Yummy!

The term 'smart nutrients' usually denotes particular vitamins or amino acids which fulfill certain neurochemically critical functions, such as the formation of neurotransmitters. Supplements, therefore, enhance cognitive abilities by making the environment more conducive to a higher rate of metabolic activity / regrowth / maintenance.

Antioxidants There are several vitamins that are classified as antioxidants, namely vitamin E, C and A (beta-carotene). Reported benefits from antioxidants are anti-aging effects and protection from damage to brain cells. Note that Vitamin A is dangerously toxic when overdosed, so taking beta-carotene (which the body turns into vitamin A) instead is recommended. Pregnant women should not supplement with vitamin A (nor beta-carotene) unless prescribed by a doctor. All of these can be purchased conveniently at a local pharmacist.

The glaring irony here is the fact that vitamin A, which is classified as an antioxidant (its main function being the renewal of the human metabolism) is also known to be toxic when taken directly. An analogy to Frank Herbert's Dune may well be appropriate, since the mysterious Water of Life on the planet Arrakis (where most of the plot of the first book of the series takes place) possesses quite similar properties. The Water of Life extends consciousness. However, since it places an immense strain on the nervous system as the brain tries to cope with an unprecedented amount and variety of multidimensional sensory input, all who have tasted it have died. All, that is, except the first book's protagonist Paul Atreides. In conclusion, both the Water of Life and vitamin A are screamingly analogous to Derrida's pharmakon.

Download a film clip of Paul Atreides witnessing the extraction of the Water of Life from a sandworm, taken from David Lynch's movie version of Frank Herbert's Dune.

B vitamins There are twelve different B vitamins, of which four have been shown to have positive effects on the nervous system. Thiamine (vitamin B1) is also considered an antioxidant.

Niacin (B3) is effective in improving memory and combating stress. Doses exceeding 50 mg may cause flushing i.e. a feeling of blood rushing to the head. Some people have also reported itching as a side-effect. These effects disappear after continued use.
Pyridoxine (B6) facilitates the formation of neurotransmitters and as such is vital for proper functioning of the brain. It has also been shown to increase life span and to decrease stress. Recommended dosage is 50-100 mg daily. Doses over 125 mg can be neurotoxic over longer periods of time and should be avoided. B6 is the only vitamin among these four B vitamins that has been shown to be toxic.
Cyanocobalamin (B12) is useful when fighting fatigue as it helps the release of energy from food. It has also been shown to facilitate learning in laboratory animals. All of these vitamins are best taken as a B-complex, so that supplementing one type of vitamin B will not cause a deficiency of another type.

Choline and Lecithin Lecithin and choline are precursors of asetylcholine which aids the transmission of electric impulses in the brain. They have been shown to improve short term memory. Both nutrients offer basically the same benefits, although choline is more efficient in that a smaller dose of choline is equivalent to a larger dose of lecithin as far as the desired effects are concerned. Both should be taken with a large amounts of B5 to help the conversion to asetylcholine, however. Choline can cause a fishy odor that can be mildly unpleasant; in large doses it may also cause diarrhea, at which point dosages should immediately be decreased. Both can be bought at health food stores as liquid or as capsules, and should be stored in the fridge in an airtight container.

Amino acids Although the use of amino acids is usually associated with body building, some of the aminos are also acknowledged to be smart nutrients. They are essential in the formation of neurotransmitters in the brain and low levels of amino acids in one's diet can result in fatigue and lack of concentration. Supplementing them should be a priority especially if one is a vegetarian on a low fat diet. Phenylalanine is the most common of brain boosting amino acids; its effects include combating stress, elevating one's mood and increasing alertness. A commonly used combination is a compound called DL-Phenylalaninen taken daily at the dose of 1000 to 1500 mg, followed by 50 mg of vitamin B5 and half a gram of vitamin C. Phenylalanine is best taken on an empty stomach, because it competes with other proteins to cross the blood-brain barrier. However, phenylalanine is known to produce undesirable and often dangerous effects when taken in conjuction with psychotic or MAO-inhibitor drugs.

There are also several other amino acids, such as Glutamine, Arginine, Tryptophan and Taurine, but the safety and utility of these variants have not been securely established. Only future research will show their viability as safe nootropics, and until then, most people will prefer more widely researched and tested supplements.

Even so, allegedly beneficial vitamins and amino acids which later turn out to be toxic (consider vitamin A and vitamin B6, to name two extreme examples) may not necessarily constitute ideal venues for cognitive enhancement.

Would smart drugs, with their carefully engineered chemical structures, be safer in that the disadvantages brought about by the existence of a certain molecule could be countered by the addition of an antagonistic (or in the very least, countereffective) molecule?

Or should one prefer natural herbs, since they have achieved homeostasis by definition? No extreme effects would be brought about by the consumption of a substance which, previously, constituted an organism sporting a functional and balanced metabolism in and of itself. Correct?

Too many questions, too little time.