It has been well established that carbohydrate (CHO) consumption before and during exercise improves exercise performance in events lasting longer than one hour, by maintaining blood glucose, high CHO oxidation rates and possibly sparing endogenous glycogen stores [1, 2]. What is less clear is the relationship between the CHO amount, type and form to maximize endurance performance.

Early studies utilized single CHO types such as glucose or glucose polymers [2, 3], but more recently the ingestion of a glucose plus fructose mixture has been shown to be more effective [1, 4–7]. Ingestion of a glucose plus fructose drink had higher exogenous CHO oxidation rates compared to glucose or fructose only drinks due to increased intestinal absorption rate from both the sodium-dependent glucose (SGLT1), fructose (GLUT5), and glucose and fructose (GLUT2) intestinal transporters [1, 6, 8]. Ingestion of a mixed CHO source allows for greater CHO absorption and utilization, which can be beneficial during prolonged exercise.

More recently, researchers have investigated whether other CHO forms (solids and semisolids) have the same benefits as a liquid. No significant metabolic or exercise performance differences have been found when consuming solid or semisolid CHO sources before-exercise [9–11]. Previously in our lab, the effects of a sport drink, sport gel, sport beans and water were studied in trained cyclists during 80-min of exercise at 75% VO₂max, showing no significant metabolic or performance differences between the commercial sport products [12]. A series of studies performed by Pfeiffer and colleagues also confirmed that the exogenous CHO oxidation rates between CHO delivery via fluid, semi-solid or solid were similar during 180-min of cycling at 58% VO₂max [5, 13].

As individuals decide to take a more whole food approach, other nutritional factors (e.g. dietary fiber) can affect CHO supplementation choice. The low digestibility of fiber can elicit an osmotic and fermentative effect in the intestinal lumen, which can have unwanted side effects such as flatulence, belching, nausea, abdominal pain and diarrhea [14]. The prevalence of gastrointestinal (GI) discomfort may increase when ingesting low digestible CHO combined with exercise, resulting in a decrease in performance. A study examining the effects of raisins versus sports gels as pre-exercise feedings in cyclists showed no significant metabolic or performance differences during a 45-min sub-maximal cycling bout at 70%VO₂max followed by a 15-min performance trial [10]. We know of no study to examine the effects of raisins versus commercial sports products in runners. GI complaints are more pronounced during running, which may be related to the greater mechanical jarring involved [15]. Reports have also noted that 83% of marathoners and 81% of endurance athletes experience some level of GI distress during training or competition [15]. Ingesting a higher fiber supplement in raisins during an endurance run may cause more GI discomfort than eating lower fiber sports products.

Therefore, the purpose of this study was to examine the metabolic and running performance effects and GI tolerance of a natural whole food product (raisins) compared to a commercial product (sport chews) and water only. It was hypothesized that the raisins and chews would elicit similar metabolic responses and both would improve running time trial performance over water only, yet because of the higher fiber content, raisins would elicit greater GI discomfort.

Our results indicate that ingestion of a natural food product, raisins, had similar performance effects as a commercial sports product in chews and both products improved running time trial performance over water only. Raisins and chews maintained a higher % of non-protein macronutrient oxidation derived from CHO over the 80-min running bout at 75% VO₂max than water only. The commercial product did cause slightly higher insulin levels and CHO oxidation rates during exercise than raisins. Raisins had a greater increase in creatine kinase during exercise than both chews and water only. Our data suggests that consuming a natural, relatively fiber-rich CHO source (raisins) had similar GI effects as a commercial product.

All treatments maintained blood glucose levels at pre-exercise values during the 80-min sub-maximal trials. However, the glucose levels during exercise were higher with the commercial product compared to water only. Similar glucose responses between carbohydrate forms is in agreement with a study examining the metabolic effects of raisins (glycemic index (GcI) = 62) versus sport gels (GcI = 88) in cyclists [10]. Even though at rest, high-GcI foods result in an elevation of plasma insulin concentrations compared to low-GcI foods, this has typically not been observed during exercise. Increased catecholamine levels typically suppress insulin release, even when CHO is consumed during exercise [18]. In our study, serum insulin levels were mostly unchanged during the exercise bout for the carbohydrate treatments and decreased during exercise in the water only trial. Insulin levels were higher for the commercial product during the first 60-min of exercise compared to both raisins and water only. This is in contrast to the study by Kern et al. where insulin levels were similar between raisins and sports gel after 45-min of cycling at 70% VO₂max [10]. The feeding protocol was different in the Kern et al. study compared to ours in that the products were fed 45-min prior to exercise (ours ~10-min prior) and not given during exercise (we supplemented every 20-min of exercise). A slightly lower GcI (GcI = 62) with the raisins compared to chews (GcI = 88) may have contributed to the lower insulin response with raisins in our study.

