Outcome

HbA1c

Average blood glucose over roughly three months. The marker long-run trials report, and the one a single supplement rarely moves far.

HbA1c measures the share of haemoglobin that has glucose attached, which reflects average blood glucose over roughly the previous three months. Because it averages, it is less sensitive to a single bad week than a fasting reading is — and less able to show a short trial’s effect.

What the published trials moved

The resistant-starch figure is the clearest pooled HbA1c result we have verified. An interval of −0.95 to −0.24 excludes zero, which is why it earns a B rather than a C despite the modest number of studies reporting this endpoint.

Why HbA1c trials are hard on supplements

A three-month marker needs a trial long enough to move it. Many supplement trials run eight to twelve weeks, which is shorter than the marker’s own averaging window — so a trial can be negative simply because it ended before HbA1c caught up with the change.

This is a structural problem, not a weakness of any ingredient. It means a null HbA1c result in a short trial is weaker evidence than the same result in a six-month trial, and it is why duration is recorded alongside every row in our tables.

How to read a 0.60 point change

HbA1c is reported as a percentage. For someone starting at 7.5%, a 0.60 point drop reaches 6.9%. For someone at 5.8%, the same absolute drop reaches 5.2%. The absolute change is the same; what it means for a person is not, because the baseline determines whether a threshold is crossed.

That is why the resistant starch review reports the baseline populations rather than only the pooled delta.

HbA1c is not the whole picture

Two people can share an HbA1c and have very different day-to-day glucose patterns — one steady, one swinging. Fasting glucose and post-meal glucose describe the shape; HbA1c describes the average. Supplement trials rarely report all three.

Facts checked: 29 October 2026 · VitalRoundup Editorial Team · How we review