While you are in hyperbolic diet the thyroid hormone drops down. It drops the T3 level and increase reverse T3 (rT3) thyroid hormone. This hormone is very crucial for the body as it use as energy but while you are on fasting our body conserve energy as much as possible. On the other hand prolong fasting increase heartburn. Least food intake reduce the production of HCL, similarly the production of bacteria will reduce. But while you are on fasting, a smell of food will can triggers the brain and stomach in production of HCL and hence heartburn and acid reflux occurs.