“What,” said Gargantua, “to drink so soon after sleep? This is not to live according to the diet and prescript rule of the physicians, for you ought first to scour and cleanse your stomach of all its superfluities and excrements.”
With Gargantua's comment about drinking just after sleeping, Rabelais mocks the food habits and medical traditions of his era. This is consistent with the four humors theory of the Renaissance, which believed that digestion should be in a specific order, starting with the cleansing of waste in someone’s body before consuming any food or liquids. Early modern dietetics placed a significant value on balancing the humors through proper eating habits, like understanding when and how to eat and drink, as Rebecca Laroche states in her article for the Folger Shakespeare Library. Though Gargantua's argument fits with these ideas, Rabelais's humorous tone suggests a criticism of too strict medical standards.
Laroche, Rebecca. "The Four Humors: Eating in the Renaissance." Shakespeare & Beyond, Folger Shakespeare Library, 14 Nov. 2017, https://www.folger.edu/blogs/shakespeare-and-beyond/the-four-humors-eating-in-the-renaissance/