Diabetes and mental ability

Monday January 5 2009

“Adults with diabetes may find their mental abilities slowing down soon after the disease appears,” The Times reported. The newspaper said that a study found that people with type 2 diabetes were noticeably worse than healthy adults in their semantic speed (working out meaning) and higher thought processes, such as planning, organising and paying attention to detail. The newspaper added that age did not seem to have an effect on mental deterioration, which suggests that the damage is done early on in the disease and then stabilises.

The story is based on a study of a small group of relatively healthy Canadian adults with mild type 2 diabetes. It found that, compared to healthy people, the patients performed worse in parts of some neuropsychological tests. However, its design means that it cannot prove that diabetes is the cause of the difference in performances. The claim that diabetics are at risk of a progressive mental slow-down are not supported by the results of this study. This question can only be answered by larger, prospective studies, which take into account the range of other factors that might be involved.

Where did the story come from?

Drs Sophie Yeung, Ashley Fischer and Roger Dixon carried out the research. Their work was funded with a grant from the National Institutes of Health. The study was published in the peer-reviewed medical journal Neuropsychology .

What kind of scientific study was this?

The researchers in this cross-sectional study compared how people with and without diabetes of different ages performed on a range of neuropsychological tests. The study compared people between 53 and 70 years of age with 71 to 90-year-olds.

The participants in this study were selected from the first wave of an ongoing larger study – the Victoria Longitudinal Study (VLS). From this group, the researchers selected all 44 people with type 2 diabetes, and a control group of 522 healthy people. The researchers then further excluded anyone who had been diagnosed with Alzheimer’s or vascular dementia, mild to moderate cognitive impairment (scoring less than 26 on the mini mental state examination), neurological conditions including Parkinson’s, cardiovascular disease or psychiatric conditions. This left them with a final sample for analysis of 41 relatively healthy adults with diabetes and 424 controls.

The researchers compared the results from various cognitive and neuropsychological tests between the people with and without diabetes, and then investigated whether age had any effect on the differences. These tests had been performed as part of the first wave of the VLS study, and included memory tests, verbal fluency, and tests of executive function (such as speed of responses, ability to suppress a first response, and inhibiting automatic responses).

What were the results of the study?

The researchers found no difference between the diabetes groups and healthy control groups in terms of episodic memory (assessed by immediate word recall tests). As expected, the younger adults performed better than the older adults.

There were also no differences between the groups in terms of their semantic memory (assessed by tests of vocabulary and fact recall), verbal fluency, reaction time or perceptual speed.

In terms of executive functioning, the controls performed better in two of four tests. The controls also performed better on semantic speed tests, though not on other neurocognitive speed tests. Although the younger groups tended to outperform the older groups, diabetes did not affect different ages in different ways.

What interpretations did the researchers draw from these results?

The researchers conclude that their study has contributed to the literature on deficits associated with mild type 2 diabetes in older adults. They say that their results show that healthy controls “significantly outperformed the diabetes group only on markers of executive functioning and speed”.

What does the NHS Knowledge Service make of this study?

According to the authors, several studies have shown that mental performance is affected by diabetes. However, there is some conflict in the published research about which neuropsychological domains are affected. The purpose of this study was to investigate which domains are involved, and which of them appear to contribute to the difference. There are several points to note when interpreting this study:

  • The researchers used a cross-sectional design to compare performance scores between people with diabetes and those without. However, cross-sectional studies cannot prove causation, and so this study cannot confirm that the difference in mental performance between people with and without diabetes was a result of their condition. It may have been caused by a number of other factors, such as individual cognitive ability at the start of the study, treatment, health related behaviours, or just general health. Other than the participants’ blood pressure, the researchers made no apparent attempt to adjust for (take into account) any other possible factors that could be responsible for the link between diabetes and cognition.
  • The number of people with diabetes in this study was actually quite small. As such, it is possible that the differences seen between these 40 adults and the control group may have arisen by chance alone. The power of the study to detect differences between groups is further diminished in the sub-groups of the different age groups.
  • Although this study found that people with diabetes did not perform any differently to healthy controls in tests of verbal episodic memory and verbal fluency, this conflicts with what other studies have found. The researchers also acknowledge that it may not be possible to generalise their findings to other populations, given that it is volunteer-based and from a small urban Canadian population of relatively healthy, well-educated adults.
  • The study has not examined young people with type 1 diabetes (as may be suggested by the photograph in The Times of a young adult injecting insulin). Therefore young people diagnosed with diabetes should not be concerned that their mental performance will deteriorate.

Overall, there is a need for larger, prospective studies to confirm whether people with diabetes experience a decline in cognitive function and, in particular, which aspects of their function are affected. These studies will also need to take into account other potential confounding factors.

Analysis by Bazian
Edited by NHS Choices