People whose blood sugar levels are in the borderline range - higher than normal, but not yet diabetic - might still have an increased risk of kidney problems, a Norwegian study suggests.

Compared to individuals with normal blood sugar, people with slightly abnormal glucose levels are more likely to have two problems associated with kidney disease - abnormal blood filtration and more of the protein albumin in the urine, the study found.

The questions, said Dr. Robert Cohen, an endocrinology researcher at the University of Cincinnati College of Medicine who wasn't involved in the study, are, "What represents abnormal `enough' blood sugar to start causing problems in the kidneys that we see with full blown diabetes, and what criteria should we be using if we want to get a head start on preventing the complications of diabetes?"

Globally, about one in nine adults have diabetes, which is often linked to obesity and aging and develops when the body can't properly use or make enough of the hormone insulin to convert sugar into energy.

While the link between full-blown diabetes and chronic kidney disease is well known, doctors disagree about how much sugar in the blood might pose a risk to people without the disease. They also disagree on how to diagnose and treat patients with only mildly abnormal blood glucose levels and whether it's reasonable to call this condition "prediabetes."

For the current study, Dr. Toralf Melsom of the University of North Norway and colleagues assessed blood sugar and indicators of kidney damage in 1,261 people aged 50 to 62 who didn't have diabetes.

Researchers looked for abnormal blood sugar by measuring blood glucose in fasting patients, and by measuring blood levels of hemoglobin A1c. This second test estimates average blood sugar over several months based on the percentage of hemoglobin - the protein in red blood cells that carries oxygen - that is coated with sugar.

At the start of the study, 595 people had slightly abnormal blood sugar levels based on U.S. guidelines for interpreting these test results, which are fairly stringent. Under guidelines favored outside the U.S. that require more sugar in the blood before glucose levels are considered elevated, only 169 people had abnormal results.

After a typical follow-up period of around five years, people with slightly abnormal blood sugar under either set of guidelines were more likely to have kidneys that were working harder to filter the blood. The condition, called hyperfiltration, is thought to contribute to kidney damage in diabetes.

Participants who had slightly abnormal baseline fasting glucose test results were also more likely to have elevated levels of albumin in the urine, indicating early kidney damage.

The subset of people with slightly abnormal blood sugar under the more restrictive guidelines used outside the U.S. were 95 percent more likely to have high blood filtration rates and 83 percent more likely to have excess albumin in the urine.

The study only included middle-aged white people, so the results might be different in other populations, the authors acknowledge in the American Journal of Kidney Diseases. There also isn't consensus on the best way to define kidney filtration in studies of large populations because nephrons, the functional waste-processing unit in the kidney, vary by age and gender, the authors note.

While the study findings may not change clinical practice, the results highlight the need for doctors to pay attention to people with slightly elevated blood sugar, said Dr. Laura Rosella, a public health researcher at the University of Toronto who wasn't involved in the study.

These people should focus on lifestyle changes such as eating better, exercising more and losing weight, she said.

"If someone adopts the necessary changes that would prevent the onset of diabetes, it is likely to protect against the progression to kidney disease as well - just like it will prevent cardiovascular disease and many cancers," Rosella added.

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