A recent study in the USA suggests links between junk food consumption and negative behaviours

greater consumption of carbonated non-diet soft drinks was associated with an increased likelihood of carrying weapons or being violent towards others.

Solnick SJ and Hemenway D. The ‘Twinkie Defense’: The relationship between carbonated non-diet soft drinks and violence perpetration among Boston high school students. Inj Prev 2011 Oct 24; [e-pub ahead of print]. (http://dx.doi.org/10.1136/injuryprev-2011-040117)

via Does Sugary Soda Make Youth Violent? – Pediatrics and Adolescent Medicine.