This research is in contrast to the other substances I like, such as piracetam or fish oil. I knew about withdrawal of course, but it was not so bad when I was drinking only tea. And the side-effects like jitteriness are worse on caffeine without tea; I chalk this up to the lack of theanine. (My later experiences with theanine seems to confirm this.) These negative effects mean that caffeine doesn’t satisfy the strictest definition of nootropic (having no negative effects), but is merely a cognitive enhancer (with both benefits & costs). One might wonder why I use caffeine anyway if I am so concerned with mental ability.
Dr. Lisa Mosconi, whose research spans an extraordinary range of specialties including brain science, the microbiome, and nutritional genomics, notes that the dietary needs of the brain are substantially different from those of the other organs, yet few of us have any idea what they might be. Her innovative approach to cognitive health incorporates concepts that most doctors have yet to learn. Busting through advice based on pseudoscience, Dr. Mosconi provides recommendations for a complete food plan, while calling out noteworthy surprises, including why that paleo diet you are following may not be ideal, why avoiding gluten may be a terrible mistake, and how simply getting enough water can dramatically improve alertness. 

Omega-3 fatty acids—DHA in particular—contribute to a healthy brain. “The brain’s membranes use these fats to improve cellular structure and brain signaling, which translates into better cognitive function,” says Vasanti Malik, ScD, a research scientist in the Department of Nutrition at the Harvard T.H. Chan School of Public Health. DHA also quells chronic inflammation that can harm brain cells and lead to cognitive decline.
Choline works best when stacked with nootropics. Stacking choline with a nootropic can also help prevent or reduce side effects. Often, people find that they get headaches when they take nootropics by themselves and that stacking them with choline helps reduce this problem. It is usually suggested to stack nootropics with a choline source, especially if you do not get enough from your diet.
Fitzgerald 2012 and the general absence of successful experiments suggests not, as does the general historic failure of scores of IQ-related interventions in healthy young adults. Of the 10 studies listed in the original section dealing with iodine in children or adults, only 2 show any benefit; in lieu of a meta-analysis, a rule of thumb would be 20%, but both those studies used a package of dozens of nutrients - and not just iodine - so if the responsible substance were randomly picked, that suggests we ought to give it a chance of 20% \times \frac{1}{\text{dozens}} of being iodine! I may be unduly optimistic if I give this as much as 10%.
Amphetamine – systematic reviews and meta-analyses report that low-dose amphetamine improved cognitive functions (e.g., inhibitory control, episodic memory, working memory, and aspects of attention) in healthy people, and in individuals with ADHD.[21][22][23][25] A 2014 systematic review noted that low doses of amphetamine also improved memory consolidation, in turn leading to improved recall of information in non-ADHD youth.[23] It also improved task saliency (motivation to perform a task) and performance on tedious tasks that required a high degree of effort.[22][24][25]