The statements made on this page are not to be taken as medical advice, they are a summarisation of various clinical studies. This information has been provided to allow you to make educated decisions regarding your own health. You are encouraged to seek advice from a medical professional before acting on this information.
CaffeineCaffeine is a staple in most pre-workouts because it is the most widely researched stimulant and the results are undeniable. The key benefits of caffeine are increasing wakefulness, attention and metabolic rate (BURN MORE CALORIES!), with the secondary benefit of increased power output when dosed 400-500mg. One study has shown that low or infrequent users of caffeine experienced an 11% increase in maximum power output after an acute dose of 400mg of caffeine, but after 21 days of this supplementation (400mg/day), these benefits reduced by 60% on average!. Although caffeine is often associated with cardiovascular diseases, overdose or infertility in men, most research shows that up to 600mg per day does not increase risk of any of the above. In fact, consumers of 100-400mg caffeine per day tend to be at a lower risk of heart disease. This may also apply to those who already suffer from cardiac arrhythmia; one meta-study shows that 600mg of caffeine per day likely doesn’t encourage recurrence of arrhythmia.
That itchy feeling after you drink a pre-workout, that’s Beta-Alanine. But don’t worry, it’s a harmless side effect of one of the most effective supplements to date. Beta-Alanine’s main purpose is to increase training capacity and endurance, which is achieved by dosing at least 4g per day for an extended period. Little research has focused on the acute effects  .
Unlike most of the common pre-workout ingredients, Citrulline has displayed both acute and ongoing benefits. Acutely, Citrulline has reliably shown to increase number of reps before failure for resistance training and increases the bioavailability of nitric oxide, which is a vasodilator, leading to MASSIVE PUMPS. When dosed regularly (daily for more than two weeks), Citrulline has been shown to decrease subjective fatigue and muscle soreness after exercise; although this effect is less notable with acute dosage, it has been observed. All studies referenced above used 6-8g dose of Citrulline Malate.
Theanine has shown effectiveness at improving the effects of caffeine by reducing anxiety and increasing mental focus. Dosage ranges from 100mg-250mg, all of which show significant effects. Theanine dosed at 200mg shows reliable decrease in subjective anxiety, which may help reduce the anxious feeling when taking high doses of caffeine. On the other hand, theanine seems to improve the cognitive benefits of caffeine but has no effect when supplemented alone.
Like theanine, Taurine has shown anti-anxiety properties when dosed acutely. These studies have only been performed on non-human subjects, although the effects are likely transferrable, making it an excellent supplement to counteract the anxiousness brought on by high doses of caffeine. The human equivalent dose for these studies is approximately 16mg/kg bodyweight. A taurine deficiency seems to lower testosterone in otherwise healthy men and supplementation may increase testosterone production. These studies have mostly been performed on healthy rats. Lastly, an acute dosage of 1660mg seems to increase fat oxidation in well trained cyclists.
Alpha - glycerophosphocholine, or Alpha GPC, is often marketed as a cognitive enhancer, however there are many other benefits to acute and ongoing supplementation. The most notable of which being increased power output. Although there has been only one study on human subjects, the results appear quite reliable and worthy of replication. The study determined that with a single dose of 600mg Alpha GPC, 40 minutes before exercise increased peak force on bench press by 14% on average . Alpha GPC also appears to significantly effect the amount of bioavailable growth hormone. One study compared 600mg dose Alpha GPC with a placebo group , each group underwent resistance training and experienced a spike in growth hormone. The placebo group experienced an average 260% increase, while the supplemented group experienced a 4400% increase. A second study observed the effect without exercise and observed a 300-400% increase for the supplemented group after 60 minutes, with no change for the placebo group. It appears that the acute effects of Alpha GPC on cognition are limited in application to fitness, although one study showed that supplemented subjects were significantly more cognitive during strenuous exercise than the placebo group. 
ALCAR has various benefits that are useful in sports supplementation. The most notable acute benefits are an increased rate of fat oxidation  and increased nitric oxide presence, making it an excellent pump and fat burning supplement with doses between 2-3g. But it is best not to over supplement this as exceeding 3g may hinder power output! On going supplementation of ALCAR at low doses (250mg) can increase nitric oxide presence in the same way as acute dosing, while larger ongoing does (2-4g) appears to significantly improve attention deficit disorders such as, ADHD, depression and chronic fatigue.
English Walnut Extract
There is little research for this highly effective stimulant. Its synthetic counterpart (DMHA or Octodrine) has been banned in sport by the World Anti-Doping Agency due to its highly psychoactive nature and the lack of knowledge regarding side effects. English Walnut Extract tends to be dosed 150-250mg in pre-workouts to good effect, although there are little to no studies to quantitively measure its optimum effective dose.
Theobromine is chemically very similar to caffeine, hence it is likely to give similar benefits when supplemented. Little research has been focused on using theobromine to improve exercise performance, but one study noted that caffeine and theobromine produced the same improvements compared to placebo group when dosed equally. This study also noted that the benefits for theobromine are most evident in later stages of testing/exercise, while caffeine improves early performance.
It is also interesting to note that supplementing theobromine alone may cause increased anxiety and lowered blood pressure. But combining with caffeine leads to an improved self-assessed contentedness and alertness, while having no effect on blood pressure.
Black Pepper Extract
This ingredient has appeared in many supplements in recent years due to its possible ability to improve fat metabolism, which has been reliably demonstrated in mice. Black Pepper extract has also been shown to significantly increase the bioavailablity of other orally ingested ingredients, thereby improving their effectiveness.
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