Caffeine is a central nervous system stimulant of the methylxanthine class. It is the world’s most widely consumed psychoactive drug. Unlike many other psychoactive substances, it is legal and unregulated in nearly all parts of the world.
Caffeine is a bitter substance that occurs naturally in more than 60 plants including
- Coffee beans
- Tea leaves
- Kola nuts, which are used to flavor soft drink colas
- Cacao pods, which are used to make chocolate products
There is also synthetic (man-made) caffeine, which is added to some medicines, foods, and drinks. For example, some pain relievers, cold medicines, and over-the-counter medicines for alertness contain synthetic caffeine. So do energy drinks and “energy-boosting” gums and snacks.
Caffeine is often talked about for its negative effects on sleep and anxiety.
However, studies also report that it has various health benefits.
Benefits of Caffeine
Caffeine may have some health benefits, but not all of these have been confirmed by research.
Caffeine may boost weight loss or prevent weight gain, possibly by:
- suppressing the appetite and temporarily reducing the desire to eat
- stimulating thermogenesis, so the body generates more heat and energy from digesting food
Weight loss products that are marketed as thermogenic may contain caffeine and ephedra, or ephedrine.
Research has not confirmed long-term results.
A 75-mg serving of caffeine can increase attention and alertness, and a 160 to 600-mg dose may improve mental alertness, speed reasoning, and memory.
However, caffeine is not a substitute for sleep.
Caffeine can improve physical performance during endurance exercise.
The European Food Safety Agency (EFSA) recognizes that caffeine can increase endurance performance, endurance capacity, and reduction in perceived exertion.
However, the effects of short-term, high-intensity exercise remain inconclusive.
Caffeine affects adenosine receptors in the brain. Coffee also contains polyphenol antioxidants, and these, too, act on various pathways.
Studies have suggested that drinking coffee may help enhance some thinking skills and slow the mental decline that comes with age.
However, more research is needed to confirm this.
Alzheimer’s and Parkinson’s disease
Research has found that lifelong caffeine consumption may reduce the risk of developing Alzheimer’s disease.
Studies have also reported that people with a higher coffee consumption have a lower risk of Parkinson’s disease.
Research from Johns Hopkins University suggests that a dose of caffeine after a learning session may help boost long-term memory.
Liver and colon
It has been suggested that caffeine enemas may help prepare the colon for an endoscopy or colonoscopy by supporting the excretion of bile through the colon wall.
Proponents claim that a caffeine enema increases the levels of glutathione, an antioxidant, and so it supports the natural processes of detoxification in the liver.
However, there is little evidence to support this theory.
Coffee consumption may help decrease the risk of cirrhosis and slow the rate of disease progression in hepatitis C infection. Observational studies have found that coffee may have protective benefits for people with hepatocellular cancer.
There is some evidence that caffeine may help protect people from an eye disorder known as blepharospasm.
This condition, caused by abnormal brain function, makes people blink incessantly and can leave them functionally blind.
Researchers have found that caffeine may help protect the lens of the eye against damage that could lead to the formation of cataracts.
Some scientists have suggested that caffeine may guard against certain skin cancers.
One team found that caffeine applied directly to the skin of mice helped prevent damaging ultraviolet (UV) light from causing skin cancer.
Others have linked the consumption of three cups of caffeinated coffee a day with a 21 percent lower risk of developing basal cell carcinoma in women, and a 10 percent lower risk in men, compared with drinking less than one cup per month.
A study of 217,883 participants analyzed the association between caffeine intake and the risk of developing kidney stones.
Those who consumed more caffeine had a lower risk of developing kidney stones.
Mouth, throat, and other cancers
In a study of 968,432 men and women, participants who drank than 4 cups of coffee a day had a 49-percent lower risk of death from oral cancer, compared with those who drank no coffee at all or only an occasional cup.
Other possible cancer-related benefits include:
- a lower risk of endometrial cancer
- a reduced risk of prostate cancer
- protection against head and neck cancer
- protection against the recurrence of breast cancer
Data for 34,670 women in Sweden without a history of cardiovascular disease indicated that women who drank more than one cup of coffee per day had a 22 to 25-percent lower risk of stroke compared with women who drank less.
Low or no coffee drinking appeared to be linked to an increased risk of stroke.
Type 2 diabetes
One longitudinal study found that participants who increased their coffee intake by more than one cup a day over a 4-year period had a 1 percent lower risk of developing type 2 diabetes compared with people who did not change their intake.
People who lowered their daily consumption by more than one cup of coffee showed a 17 percent higher risk for type 2 diabetes.
However, the reasons for the link were unclear. It may be due to lowered insulin sensitivity, meaning the body does not use the insulin produced efficiently.
The team called for more investigation before asserting that high coffee consumption lowers the risk for type 2 diabetes.
