Effects of Pre-Exercise Modified Starch Ingestion on Adipose Tissue Lipolysis and Running Performance

BACKGROUND: It is well-documented that ingesting carbohydrate prior to exercise attenuates fat oxidation. However, it is yet to be established whether this effect is primarily the result of alterations in the mobilization of free fatty acids (FFA) from adipose tissue (i.e. lipolysis). Additionally, there is evidence suggesting that the glycemic index of carbohydrate influences the magnitude of the attenuation in fat oxidation. Specifically, low glycemic index carbohydrate increases fat oxidation relative to high glycemic index carbohydrate. Whether this effect is also due to alterations in adipose tissue metabolism is unknown. Finally, as increasing fat oxidation results in sparing of endogenous carbohydrate, it is possible that pre-exercise low glycemic index carbohydrate may enhance overall energy availability, particularly late in exercise, thereby enhancing endurance performance. PURPOSE: To determine the impact of pre-exercise carbohydrate of different glycemic indices on subcutaneous abdominal adipose tissue (SCAAT) metabolism and running performance. METHODS: Ten trained male runners (mass = 67.1 ± 7.4 kg, VO2max = 63.5 ± 5.3 ml∙kg-1∙min-1, 5-km personal best = 15.9 ± 3.3 min) completed three experimental trials consisting of 30 min at 60% VO2max, 30 min at 75% VO2max, and a 5-km time trial (TT). Thirty min prior to exercise, participants consumed one of three treatment beverages: 1) 75 g low glycemic index modified waxy maize starch supplement (UCAN), 2) 75 g high glycemic index sucrose- and glucose-based supplement (G), or 3) a flavor-, color-, and texture-matched non-caloric placebo (PL). SCAAT lipolysis was assessed via microdialysis. Resting and exercise gas exchange (i.e. VO2 and fuel selection patterns) was assessed via indirect calorimetry. Glucose, insulin, catecholamine, FFA, and glycerol concentrations were analyzed in whole blood and/or plasma at rest and during exercise. Perceptual responses (i.e. gastrointestinal comfort and perceived exertion) during exercise were measured via visual analog scales. Data were analyzed via magnitude-based inferences (i.e. performance, gas exchange, and perceptual responses) and null hypothesis testing (i.e. plasma and interstitial variables; p < 0.05). RESULTS: Immediately prior to exercise, blood glucose was elevated with G vs. PL (+53.0 ± 21.3 mg∙dL-1 [SD]; p = 0.000) and G vs. UCAN (+36.6 ± 24.9 mg∙dL-1; p = 0.00007). Additionally, insulin was increased prior to exercise with G vs. PL (+33.9 ± 11.0 µU∙mL-1; p = 0.000), UCAN vs. PL (+8.7 ± 4.4 µU∙mL-1; p = 0.039), and G vs. UCAN (+25.2 ± 11.0 µU∙mL-1; p = 0.000). VO2 was increased prior to exercise with G vs. PL (+19.6% ± 12.5; likelihoods [%] increase/trivial/decrease: 98/1/0) and UCAN vs PL (10.9 ± 12.2%; 86/11/2). Carbohydrate oxidation was elevated prior to exercise with G vs. PL (+200.1 ± 89.9%; 100/0/0) and G vs. UCAN (+75.5 ± 20.0%; 99/0/0). In addition, carbohydrate oxidation was enhanced at 65% VO2max with G vs. PL (22.9 ± 17.5%; 95/5/0) and UCAN vs. PL (+75.5 ± 20.0%; 75/24/1). Fat oxidation was reduced with G vs PL (-50.1 ± 26.4%; 1/2/97) and G vs. UCAN (-121 ± 74.2%; 0/0/100) prior to exercise, and with G vs. PL (-14.6 ± 9.9%; 1/5/94) and UCAN vs. PL (-9.9 ± 6.8%; 0/10/90) during exercise at 65% VO2max. While SCAAT lipolysis increased over time during exercise, there was no treatment effect. Similarly, plasma catecholamines and glycerol also increased over time but were unaffected by treatment. There was a main effect for time (p = 0.00001) and a treatment interaction (p = 0.002) for plasma FFA; however, post hoc testing revealed no significant differences between treatments. While UCAN likely attenuated abdominal cramping (-18.2 ± 14.3 units vs. PL; -10.0 ± 10.4 units vs. G), tiredness (-6.5 ± 6.6 units vs. PL), and the effort of running (-6.2 ± 5.7 units vs. G) following 60 min running, differences in TT performance were unclear and/or trivial. CONCLUSIONS: In conclusion, pre-exercise ingestion of low glycemic index modified starch attenuated the blood glucose and insulin response to feeding. Additionally, carbohydrate ingestion reduced fat oxidation, and this effect was attenuated at rest with low glycemic index carbohydrate. Despite these effects, adipose tissue metabolism and running performance were not influenced by pre-exercise carbohydrate regardless of glycemic index.