Normal Insulin Physiology

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The healthy pancreas secretes insulin into the portal system in response to daily variations in nutrient intake and energy expenditure. Polonsky et al. carefully characterized the normal 24-h insulin secretion of healthy subjects (4). They found that daily insulin secretion was pulsatile and could be divided into two components: a constant basal secretion rate and marked increases in insulin secretion following meals (Fig. 1). The basal component of insulin release, which comprises approximately 40% of the total 24-h insulin secretion, limits glucose release from the liver and free fatty acid

From: Contemporary Endocrinology: Type 1 Diabetes: Etiology and Treatment Edited by: M. A. Sperling © Humana Press Inc., Totowa, NJ

Hour Basal Insulin Secretion Rate
Fig. 1. Mean 24-h profiles of insulin secretion rates in normal subjects. Arrows indicate meals. Basal insulin comprises about 40% of total daily secretion, while abrupt pulses of insulin secretion up to five times basal levels follow ingestion of mixed meals. (Adapted from ref. 4.)

release from adipose tissue. Following ingestion of a mixed meal, insulin secretion abruptly increases up to fivefold and then gradually falls back to the basal rate over the next 4 h before the subsequent meal. These insulin pulses, which limit postprandial excursions in plasma glucose concentration by inhibiting hepatic glucose production and increasing glucose uptake and storage, are triggered in response to various secreta-gogues from the diet, including glucose and amino acids.

Exercise and energy expenditure decrease insulin secretion. As glucose uptake and metabolism are increased with exercise, the P-cells maintain euglycemia by reducing the basal insulin secretion, thereby allowing increased glucose release from the liver (5).

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