Source of Beta cell function

{{Infobox concept | name = Beta cell function | subtitle = Clinical assessment | Gold standard = Hyperglycaemic clamp with C-peptide deconvolution | Common surrogate = HOMA-B, insulinogenic index | Confounder = Prevailing insulin sensitivity }} {{technical|talk=Readability of the indices section}} '''Beta cell function''' is the capacity of the pancreatic beta cell to secrete insulin in amounts appropriate to the prevailing glucose concentration and to the prevailing insulin sensitivity. It is not a single quantity and cannot be measured directly in a living person; every clinical measure is an inference from insulin or [[C-peptide]] concentrations under some defined stimulus.{{r|rorsman2013}} The central methodological difficulty is that secretion and sensitivity are not independent. A healthy beta cell compensates for insulin resistance by secreting more, so an absolute secretion figure that looks normal may represent substantial dysfunction in a very insulin-resistant person. Measures that do not adjust for sensitivity — including fasting insulin and HOMA-B — are therefore weak, and the disposition index, the product of a secretion measure and a sensitivity measure, was devised to address exactly this.{{r|bergman2002}} Beta cell function is relevant to this wiki chiefly because it changes under incretin therapy and after substantial weight loss, and because indices derived from it appear in trial reports as secondary endpoints where their limitations are rarely restated.{{r|drucker2018}} == What is being measured == Insulin secretion cannot be sampled at source. Peripheral venous insulin has already passed the liver, which extracts a variable 40–80% at first pass, so peripheral concentration understates secretion by an amount that itself varies between people and with pulsatility.{{r|rorsman2013}} [[C-peptide]] avoids this. It is co-secreted with insulin in equimolar amounts, is not extracted by the liver, and has predictable kinetics, so a peripheral C-peptide profile can be deconvolved into a secretion rate. This is the basis of every rigorous measure of beta-cell function. The stimulus matters as much as the analyte. Intravenous glucose isolates the beta cell from the gut; oral glucose or a mixed meal includes the [[Incretin effect|incretin]] contribution and therefore measures something closer to everyday physiology but confounded by gastric emptying. A study using an intravenous stimulus and one using a meal are not measuring the same construct. == Indices in common use == | Index | Stimulus | What it approximates | Principal weakness | |---|---|---|---| | HOMA-B | Fasting only | Basal secretory capacity | Ignores insulin sensitivity; unreliable at low fasting glucose | | Insulinogenic index | 0 and 30 min of OGTT | Early-phase response | Very sensitive to sampling time | | C-peptide index | Fasting C-peptide over glucose | Basal capacity | Confounded by renal clearance | | Disposition index | Clamp or OGTT | Secretion adjusted for sensitivity | Requires a sensitivity measure | | First-phase clamp | Hyperglycaemic clamp | Docked-vesicle pool | Labour-intensive, research only | The disposition index is conceptually the strongest of these because it embodies the hyperbolic relationship between secretion and sensitivity: for a given individual the product is approximately constant, and a fall in the product indicates genuine deterioration rather than compensation. Its practical weakness is that it inherits the error of both component measurements.{{r|bergman2002}} Reference intervals for all of these are population- and assay-specific. A HOMA-B value quoted without the insulin assay it was derived from is not interpretable, because insulin immunoassays differ in cross-reactivity with proinsulin.{{r|rorsman2013,matthews1985}} == Change under treatment and weight loss == Incretin therapy improves several indices of beta-cell function while treatment continues. Whether this constitutes a durable change in the beta cell or simply reflects amplification of secretion while the drug is present has been debated since the first trials, and the honest answer is that most published improvements have not persisted through a washout period.{{r|drucker2018}} Substantial weight loss by any means improves the disposition index, and remission studies following large sustained weight loss have reported recovery of first-phase secretion in a subset of participants — most often those with shorter diabetes duration. This is the strongest available evidence that some of the functional loss in type 2 diabetes is reversible.{{r|bergman2002}} Neither observation supports a claim that any particular drug or regimen "restores" or "heals" beta cells. That framing appears frequently in promotional material and is not supported by the trial literature; the measured changes are improvements in an index under continuing treatment or after weight loss, not demonstrations of regeneration. == References == {{reflist}} <ref name="rorsman2013">Rorsman P, Braun M. "Regulation of insulin secretion in human pancreatic islets." ''Annual Review of Physiology'' 75:155–179 (2013). PMID 22974438.</ref> <ref name="bergman2002">Bergman RN, Ader M, Huecking K, Van Citters G. "Accurate assessment of beta-cell function: the hyperbolic correction." ''Diabetes'' 51 Suppl 1:S212–S220 (2002). PMID 11815482.</ref> <ref name="drucker2018">Drucker DJ. "Mechanisms of action and therapeutic application of glucagon-like peptide-1." ''Cell Metabolism'' 27(4):740–756 (2018). PMID 29617641.</ref> <ref name="matthews1985">Matthews DR, Hosker JP, Rudenski AS, et al. "Homeostasis model assessment: insulin resistance and beta-cell function from fasting plasma glucose and insulin concentrations in man." ''Diabetologia'' 28(7):412–419 (1985). PMID 3899825.</ref> == See also == * [[Insulin secretion]] * [[C-peptide]] * [[HOMA-IR]] * [[Fasting insulin]] * [[Incretin effect]] {{DEFAULTSORT:Beta cell function}} [[Category:Incretin biology]] [[Category:Metabolic biomarkers]] [[Category:Laboratory medicine]]