Before I wrote software, I spent over a decade in sports science. This is from that life. I keep it here because the way you reason about evidence does not change when you switch fields.
There is a long-running debate about whether it is better to train on an empty stomach or after eating. Some argue that fasted exercise burns more fat and helps with weight loss. Others say the difference is not significant and that training fasted can even be counterproductive. Let's look at what the evidence actually says.
What the studies say
Some research has found that fasted exercise can increase the rate at which stored fat is burned. A study published in the British Journal of Nutrition in 2013 found that active men burned around 20% more fat when they ran fasted compared to running after a meal. The catch is that there are few studies on this, and the results are not conclusive.
What the experts say
Dr. Darryn Willoughby, who directed the Exercise and Biochemical Nutrition Laboratory at Baylor University, cautions that training fasted does not necessarily lead to weight loss. He points to a study where women lost roughly the same amount of weight and fat whether they ate before or after an hour of cardio.
When fasted training does help
After an overnight fast, the body first reaches for the carbohydrates stored in muscle and the liver. Once those run low, it shifts to stored fat, which takes longer to convert into energy. That shift is why fasted exercise can be more effective for fat oxidation specifically.
It is not for everyone, though. If you have an eating disorder, hypoglycemia, or any metabolic condition, it is worth avoiding until you have talked to a doctor or a nutritionist.
The honest conclusion: fasted training can offer some advantages for cardiovascular capacity and fat oxidation, but it carries real limitations and risks, and how much you actually lose varies from person to person. Like most things in health, the right answer depends on you, and it is a conversation to have with a professional, not a rule to copy from the internet.