In plain terms: Acarbose shows early signals in human research but the evidence is limited -- small trials, surrogate endpoints, or not yet independently replicated. It is an emerging area, not yet a proven one.
Acarbose
The evidence as the record states it
Chiasson et al., STOP-NIDDM (Lancet, 2002; PMID 12086760): multicenter, placebo-controlled randomized trial of acarbose in adults with impaired glucose tolerance, with conversion to type 2 diabetes as the primary endpoint. ACE (Holman et al., Lancet Diabetes and Endocrinology, 2017; PMID 28917545): randomized, double-blind, placebo-controlled trial in 6,522 Chinese patients who had BOTH impaired glucose tolerance and established coronary heart disease. ACE found no reduction in cardiovascular events, its primary endpoint, and did report a lower incidence of type 2 diabetes. WHY THIS IS STILL A C: the two trials do not test the same population. STOP-NIDDM enrolled impaired glucose tolerance without a cardiovascular requirement; ACE required established coronary heart disease and was run entirely in China. A second trial in a different population narrows the question rather than closing it, so Rule 03's requirement for independent replication of the same claim is not met. EBL states this rather than claiming the result stands alone. Acarbose reaches longevity lists because it extended median lifespan in the NIA Interventions Testing Program in mice. Animal lifespan data does not move a grade on this site, at any effect size. There is no randomized human trial of acarbose against mortality, healthspan or longevity, and ACE was null on cardiovascular events.
Why Grade C and not Grade B
Grade C reflects early or limited human evidence -- small trials, surrogate endpoints, or preliminary findings not yet independently replicated. The evidence points in a promising direction but does not yet meet the standard for Grade B. A single well-powered, independently replicated trial with a meaningful endpoint would move this to Grade B or higher.
What would move this grade
A well-powered randomized trial with an objective, clinically meaningful primary endpoint and at least one independent replication would support Grade B or higher.
How this was studied
Acarbose 100 mg by mouth three times daily with meals, titrated from 50 mg, in adults with impaired glucose tolerance (STOP-NIDDM). The trial ran a mean of 3.3 years. Discontinuation for gastrointestinal effects was substantially higher on acarbose than placebo, which is part of what the dose means in practice.