DISCUSSION
retinopathy, macrovascular complications, heart disease, diabetes type 2 and healthy controls., nephropathy, diabetes type 2, memory impairments, diabetes
RETINOPATHY, HEART DISEASE, NEPHROPATHY, DIABETES TYPE 2, DIABETES
The aim of the current study was to investigate whether it was possible to induce placebo effects in the insulin system through conditioning with intranasal insulin. We studied the effects of conditioning on blood glucose, insulin, and C-peptide levels in patients with diabetes type 2 and healthy controls. In addition, we studied the effects of insulin conditioning on hunger, food consumption, food approach tendencies, and memory. We found that conditioning with intranasal insulin did not affect insulin or C-peptide levels; however, conditioning affected blood glucose levels in men (and not women): men in the conditioned group had higher (i.e., more stable) glucose levels than men in the control group on day 2. This conditioned effects in blood glucose mimicked the action of intranasal insulin, as the same effects were found after the insulin administration on day 1. In addition, we found that conditioning decreased hunger in healthy controls but not in patients with diabetes type 2. We can be certain that the effects found were due to conditioning and not the carryover effects from the previous day, as there were no differences between the baselines of these measures on day 2.Intranasal insulin administration affected two of the three physiological outcomes of the study: it decreased C-peptide levels in healthy participants and stabilized (prevented from dropping) the glucose levels in patients of both sexes and healthy men. Pharmacologically conditioned effects should normally mimic the effects of the drug, even though in some cases, opposite effects can be found because of negative feedback loops (Importantly, the direction of the effect of intranasal insulin and insulin conditioning on glucose did not correspond to the hypothesized direction found in a previous study (It is also hard to say if the conditioned effect we found is beneficial for patients. We did not observe a reduction in blood glucose, which is the primary aim of most diabetes treatments. Conditioning did seem to stabilize blood glucose levels, at least during the test period in men. Instability of plasma glucose levels has been shown to promote microvascular and macrovascular complications such as retinopathy, nephropathy, and heart disease (We have also found that conditioning with intranasal insulin stabilized hunger in healthy participants, partially confirming our study hypothesis. As blood insulin levels rapidly rise after food intake and insulin penetrates the blood-brain barrier, it serves as one of the signals to the central nervous system, and particularly the hypothalamus, to stop feeding and decrease hunger (In apparent contrast, no effects of intranasal insulin or conditioning were found on the calories consumed. The total amount of calories eaten during the taste test was very low, possibly because participants knew that the experiment was almost over and they could have a larger meal shortly. For future research, we would propose a more substantial meal, for example, a lunch buffet, to measure food consumption.No effect of intranasal insulin or insulin conditioning was found on memory. This does not align with several study findings that intranasal insulin administration improves memory in both healthy controls and patients with memory impairments (The sex differences found in our study align with previous research findings of the effects of intranasal insulin. Sex differences were found in previous research on the effects of insulin on declarative and working memory and food intake (Several limitations of our study must be mentioned. First, it is important to mention that, because of technical issues, we were unable to produce a placebo spray with the same preservative as the insulin spray. Because of this, insulin and placebo sprays had different smells, even though both smells can be described as “medical.” Participants in the conditioned group might have consciously or unconsciously noticed the difference in the way the spray smelled between day 1 and day 2, even though the spray administration was preceded and followed by the administration of a strong smell of aroma oil. However, none of the participants reported noticing the difference, and moreover, when we asked them about the perceived group allocation, a majority of them were not able to correctly tell what spray they received on what day. Despite that, it is important to emphasize that such modification of a part of the CS (that constituted of the rosewood smell, smell of the spray, and the context of spray administration) might possibly have led to a diminished conditioned response. At the same time, we do not expect that this change would completely have blocked the conditioning, as it has been shown that when the CS presented during the evocation phase is slightly different from the CS presented during the acquisition phase, the conditioned effect remains present (Our study has several important implications. We demonstrated that conditioning with intranasal insulin reduces hunger in healthy participants. Hunger can be a problem not only for patients with diabetes type 2 but for populations who need to follow a diet for other health reasons. Applying intranasal insulin conditioning can help these groups of people. Importantly, we provided further evidence that glucose responses can be conditioned, not only in healthy controls but also in male patients with diabetes type 2. However, our results indicate that sex and disease-specific effects might play a role in endocrine conditioning, and better understanding of these effects is needed.