Background Metabolic outcomes in type 1 diabetes (T1D) remain suboptimal. Disease modifying therapy to prevent beta cell loss presents an alternative treatment paradigm but the impact on metabolic outcomes is unclear. We aimed to define the relationship between insulin C-peptide production as a marker of beta cell function and metabolic outcomes in new-onset T1D. Methods 21 trials of disease-modifying interventions within 100 days of T1D diagnosis comprising 1315 adults and 1396 children were combined. End points assessed were stimulated AUC C-peptide, HbA1c, insulin use and hypoglycemic events as well as composite scores (IDAAC, Beta-2). Positive studies were defined as those meeting their primary endpoint. Differences in outcomes between active and control arms were assessed using the Wilcoxon rank test. Results 6 months after treatment, 24.8% greater C-peptide preservation in positive studies was associated with a 0.55% lower HbA1c (p< 0.0001), with differences being detectable as early as 3 months. Cross-sectional analysis combining positive and negative studies was consistent with this proportionality: a 27% mean improvement in C-peptide preservation was associated with a 0.64% lower HbA1c (p=6.6 x10-11). Higher initial C-peptide levels and greater degrees of C-peptide preservation were associated with greater improvement in HbA1c. For HbA1c, IDAAC, and Beta-2 Score, sample size predictions indicated that 2-3 times as many subjects per arm would be required to demonstrate a difference at 6 months as compared to C-peptide. Detecting a reduction in hypoglycemia rates was impacted by reporting methods and required larger sample sizes. Conclusion Interventions that preserve beta cell function are effective at improving metabolic outcomes in new-onset T1D, confirming their potential as alternatives or adjuncts to insulin. We have shown that improvements in HbA1c are directly proportional to the degree of C-peptide preservation, quantifying this relationship, and supporting the use of C-peptide as a surrogate endpoint in clinical trials.