Our last article discussed caffeine's primary action in the brain: it blocks adenosine, a chemical that contributes to feelings of fatigue and sleepiness. By blocking adenosine from binding to its receptors, caffeine can increase alertness and reduce the perception of effort and pain during exercise.
But adenosine receptors are not found only in the brain. They are also found in the kidneys, where they help regulate sodium and fluid balance. When caffeine blocks adenosine A1 receptors in the kidneys, it can temporarily reduce sodium reabsorption and increase urine production [1,2]. This effect has contributed to the long-standing belief that caffeine is dehydrating. For firefighters working in hot environments, however, the relationship between caffeine and hydration is more complicated.
The good news: Research indicates that low-to-moderate amounts of caffeine do not appear to interfere with the body's ability to regulate temperature or cardiovascular function during exercise in the heat. [3–8]
Does caffeine actually make you lose more fluid?
Caffeine can increase urine production, but the amount of caffeine matters. Seal et al. (2017) studied 10 healthy men who consumed either water, a low dose of caffeinated coffee (3 mg/kg), or a higher dose (6 mg/kg). The participants consumed the drinks at rest, and researchers measured their urine output for three hours.
The higher caffeine dose increased urine production compared with both water and the lower caffeine dose. However, the 3 mg/kg dose did not result in greater overall fluid loss than water during the three-hour study period. [9]
For perspective, a 170-pound (77-kg) person consuming 3 mg/kg would consume about 230 mg of caffeine. That is roughly the amount found in some large servings (12-16 oz) of coffee.
The 6 mg/kg dose would provide about 460 mg of caffeine, a considerably larger amount.
This distinction is important: more caffeine does not necessarily mean better performance, and higher doses may have greater effects on fluid and electrolyte excretion.
What happens with regular coffee consumption?
Another study produced similar results. Killer et al. (2014) compared 50 men who consumed either four 200-mL cups of black coffee or four 200-mL cups of water each day for three days. The coffee provided approximately 4 mg/kg of caffeine. After three days, there was no significant difference between the coffee and water conditions in total 24-hour urine volume or urine concentration. [10] The coffee condition did result in greater sodium excretion, which is consistent with caffeine's temporary effect on sodium reabsorption in the kidneys.
Overall, these findings suggest that moderate caffeine intake of approximately 3–4 mg/kg does not appear to cause greater overall fluid loss at rest. [9,10] But what happens when caffeine is consumed before or during physically demanding work in the heat?
Caffeine, heat, and physical work
Firefighting can combine several factors that increase heat strain: high environmental temperatures, protective clothing and equipment, strenuous physical work, and prolonged exposure to heat. Maintaining adequate fluid balance is therefore important.
Research examining caffeine during exercise in the heat provides some guidance. Ganio et al. (2011) studied cyclists who completed prolonged exercise in both cool and hot conditions after consuming either 6 mg/kg of caffeine or a placebo. Heat reduced endurance performance, but caffeine still improved performance compared with placebo. Importantly, caffeine did not further increase measures of thermal strain, including rectal or skin temperature. [3] However, individual responses varied: 9 of 11 participants demonstrated improved performance with caffeine in the cool condition, compared with 6 of 11 participants in the hot condition.
A 2022 systematic review and meta-analysis reported similar findings. Across studies, approximately 6 mg/kg of caffeine produced a modest improvement in endurance performance during about one hour of exercise in the heat. The average increase in core temperature was small, approximately 0.10°C per hour. [4] However, one important point should not be overlooked: staying adequately hydrated is essential for maintaining fluid balance, with or without caffeine.
In studies demonstrating performance benefits from caffeine during exercise in warm and/or humid conditions, participants largely had access to adequate fluids. [5,6] One study involving heat-acclimated cyclists demonstrated why this matters. Participants exercised for two hours in the heat while consuming either water or a carbohydrate-electrolyte beverage, with or without caffeine. Fluid intake replaced approximately 97% of sweat losses. [8] Under these conditions, caffeine did not impair the body's ability to regulate cardiovascular function or core temperature. [8] Compared with trials involving no fluid intake, heart rate was lower, stroke volume and cardiac output were higher, and core temperature was maintained, regardless of whether participants consumed caffeine. [8] The practical message is straightforward: Caffeine does not appear to automatically increase heat strain or cause dehydration when fluid losses are appropriately managed.
So, how much caffeine is appropriate?
There is no single caffeine dose that is appropriate for everyone. Caffeine recommendations are based on body weight, which means the same "dose" of caffeine can represent very different amounts for different people. For example:
| Athlete | Body weight (kg) | 2 mg/kg | 3 mg/kg | 4 mg/kg | 6 mg/kg |
|---|---|---|---|---|---|
| 140 lbs. | 63.6 | 127 | 191 | 254 | 382 |
| 190 lbs. | 86.4 | 173 | 259 | 346 | 518 |
Divide body weight in pounds (lbs.) by 2.2 to convert body weight to kilograms (kg).
