The next metformin paper to look at is this one:
Beneficial effects of metformin on energy metabolism and visceral fat volume through a possible mechanism of fatty acid oxidation in human subjects and rats
Here are the RQ data from 16 healthy humans after an overnight fast and for the three hours following a mixed carbohydrate/fat meal tolerance test (type of carbohydrate and fat not specified).
Aside: Here is the test "food" description: "meal tolerance tests (592 kcal, 75g of carbohydrate, 28.5g of fat; Saraya Co., Osaka, Japan)". It's great to know that there is a company called Saraya and that they have headquarters in Osaka. But I can't even find out what sort of "meals" Saraya make. Quite how anyone might replicate this study using the methods section is beyond me. In addition to these omissions the test "meal" is repeatedly described as "cookies". Go figure. Still, let's assume the measurements of RQ is numerically accurate, fingers crossed. End aside.
These healthy people, who haven't eaten overnight, have an RQ of 0.8 and the test meal produced a downward trend in RQ indicating that the "cookies", providing roughly 50% of calories as fat, tended to increase fatty acid oxidation or decrease carbohydrate oxidation. I can't be arsed to criticise their stats methods. Let's stick with the gross changes.
After two weeks on metformin at an eventual dose rate of 500mg three times daily there is a significant fall in fasting RQ indicating an increase in non-fed fat oxidation compared to the control state.
Under metformin the "cookies" produce a rising RQ, suggesting preferential metabolism of glucose in the immediate post prandial period.
So metformin promotes fat oxidation during fasting but promotes glucose oxidation during the first three hours after a plate of "cookies".
We should see if we can explain these effects on RQ in terms of mitochondrial glycerol-3-phosphate dehydrogenase (mtG3Pdh), electron transporting flavoprotein dehydrogenase (ETFdh) and the redox state of the CoQ couple driving reverse electron transport (RET) through complex I.