Caffeine questions reviewed: hydration, fat loss, heart, pregnancy, addiction
What it found
The authors examined 14 common questions about caffeine and concluded that moderate doses do not dehydrate, may slightly increase fat burning during exercise, and that very high doses can be lethal.
They also say pregnant women should be advised to abstain.
What they found
Studies in people
Hydration at rest
In people who usually drink coffee, moderate daily caffeine (about 3 mg/kg or 250-300 mg) did not increase urine volume. Very high doses (6 mg/kg or at least 500 mg) may cause a short-term increase in urine.
Hydration during exercise
When people drank enough fluids, caffeine did not harm fluid balance during exercise. Some studies found a small rise in body temperature of about 0.2 to 0.3 degrees Celsius.
Upper vs lower body
Studies on whether caffeine helps upper or lower body strength more gave conflicting results. The authors say the effects depend on dose, individual differences, muscle group size, and activity type.
Depression and mood
Some studies found that people who regularly consume moderate caffeine are less likely to have depressive symptoms. But too much caffeine can worsen anxiety, disrupt sleep, and cause withdrawal symptoms that may worsen depression.
Sex differences
Research on sex differences in caffeine response is mixed. Some studies found no differences in metabolism or exercise performance between males and females, while others reported small differences in heart rate or blood pressure.
Habitual use
Most evidence suggests that regular caffeine consumption does not reduce the performance boost from a dose taken before exercise. Some studies suggest a higher dose of 6 to 9 mg/kg may be needed for regular consumers.
Individual response
There is large variation in how people respond to caffeine for exercise performance. One analysis suggested that only about 5% of people might be true non-responders, and even they might respond to a different dose or timing.
Bone mineral
A meta-analysis found that up to 4 cups of coffee per day (up to 400 mg caffeine) was associated with a lower risk of hip fracture compared to 9 or more cups. Another meta-analysis in women found that 330 mg of caffeine per day was linked to increased fracture risk.
Pregnancy
Observational studies consistently link caffeine intake during pregnancy to risks like miscarriage, low birth weight, and fetal growth restriction. The authors say pregnant women and those considering pregnancy should be advised to abstain.
Studies in people and animals
Body weight and fat
Short-term studies found that caffeine increased fat burning during exercise, especially at doses of 3 mg/kg or more. Longer-term studies on weight loss gave mixed results, and many used other ingredients alongside caffeine.
Heart problems
Caffeine can cause short-term increases in heart rate and blood pressure. Some studies link high caffeine intake to increased arterial stiffness and a higher risk of atrial fibrillation, but other studies suggest moderate coffee may protect against stroke and heart failure.
Other findings
Lethal dose
The FDA estimates that toxic effects like seizures can occur with rapid consumption of around 1,200 milligrams of caffeine. A fatal dose typically exceeds five grams, which is unlikely from coffee or tea.
Addiction
Caffeine withdrawal is a recognized diagnosis, but current evidence does not fully support classifying caffeine as addictive. Caffeine does not appear to release dopamine in the brain's reward center in the way addictive drugs do.
What the authors conclude
“The cumulative scientific evidence suggests that pregnant women and those considering pregnancy should be advised to abstain from caffeine.”
Also in their conclusions
- They conclude that moderate caffeine consumption (up to 400 mg per day) is not dehydrating and may slightly increase fat burning during exercise.
- They advise that pregnant women and those considering pregnancy should be advised to abstain from caffeine.
- They state that caffeine is not addictive according to current criteria, but withdrawal symptoms can occur.
How it was done
Despite a large body of research on caffeine, questions and misconceptions persist about its effectiveness and safety. The authors wanted to evaluate the evidence behind these common questions.
The authors reviewed existing scientific literature on caffeine, covering topics such as hydration, body fat, exercise performance, depression, heart problems, bone health, pregnancy, and addiction. They did not conduct new experiments.
What it can’t tell you
- This is a review of existing studies, not new experiments, so it cannot prove cause and effect.
- Many of the studies in the review were observational, which can show links but not prove that caffeine causes the outcomes.
- The authors note that more research is needed on several topics, including bone health and individual responses.
Who paid
- Conflicts
- JA is the CEO of the International Society of Sports Nutrition (ISSN), a 501c3 academic non-profit. The ISSN receives grants from companies that sell, market, and manufacture caffeine-containing sports nutrition products. JRS has no conflict of interest related to this manuscript regarding financial or business interests. Over the past 25 years, he has received grants and contracts to research dietary supplements, served as a paid consultant for industry, and received honoraria for speaking at conferences and writing lay articles about sports nutrition ingredients and topics. TNZ has no conflict in terms of financial or business interests related to the topic of this publication. He has received funding from dietary supplement companies for research, honoraria for speaking at conferences, and fees for writing articles and consulting. SMA has received grant funding from the US Department of Defense and from industry for research on caffeine and caffeine-related substances. LML is President of Nutrition, Exercise, and Wellness Associates, which receives financial support from educational, food, and/or dietary supplement companies that produce, market, distribute, or sell coffee-related products. He has received sports nutrition-related grants, as well as financial compensation for speaking at conferences and writing lay articles about sports nutrition. DC, SF, BA, JGL, MMA, CE, JR, MG, EG, JH, DA, BDM, and MS have no conflicts to declare.
- Authors work at
- Nova Southeastern University, USA; Texas Woman's University, USA; University of Central Florida, USA
The paper
- Title
- Common questions and misconceptions about caffeine supplementation: what does the scientific evidence really show?
- Type
- ReviewThe authors read earlier studies and describe what they found. It isn’t a new study.
- Evidence
- Studies in people · animals · lab
- Summarised from
- Full text
- Cite
- Antonio J, Newmire DE, Stout JR, et al (2024). Common questions and misconceptions about caffeine supplementation: what does the scientific evidence really show?. Journal of the International Society of Sports Nutrition. doi:10.1080/15502783.2024.2323919Free full textPubMed 38466174DOI
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