Tinnitus and Hyperinsulinemia

"Although the world is full of suffering, it is also full of the overcoming of it." - Helen Keller

Tinnitus, or phantom sounds in the ears like ringing, is an incredibly aggravating condition that affects millions of people. The intense level of suffering this condition can put on a life can be seen on sites such as TinnitusTalk.com. 

I've been studying Alzheimer's disease and its relation to adverse insulin receptor signaling for a couple of years now, so I was intrigued when I saw the Nature article Tinnitus and Risk of Alzheimer's and Parkinson's Disease: A Retrospective Nationwide Population-Based Cohort Study pop up on my radar last year. "Tinnitus has been implied as a 'soft' sign of neurodegenerative disease, which is characterized by progressive loss of neuronal function, such as Alzheimer's disease and Parkinson's disease," it read. The study concluded that tinnitus patients did in fact have a higher risk for developing neurodegenerative diseases like Alzheimer's and Parkinson's. Unsurprisingly, diabetes was also implicated with a higher risk for developing Alzheimer's.

I decided to go further down the rabbit hole, and I came across many published studies linking disturbances of glucose metabolism such as diabetes and insulin resistance with inner ear diseases like tinnitus. A quick list of findings:

A study released in 2004 called Hyperinsulinemia and Tinnitus: A Historical Cohort stated: "Demonstrably, the earlier the identification of a metabolic disorder, the better the clinical response, not merely as regards to tinnitus but in relation to vertigo and prevention of hearing loss. The presence of underlying metabolic disease should thus be examined in every patient presenting with cochlear or vestibular disorders (or both), by means of the 5-hour insulinemic and glycemic curves. These curves allow the diagnosis of abnormal carbohydrate metabolism much earlier than do the tests traditionally used for this purpose (fasting glycemia and 2-hour glucose tolerance)." The whole study is worth a read, but I'll just quickly add a tidbit of the researcher's conclusion, which reinforces the information stated above: "The difficulty of treating tinnitus patients (who frequently present with associated depressive or anxiety disorders) is well-known. Therefore, a thorough investigation of associated metabolic alterations using the 5-hour glycemic and insulinemic curves, as performed in this study, should be part of the routine evaluation of patients with tinnitus and may result in significant easing in symptoms and quality of life."

According to the 2007 article Tinnitus and Diabetes: "The inner ear, like the brain, is totally lacking in energy reserves. Its metabolism depends directly on the supply of oxygen and glucose from the blood supply. Alterations in glucose metabolism therefore have great potential for disturbing the workings of the inner ear."

The 2003 paper The Incidence of Hyperinsulinemia in Patients with Tinnitus and the Effect of a Diabetic Diet on Tinnitus concluded that: "Hyperinsulinemia may play an etiologic role in tinnitus and a diabetic diet may result in significant improvement in tinnitus complaints in this population."

Tinnitus is also correlated with lower levels of GABA. I'm immensely interested in the connection to the GABAergic system and diabetes. GABA, a calming inhibitory neurotransmitter, is produced in pancreatic beta cells, the same ones that get destroyed by diabetes. Subsequently, it's not too difficult to correlate the loss of pancreatic beta cells to the loss of systemic GABA. Research has shown that there is a 75% loss of beta cell function by the time a diagnostic blood sugar level of 140mg/dl is exceeded. 75%! Not that I have any functioning pancreatic beta cells myself anymore, and I'll point out it's quite the chore staying alive without them, but I would be very concerned if even a quarter of my beta cells were unknowingly gone as a non-diabetic person. One would definitely be losing systemic GABA as a result, and what symptoms might that bring on? Anxiety, depression, insomnia, other serious mood disorders, tinnitus, poor digestion... the list goes on. 

The very intriguing 2019 study Tinnitus Correlates with Downregulation of Cortical Glutamate Decarboxylase 65 Expression But Not Auditory Cortical Map Reorganization found that "tinnitus was significantly correlated with downregulation of GAD65 in the auditory cortex." GAD65, or glutamate decarboxylase 65, antibodies are a hallmark indicator of the development of Type 1 or insulin-dependent diabetes. GAD65 is prevalent in highest concentration in the brain and pancreatic beta cells. It's essentially an enzyme that catalyzes the decarboxylation of glutamate to GABA, and is therefore key in systemic GABA production. These antibodies appear years, possibly decades, before the onset of a clinical diabetes diagnosis.