Both CHO treatments produced higher RER values after 60-min of exercise, and thus greater energy contributions from CHO and less from fat compared to water only. Interestingly, the raisin treatment induced a lower energy contribution from CHO and greater from fat compared to the chews treatment. The slightly lower GcI may have decreased CHO absorption at the intestine and caused a slightly lower CHO oxidation rate with the raisins. The lower energy contribution from fat and higher from CHO with the chew treatment could have resulted from a type I statistical error, considering the small, non significant RER differences between raisins and chews during the last 20-min of exercise. Other studies support that relatively low-GcI foods do not have a different metabolic effect during exercise compared to high-GcI foods, especially when subjects receive carbohydrate supplements during exercise [10, 18].

Preventing GI distress is important for competitive endurance performance. In our study, there was remarkably little to no adverse GI effects with all treatments. Studies have found an increase in GI symptoms experienced during running, which has been attributed to the mechanical jarring involved in running and the decreased blood flow to the GI tract during exercise [15, 19]. GI blood shunting is dependent on exercise intensity, which can affect passive and active CHO absorption and delivery to the systemic circulation [20] and GI discomfort experienced during exercise. It has been found that at VO₂max, both active and passive intestinal glucose absorption is significantly reduced compared to 30% and 50% VO₂max [20]. Our subjects completed the 80-min running bout at ~75% VO₂max, which may have reduced blood flow to the GI tract. However, the lower CHO consumption rate (~0.7 g·min^-1) may have reduced the risk of developing GI discomfort. The recommended CHO consumption rate during exercise is 0.5-1.0 g·min^-1[21]. Therefore, we speculate that the exercise intensity and amount of CHO consumed allowed for adequate GI blood supply to support high oxidation efficiency and a smaller % of the ingested CHO remained in the GI tract [1].

It was hypothesized that the increased fiber content in raisins, combined with the mechanical jarring involved with running, would result in greater GI discomfort. The dietary fiber in raisins could have had an osmotic effect in the intestinal lumen resulting in abdominal pain and diarrhea [14]. Our subjects consumed ~7 g·hr^-1 fiber during the raisin treatment and had no severe GI disturbances compared to the chews and water treatments. A slight increase in belching was experienced for both the raisins and chews treatment yet, exercise performance was better in these trials than water only. There seems be a direct relationship between exercise duration and GI distress [15, 22], especially in ultramarathon distances whereby GI distress can severely limit performance [22]. It is possible that if individuals continue to consume fiber-rich CHO sources, such as raisins, during endurance events >2-hr, the combined increase in exercise duration and fiber content in the GI tract could increase the severity of GI symptoms experienced. Further study with longer distances and in actual race conditions is warranted. Another factor that can contribute to GI discomfort is the hydration status of an individual. Subjects have reported GI complaints (37.5%) while exercising in a dehydrated state (4% BW loss) [23]. Hydration status in our subjects was sufficient in all treatments (hematocrit = ~47% and BW loss = ~1.5%), which could explain the few GI complaints.

The raisin treatment elicited higher plasma CK concentrations, corrected for baseline measures, during the 80-min run. We are unsure as to the causes of the higher CK values with the raisins, but only half of the subjects had higher responses with the raisin treatment compared to water or chews. The large standard deviations in the measurement of plasma CK levels could have played a role as could higher baseline levels before treatment consumption. The subjective scoring of muscle soreness and fatigue were similar between all treatments as was time trial performance and hydration status. Thus, the CK response to exercise appeared to be dissociated from other indices of muscle damage (e.g. muscle soreness and performance impairment) [24]. It is uncertain as to what factors resulted in the higher plasma CK concentrations with raisin ingestion and further research on the potential detrimental effects of raisin ingestion with exercise durations greater than 2-hours is needed.

This study is limited in that we conducted this experiment in the laboratory instead of an actual running competition and the treatments were given to subjects while standing on the treadmill instead of while running. Since the majority of runners in competition consume CHO while still running, it is uncertain whether solid CHO can negatively affect performance times solely by the act of consumption. Previous field studies have found that semi-solid CHO intake increased running time compared to liquid CHO intake [25]. There is the possibility that chewing solid CHO sources (e.g. chews and raisins) can disrupt an individual’s breathing pattern and in combination with running could negatively affect performance.

In conclusion, our study provides evidence that solid CHO consumption during a ~100-min run allows for maintenance of blood glucose levels and improved performance compared to water only. Our data suggests that consuming a natural CHO source (raisins) within the ACSM/ADA/DC recommendations [21] is well tolerated and maintains blood glucose levels and running performance similar to a commercial CHO product (sport chews).