Therec are numerous types of sugar. From monsaccharides (simple sugars) to disaccharides (compound sugars). Glucose, a simple sugar, is the primary source of energy in the body and more specifically contributes the bulk of energy to the brain. So a deficiency in glucose levels will have a negative affect of a person mentally as well as emotionally. Excess intake of sugars however result in a plethora of problems ranging from diabetes to heart disease and, as discussed here, fatigue. So balancing bloos sugar levels is important. When you ingest a large amount of sugar your pancreas releases insulin so that sugar can enter your cells which results in a state of hypoglycemia (low blood sugar level), and from here your body releases hormones to rebalance your blood sugar levels. This is actually quite a drastic process.
Interesing point there EscapeArtist. I would categorize these things into the groups fungus, parasites and general bacteria. Parasites are becoming more commonly understood in the West after so long being confined to the domain of third world countries, supposedly. But in actual fact it is the prototypical Western diet that serves to feed parasites the best. And why shouldn't the world's best known survivors find a way to conquer the West anyway? Parasites such as tapeworms and ascaris - there are numerous types of each - release a secretion which causes the host to crave processed foods and sugar. Think of the body as a biome, and parasites endeavour to terraform it's environment so it's more inhabitable. That means higher levels of acidity and lower levels of oxygen (note that co-enzyme Q10 is a potent killer of parasites). All forms of sugar are acidic.
Before I end my post I'd like to talk a little bit about the effect of high sugar level and brain function. Sustained levels of high blood sugar has been linked to memory and learning disorders as well as depression. With regards to memory, high sugar consumption depletes the production of brain derived neurotrophic factor (BDNF) which is important in the formation of new memories and this influences learning. Lower levels of BDNF also leads to insulin resistance and metabolic syndrome. There are a lot of other health problems related to decreased BDNF worth further reading.
