GLOSSARY

Fat as Exercise Fuel describes fat serving as an energy substrate for exercise. Consuming more fat immediately before exercise does not guarantee that more useful fat energy will be delivered or that performance will improve.

Fat contributes to exercise energy through circulating fatty acids and intramuscular lipid stores. Greater fat availability, uptake or mobilization does not automatically mean a greater oxidation rate. Relative contribution and absolute rate are also different: the percentage can rise while total exercise demand changes. Oxidizing more fat during one session does not by itself establish greater long-term body-fat loss. Interpret the measured process, exercise intensity, diet and overall energy balance together.

Exercise uses a mixture of fuels whose proportions change with intensity, duration, training and diet. The crossover concept describes a shift toward greater carbohydrate contribution as intensity rises under specified conditions. It is a model of relative fuel use, not an on-off switch that turns fat metabolism off. The crossover point is not fixed across all individuals or protocols. Interpret substrate patterns with the actual workload and available fuel rather than a universal heart-rate zone.

A sustained low-carbohydrate, high-fat diet can increase reliance on fat and change fuel handling. Nutritional ketosis is a related but more specific state and is not produced by every higher-fat diet. Increased fat oxidation does not establish better performance, particularly when high rates of carbohydrate-supported work are required. Some athlete studies have found impaired exercise economy or high-intensity performance. Restoring carbohydrate after fat adaptation does not guarantee immediate reversal of every effect or the advantages of both approaches.