Whey protein isolate and glycomacropeptide decrease weight gain and alter body composition in male Wistar rats

The effect of feed protein type on body composition and growth has been examined. Evidence exists that whey protein concentrate is effective at limiting body fat expansion. The presence of caseinomacropeptide, a mixture of glycosylated and non-glycosylated carbohydrate residues, in particular glycomacropeptide (GMP) in whey protein concentrate may be important for this effect. The influence of whey protein isolate (WPI) and GMP on weight gain and body composition was examined by feeding Wistar rats ad libitum for 7 weeks with five semi-purified American Institute of Nutrition-based diets differing in protein type: (1) casein; (2) barbequed beef; (3) control WPI (no GMP); (4) WPI + GMP at 100 g/kg; (5) WPI + GMP at 200 g/kg. Body composition was assessed, and plasma samples were assayed for TAG, insulin and glucose. Body-weight gain was lower (-21 %) on the control WPI diet relative to casein, with a non-significant influence associated with GMP inclusion (-30 %), the effect being equivalent at both levels of GMP addition. Renal and carcass fat mass were reduced in the highest GMP diet when compared with WPI (P<0.05). Plasma insulin was lowered by GMP at the highest addition compared with WPI alone (-53 %; P< 0.01). Plasma TAG in the WPI + GMP (200 g/kg) group were lower (-27 %; P< 0.05) than the casein and beef groups. In conclusion, GMP appears to have a significant additional influence when combined with WPI on fat accumulation. WPI alone appears to have the predominant influence accounting for 70 % of the overall effect on body-weight gain. Mechanisms for this effect have not been identified but food intake was not responsible.