Creatine supplementation is in widespread use to enhance athletic performance, and has also show its effectiveness in the treatment of neurological, neuromuscular and atherosclerotic disease.
When the induced workload in the brain is heavy, the activated brain area may be temporarily fuel-limited. Phosphocreatine levels can decrease acutely with brain activation. These data indicate that in the initial stages of the workload the brain may be temporarily fuel-limited and suggest that brain performance might benefit from an increased supply of fuel. Creatine may similarly boost brain performance due to its role in energy metabolism. Previously, creatine has been shown to be neuroprotective in various neurological conditions. It has been shown before, that brain creatine levels in humans increase in response to mental training  and supplementation with creatine reduces mental fatigue.
Rae, Caroline, et al.  wanted to determine whether oral creatine monohydrate supplementation can increase mental performance. To get maximal increase by supplementation they examined the effect on forty-five young adults with vegetarian diets. However, it is not currently known whether brain creatine levels are lower in vegetarian subjects than omnivores although that certainly seems to be the case. The results of the study showed that oral creatine monohydrate supplementation significantly increased intelligence and improved brain function compared to placebo.
Not all studies are consistent however. A study on twenty-two young adults suggested that 6 week creatine supplementation had no effect cognitive processing. People suffering from creatine deficiency syndrome and its symptoms may result in mental retardation.
Note that creatine supplementation can only increase levels to a saturated value, beyond which excess creatine is excreted.
Rae, Caroline, et al. “Oral creatine monohydrate supplementation improves brain performance: a double–blind, placebo–controlled, cross–over trial.” Proceedings of the Royal Society of London. Series B: Biological Sciences 270.1529 (2003): 2147-2150.
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Rawson, Eric S., et al. “Creatine supplementation does not improve cognitive function in young adults.” Physiology & behavior 95.1 (2008): 130-134.
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