Meal cysteine improves postprandial glucose control in rats fed a high-sucrose meal

Whey protein, particularly the α-lactalbumin fraction, are rich in cysteine (cys) and could therefore favor postprandial glucose homeostasis by a glutathione-mediated effect. This work investigates the effects of the ingestion of an α-lactalbumin-rich whey concentrate (α-LAC) during a high-sucrose (HS) meal on postprandial glucose homeostasis in healthy rats. In the first experiment, rats received an HS meal containing 14% protein, in which the protein source was either α-LAC (HSa) or total milk proteins, alone (HS0) or supplemented with 17 mg (HS1) or 59 mg (HS2) of N-acetylcysteine (NAC). This resulted in a total cys content 3.6-fold higher in the HS1 and HSa meals and 12-fold higher in the HS2 meal, when compared to the HS0 meal. Postprandial parameters were monitored for 3 h after ingestion of the meal. The same measurements were performed on rats injected with 4 mmol/kg of buthionine sulfoximine (BSO), a specific inhibitor of glutathione synthesis. Increasing the meal's cys content dose-dependently reduced both postprandial glucose and insulin (P<.05). The inhibition of glutathione synthesis with BSO injection abrogated the beneficial effects of NAC supplementation on postprandial glucose response but did not affect those of α-LAC. These results show that (1) the substitution of α-LAC for total milk protein reduces glucose response, as does the addition of a cys donor to the meal, (2) but contrary to those of a simple cys donor, the beneficial effects of α-LAC are not entirely mediated by glutathione synthesis, suggesting additional mechanisms.