Aging exaggerates the blood glucose response to total parenteral nutrition

Objective

To determine the effect of age on the blood glucose and insulin responses to a clinical model of glucose loading (i.e., total parenteral nutrition TPN] with hypertonic glucose), in patients with a variety of conditions.

Design

A prospective cohort study.

Setting

An adult university hospital.

Patients

Seventy-one consecutive, clinically stable patients receiving central TPN, excluding those with metabolic disease or receiving relevant medications.

Intervention

None.

Main Outcome Measures

Serum levels of glucose, insulin, C-peptide and cortisol determined in peripheral venous blood obtained immediately before initiating TPN and again 48 to 96 hours later; acute physiology score (APS) and habitual level of physical activity (HAL).

Results

Serum levels of glucose, insulin and C-peptide increased following initiation of TPN (all p < 0.001). The serum glucose level during TPN administration increased as a function of both patient age and severity of illness (APS) (r² = 0.37, all p < 0.01), whereas the serum insulin level was inversely related to age and increased as a function of serum glucose, glucose rate of infusion and HAL (r² = 0.57, all p < 0.05). The serum C-peptide:insulin molar ratio did not vary with age.

Conclusions

Aging and severity of illness interact to exaggerate the increases in blood glucose that accompany TPN with hypertonic glucose. Serum insulin responses to TPN decline with aging, likely reflecting reduced insulin secretion. Diminished insulin responses may contribute to hyperglycemia and represent a diminished anabolic signal in such patients. The acutely ill elderly patient is predisposed to hyperglycemia and should be monitored carefully even when pre-TPN blood glucose values are normal.