What I recall from Lights Out....
Seratonin is released with insulin. Seratonin is responisible for happy moods. This explains why we crave sweets when depressed/stressed.
Problem: with over consumption of carbs over time, insulin receptors burn out (downregulation), which leads to insulin resistance and eventually Type II diabetes. At the same time in the brain, seratonin receptors burn out, thus the brain is incapable of feeling happy (depression).
The way the MD treats diabetes is by giving drugs to either create more insulin to bombard the already burnt out receptors with (metformin) or have the body to create new fat cells to increace the number of receptosr (Glyboride).
The way the MD treats the downregulation of seratonin receptors in the brain is similar - he presribes SSRI's, which increaase the amount of seratonin circulating in the brain by binding to the sites in the synapse where the seratonin reenters the neuron after creating the action potential in the adjoining neuron. This will work for a while, but over time, the receptors will burn out alltogether. That's why when you meet someone who has been on SSRI's for an extended period, it's like their soul is missing - they have lost the capability to really "feel".
The problrem arises when other duties of seratonin become affected - seratonin is a neurotransmitter that plays a role in impulse control. So now you have a population of people who cannot feel joy and have no impulse control - thus the black box warnings for SSRI's and suicidality.
Anyway, there are tons of studies linking diabetes and depression.