Caffeine facts vs. caffeine fiction: revealing the truths

Caffeine is one of the most effective and most commonly used ergogenic aids in sport. It’s also one of the most effective and most commonly used ergogenic aids in life! As a result, there is a lot of research on the topic, but there are also a lot of opinions about caffeine usage, sometimes founded on evidence and sometimes not. This often leads to considerable confusion.

New research

To cut through this confusion, a team of US researchers has recently carried out a thorough and extensive review of all of the existing caffeine research⁽¹⁾. In this study, published in the Journal of the International Society of Sport Nutrition, the researchers have answered the most commonly asked questions about caffeine by looking at what the complete body of data says on this topic. One question at a time, let’s take a look at what they had to say.

Question #1: Does caffeine dehydrate you at rest?

A common public perception is that caffeine intake will cause dehydration. Unfortunately, this widespread belief is based on a limited number of older research studies. Based upon more recent research, it appears that the caffeine has a very small and likely insignificant impact on hydration at dosages consumed by most humans. Now, if you decide to take extremely high dosages of caffeine, somewhere in excess of 6 mg/kg or 500 mg in total, there is the potential for increased urine output. However, even in this case, if caffeine usage is balanced out by fluid intake, this is unlikely to present a problem.

Question #2: Does caffeine dehydrate you during exercise?

Multiple studies have demonstrated that there appear to be no differences in hydration when the same exercise is performed with or without caffeine. There are many factors that influence hydration status during exercise, including individual differences in sweat rates. More than anything else, it’s fluid intake that determines hydration levels. These factors appear to be much more relevant than caffeine intake. If you consume caffeine as part of a drink, or with fluids, it’s very unlikely to cause dehydration.

Question #3: Does caffeine promote the loss of body fat?

Caffeine has been shown increase the degree of fat oxidation during exercise and at rest. However, that means a larger percentage of calories are burned from fat, not necessarily that more calories are burned. When it comes to weight loss, multiple studies have shown that caffeine can lead to more weight loss. However, these studies tend to use high dosages of caffeine in overweight populations, and often contain other thermogenic compounds (supplements that speed up calorie burning). In other populations, the impact is less clear. Regardless of whether caffeine directly inhibits fat loss, for those who are dieting, caffeine can potentially combat the lethargy and changes in alertness associated with calorie deprivation.

Question #4: Does habitual caffeine consumption influence the performance response to acute caffeine supplementation?

It is common practice by athletes to abstain from caffeine usage prior to major competitions in order to experience the full effects of caffeine use. While this practice seems to make intuitive sense, the evidence from research seems to invalidate it because caffeine habituation does not appear to have any impact on the effectiveness of caffeine. And even if caffeine habituation does have an impact on performance, it can be overcome by simply consuming more caffeine than normal!

Question #5: Does caffeine affect upper vs. lower body performance/strength differently?

There is evidence that caffeine improves both upper body and lower body performance. However, there are multiple studies and reviews showing that upper but not lower body strength is improved by caffeine ingestion - as well as studies and reviews showing lower but not upper body strength is improved. It may therefore depend more on the individual in question and the dosage use. As there has been benefit demonstrated for both, assume that caffeine works for both then try to see what works best for you.

Question #6: Is there a relationship between caffeine and depression?

The relationship between caffeine and depression is nuanced. Caffeine intake has been associated with improvements in mood and alertness, which can positively impact depression, and there is some evidence that regular intake lowers the risk of depression. However, caffeine can also lead to increased anxiety and disruption of sleep, both of which can have a negative impact on depression. Whether caffeine has a positive or negative impact will likely come down to the interaction between the dosage used, together with the tolerance and response to caffeine of the individual in question.

Question #7: Can too much caffeine kill you?

Yes - however, almost anything can kill you! It is almost impossible to die from a caffeine overdose when a healthy person consumes it in the form of naturally occurring beverages and teas. However, if consumed in the form of pill or powders, much higher quantities can be consumed, potentially leading to adverse outcomes.

Question #8: Are there sex differences regarding caffeine’s effects?

Caffeine metabolism is affected by elevations in estradiol as well as contraceptive use. However, the acute physiological response to caffeine intake does not appear to differ between males and females in most research. While there is the occasional study that suggests a potentially more favorable response in males, the overall response seems to be similar in both sexes, and both sexes can expect to improve their performance with caffeine.

Question #9: Does caffeine work for everyone? 

Research suggests that caffeine does enhance performance in all individuals. However, there is a large amount of inter-individuality as to how significant this effect is. This is likely due to genetic differences in caffeine metabolism, although known caffeine metabolism genes cannot explain these differences. Different dosages for different individuals may be required for optimal effectiveness. If you have one poor experience with caffeine, it may be a one-off event, or the dosage may need to be adjusted.