Type 2 Diabetes Is Linked to Increased Risk for Parkinson's Disease

News Author: Caroline Cassels
CME Author: Désirée Lie, MD, MSEd

Posted: 3/28/2007

March 28, 2007 -- Individuals with type 2 diabetes have an increased risk for Parkinson's disease (PD) as they age, a study reports.

Gang Hu, MD, PhD, and colleagues from the National Public Health Institute in Helsinki, Finland, found that individuals who developed type 2 diabetes had an 83% increased risk for PD compared with the general population.

Although a few cross-sectional and 2 case-control studies have examined the prevalence of diabetes among patients with PD, this is the first time a large prospective study has shown that type 2 diabetes is a risk factor for PD.

"Although the association between type 2 diabetes and PD is not as great as that of coronary heart disease or stroke, this finding is still clinically important and something clinicians should pay attention to," Dr. Hu told Medscape.

The study appears in the April 2007 issue of Diabetes Care.

18-Year Follow-Up

The prospective population-based study observed 51,552 Finnish men and women aged 25 to 74 years for a mean follow-up period of 18 years. None of the participants had a history of PD at baseline. However, 591 men and 507 women had a history of type 2 diabetes at study outset.

Compared with subjects without type 2 diabetes, individuals with diabetes were older, had higher body mass index (BMI) and systolic blood pressure (BP), had lower levels of total serum cholesterol, were less likely to smoke or drink alcohol, and were more likely to be physically inactive.

During the follow-up period, 324 men and 309 women developed incident PD. The average age of subjects at diagnosis was 64.3 years for men and 65.8 years for women.

Table. Age- and Study-Year-Adjusted Hazard Ratios of Incident PD in Subjects With Type 2 Diabetes*

	Hazard Ratio	95% CI
Men	1.78	1.01 - 3.12
Women	1.90	1.04 - 3.52
Overall	1.83	1.21 - 2.76

*PD indicates Parkinson's disease; CI, confidence interval.
Source: Diabetes Care. 2007;30:842-847.

Possible Mechanisms

Further adjustment for BMI, systolic BP, total cholesterol, education, leisure-time physical activity, smoking, alcohol consumption, and coffee and tea consumption affected the results only slightly, the authors report.

Although the mechanism of the association between type 2 diabetes and PD is poorly understood, there are several potential possibilities, Dr. Hu noted.

For instance, according to the study, some animal and in vitro research suggests insulin may play a role in regulation of brain dopanergic activity. In addition, Dr. Hu pointed out that several lifestyle factors are associated with the risk for both type 2 diabetes and PD.

However, at this point, it is unclear whether addressing type 2 diabetes risk factors would reduce PD risk. For instance, while cigarette smoking increases the risk for type 2 diabetes, some studies have suggested there is an inverse relationship between cigarette smoking and PD risk. Caffeine also is known to reduce PD risk.

Dr. Hu also pointed out that while the Health Professionals Follow-up Study found increased physical activity was associated with a reduced risk for PD in men, this association was not found among women participating in the Nurses' Health Study.

More Research Needed

In addition, although no association between total alcohol consumption and the risk for PD was found in either the Health Professionals Follow-up Study or the Nurses' Health Study, the combined analysis of these 2 pieces of research showed beer drinkers had a 30% lower incidence of PD than nonbeer drinkers.

However, Dr. Hu noted that in the current study individuals with type 2 diabetes were less likely to drink alcohol or smoke and were less physically active. In addition, there were no differences in coffee consumption between those with and without diabetes.

It is also possible, he added, that diabetes may increase PD risk partly through excess body weight.

Because PD mainly affects older individuals, said Dr. Hu, it is possible that lifestyle or other possible factors, such as insulin dysregulation, may need a longer period to show their effects on the dopaminergic system.

However, more research is needed to elucidate the biological mechanisms behind this association, Dr. Hu added.

The authors have disclosed no relevant financial relationships.

Diabetes Care. 2007;30:842-847.

Clinical Context

Type 2 diabetes is associated with an excess risk for coronary artery disease and is an independent risk factor for other complications such as neurodegenerative diseases (eg, neuropathy, stroke, dementia, and Alzheimer's disease), but little is known about the association of diabetes with PD. Some studies have reported that up to half of patients with PD have abnormal glucose tolerance.

This is a cross-sectional study that is part of a longitudinal, population-based, cohort study in Finnish men and women to examine the association between type 2 diabetes and incident PD.

Study Highlights

6 independent cross-sectional studies were carried out in 5 geographic areas of Finland in a randomly selected sample consisting of 6.6% of the population during 6 different years.
The sample was stratified by age, area, sex, and age group.
The total sample size of the 6 surveys was 53,166, and participation rate varied from 74% to 88% by year.
Assessment of type 2 diabetes was based on self-reporting and 2 nationwide registers.
World Health Organization criteria were used to define diabetes, which was a diagnosis made by clinicians.
Those who reported diabetes at baseline or a hospital discharge diagnosis of diabetes were classified as having a history of diabetes at baseline.
The diagnosis of PD was based on the National Social Insurance Institution's special reimbursement for PD drugs, as all patients with this diagnosis were eligible for free drugs.
The diagnosis was based on clinician history and physical examination with agreement by 2 independent specialist clinicians.
Follow-up of each cohort member continued until the date of diagnosis of PD, death, or December 2002.
A self-administered questionnaire, which asked about education, coffee intake, alcohol, smoking, physical activity, and tea intake, was sent to participants to complete at home.
At study sites, anthropometric measurements were made, and BP was recorded. Venous blood was drawn for cholesterol readings.
Mean age was 44 to 53 years, mean BMI was 26.3 to 27.6 kg/m², mean BP was 143/88 mm Hg, and serum total cholesterol was 6.2 mmol/L.
At baseline, 591 men and 517 women were diagnosed as having a history of type 2 diabetes.
Those with type 2 diabetes were older, had higher BMIs and BPs, had lower levels of serum total cholesterol, and were less often smokers and alcohol drinkers.
During a mean follow-up of 18 years, 324 men and 309 women were diagnosed with incident PD.
The average ages of diagnosis of PD were 64.3 years in men and 65.8 years in women.
Age- and study-year-adjusted hazard ratios of incident PD for individuals with vs without diabetes were 1.80 for men, 1.93 for women, and 1.85 for men and women combined.
Further adjustment for sex, BMI, BP, cholesterol, physical activity, and education did not significantly change the results.
After exclusion of participants diagnosed with PD, during the first 5 years of follow-up, the hazard ratio for all participants was 1.88 for PD.
The association between type 2 diabetes and PD was independent of smoking status, coffee and alcohol consumption, and body weight.
The association was present in participants aged 24 to 54 years and in smokers.
The authors noted that the mechanism for this association is currently unknown.

Pearls for Practice

Type 2 diabetes is associated with increased risk for incident PD.
The association between type 2 diabetes and PD is independent of sex, smoking, alcohol and coffee intake, and body weight.