What does your HbA1c tell you about your risk of diabetes and ageing?

HbA1c is a measure of your average blood sugar over the past two to three months. A value of 5.7% or higher in children and young adults (ages 10-19) increases the likelihood of developing type 2 diabetes later in life by four to seven times compared with peers who have an HbA1c of 5.3% or lower. Notably, HbA1c predicted that risk just as well as a fasting blood glucose measurement or an oral glucose tolerance test.

Once you have diabetes, what matters is not only how high your HbA1c is, but also how stable it remains over time. Large fluctuations are associated with a 26 to 53% higher risk of kidney function loss, regardless of the average value. Stable blood sugar control is therefore at least as important as achieving a good average value. In addition, a high HbA1c is one of nine factors that predict whether someone with diabetes will develop early kidney complications, based on data from more than 41,000 patients.

A chronically high HbA1c also has consequences for other organs. In people with type 2 diabetes who start blood-sugar-lowering medication (SGLT2 inhibitors), one observational study found that an HbA1c of 9% or higher was associated with a greater risk of dementia, particularly caused by vascular damage in the brain. This association weakened considerably after correction for other factors, so it does not yet constitute a proven cause-and-effect relationship. Serious eye complications (sight-threatening retinopathy) are also associated with a higher HbA1c, alongside diabetes duration and blood pressure.

Good news: a small study (47 participants, without a control group) showed that continuous glucose monitoring in people with type 2 diabetes who do not use insulin produced measurable improvements in blood sugar, weight, blood pressure and cholesterol. Placebo effects and extra medical attention may have played a role, so this result calls for confirmation in larger studies.

Finally: even in people who have had a kidney transplant, an HbA1c of 5.3% or higher proved sufficient to identify a high-risk group for post-operative diabetes. This was investigated retrospectively at a single hospital, but it shows that being 'still normal' on the standard scale does not automatically mean there is no elevated risk.