You are misinformed. You do use fat during the marathon however for fast runners the vast majority of energy comes through glycogen stores and lipid metabolism plays a much smaller role. Training allows you to become more efficient at storing and utilizing glycogen. See below:L.L. Spriet (2007) Regulation of Substrate Use During the Marathon. Sports Medicine, Volume 37, Numbers 4-5, 2007 , pp. 332-336(5)Elite athletes run marathons at speeds requiring between 80% and 90% V-dotO2max, and finish in times between 2:05:00 and 2:20:00. They are highly adapted to oxidise fat and must do so during training. However, they compete at such high running speeds, that CHO oxidation (also highly adapted) may be the exclusive source of energy while racing. MJ O'BRIEN, CA VIGUIE, RS Mazzeo (1993) Carbohydrate dependence during marathon running Medicine & Science in Sports & Exercise Volume 25 (9)....Therefore, total body glycogen stores were made available for combustion. All classes of energy substrates participate, but carbohydrate, not lipid, is the primary fuel for marathon running.GA Brooks, Importance of the 'crossover'concept in exercise metabolism (1997) Clinical and Experimental Pharmacology and PhysiologySUMMARY1. The ‘crossover’ concept is a model of substrate supply during exercise which makes the following predictions.2. Lipid is the major fuel (approximatel. 60%) for non-contracting skeletal muscle and the body at rest.3. Energy flux, as determined by exercise intensity, is the major factor in determining the balance of substrate utilization during exercise. Thus, moderate and greater exercise intensities increase contraction-induced muscle glycogenosis and glycolysis, increase recruitment of fast-twitch muscle fibres, increase sympathetic nervous system activity and down-regulate mitochondrial fatty acid uptake.4. Glycogen and glucose utilization scales exponentially to relative exercise power output with a greater gain in glycogen than in glucose use at high power. The relationship between free fatty acid flux and power output is an inverted hyperbola. Consequently, at high power outputs, the role of lipid oxidation is diminished.5. Factors such as endurance training, energy supply, as influenced by dietary manipulation, and prior exercise play secondary roles in determining the balance of substrate utilization during exercise.6. Comparisons of the metabolic responses in subjects engaged in activities requiring vastly different metabolic rates or comparisons of subjects of different gender, age or training status require normalization of data to total energy flux.
J.O. wrote:
Aghast wrote:You don't burn fat during a marathon. You don't burn fat when running, it is too inefficient.
You have no idea. Fat is a very important fuel in Marathon running. The more fat you can metabolize at race pace, the more glycogen you can conserve for when it's time to start racing, at around 20 miles.