that almost 50 percent of people in the United States have at least one cup of coffee per day, and about 70 percent of people drink tea in the United Kingdom. Some researchers claim that caffeine is nonaddictive, whereas other researchers declare it to be the fifth most addictive substance in the United States (following heroin, alcohol, cocaine and nicotine). Caffeine also is added as a flavoring agent to some foods and beverages.

Once consumed, caffeine quickly enters the bloodstream through the stomach and small intestine. It travels through the bloodstream and goes directly to the central nervous system. There, it stimulates certain chemicals in the brain that produce an energizing effect on the body. This effect can be felt approximately 15 to 45 minutes after consumption, and reaches a peak within 30 to 60 minutes. Caffeine does not accumulate in the bloodstream nor is it stored in the body.

The half-life of caffeine is roughly five to seven hours, meaning that it will take about that long for 50 milligrams (mg) of caffeine to be metabolized (broken down) after 100 mg have been consumed (e.g., a cup of coffee contains 60 to 160 mg of caffeine). Until the liver has completely broken down the caffeine, the central nervous system will continue to be stimulated. Once broken down, caffeine is quickly expelled from the body through urine, breast milk, saliva or semen.

Research has revealed that caffeine’s stimulating effect is due to its relationship with a chemical called adenosine, which appears to be the body’s natural sleep-enhancer. Normally, an adenosine molecule fits neatly into the receptor on a cell (like a key fitting in a lock), which turns on the cell’s activity and facilitates sleep. However, caffeine fills some of those receptors so that fewer adenosine molecules can get in to turn on the cells’ activity. Consequently, the body’s natural sleep-enhancing effect is partially blocked, and the person feels less fatigued and more alert.

Although most studies have shown no correlation between caffeine consumption and medical problems, people are encouraged to ask their physician about the potential health risks posed by caffeine use. The answer to that question may differ depending on a patient’s circumstances.

For example, a physician may advise against caffeine use in women who are pregnant or breastfeeding. Although no studies have shown a conclusive link between caffeine consumption and damage to a fetus or nursing child, experts usually err on the side of caution in these cases. In addition, some studies have indicated that consuming high levels of caffeine while breastfeeding may be associated with lower sleep levels and poor feeding in the baby.

A physician may also recommend limited use of caffeine for those who have certain types of illnesses, such as coronary artery disease, peptic ulcers, primary insomnia, generalized anxiety disorder, gastroesophageal reflux, urinary incontinence, gastritis or high blood pressure.

Physicians can inform patients about possible interactions between caffeine and other medications or supplements a patient may be taking, including:

• Antibiotics. These drugs may interfere with the breakdown of caffeine in the body, increasing the amount of time that caffeine acts on the body.
• Ephedra. This dietary supplement is known to cause serious health problems such as heart attacks, strokes or seizures. Combining ephedra with caffeine may increase the risk of such problems.
• Theophylline. Combining use of this bronchodilator (used to treat asthma) with caffeine can increase the concentration of the drug in the bloodstream, leading to symptoms such as nausea, vomiting and heart palpitations.

Other factors that may contribute to caffeine sensitivity include age, smoking habits, medication or hormone use, and health conditions such as anxiety disorders. In addition, if a beverage contains certain additional ingredients (e.g., guarana, ginseng), the stimulating effect of caffeine is increased. Although age may play some role in a person’s sensitivity to caffeine, most studies have shown that caffeine affects children and adults in a similar manner.

Research has also shown that as little as 100 mg of caffeine per day can cause some people to become tolerant to caffeine. This can also result in withdrawal symptoms such as headaches, drowsiness and difficulty concentrating when the usual amount of caffeine is not consumed. These withdrawal symptoms usually disappear within a day or two after restricting or eliminating caffeine use.

Experts generally agree that for most people, caffeine does not pose serious health risks when moderate amounts are consumed, such as between 250 and 350 mg per day. Even when a person experiences effects of caffeine consumption, such as increased heart rate, anxiety, the jitters or insomnia, these effects only last for a few hours since caffeine does not accumulate in the body.

People who do not use caffeine regularly are likely to experience positive mood effects when they consume low amounts of the substance. They may feel happy, alert, energetic and more sociable. People who regularly consume caffeine also feel a boost when the substance enters their body, but this is more likely to be a response to the caffeine’s effect of suppressing withdrawal symptoms.

