People with diabetes are about twice as likely to develop clinical depression, but the relationship also runs in the opposite direction: Palmer notes that people with depression have roughly a 60% higher risk of subsequently developing diabetes. Rather than viewing this simply as the psychological burden of living with chronic disease, he points to shared biological factors—including inflammation, elevated cortisol, insulin resistance, and impaired glucose regulation—that may help explain why the two conditions so frequently occur together.

The relationship can also become self-reinforcing. Depression is associated with poorer glucose control and greater insulin resistance in people with diabetes, while depression in diabetic patients tends to persist substantially longer. Palmer argues that treatment should therefore consider the metabolic and psychiatric conditions together. Alongside conventional care, he highlights exercise and dietary interventions, including low-carbohydrate and ketogenic diets, which can dramatically improve insulin resistance and diabetes and may also improve mood and energy.

https://www.psychologytoday.com/us/blog/advancing-psychiatry/201902/diabetes-and-depression-which-comes-first