Diet has a strong influence on our gut microbiota, even stronger than that of host genetic background. Caloric restriction is a specific diet change that can delay the development of metabolic disorders and has been shown previously to delay immune senescence, which is the shift towards more immune memory cells with age. However, the interactions among caloric restriction, the microbiota, and the immune system are not well understood. To close this gap, researchers examined the impact of fecal microbiota transfer from a woman in a research diet trial to germ-free mice. The fecal samples were collected before (AdLib) and eight weeks into a very low-calorie formulated diet (CalRes). CalRes-microbiota recipients had a restructured gut microbial community compared to AdLib-microbiota recipients. These mice also had reduced body fat accumulation and improved glucose tolerance compared to the AdLib-microbiota recipients. CalRes-microbiota mice also showed a reduction in memory cells, including intestinal effector memory CD8+ T cells, which may suggest delayed immune senescence. While these results are based on the microbiota from a single individual and need to be verified in more people and non-mouse models, this study describes potential interactions between the host immune system and their microbiota in response to caloric restriction.