Gastrointestinal tolerance to a standardized milk-based hydration strategy is similar across exercise modalities

Gastrointestinal (GI) symptoms are frequently reported during endurance exercise, and running is often assumed to provoke greater GI discomfort than cycling due to higher mechanical impact. However, most supporting evidence derives from field studies with limited control of exercise intensity, hydration volume, and beverage composition, making it difficult to isolate the independent effect of exercise modality. This study aimed to compare GI symptom burden and beverage palatability between treadmill running and stationary cycling under controlled conditions.

In a randomized crossover design, physically active adults completed treadmill running and stationary cycling under standardized conditions, including matched relative exercise intensity, duration, hydration volume, and beverage composition. A low-fat, lactose-free A2 cow's milk was used as the sole hydration beverage. GI symptoms were assessed repeatedly throughout exercise, and beverage palatability was evaluated using hedonic ratings.

Overall GI symptom burden was comparable between modalities, with no significant differences in aggregate upper GI, lower GI, or systemic scores (all p ≥ 0.34), and equivalence confirmed for upper GI and systemic regions. Although cycling was associated with higher odds of heartburn/acidity (OR = 7.40; p = 0.001), flatulence (OR = 2.45; p = 0.012), defecation urgency (OR = 231.40; p = 0.007), and headache presence (OR = 2.38; p = 0.044), these differences did not translate into higher composite symptom burden. Palatability remained acceptable and did not differ between modalities.

When exercise intensity, duration, hydration volume, and beverage composition are controlled, exercise modality does not meaningfully influence GI tolerance. Low-fat, lactose-free A2 milk was well tolerated across both modalities, supporting its use as an in-exercise hydration beverage regardless of exercise mode.