Coingestion of glucose with galactose attenuates elevations in circulating galactose concentrations typically observed following ingestion of galactose alone, and is largely attributed to elevated concentrations of glucose rather than insulin (Williams et al., 1983). This study investigated the metabolic response to galactose and whey protein coingestion and examined whether whey protein induced elevations in insulin are sufficient to lower circulating galactose concentrations.

In a randomised crossover design, twelve metabolically healthy participants (8 males, 4 females; age: 26 ± 6 years; height: 174.2 ± 8.4 cm; body mass: 68.8 ± 10.3 kg; body mass index: 22.5 ± 1.8 kg·m^-2) undertook three experimental trials. Participants consumed 300 mL water containing either 25 g galactose (GAL), 25g galactose and 25 g glucose (GAL+GLU), or 25g galactose and 25 g whey protein isolate (GAL+WHEY). On each occasion, participants arrived in an overnight fasted (~10-h) state having adhered to 24-h diet and physical activity replication. Arterialised blood samples were obtained in a fasted state, and at 15, 30, 45, 60, 90 and 120 min during the postprandial period to determine the plasma metabolite and hormone response to the test beverages. Time independent variables were analysed by one-way repeated measures ANOVA, with Bonferroni post-hoc corrections applied for multiple comparisons. Data are mean ± SD.

Peak galactose concentrations were highest in GAL (0.424±  0.445 mmol·L^-1) and were reduced in GAL+GLU (0.029 ± 0.027 mmol·L^-1; P=0.0002) and GAL+WHEY (0.057 ± 0.083 mmol·L^-1; P=0.0003), with no difference in peak concentrations between GAL+GLU and GAL+WHEY. Plasma galactose incremental area under the curve (iAUC) followed the same pattern, with GAL+GLU (1 ± 2 mmol·L^-1·120min; P=0.0014) and GAL+WHEY (1 ± 2 mmol·L^-1·120min; P=0.0008) significantly lower than that of GAL (14 ± 15 mmol·L^-1·120min). Postprandial insulin response (iAUC) did not differ significantly between GAL+GLU (2399 ± 1936 pmol·L^-1·120min) and GAL+WHEY (2191 ± 1544 pmol·L^-1·120min; P>0.9999) but were significantly higher than GAL (424 ± 286 pmol·L^-1·120min; both P<0.0001). Plasma glucose total area under the curve (tAUC) was higher in GAL+GLU (657 ± 40 mmol·L^-1·120min) than GAL (605 ± 39 mmol·L^-1·120min; P=0.0048). Glucose tAUC was reduced further in GAL+WHEY (581 ± 44) compared to GAL+GLU (P=0.0002) and GAL (P=0.0123). The postprandial plasma glucagon response (tAUC) following GAL+WHEY (1663 ± 714 pmol·L^-1·120min) was above that of GAL (630 ± 369 pmol·L^-1·120min; P<0.001) and GAL+GLU (443 ± 161 pmol·L^-1·120min; P<0.001), with GAL and GAL+GLU differing (P=0.0089). Plasma lactate (iAUC) did not differ between GAL+WHEY (48 ± 17 mmol·L^-1·120min) and GAL (69 ± 33 mmol·L^-1·120min; P=0.0576) but was significantly lower than GAL+GLU (93 ± 30 mmol·L^-1·120min; P<0.0001). GAL+GLU and GAL did not differ (P=0.0942).

Coingestion of whey protein isolate with galactose attenuates the typical rise in plasma galactose concentrations observed when galactose is ingested alone. The absence of a clear increase in plasma glucose concentrations with whey protein coingestion suggests a rise in glucose is not essential for galactose clearance, rather the data suggest a role of insulin mediated plasma galactose clearance.