We still don’t really know what it is about ultra-processed food that generates weight gain. The rate of chewing may be a factor. In Hall’s study, during the weeks on the ultra-processed diet people ate their meals faster, maybe because the foods tended to be softer and easier to chew. On the unprocessed diet, a hormone called PYY, which reduces appetite, was elevated, suggesting that homemade food keeps us fuller for longer. The effect of additives such as artificial sweeteners on the gut microbiome is another theory. Later this year, new research from physicist Albert-László Barabási will reveal more about the way that ultra-processing actually alters food at a molecular level.