Intranasal insulin reduces olfactory sensitivity in normosmic humans

Area of Science:

• Neuroscience
• Endocrinology

Background:

• The human brain, particularly the olfactory bulb, possesses high densities of insulin receptors.
• The olfactory bulb is crucial for odor processing and is the oldest part of the olfactory central nervous system.

Purpose of the Study:

• To investigate the hypothesis that enhanced brain insulin signaling modulates human olfactory processing.
• To examine the effects of intranasal insulin administration on olfactory threshold and discrimination.

Main Methods:

• A double-blind, placebo-controlled, within-subject design was employed.
• 17 participants received a single intranasal dose of either insulin (40 IU) or placebo.
• Olfactory abilities were assessed using threshold and discrimination tests; blood glucose, insulin, and cortisol levels were measured.

Main Results:

• Intranasal insulin significantly decreased sensitivity to the odorant n-butanol compared to placebo (-13%; P = .025).
• Olfactory discrimination ability remained unaffected (P = .841).
• Plasma glucose levels dropped slightly, but this was not correlated with changes in olfactory sensitivity.

Conclusions:

• Intranasal insulin impairs olfactory sensitivity to non-food odors but does not affect olfactory discrimination.
• Brain insulin signaling variations likely impact the olfactory threshold in humans.
• Further research is needed to explore insulin's effect on the perception of food-related odors.