According to Sadhguru fasting on Poornima is a good idea because whatever food is taken in becomes tamasic as he thinks that things deteriorate faster on such days. Now what are the spiritual and ayurvedic benefits given in the scriptures of fasting on Poornima (full moon)?
All the Tithis have respective over ruling Deities. I am giving few such over-lordships here as found in the VarAha PurAna.
So, fasting on Purnima is generally to appease the Moon God. All round benefits are mentioned in the following passage.
One of Lord Brahma's distinguished Manasa Putras named Sage Atri begot Soma (Moon) who too was illustrious. Prajapati Daksha married off all his twenty seven daughters to Soma. But the latter was ignoring all the daughters, excepting Devi Rohini. The rest of them complained to their father Daksha who reprimanded Soma to behave but Soma ignored the repeated warnings of Daksha. Out of exasperation, Daksha cursed Soma to gradually decline and finally disappear. In the absence of Soma, there were far reaching adverse effects in the Universe. Devas including Vanaspati felt Soma's absence and so did human beings, trees, animals and worst of all medicinal plants, herbs and so on. The Sky was dark and the Stars too lost the shine. As Devas approached Vishnu, He advised the solution of Churning the Ocean by both Devas and Danavas and latter too agreed to the proposal for the lure of securing Amrit. In the process of churning, Moon was given birth again. This had brought great relief to Devas, human beings whose Ekadasha Indriyas (Eleven Body parts), Pancha Bhutas (Five Elements), Solah Kshetra Devatas, Trees and Medicines. Even Rudras head was without Soma and so did water. Lord Brahma blessed Pournima thithi and selected the day for worship to Chandra Deva when fast is viewed propitious along with token amount of money and material as charity. That specific day Yava Bhojan (Barley food) or barley made material is considered to bless a person with Gyan (Knowledge), radiance, health, prosperity, food grains and salvation.
BTW, i am not sure what you meant by "Ayurvedic benefits".