Past research studies have attempted to probe the association between diabetes and Parkinson’s disease (PD), with varying conclusions. However, a new study published online in the journal Neurology recently set out to clarify the relationship between diabetes and PD.

Glucose, insulin, and diabetes

Glucose is a simple sugar that is a building block of most dietary carbohydrates and is the body’s major source of energy. Insulin is a chemical produced by the pancreas that regulates glucose in the body. Among other roles, it allows cells to take in glucose from the blood.
In type I diabetes, the body is unable to produce insulin. Before insulin was available as an injectable medication, patients with type I diabetes would essentially die of starvation, unable to utilize the glucose in their blood to produce energy. In type II diabetes, the body produces insulin, but the cells can’t use it effectively and blood glucose levels remain higher that they should be. This physiologic state is known as insulin resistance. When blood glucose levels are chronically too high, small blood vessels in various organs can be harmed. This can result in damage to the kidneys or the eyes, for example.

The brain and diabetes

A wealth of literature has developed over the past number of years studying the effects of diabetes on the brain. Certainly, high glucose levels can damage blood vessels in the brain and thereby increase the risk of stroke. But its effects are more widely felt than that. High glucose and insulin resistance affect many neuronal processes and contribute to inflammation in the brain. Type II diabetes also appears to increase the risk of Alzheimer’s disease and other dementias.

A new look at the link between Parkinson’s disease and diabetes

In the newly published study, the research team analyzed data of hospital admissions and identified a large group of patients who were admitted to the hospital with a diagnosis of type II diabetes, as well as another large group of patients who were admitted to the hospital without this diagnosis. Any patients in either group with a diagnosis of PD at or before their earliest admission date were not included in the trial. The research team followed each group over time and recorded how many in each group subsequently developed PD.

What the study found

The group that had type II diabetes had an approximately 30% higher chance of developing PD than those who did not. Expressed in a different way: in the study under discussion, for every 10 patients without type II diabetes who developed PD, approximately 13 patients with type II diabetes developed PD.
An additional finding of the study was that the younger the patients with type II diabetes, the higher their risk of developing PD. For the very youngest group, between the ages of 25-44, the risk of developing PD was close to four times those who did not have diabetes. The risk of developing PD was also further increased if the diabetes was characterized as complicated, defined as having caused damage to the eyes, kidneys or peripheral nerves.
To be clear, the vast majority of people with PD do not have type II diabetes and vice versa, but close epidemiologic studies of large numbers of people have identified this small increase in risk that could help us understand and possibly even better treat PD.

Why might a link exist between Parkinson’s disease and diabetes?

The study team hypothesized a few reasons why there might be an increased risk of PD in those with type II diabetes. There might be genetic abnormalities shared by the two diseases that may predispose this population to both. Type II diabetes may not have any direct link to PD, but when the two coexist, they may create a more hostile environment in the brain, thereby accelerating the neurodegenerative processes that are underway in PD.
Most intriguing is the possibility that PD and type II diabetes share common abnormal cellular processes in the brain. For example, new evidence demonstrates that insulin signaling may be abnormal in PD and neurodegenerative diseases in general. In addition, dysfunction of the mitochondria (the cell’s energy producing center), oxidative damage, and inflammatory responses, may all play a role in both diseases. The relationship between type II diabetes and PD is further dissected in this interesting article.

What about using medications for diabetes to help Parkinson’s disease?

If this is the case, and PD and type II diabetes share similar abnormalities, could medications used for diabetes have positive effects in PD? Researchers have been looking into this. A trial in PD patients using pioglitazone, a diabetes medication that increases the body’s sensitivity to insulin, was published in 2015, but showed negative results. Exenatide, a glucagon-like peptide-1 (GLP-1) agonist which enhances secretion of insulin, showed promise as a neuroprotective agent (a medicine that can protect nerves from dying) in studies done in cell culture and animals. A small double blinded, placebo controlled study of exenatide in 62 patients published in October 2017, showed positive results. Larger trials of this class of medication are necessary, but this data is very encouraging and suggests that manipulating insulin biology can have positive effects on PD.

Tips and take-aways

• There is an association between diabetes and slightly increased risk of Parkinson’s disease.
• A diabetes medication exenatide was recently shown in a small study to improve motor function in PD. Stay tuned for larger studies of this medication.
• Whether or not you have PD, if you have diabetes it is important to aim to keep your blood glucose levels within the normal range to optimize brain health.