People with bipolar disorder are three times more likely than the general population to develop type 2 diabetes. Type 2 diabetes typically occurs in adulthood and is associated with insulin resistance, as opposed to type 1, which is usually diagnosed in childhood and is associated with insulin deficiency.
In a talk at the 2015 meeting of the Society of Biological Psychiatry, researcher Tomas Hajek reported that in a large group of bipolar patients, 13% reported a history of type 2 diabetes, 21% were diagnosed with type 2 diabetes upon laboratory evaluation, and 32.2% had pre-diabetes without realizing it. Thus, about half of these patients with bipolar disorder were either affected by diabetes or at risk for it, many without knowing it.
The Bad News
Diabetes complicates the course of bipolar illness. Type 2 diabetes is associated with poorer response to treatment, atrophy of the hippocampus, cognitive impairment, and higher rates of conversion from mild cognitive impairment to full-blown dementia.
The main effect of type 2 diabetes is insulin resistance. The body produces enough insulin, but insulin’s effects at its receptors are impaired. Diabetes also causes deficits in growth factors, increases in the enzyme GSK3B, decreases in mitochondria and brain-derived neurotrophic factor (BDNF, which protects neurons), and glucose toxicity.
Recent research by Hajek and colleagues shows that diabetes has several other detrimental effects on the brain in bipolar disorder.
On magnetic resonance spectroscopy (MRS) scans, people with type 2 diabetes had lower levels of NAA, a marker of neuronal integrity, in the prefrontal cortex. This can indicate impaired functioning. People with type 2 diabetes also had lower levels of creatine, indicating impaired energy metabolism. In addition, hippocampal volume decreases with aging, and type 2 diabetes accelerated this age-related decline.