Depression and anxiety are the most common mental health conditions worldwide, making them a leading cause of disability.1 Even beyond diagnosed conditions, subclinical symptoms of depression and anxiety affect the wellbeing and functioning of a large proportion of the population.2 Therefore, new approaches to managing both clinically diagnosed and subclinical depression and anxiety are needed.
In recent years, the relationships between nutrition and mental health have gained considerable interest. Indeed, epidemiological research has observed that adherence to healthy or Mediterranean dietary patterns—high consumption of fruits, vegetables, nuts, and legumes; moderate consumption of poultry, eggs, and dairy products; and only occasional consumption of red meat—is associated with a reduced risk of depression.3 However, the nature of these relations is complicated by the clear potential for reverse causality between diet and mental health (fig 1). For example, alterations in food choices or preferences in response to our temporary psychological state—such as “comfort foods” in times of low mood, or changes in appetite from stress—are common human experiences. In addition, relationships between nutrition and longstanding mental illness are compounded by barriers to maintaining a healthy diet. These barriers disproportionality affect people with mental illness and include the financial and environmental determinants of health, and even the appetite inducing effects of psychiatric medications.Consumption of highly refined carbohydrates can increase the risk of obesity and diabetes.5 Glycaemic index is a relative ranking of carbohydrate in foods according to the speed at which they are digested, absorbed, metabolised, and ultimately affect blood glucose and insulin levels. As well as the physical health risks, diets with a high glycaemic index and load (eg, diets containing high amounts of refined carbohydrates and sugars) may also have a detrimental effect on psychological wellbeing; data from longitudinal research show an association between progressively higher dietary glycaemic index and the incidence of depressive symptoms.6 Clinical studies have also shown potential causal effects of refined carbohydrates on mood; experimental exposure to diets with a high glycaemic load in controlled settings increases depressive symptoms in healthy volunteers, with a moderately large effect.7
Although mood itself can affect our food choices, plausible mechanisms exist by which high consumption of processed carbohydrates could increase the risk of depression and anxiety—for example, through repeated and rapid increases and decreases in blood glucose. Measures of glycaemic index and glycaemic load can be used to estimate glycaemia and insulin demand in healthy individuals after eating.8 Thus, high dietary glycaemic load, and the resultant compensatory responses, could lower plasma glucose to concentrations that trigger the secretion of autonomic counter-regulatory hormones such as cortisol, adrenaline, growth hormone, and glucagon.5 9 The potential effects of this response on mood have been examined in experimental human research of stepped reductions in plasma glucose concentrations conducted under laboratory conditions through glucose perfusion. These findings showed that such counter-regulatory hormones may cause changes in anxiety, irritability, and hunger.10 In addition, observational research has found that recurrent hypoglycaemia (low blood sugar) is associated with mood disorders.9
The hypothesis that repeated and rapid increases and decreases in blood glucose explain how consumption of refined carbohydrate could affect psychological state appears to be a good fit given the relatively fast effect of diets with a high glycaemic index or load on depressive symptoms observed in human studies.7 However, other processes may explain the observed relationships. For instance, diets with a high glycaemic index are a risk factor for diabetes,5 which is often a comorbid condition with depression.4 11 While the main models of disease pathophysiology in diabetes and mental illness are separate, common abnormalities in insulin resistance, brain volume, and neurocognitive performance in both conditions support the hypothesis that these conditions have overlapping pathophysiology.12 Furthermore, the inflammatory response to foods with a high glycaemic index13 raises the possibility that diets with a high glycaemic index are associated with symptoms of depression through the broader connections between mental health and immune activation.