<p><i>Wolbachia</i>, a bacterial endosymbiont, acts as an obligate nutritional mutualist in the bed bug, <i>Cimex lectularius</i>. <i>Wolbachia</i> in <i>C. lectularius</i> (<i>w</i>Cle) supplements B-vitamins, namely riboflavin (B2) and biotin (B7), which are deficient in the bed bug’s diet of vertebrate blood. Experimental elimination of <i>w</i>Cle significantly impairs fitness in bed bugs, resulting in slow development, low egg production and egg hatch rate, and smaller adult body size. Although this obligatory symbiosis has been well-documented, the specific physiological mechanisms by which <i>w</i>Cle-supplemented B-vitamins promote bed bug fitness remain unclear. We hypothesized that B-vitamin deficiency impairs digestion in aposymbiotic bed bugs, and in this study we investigated the effects of <i>w</i>Cle elimination on three digestive processes in the bed bug – diuresis, erythrocyte (red blood cell) lysis, and protein catabolism. Our results show that <i>w</i>Cle elimination significantly slows both diuresis and protein catabolism. We also demonstrate that riboflavin is critical for the breakdown of hemoglobin, the main protein component of red blood cells, but not albumin, the main protein component of plasma. We propose that the lack of <i>w</i>Cle<i>-</i>supplemented riboflavin results in systemic protein deficiency, driving various fitness-related deficits in aposymbiotic bed bugs. These findings enhance our understanding of bed bug digestive physiology and the <i>w</i>Cle-bed bug nutritional mutualism, with broader implications for other blood-feeding arthropods.</p>