Brain's main energy source is glucose. It uses about 20% of total glucose . Brain hypoglycemia causes depressive-like behaviors in mice through adrenergic pathways .
When it comes to humans, here is a study that claims low glucose leads to increased aggression in married couples (see this too):
Self-control requires energy, part of which is provided by glucose. For 21 days, glucose levels were measured in 107 married couples. To measure aggressive impulses, each evening participants stuck between 0 and 51 pins into a voodoo doll that represented their spouse, depending how angry they were with their spouse. ... As expected, the lower the level of glucose in the blood, the greater number of pins participants stuck into the voodoo doll, and the higher intensity and longer duration of noise participants set for their spouse .
However, the conclusion is disputed:
Bushman et al.'s study does not demonstrate that fluctuations in blood glucose affect individuals' self-control abilities. As an important consequence, there is no reason to assume that giving couples a sugary “boost to their self-control energy” (p. 3) will reduce intimate partner violence. Because the glucose model of self-control lacks empirical foundation, it does not qualify as a framework for scientifically based intervention strategies .
What is sure, is that hypoglycemia activates sympathetic nervous system:
... the neurogenic symptoms of hypoglycemia are largely the result of sympathetic neural, rather than adrenomedullary, activation .
Hypoglycemia increases plasma levels of both epinephrine and norepinephrine. These catechols are released primarily from the adrenal medulla. However, it is well documented that hypoglycemic increases muscle sympathetic nerve activity, and that both alpha and beta adrenergic activity increase .
And this leads to behavioral changes (at least in animals):
Noradrenaline is involved in many different functions, which all are known to affect behaviour profoundly. ... Part of these effects may arise in indirect ways that are by no means specific to aggressive behaviour, however, they are functionally relevant to it. Other effects may affect brain mechanisms specifically involved in aggression. Hormonal catecholamines (adrenaline and noradrenaline) appear to be involved in metabolic preparations for the prospective fight; the sympathetic system ensures appropriate cardiovascular reaction, while the CNS noradrenergic system prepares the animal for the prospective fight. ... It appears that neurons bearing postsynaptic alpha2-adrenoceptors are responsible for the start and maintenance of aggression, while a situation-dependent fine-tuning is realised through neurons equipped with beta-adrenoceptors .
- Wikipedia contributors, "Human brain," Wikipedia, The Free Encyclopedia, http://en.wikipedia.org/w/index.php?title=Human_brain&oldid=615456836 (accessed July 6, 2014).
- Park MJ, Yoo SW, Choe BS, Dantzer R, Freund GG. Acute hypoglycemia causes depressive-like behaviors in mice. Metab. Clin. Exp. 2012 Feb;61(2):229-36. doi: 10.1016/j.metabol.2011.06.013. PubMed PMID: 21820138.
- Bushman BJ, Dewall CN, Pond RS, Hanus MD. Low glucose relates to greater aggression in married couples. Proc. Natl. Acad. Sci. U.S.A. 2014 Apr 29;111(17):6254-7. doi: 10.1073/pnas.1400619111. PubMed PMID: 24733932.
- Lange F and Kurzban R (2014) Sugar levels relate to aggression in couples without supporting the glucose model of self-control. Front. Psychol. 5:572. doi: 10.3389/fpsyg.2014.00572
- DeRosa MA, Cryer PE. Hypoglycemia and the sympathoadrenal system: neurogenic symptoms are largely the result of sympathetic neural, rather than adrenomedullary, activation. Am. J. Physiol. Endocrinol. Metab. 2004 Jul;287(1):E32-41. doi: 10.1152/ajpendo.00539.2003. PubMed PMID: 14970007.
- Hoffman RP. Sympathetic mechanisms of hypoglycemic counterregulation. Curr Diabetes Rev. 2007 Aug;3(3):185-93. PubMed PMID: 18220670.
- Haller J, Makara GB, Kruk MR. Catecholaminergic involvement in the control of aggression: hormones, the peripheral sympathetic, and central noradrenergic systems. Neurosci Biobehav Rev. 1998;22(1):85-97. PubMed PMID: 9491941.