A recent study from the Netherlands shows a temporal link
By Amelia Williams 
The rate of depression in people living with type 2 diabetes is almost double that of the general population. Living with both conditions places a significant emotional and practical burden on those affected. It also has negative impacts on overall health and quality of life, including reduced self-care, suboptimal glucose levels and increased risk for diabetes-related complications. So, it is vital to establish the reason for this link to determine potential prevention and treatment options.
One proposed hypothesis is that biological mechanisms may be responsible for the development of depression in people with type 2 diabetes. Specifically, that hyperglycaemia (high glucose levels) and insulin resistance, which are key features of type 2 diabetes, may cause biological changes in the brain that lead to depression. NB. Insulin resistance is when cells in the muscles, fat and liver don’t respond well to insulin, which makes it harder to take up glucose from the bloodstream. Prior research has shown that associations between these factors do exist. However, most of this research has assessed this relationship at only one time point, and with individuals who already had depressive symptoms. This means that it is not appropriate to conclude that hyperglycaemia and insulin resistance cause depression.
A recent study  published in Diabetologia used data from the population-based Maastricht Study and has been able to provide some important insights on this topic. The aim of the study was to investigate whether hyperglycaemia and insulin resistance were associated with the development of depression over time. Markers of hyperglycaemia, insulin resistance and depressive symptoms were assessed at baseline in participants who did not have pre-existing depressive symptoms. Following this, assessment of depressive symptoms was completed annually over four years.
The study found that:
- individuals with hyperglycaemia were more likely to develop depression at follow-up
- higher levels of hyperglycaemia further increased the risk of depression
- insulin resistance was associated with depression among individuals who also had cardiovascular risk factors
Overall, this study provides evidence that hyperglycaemia is involved in the development of depression over time in people with type 2 diabetes. This is a valuable finding, as it shows that reducing hyperglycaemia may be a viable option for preventing and treating depression in this population. However, additional research is needed to confirm these findings. While the role of insulin resistance in the development of depression remains unclear, the findings of this study point to potential avenues for future research to determine the role of cardiovascular risk factors in this relationship.
If you woud like to read more about depression and diabetes, check out our other blogs  on this topic.
If you would like to know more about managing diabetes and depression, check out this NDSS factsheet ‘Diabetes and depression’: https://www.ndss.com.au/wp-content/uploads/fact-sheets/fact-sheet-diabetes-and-depression.pdf 
Geraets  AFJ, Köhler  S, Muzambi  R, et al. The association of hyperglycaemia and insulin resistance with incident depressive symptoms over 4 years of follow-up: The Maastricht Study.  Diabetologia. 2020 Nov;63(11):2315-2328. doi:10.1007/s00125-020-05247-9