Exploring the Impact of Micronutrients on Internalizing Symptoms

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It is estimated that at least 30% of the global human population is deficient in essential micronutrients such as vitamins and minerals (Ramakrishnan, 2002). Adequate nutrition represents an important aetiological component of many physical illnesses. The dietary intake of micronutrients has been shown to prevent serious physical illness, for instance, cardiovascular disease and diabetes (Park & Lee, 2011). To date however, the role of micronutrient deficiency in heightening the risk of developing mental disorders still requires confirmation. According to epidemiological research, the regular consumption of unhealthy foods, such as junk foods or highly processed and refined foods, is increasing, particularly in young populations (Adair & Popkin, 2005). The diets of young adults, particularly university students living away from home, has been shown to be of poorer nutritional quality compared to those living with their family (Larson et al., 2011, Papadaki et al., 2007). Micronutrient deficiencies are a common feature of 'Western' diets (i.e. high intakes of processed and refined foods) in comparison to more 'traditional' dietary patterns, such as those consisting of higher consumptions of whole and natural foods (Carrera-Bastos, Fontes, O'Keefe, Lindeberg, & Cordain, 2011). Recent research has shown that healthy dietary patterns are associated with a lower incidence of internalising symptoms (Jacka et al., 2010, 2011a, 2011b, 2013a, 2013b, O'Neil et al., 2014, Sanchez-Villegas et al., 2009, Sanhueza, Ryan, & Foxcroft, 2013). For instance, Jacka et al. (2010) found that Western diets were associated with increased risk of developing depressive disorders, whereas more traditional dietary patterns are associated with a lower incidence of depression in adult participants. There is evidence to suggest that the dietary intake of specific micronutrients, such as zinc, magnesium, and B-group vitamins, are inversely associated with depressive symptoms (Jacka et al., 2012, Maes et al., 1997, Maserejian et al., 2012, Murakami et al., 2008, 2010, Roozbeh et al., 2011, Stanis¿awska et al., 2013, Sanchez-Villegas et al., 2009, Yary & Aazami, 2011). However, what specific micronutrient deficiencies are associated with an increased risk of anxiety and depressive disorders respectively still remains relatively unclear. Several psychobiological theories have been proposed to explain the biochemical abnormalities evident in internalising disorders, such as depression and anxiety. For instance, inflammation and heightened immune function have been identified as psychophysiological features of depressive disorders (Dale et al., 2015, Maes, 1993, 2011, Maes et al., 1992, 1996, 1997, Postolache, 2012, Serafini, 2012). Similarly, dysregulation of neurotransmitters, such as serotonin and GABA, has been observed in both depressive and anxiety disorders (Baumgartner et al., 2012, Guilarte, 1993, Kruiemann et al., 1987). Additionally, specific micronutrients have been shown to be essential for the regulation of immune function (Szewczyk, Kubera, & Nowak, 2011), effective inflammatory processes (Fraker & King, 2004, Szewczyk et al., 2011) , and neurotransmitter synthesis (Georgieff, 2007). Thus it is possible that micronutrients may play a role in the aetiology of mental disorders. Given that most mental disorders begin before adulthood, further exploration of the role of micronutrients in psychopathology, particularly in early life, is important as these may be used to identify potential individuals at risk and to modify risk. In this review, I aim to critically evaluate the existing observational and experimental literature exploring the role of micronutrients in the development and maintenance of internalising disorders in children, adolescents, and adults.