The role of tryptophan metabolism and food craving in the relationship between obesity and bipolar disorder

Abstract

Background & aims: Individuals with bipolar disorder (BD) have a significantly increased risk of obesity-related conditions. The imbalance between food intake and energy expenditure is assumed to be a major risk factor for obesity in BD. This study analyzed food craving in relation to anthropometric, metabolic, and neurobiological parameters in a well-characterized cohort of euthymic individuals with BD.

Methods: One-hundred-thirty-five patients completed the Food-Craving Inventory assessing four categories of food craving (fat, fast-food, sweets and carbohydrate craving). Additionally, clinical, metabolic and anthropometric parameters were assessed.

Results: Higher levels of fat craving were observed in males, versus females, with BD. High levels of carbohydrate craving positively correlated with kynurenine and the kynurenine-to-tryptophan ratio. Higher serum nitrite and neopterin levels were related to fat craving. Parameters of fat metabolism (triglycerides, high-density lipoprotein) were associated with fat and fast-food craving. Anthropometric measures of obesity (e.g. body mass index, waist-to-hip-ratio) were not related to food craving.

Conclusions: Overweight/obese individuals with BD show an increased driving of tryptophan down the kynurenine pathways, as indicated by an increase in the serum kynurenine-to-tryptophan ratio. The driving of tryptophan down the kynurenine pathway is mediated by immune-inflammatory activity and stress. The correlation of increased kynurenine with food craving, especially carbohydrate craving, probably indicates a regulatory deficit in the maintenance of chronic inflammatory processes in obesity and BD. Food craving seems to be of clinical importance in the treatment of metabolic disturbances in BD, although not associated with anthropometric measures of obesity. Rather, food craving correlates with blood metabolic parameters and an increased activation of the kynurenine pathway, both of which are linked to higher affective symptomatology and the development of cardiovascular diseases.

Keywords: Bipolar disorder; Food craving; Obesity; Overweight; Tryptophan breakdown.