What is 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.

Caffeine also has several metabolic effects, including:

  • Increases movement of food through the intestinal tract
  • Releases free fatty acids from adipose (fat) tissue
  • Affects kidneys and increases urination

Health impact of caffeine

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.
Children also should limit their intake of beverages that contain caffeine, including sodas and sports drinks. High levels of caffeine affect children in ways that are similar to the effects on adults. The United States does not have maximum intake guidelines for caffeine in preschool children, but Canada recommends that preschool children avoid consuming more than 45 milligrams (mg) of caffeine per day. Experts report that caffeine will not stunt a child’s growth.

Caffeine’s impact varies from person to person. Some people are not greatly affected, while others may be especially susceptible to the effects of caffeine use. Several factors may be involved in increased sensitivity to caffeine.

They include:

  • Body mass. People with smaller body mass feel the effects of caffeine more quickly.
  • History of caffeine use. People who do not regularly use caffeine may be more susceptible to its effects. Regular use of caffeine helps an individual to build up a tolerance to the substance.
  • Stress. All forms of stress increase a person’s susceptibility to caffeine.

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
Extraordinarily high caffeine intake (150 mg per kilogram of body weight) can lead to serious health consequences. Even moderate caffeine intake can create symptoms of caffeine overdose in certain people who are highly sensitive to caffeine. People who drink high levels of caffeine often have trouble sleeping at night. They wake in the morning feeling fatigued and drink coffee or another caffeinated beverage throughout the day to help them stay awake. This in turn causes them to again have trouble falling asleep, creating a cycle that leaves a person increasingly tired as the cumulative effects of sleeplessness build up.

Symptoms of caffeine overdose include:

  • Flushed face
  • Frequent need to urinate (diuresis)
  • Rapid heart rate (tachycardia)
  • Restlessness or feeling jittery
  • Shakiness
  • Throwing up (vomiting)
  • Trouble sleeping (insomnia)
  • Upset stomach (nausea)

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.

Sources of caffeine

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]):

Dark (semi-sweet) chocolate, 1 oz.5 to 35
Unsweetened chocolate, 1 oz.26
Milk chocolate, 1 oz.1 to 15
Coffee flavored yogurt, 6 oz.35
Coffee ice cream, 1/2 cup20 to 30
Chocolate ice cream, 1/2 cup2

Brewed coffee, 8 oz.60 to 160
Instant coffee, 8 oz30 to 120
Espresso, 2 oz100
Brewed imported tea, 8 oz.25 to 110
Brewed American tea, 8 oz.20 to 90
Energy drinks, 8 oz.35 to 90
Functional waters, 8 oz.20
Iced tea, 8 oz.6 to 60
Caffeinated water, 8 oz.30 to 60
Soft drinks or soda, 8 oz.15 to 50
Instant tea, 8 oz.24 to 31
Chocolate milk, 8 oz.2 to 7
Decaffeinated coffee, 8 oz.2 to 4

Aspirin, 1 tablet0
Appetite suppressant, 1 tablet200
Medication to help people stay alert, 1 tablet100 to 200

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.

Reducing caffeine intake

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)
Withdrawal symptoms can begin as soon as 12 hours after the last dose of caffeine. They usually peak within 20 to 48 hours and can last up to one week, although they generally go away after a day or two of eliminating caffeine use.

Some people choose to quit caffeine abruptly and then wait out the withdrawal symptoms. Others choose to avoid the withdrawal symptoms altogether by slowly weaning themselves from caffeine over a period of two to four weeks. For example, people may drink beverages that are half-caffeinated and half-decaffeinated, or drink decaffeinated beverages every other day.

Another popular strategy is caffeine fading, in which people drink one-half cup less of the caffeinated beverage every day. By gradually reducing their daily caffeine intake, people can eventually reach their goal of drinking only caffeine-free beverages without experiencing withdrawal effects.

Some people have raised concerns about the chemical process by which coffee is decaffeinated. Although water is sometimes used to slowly remove caffeine from coffee beans, the use of chemicals is a popular strategy for decaffeinating coffee. Within the next few years, genetically engineered coffee beans may provide the same “decaf” benefits without the chemical treatment. In the meantime, people who are concerned about the chemical process and still wish to cut down might benefit from drinking herbal tea, juice, water, milk or other caffeine-free drinks.

Other tips for limiting caffeine intake include:

  • Keep track of how much caffeine is consumed each day. For example, patients are urged to track the number of cups of coffee or cans of soda they drink.
  • Gradually reduce the amount of caffeine consumed each day. People can replace caffeinated drinks with decaffeinated alternatives, which are available for coffee, tea and soda.
  • Brew tea for shorter periods of time, or choose herbal teas (which do not have caffeine).
  • Try to choose over-the-counter medications that are caffeine-free. Reading the packaging labels can reveal whether the drug has caffeine or not.

Questions for your doctor regarding caffeine

Preparing questions in advance can help patients to have more meaningful discussions regarding their conditions. Patients may wish to ask their doctor or registered dietitian the following questions related to caffeine:

  1. How much caffeine is too much for me?
  2. Is there a reason I should avoid caffeine?
  3. How can I tell if I have a high sensitivity to caffeine?
  4. I’m pregnant/nursing – how much caffeine can I safely consume?
  5. Do the caffeinated products I consume every day interfere with any of my current medication?
  6. Are there certain over-the-counter medications that may contain caffeine that I should avoid?
  7. Can you recommend alternatives for some of my favorite foods that contain caffeine?
  8. How much caffeine should my child consume?
  9. What tips can you give me for reducing my caffeine intake?
  10. Is decaf coffee a good alternative for me? Are there any other alternatives?

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