Increasing serum betahydroxybutyrate concentrations during the 72-hour fast: evidence against hyperinsulinemic hypoglycemia.

OBJECTIVE: We have determined whether the behavior of betahydroxybutyrate (BOHB) during the 72-h fast of persons without evidence of hyperinsulinemic or any form of organic hypoglycemia might provide indicators of a negative fast.

DESIGN: Twenty-one patients with surgically confirmed insulinoma and 34 patients with negative 72-h fasts had BOHB measured every 6 h until Whipple's triad in the former and until 72-h in the latter.

RESULTS: Quadratic regression curves of BOHB from the negative fasts showed the typical curve to be flat initially, then increase in a manner that was roughly linear. Using time-specific medians, the changes were: 12-18 h, 0% increase; 18-36 h, 333% increase; 36-54 h, 210% increase, and 54-72 h, 167% increase. In contrast, patients with insulinoma had suppressed BOHB concentrations. Two successive BOHB values in excess of the 18-h level seemed to portend a negative fast. By using the previously published criterion of BOHB more than 2.7 mmol/liter (a surrogate for hypoinsulinemia and thereby an indicator of a negative fast), 74% of persons with a negative fast reached this level before the 72-h point.

CONCLUSIONS: Serial measurements of BOHB during the 72-h fast have the potential to provide not only clues during the fast that it will ultimately be negative, but also the opportunity to truncate the fast if the endpoint BOHB criterion for a negative fast is met before 72 h.