New research suggests glucose test may predict cognitive decline

The higher a subject's results on a 2-hour glucose tolerance test, the more likely they were to show memory declines 10 years later.

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The Study

Oral Glucose Tolerance Test Predicts Episodic Memory Decline: A 10-Year Population-Based Follow-up Study 

Where: University of Turku and Turku University Hospital, Turku, Finland

Published: August 15, 2021, Diabetes Care

The Takeaway

People with signs of insulin resistance are more likely to develop cognitive decline in the future.

What It Looked At

The study investigated whether the results of a 2-hour oral glucose tolerance test (OGTT) can predict the odds of memory decline 10 years later. The study participants were 961 Finnish adults age 45-74with a wide range of ages and health states.

In an OGTT, subjects consume a drink with 75g of glucose and have their blood sugar tested at set intervals—typically 30 minutes, one hour, and two hours—to determine how the body processes glucose. It is the most common glucose tolerance test and is often used to diagnose conditions like prediabetes and diabetes.

To measure cognitive health, participants completed an exercise where they attempted to learn and recall a set of words, and another that tested verbal fluency in conversation. These tests are often used to assess early signs of Alzheimer’s. The researchers then performed the same tests on the subjects 10 years later.

Previous research has found that metabolic conditions like diabetes are potential risk factors for dementia, Alzheimer’s, and cognitive impairment.

What It Found

Researchers found that higher two-hour glucose levels in the OGTT in people older than 54 correlated to a decline in the word recall exercise in the follow-up study 10 years later.

“The association was found for the total range of glucose values,” says lead researcher Laura Ekblad. “The higher the 2-h glucose value, the lower the score on word-list delayed recall.”

The study also showed a correlation between fasting glucose ranges and decline in the word-learning exercise, once adjusted for age, sex, and education.

Why It Matters

A large body of research has established connections between metabolic challenges and cognitive decline. The links are so clear that Alzheimer’s disease is sometimes called Type 3 diabetes. Other studies have shown that impaired glucose control can be an early warning sign of cognitive decline, and diabetes significantly increases the risk of cognitive disorders.

Several mechanisms are likely at play, including insulin resistance in the brain (an organ that is highly dependent on insulin), mitochondrial damage to brain cells, which can affect energy production in the body, amyloid plaque build-up which affects parts of the brain concerned with memory and comprehension. Another factor is oxidative stress which can affect cognitive abilities, and speed up brain aging.

This is a small study, showing a fairly small effect. But what’s notable is the use of the OGTT as a predictive diagnostic tool. The authors found in previous research that an insulin test known as HOMA-IR has been shown to predict cognitive decline, but HOMA-IR is not part of a typical testing regimen. OGTT is more common and easily obtained, so it could be a valuable tool in identifying people at risk of cognitive decline early on when interventions can still be effective. The authors point out that given the very slow development of many of the components of cognitive impairment, like amyloid buildup or vascular damage, it’s more important to identify risk factors in middle age than in older adults.