In eusocial honey bees, Apis mellifera, diet, gut microbiota and nestmates can all contribute to the health of freshly-emerged individual workers, but their relative importance for longevity and body weight is currently unknown. Here, we show that diet is most relevant, followed by gut microbiota and the presence of nestmates. Freshly emerged workers were randomly assigned to eight treatments (with or without honey/pollen, protein-substitute lactalbumin, antibiotic tetracycline and nestmates for 24 h) and maintained under standardized laboratory conditions. Longevity and food consumption were measured daily and fresh body weight was assessed at day seven. The data show a significantly better survival and a higher body weight in workers supplied with honey/pollen. Survival was higher in the lactalbumin treatments compared to the ones restricted to sucrose only, but lower compared to those with honey/pollen, highlighting the importance of micronutrients. In contrast, antibiotic treatment had a significant negative effect on longevity and body weight, which may be explained by inactivated gut microbiota and/or toxicity of the antibiotics. There was no positive effect of nestmates, probably due to the short exposure period. In contrast, nestmates showed a negative effect on survival in antibiotic-treated workers, possibly by transmitting pathogens and antibiotic-induced gut dysbiosis. In conclusion, a macro- and micronutrient-rich diet appears to be the key to individual honey bee worker health. Providing an optimal diet and possibly gut microbiota appears to be a promising way to promote managed A. mellifera health.