Provided herein are methods for diagnosing pre-diabetes or type 2 diabetes based on faecal or colonic samples and determining levels of butyric acid and one or both or acetic acid and propionic acid in the samples, and thereby determining the molar ratio of acetic acid to butyric acid, propionic acid to butyric acid and/or acetic acid to propionic acid to butyric acid. Also provided are methods for increasing the intestinal, colonic or faecal molar ratio of acetic acid to butyric acid, the intestinal, colonic or faecal molar ratio of propionic acid to butyric acid and/or the intestinal, colonic or faecal molar ratio of acetic acid to propionic acid to butyric acid in subjects with type 2 diabetes.
