New analyses of both observational and genetic data have provided “convincing evidence” that type 2 diabetes is associated with an increased risk for Parkinson’s disease.
Dr. Alistair Noyce
“The fact that we see the same effects in both types of analysis separately makes it more likely that these results are real – that type 2 diabetes really is a driver of Parkinson’s disease risk,” Alastair Noyce, PhD, senior author of the new studies, said in an interview.
The two analyses are reported in one paper published online March 8 in the journal Movement Disorders.
Dr. Noyce, clinical senior lecturer in the preventive neurology unit at the Wolfson Institute of Preventive Medicine, Queen Mary University of London, explained that his group is interested in risk factors for Parkinson’s disease, particularly those relevant at the population level and which might be modifiable.
“Several studies have looked at diabetes as a risk factor for Parkinson’s but very few have focused on type 2 diabetes, and, as this is such a growing health issue, we wanted to look at that in more detail,” he said.
The researchers performed two different analyses: a meta-analysis of observational studies investigating an association between type 2 diabetes and Parkinson’s; and a separate Mendelian randomization analysis of genetic data on the two conditions.
They found similar results in both studies, with the observational data suggesting type 2 diabetes was associated with a 21% increased risk for Parkinson’s disease and the genetic data suggesting an 8% increased risk. There were also hints that type 2 diabetes might also be associated with faster progression of Parkinson’s symptoms.
“I don’t think type 2 diabetes is a major cause of Parkinson’s, but it probably makes some contribution and may increase the risk of a more aggressive form of the condition,” Dr. Noyce said.
“I would say the increased risk of Parkinson’s disease attributable to type 2 diabetes may be similar to that of head injury or pesticide exposure, but it is important, as type 2 diabetes is very prevalent and is increasing,” he added. “As we see the growth in type 2 diabetes, this could lead to a later increase in Parkinson’s, which is already one of the fastest-growing diseases worldwide.”
For the meta-analysis of observational data, the researchers included nine studies that investigated preceding type 2 diabetes specifically and its effect on the risk for Parkinson’s disease and progression.
The pooled effect estimates showed that type 2 diabetes was associated with an increased risk for Parkinson’s disease (odds ratio, 1.21; 95% confidence interval, 1.07-1.36), and there was some evidence that type 2 diabetes was associated with faster progression of motor symptoms (standardized mean difference [SMD], 0.55) and cognitive decline (SMD, −0.92).
The observational meta-analysis included seven cohort studies and two case-control studies, and these different types of studies showed different results in regard to the association between diabetes and Parkinson’s. While the cohort studies showed a detrimental effect of diabetes on Parkinson’s risk (OR, 1.29), the case-control studies suggested protective effect (OR, 0.51).
Addressing this, Dr. Noyce noted that the case-control studies may be less reliable as they suffered more from survivor bias. “Diabetes may cause deaths in mid-life before people go on to develop Parkinson’s, and this would cause a protective effect to be seen, but we believe this to be a spurious result. Cohort studies are generally more reliable and are less susceptible to survivor bias,” he said.
For the genetic analysis, the researchers combined results from two large publicly available genome-wide association studies – one for type 2 diabetes and one for Parkinson’s disease to assess whether individuals with a genetic tendency to type 2 diabetes had a higher risk of developing Parkinson’s.
Results showed an increased risk for Parkinson’s in those individuals with genetic variants associated with type 2 diabetes, with an odds ratio of 1.08 (P = .010). There was also some evidence of an effect on motor progression (OR, 1.10; P = .032) but not on cognitive progression.
On the possible mechanism behind this observation, Dr. Noyce noted type 2 diabetes and Parkinson’s have some similarities in biology, including abnormal protein aggregation.
In the study, the authors also suggest that circulating insulin may have a neuroprotective role, whereas systemic and local insulin resistance can influence pathways known to be important in Parkinson’s pathogenesis, including those that relate to mitochondrial dysfunction, neuroinflammation, synaptic plasticity, and mitochondrial dysfunction.
Dr. Noyce further pointed out that several drugs used for the treatment of type 2 diabetes have been repurposed as possible treatments for Parkinson’s disease and are now being tested for this new indication. “Our results support that approach and raise the idea that some of these drugs may even prevent Parkinson’s in people at risk,” he said.
Most people who have type 2 diabetes won’t get Parkinson’s disease, he added. Other outcomes such as heart disease, kidney disease, and microvascular complications are far more likely, and the main aim of preventing and treating type 2 diabetes is to prevent these far more common outcomes. “But our data suggests that this could also have a possible benefit in reducing future Parkinson’s risk,” he said.
Not on the horizon at present is the possibility of screening patients with type 2 diabetes for signs of early Parkinson’s, Dr. Noyce said.
“There isn’t a test for identifying presymptomatic neurodegenerative diseases such as Parkinson’s yet, but perhaps in the future there will be, and type 2 diabetes may be one risk factor to take into account when considering such screening,” he added.
This work was financially supported by grants from The Michael J. Fox Foundation; the Canadian Consortium on Neurodegeneration in Aging (CCNA); the Canada First Research Excellence Fund (CFREF), awarded to McGill University for the Healthy Brains for Healthy Lives (HBHL) initiative; and Parkinson Canada, and the Intramural Research Program of the NIH, National Institute on Aging.
Dr. Noyce reports grants from the Barts Charity, Parkinson’s UK, Aligning Science Across Parkinson’s and Michael J. Fox Foundation, and the Virginia Keiley Benefaction; and personal fees/honoraria from Britannia, BIAL, AbbVie, Global Kinetics Corporation, Profile, Biogen, Roche, and UCB outside of the submitted work.
A version of this article first appeared on Medscape.com.