Sources of Caffeine
Caffeine occurs naturally in the leaves, seeds, or fruit of more than 60 plant species, including:
- coffee beans
- tea leaves and buds
- dola nuts
- cacao beans
- guarana seeds
- yerba mate leaf
Caffeine in plants acts as a natural pesticide. It paralyzes and kills insects that attempt to feed on them.
Caffeine features in tea, coffee, and chocolate, and it is regularly added to gum, jelly beans, waffles, water, syrup, marshmallows, sunflower seeds, and other snacks.
The FDA recommends that healthy adults limit their caffeine intake to a maximum of 400 milligrams (mg) a day, about 4 or 5 cups of coffee. This amount is not associated with negative effects.
There is no set limit for children, but the American Academy of Pediatrics (AAP) discourages the consumption of caffeine and other stimulants by children and adolescents.
The amount of caffeine included in some common foods and beverages are:
- One 8-ounce cup of coffee: 95 to 200 mg
- One 12-ounce can of cola: 35 to 45 mg
- One 8-ounce energy drink: 70 to 150 mg
- One 8-ounce cup of tea: 14 to 60 mg
Decaffeinated cola and soft drinks contain no caffeine, but decaffeinated coffee is not caffeine-free.
“Energy drinks” contain varying amounts of caffeine.
Additional products are now appearing on the market, from “psyched up” oatmeal to “wired” waffles.
These have raised concerns, especially regarding the potential impact on children and adolescents.
The Food and Drug Administration (FDA) considers caffeine to be both a drug and a food additive. They recommend a maximum intake of 400 mg a day.
In prescription and over-the-counter (OTC) medicines, caffeine is used to treat tiredness and drowsiness, and to improve the effect of some pain relievers.
It belongs to a group of medicines called central nervous system (CNS) stimulants.
Foods containing caffeine can help restore mental alertness.
Caffeine’s use as an alertness aid should only be occasional. It is not intended to replace sleep and should not regularly be used for this purpose.
In the United States (U.S.), more than 90 percent of adults use caffeine regularly, with an average consumption of more than 200 milligrams of caffeine per day. This is more caffeine than in two 6-ounce cups of coffee or five 12-ounce cans of soft drink.
Much of the published research about caffeine suggests that it is beneficial, in moderation.
However, some studies highlight the potentially harmful effects of caffeine.
Research published in 2016 found that, in 234 middle school students in Korea, a higher caffeine intake was linked to a higher weight, lower academic achievement, and a higher risk of severe depression.
However, whether caffeine leads to depression or depression causes people to consume more caffeine remains unclear.
People with type 2 diabetes report that their blood glucose levels rise after consuming caffeine.
There is some evidence that caffeine may impair insulin action, leading to a small but detectable rise in blood sugar levels, particularly after meals.
Studies have suggested that more than 300 mg a day of caffeine, or the amount equal to around three cups of coffee, could lead to:
- loss of pregnancy
- delayed fetal growth
- abnormal fetal heart rhythm
According to the National Institutes of Health (NIH), the weeks before pregnancy also count. Research shows that if both parents consume more than two caffeinated drinks a day in the weeks before they conceive, a loss of pregnancy may be more likely.
Women should limit their caffeine intake to 200 mg or less during pregnancy.
Some research suggests that caffeine may reduce muscle activity in the Fallopian tubes, which carry eggs from the ovaries to the womb.
This could mean, say the study authors, that caffeine reduces a woman’s chances of becoming pregnant by about 27 percent.
Caffeine passes into breast milk in small amounts, and it may build up in the nursing infant.
Infants whose mothers drink large amounts of caffeinated beverages may be jittery and have trouble sleeping.
An additional intake of caffeine may trigger a gout attack in people with the condition.
Drinking six or more caffeinated beverages in 24 hours has been associated with an almost four-fold increase in the risk of recurrent gout attacks.
A study that looked at 1,356 women found that those with an intake of 329 mg of caffeine a day, equivalent to about three cups of coffee or more, had a 70-percent higher chance of bladder problems.
Consuming caffeine 3 and even 6 hours before bedtime can significantly disrupt sleep. Up to 6 hours before bedtime, caffeine can reduce objectively measured total sleep time by more than 1 hour.
A population-based study found that dietary and medicinal caffeine consumption may be a modest risk factor for triggering chronic daily headaches, regardless of headache type.
A study published in the journal Menopause found that women who consumed caffeine during menopause were more likely to have hot flashes and night sweats.
Other adverse effects
Caffeine’s main effect on the body is an increased temporary sense of wakefulness and alertness, but it can also cause uncomfortable symptoms.
Consuming over 400 mg of caffeine a day can lead to:
- jitters and shakes
- disrupted sleep
- fast or uneven heartbeat
- high blood pressure
- nervousness or anxiety
- stomach upset, diarrhea, and nausea
- muscle tremors
Caffeine increases the release of acid in the stomach, sometimes leading to an upset stomach or heartburn.
