This chapter – focussing on adults – concerns the effects on brain and behaviour of deficiencies in vitamins, minerals and macronutrients, which cannot be synthesised in the body. Section 9.2 examines the neurobehavioural consequences of hypovitaminosis (intake below that recommended) and deficiency for each vitamin, including thiamine (Wernicke’s encephalopathy and Korsakoff’s syndrome), NAD (pellagra) and folate (depression). Section 9.3 covers mineral deficiencies, with notable impacts from iodine (hippocampal impairment and links to neurodegeneration in later life), selenium (hippocampal impairment) and zinc (depression). Section 9.4 examines the two essential macronutrient deficiencies. One covers omega-3 and omega-6 fatty acids, with deficiency linked to depression and neurodegeneration. The other covers the essential amino acids and the brain’s unique deficiency detection mechanism. Depression seems to be a common consequence of deficiency, and deficiency in mid-to-later life seems to link to neurodegeneration, but supplementation generally of individual micronutrients has not revealed much benefit in this regard.