Benefits
Post-meal blood sugar
This comes from the L-arabinose ingredient, not from Prenulin. In a randomised double-blind crossover study, 15 healthy men drank 75 g of sugar in 300 ml of water with different amounts of L-arabinose added. At the highest amount (4%), the blood sugar peak was about 11% lower, the insulin peak was about 33% lower and came later, and insulin over the whole test was about 23% lower. That is one small study of one ingredient using a single sugary drink, not a test of the finished product.
Sugar metabolism
L-arabinose slows the digestion of table sugar (sucrose) by partly blocking the gut enzyme that splits it, which is what the healthy-volunteer study measured. The chromium picolinate study cited here is a separate piece of research: 600 micrograms a day for four months in 71 people who already had type 2 diabetes with poor control. In that group, average HbA1c fell 1.90 points versus 1.00 in the comparison group and fasting glucose fell 31.0 versus 14.0 mg/dl. That trial was single-blind rather than double-blind, blood fat changes were not significant, and it was done in people with a diagnosed disease, so it does not describe what chromium does for a healthy person.
Carbohydrate handling (blood sugar, not weight)
Prenulin is marketed for softening the impact of sweet and starchy foods, but the cited research does not show a weight benefit. The sugar-drink study measured appetite and how much food people ate afterwards and found no significant change in either, and no significant change in blood triglycerides. Nothing cited here shows an effect on body weight.
Mechanism of action
Sucrase inhibition
L-arabinose inhibits the sucrase enzyme, slowing the breakdown of sucrose into absorbable sugars and blunting the post-meal glucose rise.
Chromium and insulin
Chromium is a mineral that is thought to help insulin work, so cells can take up glucose from the blood. This is the proposed explanation rather than something the studies cited here measured directly.