Evidence for cephalic phase insulin release in humans: A systematic review and meta-analysis

The initial release of insulin in response to food stimuli acting on receptors in the head and oropharynx is called the cephalic phase of insulin secretion. Insulin has been shown to act centrally to regulate food intake and glucose metabolism and the cephalic phase of insulin secretion may contribute to these functions. Though well documented in laboratory animals, the existence of cephalic phase insulin release in humans has recently come into question. We therefore performed a systematic review and meta-analysis of studies of cephalic phase insulin release in humans. Efficacy outcomes included any change in circulating insulin levels in healthy human volunteers post any food stimulus as compared to baseline or control in a time period of no longer than 10 min. Primary outcome: The overall pooled effect size estimate for cephalic phase insulin release was 0.47 [0.36, 0.58] p-value <0.0001. Secondary outcomes: A random effects meta-analysis with an added moderator for type of stimulus presentation (one, two, four or five sensory qualities) and type of stimulus offered (liquid, solid formulation) also significantly influenced results p = 0.0116 and p = 0.0024 respectively, while sex had no significant effect. Sensitivity Analysis: More restrictive analyses only including studies that used non-ingestive stimuli (p = 0.0001), or studies that reported insulin values within 5 min post stimulus presentation (p < 0.0001) still showed significant positive overall effect size estimates. In summary, our analysis shows that there is evidence for the presence of cephalic phase insulin secretion in humans. Secondary analyses suggest that the type and presentation of stimulus may significantly influence cephalic phase insulin secretion, while sex had no significant effect on cephalic phase insulin secretion.