Fasted training usually means morning exercise after an overnight fast. The body may use a higher proportion of fat during that session, but this does not automatically produce greater fat loss over weeks and months. Energy balance and adherence matter more.
The short answer
Fasted training is tolerable for some people and sessions, but it has no magical fat-loss advantage and may reduce the quality of demanding workouts.
Which sessions fit best?
Walking or short easy training is tolerable without food for many people. Long endurance work, hard intervals, high-volume lifting, or record attempts usually benefit from prior fuel.
Warning signs
Stop and assess dizziness, shaking, unusual weakness, nausea, or poor concentration. Diabetes, pregnancy, an eating-disorder history, or glucose-altering medication require professional guidance.
A practical test
Compare two similar sessions, one fasted and one after a light snack. Record output, repetitions, effort, and later hunger, then choose the approach that supports performance and adherence.
Practical checklist
Do not test unfamiliar food on an important training or competition day.
Adjust amount and timing to gut tolerance and session goals.
The quality of the whole daily diet matters more than one meal.
References
Nutrition and Athletic Performance — Academy of Nutrition and Dietetics, Dietitians of Canada and ACSM — https://pubmed.ncbi.nlm.nih.gov/26920240/
ISSN Position Stand: Nutrient Timing — https://pubmed.ncbi.nlm.nih.gov/28919842/