Sports Nutrition Lab

How the model works: energy use comes from intensity (% of VO2max) and fitness for a 70 kg athlete (about 5 kcal per litre of oxygen). The carbohydrate share follows a crossover curve that moves to higher intensities with training. Carbohydrate comes from muscle glycogen and blood glucose; the liver and any carbohydrate eaten during exercise keep blood glucose supplied. Starting stores: glycogen in the working leg muscles about 250 to 380 g depending on training (whole-body muscle holds more; about 35% more after loading); liver glycogen about 55 g after an overnight fast, 90 g after a meal. The athlete fatigues when muscle glycogen runs very low, when the liver can no longer keep blood glucose up, or, above about 80% VO2max, from fast-system by-products. Simplified for learning.

A teaching model. Sweat rate rises with intensity, temperature and humidity (typical range about 0.3 to 2.5 L per hour). One litre of sweat is about 1 kg of body mass. Sweat sodium is set at about 900 mg per litre (1,500 mg for a salty sweater). Blood sodium is estimated from total body water (about 60% of body mass) and sodium gained or lost; core temperature rises with intensity, heat, humidity and dehydration. Real people vary widely. Not medical advice.

The picture updates with the answer you type. The worked solution builds step by step; units that cancel are struck out in red. All targets are ranges from sports nutrition guidelines used for practice, not personal plans.