These numbers illustrate why simply saying "take 200 mg of caffeine" does not tell the whole story. For some people, 200 mg may represent a relatively low dose. For others, it may represent a higher dose relative to body size.
Caffeine intake can also add up quickly. A firefighter might have coffee before a shift, an energy drink later in the day, and another caffeinated product during an overnight shift. Based on the type and size of the caffeinated products the total intake can be substantially higher than intended.
Don't forget about caffeine's half-life. Caffeine also stays in the body for several hours. Caffeine’s typical half-life is approximately 4–6 hours, although there is considerable individual variation. For example, if someone consumes 200 mg of caffeine at 5 p.m., approximately 100 mg could still remain in the body around 9–11 p.m.
That matters for firefighters working overnight shifts because caffeine consumed late in the shift may interfere with sleep after returning home. And sleep is an important part of recovery. Therefore, when deciding whether to use caffeine, consider more than just the immediate effects on alertness or performance:
· How much caffeine is in the amount of coffee or energy drink you are consuming?
· How much caffeine have you already consumed, and when did you consume it? How physically demanding is the work, and what are the environmental conditions?
· Are you replacing the fluids you are losing?
· When do actually need caffeine, or do you need to sleep?
The TLDR (Too Long Didn’t Read) Version:
Caffeine is not automatically dehydrating. Research suggests that low-to-moderate caffeine intake does not cause meaningful additional fluid loss at rest and does not appear to impair thermoregulation during exercise in the heat when fluid losses are appropriately managed. [3–10] However, higher doses can have a greater diuretic (fluid) and natriuretic (sodium) effect, and caffeine intake can accumulate throughout the day.
For firefighters, a practical caffeine strategy should be considered:
1. Dose
More caffeine is not necessarily better. Consider caffeine intake relative to body size and individual response.
2. Hydration
Caffeine should not replace a sound hydration strategy. When working in the heat, replace fluids lost through sweating.
3. Total daily intake
Count caffeine from all sources, including coffee, energy drinks, pre-workout products, supplements, and other caffeinated beverages.
4. Individual response.
People respond differently to caffeine. A strategy that works well for one firefighter may not work well for another.
5. Timing
Because caffeine can remain in the body for several hours, late-shift caffeine may interfere with sleep and recovery.
The takeaway is not that firefighters should avoid caffeine. Instead, use caffeine strategically, while keeping hydration, total intake, and sleep in the equation.
References
[1] Rieg T, Steigele H, Schnermann J, et al. Requirement of Intact Adenosine A1 Receptors for the Diuretic and Natriuretic Action of the Methylxanthines Theophylline and Caffeine. The Journal of Pharmacology and Experimental Therapeutics. 2005;313(1):403–409.
[2] Vallon V, Osswald H. Adenosine Receptors and the Kidney. Handb Exp Pharmacol. 2009;(193):443–470.
[3] Ganio MS, Johnson EC, Klau JF, et al. Effect of ambient temperature on caffeine ergogenicity during endurance exercise. Eur J Appl Physiol 2011;111(6):1135–1146.
[4] Naulleau C, Jeker D, Pancrate T, et al. Effect of Pre-Exercise Caffeine Intake on Endurance Performance and Core Temperature Regulation During Exercise in the Heat: A Systematic Review with Meta-Analysis. Sports Med. 2022;52(10):2431–2445.
[5] Millard-Stafford ML, Cureton KJ, Wingo JE, et al. Hydration during Exercise in Warm, Humid Conditions: Effect of a Caffeinated Sports Drink. International Journal of Sport Nutrition and Exercise Metabolism. 2007;17(2):163–177.
[6] Beaumont RE, James LJ. Effect of a moderate caffeine dose on endurance cycle performance and thermoregulation during prolonged exercise in the heat. Journal of Science and Medicine in Sport. 2017;20(11):1024–1028.
[7] Ely BR, Ely MR, Cheuvront SN. Marginal Effects of a Large Caffeine Dose on Heat Balance During Exercise-Heat Stress. International Journal of Sport Nutrition and Exercise Metabolism. 2011;21(1):65–70.
[8] Del Coso J, Estevez E, Mora-Rodriguez R. Caffeine during Exercise in the Heat: Thermoregulation and Fluid-Electrolyte Balance. Medicine & Science in Sports & Exercise. 2009;41(1):164–173.
[9] Seal AD, Bardis CN, Gavrieli A, et al. Coffee with High but Not Low Caffeine Content Augments Fluid and Electrolyte Excretion at Rest. Front Nutr. 2017;4:40.
[10] Killer SC, Blannin AK, Jeukendrup AE. No Evidence of Dehydration with Moderate Daily Coffee Intake: A Counterbalanced Cross-Over Study in a Free-Living Population. PLOS ONE. 2014;9(1):e84154.