Fate of exogenous glucose during exercise of different intensities in humans

Substrate preference in aminophylline-stimulated thermogenesis

The present study investigated the suitability of different substrates on aminophylline (AMPY)-induced thermogenesis in rats during cold exposure. Feeding of distilled water 60 min prior to cold exposure in two-day fasted rats resulted in the lowest total heat production and final body temperature in both saline- and AMPY-treated groups. Feeding of 5 ml Intralipid (2 Kcal/ml), a triglyceride mixture, did not improve thermogenesis beyond the control levels. However, feeding of isocaloric substitutes of sucrose elevated significantly the total thermogenesis by 7.9% and 7.4% and final body temperature by 2.23 and 1.61 degrees C, respectively, in saline- and AMPY-treated groups. The increase in thermogenesis by sucrose is not due to its thermic effect. It is concluded that sucrose, in combination with AMPY, may be of value in improving resistance to cold.

Influence of glucose and fructose ingestion on the capacity for long-term exercise in well-trained men

The aim of the present study was to examine the influence of glucose and fructose ingestion on the capacity to perform prolonged heavy exercise. Eight well-trained healthy volunteers exercised on a bicycle ergometer at 68 +/- 3% of their VO2 max until exhaustion, on three occasions, with 8-day intervals. During the exercise they ingested either glucose (250 ml, 7%), fructose (250 ml, 7%) or water (250 ml) every 20 min in a double-blind randomized study design. Arterial blood samples were collected at rest and during exercise for the determination of substrates and hormones. Muscle glycogen content (m. quadriceps femoris) was measured before and after exercise. The duration of exercise lengthened with repeated exercise (3rd test: 136 +/- 13 min v. 1st test: 110 +/- 12 min, P less than 0.01). Corrected for the sequence effect, total work time until exhaustion was significantly longer with glucose (137 +/- 13 min) than with either fructose (114 +/- 12 min) or water (116 +/- 13 min) (both P less than 0.01). When glucose or fructose was ingested, the arterial plasma glucose concentration was maintained at the normoglycaemic level; with water ingestion, plasma glucose values fell during exercise in seven subjects and remained at the resting level in the eighth subject. The muscle glycogen concentration was 467 +/- 29 mmol kg d.w.-1 at rest and fell to approximately half the initial value at exhaustion. In the subgroup of seven subjects in whom glucose values decreased with water intake, the mean rate of glycogen degradation was significantly lower (P less than 0.05) with the ingestion of glucose (1.3 +/- 0.4 mmol kg d.w.-1 min-1) as compared to fructose (2.1 +/- 0.5 mmol kg d.w.-1 min-1) or water (2.3 +/- 0.5 mmol kg d.w.-1 min-1). Intermittent glucose ingestion (3 X 17.5 g h-1) during prolonged, heavy bicycle exercise postpones exhaustion and exerts a glycogen-conserving effect in the working muscles. In contrast, fructose ingestion during exercise maintains the glucose concentration at the basal level but fails to influence either muscle glycogen degradation or endurance performance.

Effect of physical training on utilization of a glucose load given orally during exercise

The effect of a 6-wk training period on the oxidation of a 100-g glucose load given orally during exercise was investigated in six healthy male volunteers. The subjects were submitted before and 24 h after the training program to a 105-min exercise bout (performed at about 40% of the pretraining VO2max) followed by a 90-min resting period. Naturally labeled [13C]glucose was given 15 min after the beginning of exercise. Exogenous glucose oxidation was derived from 13CO2 measurements in expired air, and total glucose and lipid oxidation were evaluated by indirect calorimetry. Training (60-min bicycling 5 days a week at 30-40% VO2max) resulted in a 29% increase in VO2max. During the 15 min of exercise that preceded glucose ingestion, the rate of total carbohydrate oxidation was slightly decreased after training, whereas the rate of lipid oxidation was slightly increased. Training did not affect the response of blood glucose, plasma insulin, or plasma free fatty acids to the glucose ingested during exercise; in contrast, the circulating levels of epinephrine, glycerol, and lactate were significantly reduced after training. Substrate utilization measurements revealed similar oxidation rates of carbohydrates (106.9 +/- 2.7 before vs. 100.2 +/- 4.7 g/3 h after training) and of lipids. However, detailed analysis revealed a significant 17% increase in exogenous glucose oxidation, thus indicating a significant sparing of endogenous carbohydrates. In conclusion, physical training induces a modest but significant increase in the oxidation of an oral load of glucose given during subsequent exercise of moderate intensity, a phenomenon reinforcing the sparing of endogenous carbohydrate stores.

The marathon: dietary manipulation to optimize performance

Despite pronounced involvement of the cardiovascular and respiratory systems during the marathon, the limiting factor for this event revolves around the supply and utilization of intramuscular and extramuscular fuel reserves. The single most consistently observed factor contributing to fatigue at work intensities selected by marathon runners is the depletion of muscle's endogenous carbohydrate, glycogen. Dietary manipulations which reduce the rate of muscle glycogen degradation will, therefore, spare this important fuel and delay fatigue. The purpose of this paper will be to review dietary factors which are pertinent to fuel utilization in order to optimize marathon performance.