Hydration With A2 β-casein Milk: Gastrointestinal Tolerance During Cycling And Running

Hydration is crucial for performance and health during physical activity, though some beverages like milk may cause gastrointestinal (GI) discomfort, which can be aggravated by exercise type. A2 milk, however, is reportedly easier to digest. PURPOSE: To evaluate GI symptoms consuming A2 milk during moderate-to-high intensity cycling and running, comparing symptom severity between activities. METHODS: Eleven active participants (18-40 years) completed two sessions, consuming 16 mL/kg body mass of lactose-free, reduced-fat A2 β-casein milk before and during either cycling or treadmill running at 70-80% HRmax for 60 min. Hedonic ratings were recorded pre-exercise, at 30, and 60 minutes, and GI symptoms were assessed upon arrival, pre-exercise, at 30, and 60 minutes. RESULTS: Fluid intake averaged 973.99 ± 196.23 mL for running and 971.07 ± 201.91 mL for cycling (p > .05). GI symptoms differed significantly between conditions (p < .05), with higher flatulence during cycling (0.9 ± 1.6, 0-5) (mean ± SD, min-max) than running (0.1 ± 0.4, 0-3) on a 0-9 scale; other GI symptoms showed no differences. Symptom severity varied over time (p < .001): belching increased from arrival (0.1 ± 0.4, 0-2) to pre-exercise (0.6 ± 1.0, 0-3), 30 min (3.2 ± 2.3, 0-8), and 60 min (3.7 ± 2.3, 0-9). Similarly, thickened or cut saliva rose from arrival (0.04 ± 0.2, 0-1) to pre-exercise (0.8 ± 1.3, 0-4), 30 min (2.3 ± 1.8, 0-6), and 60 min (2.4 ± 1.9, 0-5). Hedonic ratings declined significantly (p < .05) from arrival to pre-exercise (7.6 ± 0.8, 6-9), 30 min (6.4 ± 1.4, 4-8), and 60 min (6.1 ± 1.5, 3-8) during running, and from arrival to pre-exercise (8.2 ± 0.6, 7-9), 30 min (6.8 ± 1.2, 5-9), and 60 min (6.4 ± 2.0, 3-9) during cycling, with no differences between conditions (p > .05). CONCLUSIONS: While flatulence was slightly higher during cycling, most GI symptoms and palatability showed no significant differences between activities. Instead, symptoms were largely time-dependent: belching and thick/stringy saliva increased and palatability declined progressively over the 60-minute period. These findings suggest that the effect of lactose-free, reduced-fat A2 milk on GI tolerance and enjoyment depends more on exercise duration than activity type. Funded by Dos Pinos Milk Producers Coop. of Costa Rica under a cooperation agreement with University of Costa Rica. Supported by: Funded by Dos Pinos Milk Producers Coop. of Costa Rica under a cooperation agreement with University of Costa Rica for research project VI-838-C4-325.