Intramyocellular lipids (IMCL) and muscle glycogen provide local energy during exercise (EX). The objective of this study was to clarify the role of high versus low IMCL levels at equal initial muscle glycogen on fuel selection during EX. After 3 h of depleting exercise, 11 endurance-trained males consumed in a crossover design a high-carbohydrate (7 g kg(-1) day(-1)) low-fat (0.5 g kg(-1) day(-1)) diet (HC) for 2.5 days or the same diet with 3 g kg(-1) day(-1) more fat provided during the last 1.5 days of diet (four meals; HCF). Respiratory exchange, thigh muscle substrate breakdown by magnetic resonance spectroscopy, and plasma FFA oxidation ([1-(13)C]palmitate) were measured during EX (3 h, 50% W (max)). Pre-EX IMCL concentrations were 55% higher after HCF. IMCL utilization during EX in HCF was threefold greater compared with HC (P < 0.001) and was correlated with aerobic power and highly correlated (P < 0.001) with initial content. Glycogen values and decrements during EX were similar. Whole-body fat oxidation (Fat(ox)) was similar overall and plasma FFA oxidation smaller (P < 0.05) during the first EX hour after HCF. Myocellular fuels contributed 8% more to whole-body energy demands after HCF (P < 0.05) due to IMCL breakdown (27% Fat(ox)). After EX, when both IMCL and glycogen concentrations were again similar across trials, a simulated 20-km time-trial showed no difference in performance between diets. In conclusion, IMCL concentrations can be increased during a glycogen loading diet by consuming additional fat for the last 1.5 days. During subsequent exercise, IMCL decrease in proportion to their initial content, partly in exchange for peripheral fatty acids.

During this 3-h exercise, energy supplies from fat and CHO were similar in both genders, and males as well as females reduced their IMCL stores significantly. The larger contribution of IMCL during exercise in males compared with females could either be a result of gender-specific substrate selection, or different long-term training habit.

Although children commonly travel to high altitudes, their respiratory adaptation to hypoxia remains elusive. Therefore, in the present study respiratory inductive plethysmography, pulse oximetry (Sp,O 2 ) and end-tidal CO 2 tension (PET,CO 2 ) were recorded in 20 pre-pubertal children (aged 9-12 yrs) and their fathers during 1 night in Zurich (490 m) and 2 nights at the Swiss Jungfrau-Joch research station (3,450 m) following ascent by train within ,3 h.In children, mean¡SD nocturnal Sp,O 2 fell from 98¡1% at 490 m to 85¡4 and 86¡4% at 3,450 m (nights 1 and 2, respectively); PET,CO 2 decreased significantly from 37¡6 to 32¡3 and 33¡4 mmHg (3,450 versus 490 m). In adults, changes in nocturnal Sp,O 2 and PET,CO 2 at 3,450 m were similar to those in children.Children spent less time in periodic breathing at 3,450 m during night 1 and 2 (8¡11 and 9¡13%, respectively) than adults (34¡24 and 22¡17%, respectively), and their apnoea threshold for CO 2 was lower compared with adults (27¡2 and 30¡2 mmHg, respectively, both nights). Sp,O 2 , PET,CO 2 and time in periodic breathing at altitude were not correlated between children and their fathers.In conclusion, children revealed a similarly reduced nocturnal O 2 saturation and associated hyperventilation at high altitude as adults but their breathing pattern was more stable, possibly related to a lower apnoea threshold for CO 2 .

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