Effect of cold exposure on fuel utilization in humans: plasma glucose, muscle glycogen, and lipids

Abstract

The relative roles of circulatory glucose, muscle glycogen, and lipids in shivering thermogenesis are unclear. Using a combination of indirect calorimetry and stable isotope methodology ([U-13C]glucose ingestion), we have quantified the oxidation rates of these substrates in men acutely exposed to cold for 2 h (liquid conditioned suit perfused with 10 degrees C water). Cold exposure stimulated heat production by 2.6-fold and increased the oxidation of plasma glucose from 39.4 +/- 2.4 to 93.9 +/- 5.5 mg/min (+138%), of muscle glycogen from 126.6 +/- 7.8 to 264.2 +/- 36.9 mg glucosyl units/min (+109%), and of lipids from 46.9 +/- 3.2 to 176.5 +/- 17.3 mg/min (+376%). Despite the observed increase in plasma glucose oxidation, this fuel only supplied 10% of the energy for heat generation. The major source of carbohydrate was muscle glycogen (75% of all glucose oxidized), and lipids produced as much heat as all other fuels combined. During prolonged, low-intensity shivering, we conclude that total heat production is unequally shared among lipids (50%), muscle glycogen (30%), plasma glucose (10%), and proteins (10%). Therefore, future research should focus on lipids and muscle glycogen that provide most of the energy for heat production.

Does non-exercise activity thermogenesis contribute to non-shivering thermogenesis?

“…Non-shivering thermogenesis is documented to occur in humans whereby energy expenditure increases with body cooling (Paolone and Paolone 1995;Haman, Peronnet et al 2002;van Ooijen, van Marken Lichtenbelt et al 2005). However the mechanism of non-shivering thermogenesis is unknown.…”

Effects of ingesting [13C]glucose early or late into cold exposure on substrate utilization

“…Therefore, using indirect calorimetry and isotopic methodologies in non-cold-acclimatized men, the main purpose of this study was to determine whether ingesting glucose at a moment coinciding with the maximal shivering intensity could increase the utilization rate of the ingested glucose. 13 C-enriched glucose was ingested (800 mg/min) from the onset (G0) or after 60 min (G60) of cold exposure when the thermogenic rate was stabilized to low-intensity shivering (ϳ2.5 times resting metabolic rate). For the same quantity of glucose ingested, the oxidation rate of exogenous glucose was 35% higher in G60 (159 Ϯ 17 vs. 118 Ϯ 17 mg/min in G0) between minutes 60 and 90.…”

“…During low-intensity shivering, there is a substantial delay in establishing a metabolic steady state [ϳ60 min (1,13)] compared with exercise (ϳ2-3 min). The limited muscle activity early during cold exposure could drastically limit contractionmediated glucose uptake in skeletal muscles and ultimately the utilization or storage of exogenous glucose.…”

“…The last evening meal was standardized (3,220 kJ or 770 kcal, 42% CHO, 28% fat, and 30% protein), and subjects were asked to report to the laboratory at 0730 h the next morning after a 12-14 h fast. Over the 2 days preceding the experiment, ingestion of CHO from plants naturally rich in 13 C (C4 photosynthetic cycle) was avoided to maintain low 13 C background enrichment in plasma glucose and expired CO2 (26). On their arrival in the laboratory, subjects wearing only shorts were weighed and instrumented with thermal probes.…”

“…Changes in Ḣ were calculated by indirect respiratory calorimetry corrected for protein oxidation (as described in Ref. 13). Mean skin temperatures (T skin) were monitored continuously before and during cold exposure using heat flux thermocouples [Concept Engineering, Old Saybrook, CT; areaweighted equation from 12 sites: forehead, chest, biceps, forearm, abdomen, lower and upper back, front and back calf, quadriceps, hamstrings, and hand (16)].…”