How Does Caffeine Affect the Body?
Shanthi Appelo, MS, RD
| 4 min read
Coffee is more than just a drink for most Americans – it’s a daily ritual. It’s part of our morning or afternoon routines and serves as the centerpiece of social get-togethers and work meetings. Coffee is ingrained in American culture, as evidenced by the 73% of Americans who say they drink at least one cup of coffee each day. An obvious ingredient in coffee is even more popular than the drink itself, as a staggering 94% of Americans polled in a 2023 survey say they drink caffeinated beverages.
Caffeine is a stimulant that affects the body’s central nervous system. It’s found naturally in some foods like coffee, tea and certain chocolates. It’s also added to other beverages and foods to sell as an energy-boost. For many, caffeine helps with waking up in the morning and allows for better concentration throughout the day. But for those sensitive to caffeine, it may contribute to headaches, a rapid heart rate or trouble sleeping, as well as other issues.
Digesting caffeine: What does caffeine do to your body?
The stimulant activity of caffeine is not limited to the brain, but the colon too. Caffeine stimulates muscle contractions in the colon. These contractions promote peristalsis, or the contraction and relaxation of muscles, which can help promote a bowel movement.
On the flipside, caffeine doesn’t cause everyone to have a bowel movement. Caffeine is a diuretic, which can increase urination and thereby cause constipation.
Caffeine takes a long time to digest. Between five and six hours after drinking a caffeinated beverage, about half of the caffeine remains in the system. This is known as a half-life. It can take up to 10 hours for caffeine to completely leave a person’s bloodstream. Just like caffeine helps some people wake up in the morning, it can also make it more difficult to sleep at night. To avoid sleep disruption, some may need to avoid caffeine up to eight hours before bedtime.
Does coffee dehydrate you or hydrate you?
The myth that coffee causes dehydration is just that – a myth. But that doesn’t mean it is a sufficient source of hydration, either. Coffee doesn’t move the needle in either direction.
Despite being a diuretic, coffee is made up of approximately 95% water, so the fluid in the beverage makes up for the frequent urination it triggers. Conversely, an individual is not at risk of being dehydrated by coffee if they drink reasonable amounts of it per day. A person runs the risk of being dehydrated if they only drink caffeinated beverages throughout the day and consume little to no water.
About 400mg each day is believed to be a safe level for most adults, according to the U.S. Food and Drug Administration. This equals about four cups of coffee, 10 cans of cola or two energy shot drinks.
How caffeine affects the heart
Research examining the link between high caffeine consumption and heart disease is conflicting. Drinking coffee raises blood pressure, but that effect is usually temporary, as you are drinking it, and doesn’t normally lead to hypertension.
As for energy drinks, brands contain varying amounts of caffeine. Drinks with high caffeine content can add up to unhealthy levels for those who drink several a day. More than just a jolt of caffeine, energy drinks often contain additional chemicals that can increase alertness. Energy drinks are considered dietary supplements and are not regulated by the U.S. Food and Drug Administration, meaning they’re not tested for safety or ingredients. The combination of excess caffeine and chemicals can be a perfect storm for people with preexisting heart conditions. According to a 2024 Mayo Clinic study, patients prone to cardiac arrhythmias (irregular heartbeats) due to genetic heart disease could be at a higher risk of arrhythmia when consuming energy drinks.
Those with high blood pressure or who have had a heart attack should discuss caffeine consumption with their physician.
Who should avoid caffeinated drinks?
Coffee consumption can pose issues for some individuals with gastrointestinal conditions including:
- Crohn’s disease
- Irritable bowel syndrome
- Ulcerative colitis
For individuals with these conditions, it’s best to avoid caffeine during a flareup. The acidic nature of coffee can also disrupt the intestinal lining and exacerbate symptoms for those with gastritis or reflux.
Pregnant and breastfeeding women should talk with their doctor about limiting their caffeine intake to 200mg daily, or about two cups of coffee.
The American Academy of Pediatrics discourages all children under the age of 12 from drinking caffeinated beverages. Adolescents aged 12 to 18 should limit consumption to 100mg per day, which is the equivalent of two to three cans of cola or one cup of coffee.
Shanthi Appelö is a registered dietitian and health and wellness spokesperson at Blue Cross Blue Shield of Michigan. For more health tips and information, visit AHealthierMichigan.org.
Photo credit: Getty Images
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