So here goes.
Basically your brain needs glucose as fuel to function, which the body sources from carbohydrate, and doesn't much care what sort except that it be regular and continuous.
However, if you rely on simple carbohydrates like a bar of chocolate or a packet of crisps, to the exclusion of much else, then you get a carbohydrate surge which induces an insulin surge, which can then cause the sort of mood swings that make concentration difficult. Or you get a headache. Or an energy slump. All of which is much more noticeable in children. Complex carbohydrates - oatmeal, wholegrains, bananas, pulses and beans, nuts, brown rice - are a better option, as they also contains lots of other beneficial nutrients.
So to keep the brain concentrating, complex carbohydrates and adequate hydration (because the brain is 90% water and is happiest well-watered) is a good place to start.
After complex carbohydrates and water, here are my other recommendations.
Eggs, for example, which contain the micronutrient choline (an essential precursor for acetylcholine, an important neurotransmitter in the brain) in the yolks, and also found in soybeans, lentils, oats, sesame seeds and other sources. One of the benefits of combining protein like an egg, with complex carbohydrate like wholegrain bread, is that this also helps manage insulin surges.
You've heard it before, but antioxidants help mop up free radicals, the damaging by-product of cellular activity in our bodies. Fresh fruit and vegetables - lots of them, the more brightly coloured the higher they are in antioxidants. So this is why blueberries, pomegranates, sweet potato, beetroot, spinach, etc. sometimes get dubbed 'superfoods' because they are high in antioxidants. Aim for a rainbow diet. And green tea, also high in antioxidants - my daily favourite is Mighty Leaf's green tea with jasmine.
And because I also believe in a little of what you fancy doing you good... I'm pleased to report that dark chocolate is high in antioxidants. My treat is Green & Blacks organic dark chocolate with ginger... who knew something so delicious could also be good for you.
Essential fatty acids
From oily fish, with the recommendation that you eat at least two portions a week. Essential fatty acids (EFAs) are just that, essential and you can't manufacture them yourself. What you're aiming for is omega-3 EPA and omega-3 DHA. Forget omega-6, from vegetable sources, the majority of Western diets are supersaturated with omega-6 already, and it competes with omega-3 for absorption.
This is the most widely consumed pharmacologically active substance in the world - trimethylxanthine, to give it its chemical name - but in small doses can give you a short-term kick helping your brain to focus. However, it will also stimulate your central nervous system and in excess will affect your sleep patterns, give you a racing heart and possibly palpitations, and it's addictive. Make caffeine your friend and not your enemy. Drink freshly brewed coffee and limit it to a couple of cups a day at the most, and avoid drinks like Red Bull like the plague.
I'm not big on supplements, which should always be used to supplement a good diet rather than compensate for a poor one. Eating a balanced and varied diet, with lots of the above, should help ensure all the nutrients and micronutrients you ordinarily need, and forestall the necessity for masses of supplements. However, there are times when supplementation might be a useful option. Omega-3 EFAs are one nutrient that can be hard to get enough of even from a good diet, but choose your omega-3 supplements with care because there are lots out there that actually deliver very little active ingredient. Vitamin B can sometimes be a little deficient, especially as we age and our ability to absorb it from our diet deteriorates. Dietary iron is sometimes deficient in women of reproductive age, giving rise to sub-clinical iron deficiency anaemia which can cause symptoms of fatigue, muddled thinking and forgetfulness - and short-term supplementation to improve haemoglobin levels can be helpful.
And your most important meal of the day? Breakfast. You've heard that before, too - but there's no doubt about it, you'll concentrate better if you eat breakfast.