Background: An increasing number of clinical trials, including our randomized phase III trial (NCT05503108), are investigating the potential benefit of short-term fasting using a fasting mimicking diet (FMD) on the anti-tumor effect of chemotherapy. Preclinical research suggests that FMD promotes infiltration of CD8+ T-cells, while decreasing regulatory T-cells (Treg). Extensive immunoprofiling is needed to investigate the immunomodulatory effects of FMD alone and guide ongoing translational trials on FMD as an adjunct to chemotherapy in patients with cancer. Methods: FIND is a prospective pilot study, which included 9 healthy volunteers who followed a 4-day FMD every 3 weeks, 2 times (NCT04833439). Peripheral blood samples were collected in a fed state (baseline), after overnight fasting (overnight) and directly after the second cycle of FMD (final). Metabolic markers (IGF1, glucose, ketone bodies) to ensure ketosis were measured in the plasma. The RNA from white blood cells was isolated and transcriptomics was performed with the Nanostring IO360 human Pancancer panel, and analyzed by nSolver Advanced analysis, Graphpad Prism and String analysis. Statistical testing was done using Wilcoxon signed-rank tests in R-studio. In addition, 40-plex spectral AURORA flow cytometry for immune cell composition with FlowJo analysis was performed. This study was approved by the local Medical ethical committee (NL76033.058.21). Results: Paired analysis of the Nanostring data identified 56 differentially expressed genes (DEG) comparing baseline and final samples initially, of which 19 genes showed at least 1.2 log² fold-change in at least half of the participants. String analysis indicated that the DEGs were associated with fatty-acid metabolism, T-cell and myeloid cell function and immune regulatory pathways. Cell typing showed an increase in cytotoxic NK CD56dim cells (p=0.031) and a decrease in Tregs decreased (p=0.063) after FMD. In-depth analysis of gene specific pathway enrichment and immune cell profiling by flow cytometry will be presented. Conclusions: The expression profile of 56 genes in circulating immune cells changed after 2 FMD cycles, next to fatty-acid metabolism affecting the composition and function of both T-cells and myeloid cells and their regulation. Altogether, FMD induced immunological changes that could support cancer treatment and guide immunomonitoring in future trials evaluating the effects of FMD in cancer treatment. Clinical trial information: NCT04833439.