No Effect of High-Amylose Maize Starch on Pre- or Postexercise Hydration Status, Thermophysiological Strain, or Exhaustive Exercise Performance During Exertional Heat Stress in Endurance Runners.

High-amylose maize starch (HAMS), containing fermentable resistant starch (RS), is suggested to improve fluid absorption/retention in athletes but lacks supporting evidence and may increase gastrointestinal symptom (GIS) risk. This study investigated the use of HAMS in pre- and postexercise hydration protocols on hydration status and GIS before, during, and after exertional heat stress (EHS), and thermophysiological strain and exhaustive exercise performance during EHS. Ten endurance-trained runners (eight men and two women) completed two trials in randomized order, consuming a HAMS beverage (12.7 g RS) or placebo 12 hr pre-EHS as part of a 25-ml/kg hyperhydration protocol. Participants then completed 2 hr of steady-state running (60% maximum oxygen uptake, ∼30 °C, 35% relative humidity) followed by a time-to-exhaustion performance test. Post-EHS, participants consumed a HAMS recovery beverage (6.2 g RS) or placebo while replacing 150% of their fluid deficit over 3 hr. HAMS did not influence net fluid balance pre-EHS (HAMS: +44 ± 642 ml, placebo: +210 ± 697 ml) or post-EHS (HAMS: +1,636 ± 687 ml, placebo: +1,662 ± 697 ml; p = .40). Changes in plasma volume and osmolality, and thermophysiological variables, did not differ between trials (all p > .05). Time-to-exhaustion performance test was unaffected by the intervention (HAMS: 1,190 ± 508 s, placebo: 1,343 ± 631 s; p = .10). GIS incidence and severity scores were similar in both trials (all p > .05). A beverage containing RS from HAMS 12 hr before EHS, or immediately following EHS, does not alter retention of water volumes recommended for preexercise hyperhydration or postexercise rehydration nor GIS pre-, during, or post-EHS. As a result, no changes in hydration status, thermophysiological strain, or performance were observed.