Acute load-dependent effects of oral whey protein on gastric emptying, gut hormone release, glycemia, appetite, and energy intake in healthy men.

Randomized, double-blind, crossover study in healthy, lean men at the Royal Adelaide Hospital, Australia. Twenty men were recruited and 18 completed all 3 study days. The active intervention conditions were the 30 g and 70 g whey protein isolate loads.

Two acute oral whey protein isolate drinks were tested on separate study days: a low-load drink containing 30 g and a high-load drink containing 70 g, each in 450 mL total volume. Both were compared with a low-energy cordial/water control in a randomized, double-blind, crossover design.

Both whey protein loads altered gastric emptying, gut hormone responses, and short-term appetite, and both suppressed buffet energy intake versus control. Gastric emptying was slower with whey protein, with T50 of 26 6 3 min for 30 g and 65 6 9 min for 70 g versus 12 6 0.5 min for control (P < 0.001). Buffet intake was lower after both loads, at 1027 6 81 kcal with 30 g and 998 6 71 kcal with 70 g versus 1174 6 91 kcal with control (P < 0.01), while total energy intake was not significantly different (P = 0.07). Higher protein load produced more sustained hormone responses, including higher GLP-1 and later peaks for several hormones, but did not produce greater suppression of intake than the 30 g load. Overall, the findings support a threshold effect around 30-40 g of protein for reducing intake when a meal follows within a few hours.

Small sample size and a narrow population of healthy, lean men limit generalizability. The study was acute and tested only short-term responses across three test days, so it does not address longer-term outcomes. Two participants were excluded, and gastric emptying data from 2 participants were also excluded because of intragastric air and suboptimal image quality.