Objective: confirm the efficacy and feasibility of daily ingested encapsulated freeze-dried autologous (own)faecal matter on the preservation of residual beta cell function as assessed by C-peptide release upon amixed meal test (MMT) in recently diagnosed type 1 diabetes mellitus (T1D). Study design: double-blind placebo-controlled study Study population: n=110, recently diagnosed (\<100 days of diagnosis) patients with T1D, aged 18-45 years, BMI 18-30 kg/m2, male/female. Intervention: After inclusion and randomisation individuals will receive for 6 months either placebo or freeze-dried autologous encapsulated FMT in a 1:2 ratio. Subsequently, participants with be followed for 6 months whether beta cell preservation was durable after cessation of treatment. Main study parameters/endpoints: The primary endpoint is long-term preservation of beta cell insulin secretion capacity as assessed by stimulated C-peptide AUC0-120minresponse upon MMT (at0, 6 and 12months).The secondary endpoint pertains to changes in post-meal urinary C-peptide levels, plasma biochemistry (HbA1c levels),glucose time-in-range and subsequentexogenous insulin dose use at 0, 6 and 12months. Nature and extent of the burden and risks associated with participation,benefit and group relatedness: This study is considered a low-risk study, 3MMTs will be performed, for which 70 ml of blood samples will be drawn each visit. As of today, no severe adverse events as result of FMT have been reported in this centre and in the ENCAPSULATE trial investigating the feasibility and safety of this approach participants only reported some minor and transient constipation. In addition, the use of autologous faeces comes with a lower(absent)risk for transmitting any unknown pathogens compared to an allogenic FMT. As there currently is no widely applied therapy to preserve beta cell function in type 1 diabetes, encapsulated autologous FMT can have a potential benefit for the participants.