Caffeine can interfere with the sleep cycle. Sleep loss is cumulative, and even small nightly decreases can add up and disturb daytime alertness and performance.
Some medicines may interact with caffeine.
These include some:
- Antipsychotics, such as clozapine
- Some antidepressants
- Carbamazepine, as caffeine could increase the risk of seizures
Caffeine may also interact with diuretics, estrogens, valproate, and some other medications.
A number of herbs and supplements can interact with caffeine to varying degrees:
A number of myths surround caffeine consumption. Have a look at some of them.
1. Is caffeine addictive?
In 2013, the American Psychiatric Association (APA) added caffeine withdrawal to the list of recognized conditions in the Diagnostic and Statistical Manual of Mental Disorders (DSM-V). However, not all caffeine consumers have withdrawal symptoms if they stop consuming caffeine.
People who suddenly stop drinking coffee may experience symptoms about 12 to 24 hours after quitting. These peaks after 20 to 48 hours before disappearing. Gradually reducing caffeine intake over a period of days does not trigger these symptoms.
Unlike other drugs, caffeine has not been shown to activate the pathways in the brain that are related to addiction.
Therefore, caffeine is not considered an addictive substance.
2. Is caffeine a diuretic?
Caffeine has been associated with increased urinary volume and frequency, causing the body to lose water and electrolytes such as potassium and sodium.
However, researchers have not found a significant difference in fluid loss among people who drink or do not drink coffee.
One team concluded that: “Coffee, when consumed in moderation by caffeine habituated males provides similar hydrating qualities to water.”
Additional water loss may occur if a person consumes more than 250 mg a day, but the fluid consumed with the drink is likely to make up for any loss.
3. Does coffee cause osteoporosis?
Caffeine may affect the way the body absorbs calcium, and this has raised concerns that drinking coffee can lead to osteoporosis.
However, this has not been confirmed by research.
A Swedish study of data for over 60,000 women found that: “High coffee consumption was associated with a small reduction in bone density that did not translate into an increased risk of fracture.”
Women with a good intake of calcium through their diet are unlikely to be at risk of osteoporosis as a result of drinking coffee.
4. Does caffeine sober you up?
People who have drunk too much alcohol often turn to coffee or an energy drink to sober them up.
However, caffeine does not sober a person up or make them fit to drive. It may make them more alert, but it does not reverse the poor judgment and other effects associated with alcohol.
In fact, it could be more dangerous because, without the drowsiness, a person is more likely to believe they are sober, which could lead to hazardous activities such as driving home or consuming more alcohol.
A person is unlikely to die from consuming too much caffeine in the diet. It is estimated that it would take 149 or so cans of caffeinated energy drink to kill an average adult male. Vomiting would most certainly occur before a person had a chance to consume a fatal overdose of caffeine from dietary sources.
However, pure caffeine is a powerful stimulant, and very small amounts can lead to an accidental overdose. A single teaspoon of pure caffeine is roughly equivalent to 28 cups of coffee.
In addition to the usual adverse effects from too much caffeine, high doses can lead to:
- heart palpitations and rapid heart beat
- nausea and vomiting
- cardiac arrest
Following the deaths of two young men from an overdose of pure caffeine sold over the internet, the FDA urges people “to avoid pure, powdered caffeine.” They also call on parents to be aware that these products may attract young people.
Whether consumed as food or as medicine, the blood and body tissues absorb caffeine within around 45 minutes. It reaches peak level in the blood within 1 hour and remains there for 4 to 6 hours.
While there, caffeine changes the way the brain and body works.
Caffeine has a similar structure to adenosine, a chemical that is present in all human cells.
In the brain, adenosine acts as a CNS depressant.
Adenosine promotes sleep and suppresses arousal by slowing down nerve activity. Adenosine binding also causes blood vessels in the brain to dilate, to increase oxygen intake during sleep. When awake, the levels of adenosine in the brain rise each hour, making the brain and the body less alert.
To a nerve cell, caffeine looks like adenosine, and caffeine binds to the adenosine receptors.
However, unlike adenosine, it does not decrease the cell’s activity. As caffeine utilizes all the receptors adenosine binds to, the cells can no longer sense adenosine. As a result, instead of slowing down because of the adenosine level, cellular activity speeds up.
Caffeine blocks adenosine’s ability to open up the brain’s blood vessels, causing them to constrict. This is why caffeine is used in pain relief medicine for headaches. If the headache is vascular, relief comes as the caffeine narrows the blood vessels.
The blocking of adenosine causes excitatory neurotransmitters to increase in the brain. The pituitary gland notices this increased activity and releases hormones that tell the adrenal glands to produce epinephrine.