Caffeine is the most widely used stimulant in the world and is present in a variety of products including beverages such as coffee, tea, colas, energy drinks, food products containing chocolate, some energy bars, and over-the-counter medications sold to increase alertness. Dietary supplements may contain herbal sources of caffeine such as guarana, kola nut, Yerba mate, green or black tea, and cacao pods used to make chocolate products. A cup (eight ounces) of coffee contains approximately 100 mg of caffeine, while colas contain about half of that amount. Following consumption, caffeine concentrations peak after about one hour, although this can be shortened if caffeine is ingested by chewing gum, since it will be absorbed in the mouth.1 Caffeine can affect multiple biological processes but most notable are its effects on alertness, vigilance, and physical performance. Caffeine blocks receptors in areas of the brain that control arousal and motivation.
Dose Range and Upper Limit
Food and Nutrition Board DRI:
RDA/AI: Not relevant.
Upper Limit: Not relevant.
Doses Used In Randomized Clinical Trials: Multiple 200 mg doses during the late evening, overnight, and early morning periods totaling 800 mg per day have been administered during trials involving three days of sleep loss.2 Typical single acute doses to improve cognitive function approximate 200 mg,3 whereas two to six mg/kg body weight doses are used to enhance physical performance.4,5
Toxicology Data: The acute lethal dose in adult humans has been estimated to be 10 grams6; however there has been at least one report of death following intake of lesser amounts (i.e., 6.5 g) of caffeine.7
Benefits and Risks
Evaluation of Potential Benefits
Moderate daily consumption of 400 mg has been deemed safe for healthy adults and non-pregnant/non-lactating women.8 Caffeine in moderate doses up to 200 mg has improved cognitive performance in rested, sleep-deprived, and fatigued individuals.3 Doses from two to six mg/kg of body weight are associated with improved physical performance5,7; a smaller dose typically will exert a longer-lasting and larger effect for individuals who do not normally consume caffeine.9 Smaller doses can also exert a positive effect on physical performance when consumed during exercise10 and/or for re-dosing prior to a subsequent physical challenge.11 Also, caffeine may help maintain a positive subjective experience and lower perceived exertion during extended exercise.12
Potential Detrimental Effects on…
Military Performance: Higher doses of caffeine may induce mild tremor that could negatively impact marksmanship. Caffeine consumption has a mild increased diuretic effect but no long-term impact on hydration or work performance in the heat.13,14 A large caffeine dose of nine mg/kg given to non-heat-acclimated and non-habitual caffeine users during exercise in a hot, dry environment resulted in only minor shifts in heat balance.15
Doses of caffeine over 600 mg/day can cause significant side effects including tachycardia, tremors, insomnia, nervousness, GI upset, chest pain, and arrhythmias.7
Higher doses may also produce gastrointestinal distress. However, no negative effects have been reported in sleep-deprived and fatigued individuals following moderate caffeine consumption; rather, improved speed of target acquisition has been noted.16,17
Military Survivability: No data found.
Other Health Risks
High caffeine intake is not recommended during pregnancy or periods of lactation, and it may interfere with calcium absorption. Caffeine can also increase resting heart rates and blood pressure, which might impact long-term cardiovascular health.
Interactions with Medications or Other Bioactive Substances
Some studies have found that combining caffeine with alcohol can have detrimental effects on mood and cognitive performance and also can diminish the subjective signs of alcohol intoxication.18-20 The FDA has issued a warning that the caffeine added to malt alcoholic beverages was an “unsafe food additive.”21 Several states have banned the sale of energy drinks.
Caffeine is often found in energy drinks containing other products such as taurine (see monograph on energy drinks in this series) and in some multi-ingredient weight-loss supplements. Caution is advised since the impact of these other ingredients on the effects of caffeine has not been systematically studied.
Headache, irritability, and mood swings can accompany sudden withdrawal from regular caffeine use in about half of the population. These effects usually subside in a couple of days and may be alleviated if caffeine use is reduced gradually rather than abruptly.3
Concern and Benefit Estimate (see Dietary Supplement Risk Matrix)
Benefit potential: Moderate
Safety concern: Low
Numeric score: 4
Moderate daily consumption of up to 400 mg of caffeine has been deemed safe for healthy adults and non-pregnant/non-lactating women.8 Caffeine in moderate doses up to 300 mg can improve cognitive performance in rested, sleep-deprived, and fatigued individuals.3 As caffeine is present in a variety of foods and supplements, military personnel may want to monitor their daily intakes.
