The researchers - including members from Ohio State University (OSU) in Columbus - report their findings in the journal Scientific Reports.
Co-lead author Judith Schwartzbaum, associate professor of epidemiology at OSU, says, "Diabetes and elevated blood sugar increase the risk of cancer at several sites including the colon, breast, and bladder."
However, this is not the case with brain cancer such as glioma, she adds. Instead, "these rare malignant brain tumors are more common among people who have normal levels of blood glucose than those with high blood sugar or diabetes."
The new study builds on previous work led by Prof. Schwartzbaum that found a link between high blood sugar and reduced risk of meningioma - a nonmalignant brain tumor.
Glioma is one of the most common cancers that arise in the brain. It accounts for nearly a quarter of all primary brain tumors and three quarters of all malignant tumors.
Indicators of pre-symptomatic glioma much needed
Glioma begins in the glia cells in the brain. Glia cells are more abundant and very different from neurons or nerve cells.
Fast facts about brain tumor
- Around 80,000 people per year are diagnosed with primary brain tumors in the United States.
- Of these, around 53,000 are expected to be nonmalignant.
- The most common age at which primary brain tumor is diagnosed in the U.S. is 59 years.
Glia cells do not send the electrical signals that we normally associate with brain function as neurons do; instead, they protect, maintain, and support neurons.
Glioma is typically diagnosed in middle age. The researchers explain that there are currently no treatments capable of ensuring long-term survival, although some options are currently being researched.
The average survival for patients diagnosed with glioblastoma - the most common adult form of glioma - is only 14 months.
Also, as symptoms typically occur only 3 months prior to diagnosis, the authors note that "it is therefore important that indicators of the preclinical tumor be identified before the onset of symptoms."
The new study examines data from two large long-term investigations: AMORIS and Me-Can, which together totaled 797,945 participants among whom 812 developed gliomas.
The team analyzed the data on blood sugar levels and diabetes - including that related to the period preceding diagnosis - and how this related to subsequent diagnosis of glioma.
'Could the glioma itself affect blood glucose levels?'
The results showed that participants with high blood sugar and diabetes were less likely to develop glioma. Also, the link was strongest in the year leading up to the diagnosis.
Prof. Schwartzbaum says that this result got the team wondering: "Why is the association between blood glucose levels and brain cancer the opposite of that for several other cancerous tumors?"
In their paper, they discuss a number of factors that might shed some light on this question.
For example, could the tumor itself affect blood sugar levels? It is known, for instance, that insulin-like growth factor is related to recurrence of glioma, and people with diabetes have less of it than people without diabetes.
Another explanation might arise from looking at how the brain uses glucose. Although it accounts for a mere 2 percent of body mass, the human brain consumes around 20 percent of available glucose, says Prof. Schwartzbaum.
It is up to further studies to investigate these questions and see if there is anything in the relationship between blood and brain tumors that can be used to develop new treatments for brain cancer.
"Our research raises questions that, when answered, will lead to a better understanding of the mechanisms involved in glioma development."
Prof. Judith Schwartzbaum