<p>Emerging biomedical fields are frequently communicated in promissory language—expressing hope, and sometimes even hype, about their potential to solve current unsolvable public health challenges and thus contribute to a better future. In this article, we explore the language used to depict gut microbiome research as a promissory field: what does this language “do,” and what might the potential implications be for creating what Lauren Berlant terms <i>cruel optimism</i>? To answer these questions, we conduct a reflexive thematic analysis of promises of holism in academic articles and public science communication books about gut microbiome research. We argue that there is a strong match between current public health challenges and the promises of microbiome research, such that public health contexts may amplify the alluring future that microbiome research is depicted as promising. Furthermore, we argue that this promissory language is formulated in broad terms, making it difficult to pinpoint when a promise has been fulfilled—or when it did not live up to its potentials—thus creating fertile grounds for cruel optimism. The exact forms that this cruel optimism might take is too early to outline. Nonetheless, it is key to understand that the promissory language around microbiome research conveys values about the scope of impact that microbiome research is ascribed to have, with implications for how the body, self and health can be studied in its social, cultural, historical as well as biological mediations. Additionally, we argue that some of the promises of holisms exemplify an illusory holism, reactualizing the divides that the promises otherwise appear to disarm. As such, we situate microbiome research, and the promissory language that invokes holism, as part of an unsettled biomedical history.</p>