I have been living in poverty for the last year as a college student. There have been points where I could only spend $6 on food a week. When I would have money again, the thing that would rejuvenate me the most is eating globs of fat off of steaks and pork shoulders. I wouldn't even realize what a slump I was in, but in one day of high fat consumption, I would feel a tremendous mental boost in cognition and awareness. Other foods wouldn't cause this. I figured out over time that they key is fat. After that, I have been maximizing my fat intake in order to maintain my brain alert despite eating so little. I live almost exclusively off of pork and rice, and I don't feel like I'm starving. The high protein intake has also helped me maintain my physique, though it continues to deteriorate since I can not workout in such a calorie starved state.
Fat is necessary for proper brain function. Your brain is mostly made of fat. Also, animals store minerals and some vitamins in fat. Eating globs of fat off an animal is actually nutritious. Processed fats have this removed and therefore do not provide the benefits natural fat provides.
I have found that a few beers in me provides a pick-me-up that allows to me to focus on difficult tasks with no impairment. I think 3 beers is the cut off. Coffee makes me feel physically ill. There is a guy named Dave Asprey who claims almost all coffee is sold with mold on it, which causes the sick feeling. He seems like somewhat of a scam artist, but there is some truth to somethings he says.
Salt is another key. You need salt for your brain to function properly. High salt intake does not cause cardiovascular problems. This was essentially just made up. Supposedly sea salt contains trace minerals that are hard to obtain from other sources. I have not researched that one.
Cholesterol intake is essential for testosterone production. Cholesterol is a testosterone precursor and is converted to it in your body. I always maximize my cholesterol intake. Testosterone is essential for brain function. You need a proper hormone balance to function correctly.
Low vitamin d causes depression.
These are the basics. The rest is to avoid poisons.
Avoid eating anything out of cans. They coat the inside of cans with a plastic to keep it from rusting. This leaches into the food, thus poisoning you.
Don't drink out of plastic. Plastic bottles have a hormone, BPA, which leaches into the container. This is poison.
Don't microwave things in plastic containers and eat them. This increases the amount of chemicals leached.
Don't drink anything containing aspartame. This is in most sugar-free products, particularly soda. It is a neurotoxin that is proven the be the culprit behind Gulf War Syndrome.
Avoid drinking tap water. Let it sit for a few days before drinking to let the chemicals in it evaporate out and then filter it to get the metals out. Flouride is a poison they put in the water, along with other chemicals. Besides this, in the US, the infrastructure is poorly maintained, and pipes are all leaking and rusting. They are leaching metals and soil into the water.
Don't eat corn, soy, canola, cottonseed oil, or other gmo crops. GMO crops are not safe. The studies proving its safety are proven to have labeled test subjects that were eating GMOs as not eating GMOS. All the studies proving its safety are funded by Monsanto, its producer. The ones that are not funded by Monsanto say it is not safe. Some of their crops are genetically engineered to produce pesticide in every cell. They claim this pesticide is safe for humans, yet, the pesticide is labeled as toxic on the label if you buy it on its own. Some are herbicide resistant. They are sprayed with herbicide that contains a toxin, dioxin, as a bi-product of manufacture. This chemical is highly toxic and what causes birth defects in Vietnam. The Agent Orange sprayed in Vietnam contained dioxin as a bi-product of manufacture. The company that produces glyphosate, the herbicide the GMOs are resistant to, also produced agent orange and discovered glyphosate in their research. They are chemically related and even contain dioxin as a common bi-product. GMOs also can not be grown where they are and therefore require fertilizer. The soil is devoid of all nutrients but the bare basics needed to make the plants grow. These crops are much less nutritious than crops grown other ways.
Don't eat excessive amount of sugar.
Don't take vaccines containing thiomersal. This is organometallic compound with mercury as component atom. Shooting up mercury is not good.
Do not reuse oil. This changes the molecular structure of the oil into a more toxic oil.
That's all I can think of off the top of my head. I think these guidelines are pretty simple and easy to follow. I also kept this as short as possible and did not include non-health related reasons for abstaining from some of these substances.