People who consume large amounts of caffeine at once often experience temporary negative effects, such as anxiety, nervousness and an upset stomach. Some studies also indicate that high levels of caffeine may cause an increase in anxiety levels and panic attacks.

For years, researchers have looked for potential links between caffeine use and health conditions such as cancer, fibrocystic breast disease, cardiovascular disease, cholesterol levels, ulcers, inflammatory bowel disease, infertility, birth defects and osteoporosis. To date, no scientific evidence has emerged that links caffeine use to any of these disorders, according to the American Dietetic Association.

The U.S. Food and Drug Administration has classified caffeine as generally recognized as safe (GRAS) since 1958, and the American Medical Association also has stated its belief that caffeine is safe when used in moderation.

Drinking two to three cups of coffee has been shown to raise blood pressure in people who do not have high blood pressure. The reason for this remains unclear. However, there is no evidence that this rise in blood pressure is permanent or that it causes significant health risks. The rise in blood pressure associated with caffeine use lasts only a few hours and is equivalent to the rise experienced after climbing a flight of stairs.

The long-term effect of caffeine on blood pressure remains unclear, although many experts do not believe that the substance poses significant health risks related to blood pressure.

Drinking coffee or other caffeinated beverages also has a diuretic effect, meaning that it causes water loss through increased urination. However, the water that is lost is usually balanced by the amount of fluid obtained from the beverage.

Consuming caffeine slightly increases the loss of calcium through urine and feces. The amount of calcium lost with each cup of coffee is equal to the amount of calcium contained in 1 teaspoon of milk. Thus, adding milk to coffee easily restores the calcium lost through caffeine use. People who consume caffeine in moderation do not raise their risk of osteoporosis.

Although caffeine is safe when moderately used, increased levels of caffeine may have certain health consequences. People who drink more than 500 to 600 mg of caffeine a day (approximately four to seven cups of coffee) may experience the following symptoms:

• Abnormal heart rhythms
• Anxiety
• Headaches
• Irritability
• Muscle tremors
• Nausea, diarrhea or other gastrointestinal problems
• Restlessness
• Sleeplessness

More severe cases of caffeine overdose can also include symptoms such as delirium (acute mental confusion) and visual disturbances. In rare cases, caffeine overdose can also result in death.

Varying amounts of caffeine are present in over 200 foods, drinks and nonprescription medications.

Adults take in more caffeine from coffee than from any other dietary source. Children tend to get most of their caffeine through soft drinks. By some estimates, up to 70 percent of soft drinks contain caffeine, including non-cola drinks such as root beer and orange soda.

Caffeine also can be found in chocolate milk, cocoa and milk chocolate candies. However, the amount of caffeine in these foods is often quite low and often does not impact a person’s mood or behavior. In contrast, some foods do contain significant amounts of caffeine. These include dark chocolate candies and some coffee ice creams and yogurts.

Finally, some medicines contain significant amounts of caffeine. These include stimulant medications and some analgesics.

Examples of caffeine sources are presented below (in milligrams [mg]):

The U.S. Food and Drug Administration (FDA) does not require food labels to disclose how much caffeine a product contains. In fact, caffeine must be listed as an ingredient in the only if it is added to a food or beverage. If caffeine naturally occurs in the product (e.g., tea, coffee, chocolate), it does not have to be listed as an ingredient.

Many new energy drinks on the market combine high concentrations of sugar and caffeine with stimulants such as guarana. The health claims found on the labels of these drinks are not regulated in the United States. These drinks and many products containing these stimulants are banned in other countries such as Canada, France, Norway and Denmark.

Generally, people are encouraged to gradually cut down on caffeine. Abruptly quitting caffeine altogether (going “cold turkey”) produces a rebound effect in which the body becomes overly sensitive to adenosine (the natural sleep-enhancer). Consequently, adenosine has a more powerful sleep-enhancing effect, causing blood pressure to plunge and leading to a variety of other withdrawal symptoms such as:

• Depressed mood
• Headache, often severe
• Irritability
• Nervousness
• Restlessness
• Sleepiness
• Throwing up (vomiting)
• Trouble concentrating
• Upset stomach (nausea)