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Diabetes patients could face an increased risk of cancer, heart attacks and strokes by taking higher doses of insulin, Welsh experts have warned.

In a report published today in the journal of Diabetes, Obesity and Metabolism, researchers from Cardiff University School of Medicine uncovered the link after analysing thousands of NHS records.

The paper suggests insulin has growth-promoting effects which could be to blame for an increased rate of tumour development and detection.

It also found that higher insulin doses could cause harmful side-effects such as thickening of the arteries and severely irregular heart rhythms.

Researchers identified the trend after scrutinising the medical history of 6,484 patients with type 2 diabetes extracted from the UK Clinical Practice Research Datalink (CPRD).

Patients were on average aged 64 at the beginning of the study in 2000 and were followed for an average of three years from their first exposure to insulin.

Despite not knowing the exact insulin dosage each patient had received during their treatment, researchers calculated insulin dosage per day based on the quantity prescribed and the patients’ body weight.

The study divided the large cohort into four patient groups defined by the dose of insulin received, ranging from below 0.5 units per kilogram of body weight per day to doses exceeding 1.5 units.

Principal Investigator, Professor Craig Currie, from Cardiff University School of Medicine, said: “When compared to patients who received doses of less than 0.5 units, our findings indicate that patients receiving doses of between 1 to 1.5 units and more than 1.5 units were associated with a much greater death rate over time.

“An estimated 15% of patients taking a dose of between 1 and 1.5 units had a 40% increased risk of death compared with the under-0.5 unit group.

“And the 5% of patients receiving more than 1.5 units had a 75% increased risk of death when compared with the same group.”

Professor Currie has warned that the reasons behind the findings are still not entirely clear.

He added: “It’s plausible that patients on higher doses of insulin had pre-existing illnesses before they started on insulin, which could account for the spike in instances of death and cancer.

“Retrospective observational studies can only convey possible associations with adverse events such as these.

“It is also important to note that these findings do not relate to type 1 diabetes. These patients are typically much younger and absolutely require insulin.

“What this study does show is a need for prospective randomised controlled trials to investigate the cause behind these patterns to provide conclusive evidence as to the real effect insulin is having on the thousands of patients who rely on glucose controlling medication.”

Of the 6484 patients whose records were analysed, 1,110 patient deaths were recorded, as were 352 first major cardiovascular events and 382 cases of newly diagnosed cancer.

Insulin is an injectable medication used to lower blood glucose levels in type 1 and type 2 diabetes, taken in tandem with other medications or as a single therapy.

This study looked at patients with type 2 diabetes who administered insulin as their only treatment for the condition.

Simon O’Neill, Diabetes UK’s Director of Health Intelligence, said: “As diabetes is a progressive condition, those on larger doses of insulin may have had the condition for longer and have other underlying health problems that we do not understand and so we would need further, more robust, research before we could be confident that insulin really does increase risk of early death.

“But while the real-world relevance of this study is unclear, what is beyond doubt is that if you have type 2 diabetes then maintaining consistently low blood glucose levels is vital for reducing risk of devastating complications such as blindness, amputation and stroke and, ultimately, early death.

"This is why insulin is an essential treatment and we would stress that is incredibly important that people who have been prescribed insulin by their doctor continue to take it.”