The reason is that before that, the body will give priority to consuming carbohydrates. After this time, when carbohydrates are gradually exhausted, the body will start to consume fat (or the proportion of fat consumption will gradually increase at this time. )
In the early stage of exercise, such as the first 20 minutes (this time varies from person to person, because everyone's physique is different), they do consume more carbohydrates and less fat. In the late stage of exercise, that is, about 20 minutes, fat consumption is more and carbohydrate consumption is less. Therefore, it takes more than 20 minutes /30 minutes to exercise to achieve the effect of reducing fat.