Diet can have a profound effect on brain health. One example of this is epilepsy. It turns out that fasting can reduce epileptic seizures- in fact this was originally documented by the Greek physician Hippocrates, in the 5th century BC- but obviously that's not a viable long-term treatment, since eventually you have to eat or you starve. However, it's possible to mimic the state of fasting if you cut your carbohydrate consumption- the body burns fat, instead of sugar, just as it does in a fast. Using low carbohydrate diets- either a fat-heavy ketogenic diet or the induction phase of the Atkins diet- it's possible to reduce seizure frequency in most people with epilepsy. Often it's effective where drugs fail, and a small percentage of people- around 15% actually see seizures eliminated, sometimes permanently, even after they discontinue the diet. In other words, in a small number of patients, diet can actually cure a severe neurological disease like epilepsy.
A few years ago some psychiatrists speculated that it might work for bipolar disorder as well. The thing is, drugs that work for epilepsy also work for bipolar, suggesting they are somehow related. This was purely speculation at the time, but there are now a couple of documented cases of people suffering from bipolar who have been successfully treated with low-carbohydrate diets- and they claim it works better than the drugs.
The implications are profound. Some psychiatric and neurological disorders may in fact be metabolic disorders, perhaps in part caused by diet. There's been a big push in the past few decades to focus on DNA as the answer to everything, but there's a huge environmental component to these disorders. Twin studies show that if one twin has epilepsy, the odds of the other getting it are only around 50%. So even with identical DNA, and being raised in a similar environment, they only have about a fifty-fifty chance of getting the disorder... clearly genetics aren't destiny. What we really need is a better understanding of the environmental effects that cause one person to get a neurological disease, while the another stays healthy. Throwing drugs with severe side effects at people after they get sick is a good business model for pharmaceutical manufacturers, but what we really need to do is prevent people from getting these disorders in the first place.
Last, the observation that low-carbohydrate diets can be effective in treating severe neurological and psyhicatric diseases... well, it has disturbing implications for modern, high-carbohydrate diets.