Highly concentrated sports boosters contain up to 500 mg, 10 g are considered fatal for humans . Caffeine and theine are the same molecule, yet theine is considered healthier. Until 2004, caffeine was on the Olympic Committee's doping list, but no longer is.
The stimulant caffeine was first isolated from coffee beans in 1819. We've been talking about it for 200 years, without a satisfactory result.
Does caffeine really bring athletes higher, faster and further? How much does it take to crash?
Booster before training, protein shake after training
Most boosters contain caffeine, often several 100 mg. While strong black coffee ends up at about 80 mg per cup. Pre-workout caffeine is supposed to provide the last bit more focus and energy. So if you train in the evening or very early, it is going to get you out of fatigue and into using your full potential.
The result: caffeine increases alertness, blood pressure, breathing, heart rate, and metabolism. Sounds good?
Let me explain why caffeine still should be part of your average booster.
You would never guess how caffeine works!
The whole mechanism starts with energy. Your energy.
ATP is the energy molecule of the cell. It stores pure energy that your body uses to perform all its functions. This concentrated power comes from the foods you consume daily. At the end of a long series of reactions, ATP is formed, containing all the energy of glucose.
The body forms this molecule in all cells and can exchange it for performance. Whenever it is consumed in a chemical reaction, ADP remains. Or adenosine diphosphate.
So if we consume our energy currency, another molecule remains. The more of it is formed, the more we have achieved. Imagine, every time you spend money, you get a voucher in return, which certifies that you have spent money. This voucher would be the ADP.
If you accumulate a lot of vouchers, you'll see how much you’ve spent.
AND…You stop spending so much. Without the metaphor: You have consumed a lot of energy and get more tired the more ADP you form.
Receptors are responsible for this. ADP binds to them and causes fatigue at the end of yet another signaling chain reaction. Like this, the body can regenerate after strenuous tasks.
Now caffeine comes into play.
It resembles ADP in its chemical structure. Since only certain molecules bind to receptors, caffeine tricks your body by posing as adenosine and blocking its receptors. ADP can't bind and we don't get tired. But only in low concentrations of caffeine.
How does caffeine work at different levels of stress?
Endurance sports - aerobic
It has already been proven in many studies that caffeine can increase performance in aerobic endurance sports . The improvement of the mental parameter alertness also plays an important role here.
The blood circulation, which is important for a good performance in oxygen-requiring endurance sports, is rather restricted by caffeine. In a Swiss study  it was shown that 200 mg caffeine (two cups of coffee) reduce the increase in blood flow to the cardiovascular system which is heightened by exercise. This limits physical performance in endurance sports.
On the negative side, diureses (up to 30% higher urine volume) after caffeine intake partially cause dehydration and increased excretion of minerals such as sodium . Since the loss of water and minerals in endurance sports is one of the top problems, caffeine can exacerbate an existing problem.
Particularly in endurance sports, the intake of dangerously high amounts of caffeine is more frequent, which is why the substance was temporarily (albeit only in very high amounts) on the doping list.
Intensive strength and team sports - anaerobic
Studies indicate an increase in strength in bodybuilding and intensive cycling and running through caffeine. This improvement was measured in trained and untrained individuals using more than 5 mg caffeine per kg body weight. However, the tested individuals were not used to caffeine. Lower doses or use on caffeine tolerant subjects were not effective .
Professional footballers of the Brazilian Football Association were tested during intensive interval training. The test group was given 5 mg caffeine per kg body weight, the control group lactose for comparison. After warming up for 20 minutes and doing an interval training for 45 minutes, blood was taken.
After training with caffeine, the adrenaline level was increased and more white blood cells were detected in the blood circulation. This in combination is equivalent to an inflammatory reaction. The scientists suspected that more small muscle injuries occurred in the caffeine group and therefore the concentration of blood cells and platelets increased. Even during normal physical activity, this increase is 35%, compared to a reported increase in white blood cell of another 38%  when taking caffeine.
For hobby athletes, the negative consequences of high caffeine consumption outweigh the positive results of increased performance .
Three problems stand out in particular
Habituation and Dependence
Regular caffeine consumption results in getting used to the stimulation. More adenosine receptors are formed, so that the effect of caffeine is weakened. The only effect that often remains is the anti-sleep effect. Discontinuation of caffeine over one month ends the tolerance .
Individuals used to caffeine stimulation are no longer stimulated as strongly as non-habitual caffeine consumers . This would also reduce a positive effect on athletic performance over time.
Stopping regular caffeine consumption leads to physical withdrawal, which is characterised by restlessness, insomnia, headaches, vision or hearing disorders or cardiac arrhythmia.
If caffeine is taken above the individual tolerance range, circulatory problems can result from a reduced supply of glucose to the brain . The heart rate can increase sharply and, in the worst case, fainting or anxiety can occur.
After strong excitation, an energy low often follows, when the effective time span has been exceeded. A stimulation by caffeine lasts 5-6 hours, the half-life is 2-3 hours. So that you would have to replenish regularly to stay in the caffeine high.
Many people wish for a permanent stimulation, which would be predatory on the body, as it needs alternating phases of activity and relaxation.
Take home message
Instead of a continuous stimulation with a subsequent low or withdrawal symptoms, a healthy energy metabolism should be promoted by sufficient nutrient supply and regeneration.
You don’t have to get rid of your daily coffee. But pure, concentrated coffein in sports boosters can pose a health risk. Physically and mentally.
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 Robert M. Julien: Drogen und Psychopharmaka. Titel der Originalausgabe A primer of drug action – a concise and nontechnical guide to the actions, uses, and side effects of psychoactive drugs. Urban & Fischer Verlag, 2002, ISBN 3-437-21706-2.
 Caffeine Decreases Exercise-Induced Myocardial Flow Reserve, Mehdi Namdar, Pascal Koepfli, Renate Grathwohl, Patrick T. Siegrist, Michael Klainguti, Tiziano Schepis, Raphael Delaloye, Christophe A. Wyss, Samuel P. Fleischmann, Oliver Gaemperli, Philipp A. Kaufmann
Journal of the American College of Cardiology Jan 2006, 47 (2) 405-410; DOI: 10.1016/j.jacc.2005.08.064
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Effect of caffeine supplementation on haematological and biochemical variables in elite soccer players under physical stress conditions
British Journal of Sports Medicine 2007;41:523-530.
 Grgic J, Grgic I, Pickering C, et al Wake up and smell the coffee: caffeine supplementation and exercise performance—an umbrella review of 21 published meta-analyses. British Journal of Sports Medicine Published Online First: 29 March 2019. doi:10.1136/bjsports-2018-100278
 Zwyghuizen-Doorenbos A, et al. Effects of caffeine on alertness. Psychopharmacology (Berl). (1990)