Optimisation of carbohydrate supplementation during endurance exercise in temperate and warm environments

The provision of exogenous carbohydrate during endurance exercise is a recognised strategy to improve performance, through maintenance of blood glucose and liver glycogen concentrations, stimulation of the central nervous system (CNS) through oral exposure aiding mental and cognitive stimulation and ultimately delaying fatigue. Carbohydrate intake recommendations have been established offering guidance on sugar dose and type (glucose-to-fructose ratios) varying on exercise duration. However, after consideration there are gaps in knowledge and a paucity of evidence in some situations required to help optimise carbohydrate intake strategies for athletes.

Recently, food first approaches have been advocated but there is limited data relating to food-based carbohydrate intake and endurance performance and are omitted from current recommendations as highlighted by our recent systematic review (Review A). In our review 15 articles were included and showed no difference in cycling (n=14) or running (n=1) performance between carbohydrate supplements (i.e., gels and drinks) compared to foods (i.e., bananas, raisins, and potatoes). There are health implications associated with artificial/processed fructose sources including dental and oral health; however, fructose is naturally occurring in fruit and may offer a healthier alternative. The one potential drawback from using food-based or natural carbohydrate supplements during endurance exercise is the increased likelihood of gastrointestinal (GI) symptoms due to plant components such as fibre being present. Therefore, the first aim of this thesis was to investigate endurance performance (cycling; Chapter 3 and running; Chapter 4) and GI comfort using a natural fructose source (apple puree) compared to an isocaloric but highly processed fructose (crystalline fructose). In chapter 3, nine trained male cyclists (age 24 ± 7 years; V̇O2peak 65 ± 6 mL/kg/min) completed a familiarisation and two experimental trials (60 g/h carbohydrate, 120 min at 55% Wmax and ~15 min time trial). Subjective GI measures made throughout the preload and at the end of the time trial. In chapter 4, eleven trained runners (9 males, 2 females; age 32 ± 8 y, 89:53 ± 13:28 min half-marathon personal record) completed a familiarisation (8 miles) and two experimental trials (13.1 miles) on an outdoor running course ii (temperature AP: 16.7 ± 2.7°C; GF: 16.5 ± 2.7°C, relative humidity AP: 77.4 ± 11.0%; GF: 73.3 ± 12.0%, WBGT AP: 16.0 ± 2.9°C; GF: 15.3 ± 3.4°C, and wind speed GF: 1.4 ± 0.8 m/s; AP: 1.2 ± 1.0 m/s), with blood and urine samples collected before and after the run. Subjective GI measures made throughout the run. There was no performance difference in either a short cycling time trial (~15 min; GF 792 ± 68 s, AP 800 ± 65 s; P=0.313) or half-marathon running performance (AP: 89:52 ± 09:33 min, GF: 88:44 ± 10:09 min; P=0.684). Additionally, there was no elevation of GI symptoms in the natural fructose/apple puree trial.

Athlete endurance performance is recognised to be impaired in the heat and alterations in carbohydrate metabolism have been described with increased reliance on endogenous carbohydrate as described in our systematic review (Review C). However, despite research suggesting alterations in carbohydrate utilisation in the heat (Review C), current carbohydrate recommendations do not account for different environmental conditions and may not be applicable for exercise in warm environments. Thus, the aim of the investigation of Chapter 5 was to establish whether cycling performance and exogenous carbohydrate oxidation were reduced in a warm (32 °C; 50% RH) compared to a temperate environment (19 °C; 50% RH) and to investigate GI symptoms. Ten trained male cyclists/triathletes (36 ± 6 y; 55 ± 6 mL/kg/min) completed a V̇O2peak/ familiarisation trial and two experimental trials in 19°C (TEMP) and 32°C (WARM), involving 2h at ~50% Wpeak (preload) and an ~15 min time trial (TT) with fan-provided airflow (~29 kph in preload, ~35 km/h in TT) covering the cyclist. A glucose drink containing [U13C]-glucose was consumed every 20 min in preload (72 g/h). Water was provided ad libitum. Subjective GI symptoms were measured through the preload and at the end of the time trial. Results showed impaired cycling performance (TEMP 819 ± 47 s; WARM 961 ± 130 s; P=0.002) and a reduction in exogenous carbohydrate oxidation (minimum estimate of exogenous oxidation efficiency TEMP 56 ± 13%; WARM 49 ± 13%; P=0.021) in the warm condition but with no difference in GI symptoms (TEMP 1 ± 1; WARM 1 ± 2; P=0.718).

Although different carbohydrate formats (gels, drinks, bars) can be used interchangeably in temperate environments as demonstrated in our systematic review (Review B) the same choice may not apply in warm conditions due to the increased likelihood for hypohydration. In the review only one out of 22 studies demonstrated a reduction in endurance performance with consumption with bars compared to drinks and gels. Overall, the review concluded there was no differences in endurance performance between different carbohydrate formats but nearly all studies were conducted in an ambient environment. Chapter 6 investigated the impact on performance and GI symptoms with ingestion of carbohydrate gels versus carbohydrate drinks in a warm environment (32 °C). The study demonstrated impaired cycling performance in the gel trial compared to the drink (GEL 896 ± 121 s, DRINK 848 ± 82s, P=0.027), most likely explained by the differences in hydration status (GEL 1.73 ± 0.66 %, DRINK 0.55 ± 0.8%; P=0.001). There was no difference in reported GI symptoms (overall GI comfort GEL 2 ± 2, DRINK 2 ± 2; P=0.664).

Another potential strategy to improve carbohydrate uptake and reduce experienced GI symptoms is co-ingestion with other ingredients. One such ingredient is l-citrulline since it is involved with the production of nitric oxide and the resultant increased vasodilation may help promote carbohydrate absorption and uptake from the GI tract as well as reduce associated GI symptoms. Thus, the aim of Chapter 6 was to determine whether peri-exercise co-ingestion of l-citrulline (6 g/h) and carbohydrate (60 g/h) improved cycling performance and glucose utilisation with reduced GI symptoms. Coingestion did not improve performance (CHO+CIT 923 ± 70s, CHO 946 ± 90 s; P=0.279) and there was no difference in GI symptoms between trials (overall GI comfort CHO+CIT 1 ±2, CHO 1 ± 1; P= 0.804). However, exogenous carbohydrate oxidation was reduced when l-citrulline was co-ingested (minimum estimate of exogenous oxidation efficiency CHO 54 ± 10%; CHO+CIT 47 ± 8%; P=0.004). The mechanism is unclear and requires further investigation.

From the work described in this thesis, it can be concluded that natural fructose sources can be effectively used to fuel endurance performance (cycling and running) without elevating GI symptoms (Chapters 3 and 4). Exercise in warm conditions impairs cycling performance despite appropriate facing airflow and is associated with reduced carbohydrate uptake from exogenous sources (Chapter 5). Additionally, carbohydrate drink consumption resulted in improved performance compared to carbohydrate gels in a warm environment and, therefore, integrated fuel and fluid strategies may need to be considered for exercise in the heat (Chapter 6). Finally, there does not seem to be any additional benefit of co-ingesting l-citrulline with carbohydrate during exercise (Chapter 7). There were no increased GI symptoms when exercising in the heat (Chapter 5) with either gel or drink consumption (Chapter 6), or with co-ingestion with l-citrulline (Chapter 7).