Patients with newly diagnosed type 2 diabetes were significantly more likely to show poor impulse control in psychological testing than healthy people, researchers said.
In the standard Go/NoGo test of impulse control, newly diagnosed diabetics made about 50% more errors of commission than normal controls, regardless of whether they were overweight, Yasuhiko Iwamoto, MD, of Tokyo Women's Medical University in Japan, and colleagues reported in the open-access, online journal BioPsychoSocial Medicine
The differences were not attributable to cognitive impairment, the researchers concluded, because diabetic patients performed as well as controls on the Wisconsin Card Sorting Test of executive function.
- Explain to interested patients that excessive eating and the resultant weight gain greatly increase the risk of type 2 diabetes.
- Explain that this small study suggested that diabetic patients were more likely to have poor impulse control, but the study design did not permit a firm conclusion about causality.
"Our results showed that middle-aged, newly diagnosed, and medication-free patients with type 2 diabetes have a particular neuropsychological deficit in inhibitory control of impulsive response, which is an independent effect of diabetes apart from being overweight," Iwamoto and colleagues wrote.
They suggested the findings could help explain why diabetic patients find it difficult to make the recommended lifestyle adjustments such as avoiding high-fat foods and maintaining daily exercise.
The researchers explained that decision-making about daily activities relies on brain functions in different cerebral regions, mixing predictions of future rewards and punishments, inhibition of impulsive responses, and executive functions.
Overeating, they explained, occurs when the prospect of immediate reward overwhelms inhibitions that derive from awareness of negative consequences.
"In such conditions, rapid reward prediction or impulsive response to environmental stimuli prevails over the preparations by executive function," Iwamoto and colleagues asserted.
Earlier studies had indicated that reward predictions by overweight individuals tend to be higher than those of normal weight people, and their impulse control was generally lower.
Consequently, the Japanese researchers sought to test diabetic patients for performance on psychological tests that measure these functions.
The Go/NoGo test for impulse control involved showing participants one of two letters, N or H, with instructions to press a button when they saw the N but not H. Pressing the button in response to H was an error of commission, and failing to press it when shown the N was an error of omission.
The test also measured reaction times, including slowed responses that sometimes followed errors.
Prediction of future rewards was evaluated with so-called reversal and extinction tasks.
In the former, participants won points for correctly switching images on a computer screen that randomly replaced each other. The extinction task was structured the same way, except that participants stopped winning points for executing the reversal after nine correct responses; at that point, they received points for not responding to the stimulus.
As on a TV game show, correct responses were signaled with a pleasant chime sound, whereas errors were announced with a buzzer.
Participants were also assessed for clinical depression and for standard laboratory measures of glycemia and insulin resistance.
A total of 27 newly diagnosed type 2 diabetic patients and 27 non-diabetic controls participated. All participants in both groups were men, and none of the diabetic patients were taking medications for diabetes. The diabetic group included 16 who were overweight (mean BMI 29.8). There were 11 overweight controls (mean BMI 27.6).
Response inhibition in the Go/NoGo test was significantly decreased in the diabetic patients, the researchers reported. In a combined measure of commission and omission errors, labeled d', diabetic patients had a mean value of 2.55 compared with 3.22 for controls (P=0.001).
The difference was most pronounced for errors of commission, with a mean of 10 for patients versus about 6 for controls (P=0.002).
The researchers found a significant interaction between Go/NoGo performance and glycated hemoglobin levels, with an r2 value of 0.287 for d' versus HbA1c (P=0.024).
Scores did not differ significantly by weight, although there was a trend toward reduced impulse control in overweight participants.
Diabetes did not affect reaction times, overall or after errors, but weight did affect them, with faster reaction times in overweight participants.
Iwamoto and colleagues also found that diabetes status did not affect scores on the reversal and extinction tests. Overweight participants made about 40% more errors on the extinction test compared with normal-weight individuals (P=0.029) but not on the reversal test.
Achievement scores on the Wisconsin Card Sorting Test were similar in all patient groups stratified by weight and diabetes status.
So-called perseverative errors (involving continuous repetition of a response) appeared more common in normal-weight diabetic participants, but rates of these errors varied widely among individuals and the group difference was not statistically significant.
Iwamoto and colleagues cautioned that their cross-sectional data did not allow any conclusions about causality. But that didn't stop them from speculating on a possible causal relationship.
"Our study included only newly diagnosed patients with type 2 diabetes, suggesting the possibility that the neuropsychological deficits in response inhibition may contribute to the behavioral problems leading to chronic lifestyle-related diseases, such as type 2 diabetes," they wrote.
However, they acknowledged that the causal arrow could point in the other direction -- that "metabolic changes with diabetes affect brain functions and cause neuropsychological deficits."
Indeed, the researchers observed, some earlier studies have found that metabolic improvements in diabetic patients lead to improved cognitive performance.
"Further longitudinal studies will be useful to detect progression or improvement of neuropsychological deficits associated with metabolic change," Iwamoto and colleagues wrote.
They also recommended more studies into the potential causal role of impulsivity in development of type 2 diabetes. If confirmed, psychobehavioral interventions aimed at improving impulse control could be beneficial in preventing or treating the disease, they wrote.
Among limitations they identified in the study were the exclusion of women, a lower average BMI than in many studies of obese individuals, and the study's cross-sectional design, involving patients already diagnosed with diabetes.
The study received support from the Japanese Ministry of Health, Labor, and Welfare.
The authors declared they had no competing interests.