The Effects of Blood Glucose Levels on Driving Behaviour and Executive Functioning in Young People with Type 1 Diabetes

Dr Joanna M. McClintock, Dr Robert B. Isler,Dr Nicola J. Starkey,

This study received Ethical Approval from the Health and Disability Ethics Committee (Ref:
This study was made possible by grants from Grassroots Trust and the Waikato Medical Research


Hyperglycaemia (high blood glucose levels) has been demonstrated to impact negatively on executive functioning skills, however, the impact on everyday risky activities such as driving is unknown. Hazard perception is an executive functioning skill recognized as being a predictor of vehicle crashes and driving safety (Isler & Starkey, 2011).

Type 1 diabetes is an autoimmune condition resulting in the body’s inability to produce insulin, a hormone that breaks down glucose in the blood. Without insulin, the body is not able to metabolise glucose for energy which results in high levels of glucose in the blood, which is known as hyperglycaemia (International Diabetes Federation, 2011). For people with diabetes who experience hyperglycaemia, there is likely not enough insulin in their system to transfer the blood glucose to the brain for energy; thus their ability to utilise executive functioning skills becomes impaired.

Executive functions are a set of cognitive skills involving the ability to control and planfully apply one’s own mental skills (Anderson, 2008; Luria, 1973). They also include the ability to sustain or flexibly redirect attention, the inhibition of inappropriate behavioural or emotional responses, the planning of strategies for future behaviour, the initiation and execution of these strategies, and the ability to flexibly switch among problem-solving strategies (Martin, 2006; Weinberger et al., 2005). Driving is an everyday activity that relies heavily on higher-level cognitive skills, particularly those of executive function, including judgment, attention, planning, anticipating consequences, and hazard perception (Kurzthaler et al., 2005, Isler & Starkey, 2011).

The New Zealand Transport Authority (2013) and the American Diabetes Association (2014) acknowledge that metabolic control impacts on the cognitive functions associated with driving. The majority of research examining driving safety and diabetes has focussed on the impacts of hypoglycaemia (e.g., Cox, Gonder-Frederick, & Clarke, 1993; Cox, Gonder-Frederick, Kovatchev, Julian, & Clarke, 2000; Quillian, Cox, Gonder-Frederick, Driesen, & Clarke, 1994).

Hyperglycaemia is recognised by the American Diabetes Association (2014) to impact on driving abilities; however, there is not enough research to justify guidelines around driving and hyperglycaemia. The New Zealand Transport Association’s advice that “you shouldn’t drive if you are severely hyperglycaemic”, however, do not provide any specifics as to a definition of hyperglycaemic. Studies that have looked at hyperglycaemia and driving have primarily used adult populations and have often included participants with type 2 diabetes (Cox, Ford, Ritterband, Singh, & Gonder-Frederick, 2011; Sommerfield, Deary, & Frier, 2004). These studies found that hyperglycaemia detrimentally impacts on the cognitive skills involved in driving. Although we can learn from these studies the impact of diabetes is recognised to be cumulative. Therefore, the longer you have diabetes the higher the risk of complications, including cardiovascular, retinopathy, and neuropathy which will all impact on driving safety (DCCT Research Group, 1993). In comparison, young people are novice drivers, are at much less risk of micro and macro vascular diabetes-related complications and therefore may provide a clearer picture of the impacts of hyperglycaemia (without diabetesrelated complications) on executive functioning and driving safety.

The aim of this project was to investigate the effect of hyperglycaemia (high blood glucose levels) on executive functioning, and higher-level driving skills in young people with Type 1 diabetes. We hypothesised that acute hyperglycaemia would have a negative effect on performance on tests of executive functions and safe driving skills in young people with Type 1 diabetes compared to performance when blood glucose levels were within the target range.


Of 119 potential patients with type 1 diabetes of the Waikato Regional Diabetes Service 14 young people participated in the study. Each participant attended two assessment sessions (approximately 2 hours long). One assessment session was when the participant had a blood glucose level between 4 mmol/L – 9.4 mmol/L (euglycaemic) and the second when the participant had a blood glucose level above 15 mmol/L (hyperglycaemic). As can be seen in Table 1, the majority of participants were male. Half of the participants had their full licence and the majority had not had a car crash nor had they received warnings or convictions for their driving. The mean HBa1c suggest that participants overall metabolic control was suboptimal, with only 2 participants having and Hba1c of less than 58 mmol/mol.

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