European journal of applied physiology and occupational physiologyJan 1, 1994

How Eating or Infusing Glucose Affects Fuel Use During Long Exercise

Circadian Biology Weekly Brief ↗PubMed ↗DOI ↗

Updated Mar 29, 2026

Abstract

Glucose ingestion significantly increased plasma insulin concentrations to 12.9 mU.l-1 compared to 4.8 mU.l-1 with glucose infusion.

Total rates of plasma glucose oxidation were higher with carbohydrate ingestion (9.5 mmol.125 min-1 kg FFM-1) compared to glucose infusion (6.2 mmol.125 min-1 kg FFM-1).
Carbohydrate oxidation rates also increased with ingestion, reaching 37.2 mmol.125 min-1 kg FFM-1 versus 24.1 mmol.125 min-1 kg FFM-1 with infusion.
Fat oxidation contributed more to energy production during glucose infusion (51%) than with carbohydrate ingestion (18%).
Both methods maintained similar plasma glucose concentrations, with ingestion at 5.3 mmol.l-1 and infusion at 5.0 mmol.l-1.

Simplified

To determine if bypassing both intestinal absorption and hepatic glucose uptake by intravenous glucose infusion might increase the rate of muscle glucose oxidation above 1 g.min-1, ten endurance-trained subjects were studied during 125 min of cycling at 70% of peak oxygen uptake (VO2peak). During exercise the subjects ingested either a 15 g.100 ml-1 U-14C labelled glucose solution or H2O labelled with a U-14C glucose tracer for the determination of the rates of plasma glucose oxidation (Rox) and exogenous carbohydrate (CHO) oxidation from plasma 14C glucose and 14CO2 specific activities, and respiratory gas exchange. Simultaneously, 2-3H glucose was infused at a constant rate to measure glucose turnover, while unlabelled glucose (25% dextrose) was infused into those subjects not ingesting glucose to maintain plasma glucose concentration at 5 mmol.l-1. Despite similar plasma glucose concentrations [ingestion 5.3 (SEM 0.13) mmol.l-1; infusion 5.0 (0.09) mmol.l-1], compared to glucose infusion, CHO ingestion significantly increased plasma insulin concentrations [12.9 (1.0) vs 4.8 (0.5) mU.l-1; P < 0.05], raised total Rox values [9.5 (1.2) vs 6.2 (0.7) mmol.125 min-1 kg fat free mass-1 (FFM); P < 0.05] and rates of CHO oxidation [37.2 (2.8) vs 24.1 (3.9) mmol.125 min-1 kg FFM-1; P < 0.05]. An increased reliance on CHO metabolism with CHO ingestion was associated with a decrease in fat oxidation. Whereas the contribution from fat oxidation to energy production increased to 51 (10)% with glucose infusion, it only reached 18 (4)% with glucose ingestion (P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)

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How Eating or Infusing Glucose Affects Fuel Use During Long Exercise

Abstract

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