- Kamimori GH, Karyekar CS, Otterstetter R, Cox DS, et al. The rate of absorption and relative bioavailability of caffeine administered in chewing gum versus capsules to normal healthy volunteers. Int. J. Pharm. 2002;234(1-2):159-67.
- McLellan TM, Kamimori GH, Voss DM, Tate C, et al. Caffeine Effects on Physical and Cognitive Performance During Sustained Operations. Aviat. Space Environ. Med. 2007;78(9):871-7.
- Lieberman HR, Tharion WJ, Shukitt-Hale B, Speckman KL, et al. Effects of caffeine, sleep loss, and stress on cognitive performance and mood during U.S. Navy SEAL training. Psychopharmacology. 2002;164(3):250-61.
- Goldstein ER, Ziegenfuss T, Kalman D, Kreider R, et al. International society of sports nutrition position stand: caffeine and performance. Journal of the International Society of Sports Nutrition. 2010;7(5):15.
- Jenkins NT, Trilk JL, Singhal A, OConnor PJ, et al. Ergogenic Effects of Low Doses of Caffeine on Cycling Performance. Int. J. Sport Nutr. Exerc. Metab. 2008;18(3):328-42.
- Gilman A, Goodman LS, eds. The Pharmacological Basis of Therapeutics: Fifth Edition. New York, NY: Macmillan Publishing; 1975.
- Jellin J, Gregory, PJ, eds. Caffeine. Natural Medicines Comprehensive Database 2011; http://www.naturaldatabase.com.
- Nawrot P, Jordan S, Eastwood J, Rotstein J, et al. Effects of caffeine on human health. Food Additives and Contaminants Part A - Chemistry Analysis Control Exposure & Risk Assessment. 2003;20(1):1-30.
- Bell DG, McLellan TM. Exercise endurance 1, 3, and 6 h after caffeine ingestion in caffeine users and nonusers. J. Appl. Physiol. 2002;93(4):1227-34.
- Cox GR, Desbrow B, Montgomery PG, Anderson ME, et al. Effect of different protocols of caffeine intake on metabolism and endurance performance. J. Appl. Physiol. 2002;93(3):990-9.
- Bell DG, McLellan TM. Effect of Repeated Caffeine Ingestion on Repeated Exhaustive Exercise Endurance. Medicine & Science in Sports & Exercise. 2003;35(8):1348-54.
- Backhouse SH, Biddle SJH, Bishop NC, Williams C. Caffeine ingestion, affect and perceived exertion during prolonged cycling. Appetite. 2011;57(1):247-52.
- Armstrong LE. Caffeine, Body Fluid-Electrolyte Balance, and Exercise Performance. Int. J. Sport Nutr. Exerc. Metab. 2002;12(2):189-206.
- Roti MW, Casa DJ, Pumerantz AC, Watson G, et al. Thermoregulatory Responses to Exercise in the Heat: Chronic Caffeine Intake Has No Effect. Aviat. Space Environ. Med. 2006;77(2):124-9.
- Ely BR, Ely MR, Cheuvront SN. Marginal Effects of a Large Caffeine Dose on Heat Balance During Exercise-Heat Stress. Int. J. Sport Nutr. Exerc. Metab. 2011;21(1):65-70.
- Tharion WJ, Shukitt-Hale B, Lieberman HR. Caffeine Effects on Marksmanship During High-Stress Military Training with 72 Hour Sleep Deprivation. Aviation, Space, and Environmental Medicine. 2003;74(4):309-14.
- Tikuisis P, Keefe AA, McLellan TM, Kamimori G. Caffeine Restores Engagement Speed But Not Shooting Precision Following 22 h of Active Wakefulness. Aviat. Space Environ. Med. 2005;76(1):771-6.
- Curry K, Stasio MJ. The effects of energy drinks alone and with alcohol on neuropsychological functioning. Human Psychopharmacology - Clinical and Experimental. 2009;24(6):473-81.
- Oteri A, Salvo F, Caputi AP, Calapai G. Intake of Energy Drinks in Association With Alcoholic Beverages in a Cohort of Students of the School of Medicine of the University of Messina. Alcoholism - Clinical and Experimental Research. 2007;31(10):1677-80.
- Ferreira SE, de Mello MT, Pompeia S, de Souza-Formigoni MLO. Effects of Energy Drink Ingestion on Alcohol Intoxication. Alcoholism - Clinical and Experimental Research. 2006;30(4):598-605.
- U.S. Food and Drug Administration (FDA). FDA Warning Letters issued to four makers of caffeinated alcoholic beverages: U.S. Food and Drug Administration 2010.