This 2021 paper reported results of two related human clinical trials:

“Lean and prediabetic overweight/obese men were included in two randomized crossover studies. In one study, participants received supplements of either long-chain inulin+resistant starch (INU+RS), INU or maltodextrin (placebo, PLA) the day prior to a clinical investigation day. The second trial studied beta glucan+RS (BG+RS) versus BG and PLA.

1. In lean men, INU+RS increased breath hydrogen and fasting plasma butyrate, which was accompanied by increased energy expenditure, carbohydrate oxidation, and peptide YY, and decreased postprandial glucose concentrations compared to PLA.

In prediabetic men, INU+RS increased plasma acetate compared to INU or PLA, but did not affect metabolic parameters.

The three supplements were:

• INU: 12 g long-chain inulin in combination with 5.43 g maltodextrin to make it isocaloric.
• INU+RS: 12 g of long-chain inulin in combination with 9.39 g 80% resistant starch RS2 granular potato starch.
• PLA: 11.43 g maltodextrin.

2. BG+RS increased plasma butyrate compared to PLA in prediabetic individuals, but did not affect other fermentation/metabolic markers in both phenotypes.

The three supplements were:

• BG: 35.25 g 34% yeast beta glucan with 5.43 g maltodextrin to make it isocaloric.
• BG+RS: 35.25 g 34% yeast beta glucan in combination with 9.39 g 80% RS2 granular potato starch.
• PLA: 11.43 g maltodextrin + 13.1 g protein and 4.58 g fat, same type and amounts as in the beta glucan product.

Effects of one-day consumption with a ‘slowly fermentable’ complex INU or BG alone, or INU or BG combined with a more ‘rapidly fermentable’ RS on substrate and energy metabolism were studied. These fiber mixtures were selected based on a high distal colonic acetate and total SCFA production in a validated in vitro model of the human colon.

Further research should study whether longer-term supplementation periods are required to elicit beneficial metabolic health in prediabetic individuals.”

https://www.tandfonline.com/doi/full/10.1080/19490976.2021.2009297 “Fiber mixture-specific effect on distal colonic fermentation and metabolic health in lean but not in prediabetic men”

The Discussion section related these findings to other research. Not sure how these researchers determined a trial time period. As Reviewing clinical trials of broccoli sprouts and their compounds pointed out:

“Biomarkers of effect need more time than biomarkers of exposure to be influenced by dietary treatment.”