CARBOHYDRATE FEEDING DURING ENDURANCE PERFORMANCE

What you will learn:

A needs analysis of triathlon performance
Fundamentals of carbohydrate feeding during endurance exercise
Critical analysis of existing sports drinks & products
My recommended sports drinks & products based on the existing research
You will also learn some scientific jargon – always a bonus!

Who is this applicable to:

Triathletes
Endurance athletes
Possibly team-sport athletes
Anyone interested in carbohydrates during exercise
Anyone interested in nutrition science?

Who should not read this:

Low carbohydrate advocates
Clean eaters

Before we go anywhere with this, we must look at physiological demands of the sport of triathlon in order to gain an understanding of the nutritional requirements.

TRIATHLON

The sport of triathlon requires athletes to sequentially swim, cycle and run over various distances – too much like hard work to me – therefore the physiological demands and nutritional challenges differ greatly from race to race. A case study revealed the carbohydrate energy expenditure alone was 15,762 kJ (~3,767 kcal) during the Grand Columbian Half Ironman Triathlon, and muscle glycogen concentrations were reduced to 36.6 mmol/kg wet wt -1/hr-1 from an initial 227.1 mmol/kg wet wt -1/hr-1 (1) – obviously a pretty successful carb load, something that will be covered in future blog posts.

Similarly, Noakes (2000) demonstrated near complete glycogen depletion following the 4.5-hour cycling stage of a simulated Ironman Triathlon, where elite male triathletes exercised at an intensity of 71% vo2max. It was interesting to note that following the 180km cycle, glycogen stores were almost completely depleted, although the triathletes were able to maintain running speeds of 16km/h for the remaining 160-minutes, at an intensity of ~66% vo2max, possibly due to the increased rate of intramuscular triglyceride oxidation (aka fat burning) of the elite participants (3). This study, among others clearly demonstrates the elevated energy and carbohydrate requirements of triathletes, in that regards the ingestion of carbohydrate pre- and during exercise will be have a performance enhancing effects.

SPORTS DRINKS

The sports drink industry is projected to reach $2bn by 2016, this phenomenal figure highlights the rapid rise in popularity of such drinks, essentially designed to delay the onset of fatigue and enhance athletic performance through the delivery of carbohydrate and fluid simultaneously. When an industry is this financially powerful it is necessary to critique it thoroughly and analyse the efficacy of its product. So in the following article we will do exactly that. Initially we will look at the carbohydrate content, and examine which drinks are most suitable from that perspective. In part two of this short series we will go on to explore the efficacy of the additional ingredients in modern sports drinks.

FATIGUE

Fatigue is generally defined as a decrease in force production (4); although the precise cause of fatigue during endurance exercise of moderate intensity (65 – 75% vo2max) remains unknown (5) several factors are thought to be responsible, predominantly the depletion of glycogen stores, reduced blood glucose concentrations and hypoglycemia.

Existing evidence suggests that the ingestion of carbohydrate during a prolonged exercise task is effective in increasing endurance and athletic performance through:

Improved maintenance of blood glucose concentrations
Increased rates of carbohydrate oxidation
Sparing on endogenous carbohydrate stores (stores within the body)

Carbohydrate oxidation simply refers to the energy obtained from the metabolism (breakdown) of carbohydrate, obviously important during exercise.

CARBOHYDRATE FEEDING DURING ENDURANCE PERFORMANCE

A systematic review with meta-analysis of the existing carbohydrate during exercise (time trial) data concluded that the consumption of 30 – 60 g/hr carbohydrate in a 6 – 8% glucose concentration during exercise greater than 1-hour maintains blood glucose concentrations and high carbohydrate oxidation rates, and may improve time trial performance by ~2% (6). An improvement of 2% in sport of any kind is significant, an improvement of 2% in the 2013 ITU World Triathlon Grand Final in London was the difference between 1st and 26th place (International Triathlon Union, 2012). This clearly highlights the significance of carbohydrate during endurance exercise.

Such carbohydrate intakes achieve the maximal 1 – 1.1 g/min-1 carbohydrate oxidation rates, with higher intakes associated with increased incidence of gastrointestinal symptoms (stomach issues) and little additional effect.

Carbohydrate oxidation from glucose alone appears to be limited to rates of 1 – 1.1 g/min-1 owing to the fact that the sodium-dependant glucose transporter (SGLT1) becomes saturated at rates of 1 g/min (7, 8). More recent data however has illustrated a role for multiple-transportable carbohydrates, where multiple transport mechanisms can be utilised at the same time. Fructose utilises the transporter GLUT5, so saturating SGLT1 through glucose ingestion with the addition of fructose will allow for the delivery and oxidation of more carbohydrate per minute – score! Research has confirmed this, with observed carbohydrate oxidation rates around 65% greater than previously thought to be maximal (1 – 1.1 g/min from a single source). Such blends often utilise a 2:1 ratio glucose:fructose or maltodextrin:fructose, and can result in exogenous carbohydrate oxidation rates of 1.75 g/min.

These multiple-transportable carbohydrate formulations also offer the advantage of:

Enhanced fluid delivery
Improved oxidation efficiency
Reduced likelihood of gastrointestinal distress (9).

The increased rates of carbohydrate oxidation with multiple-transportable carbohydrates do result in improved time trial performance too, one study concluded the ingestion of a glucose:fructose (2:1 ratio) beverage resulted in an 8% improvement in time trial performance compared to the ingestion of a glucose only beverage, both beverages delivered carbohydrate at equal rates of 1.8 g/min (10). So sports drinks that contain both glucose and fructose ingested at high rates may be more beneficial than those that contain glucose alone (11).

To achieve such high oxidation rates, large dosages of carbohydrate are required; such large doses may not be practical or even possible during competition. Recent research investigating the dose-response relationship between carbohydrate and endurance performance suggested an intake of 78 g/hour in an 1:1:1 glucose-fructose-maltodextrin formulation was optimal during the ~160-minute cycling time trial employed in that study. I’d agree, such an intake aligns with the previous evidence. Therefore, for triathlon performance the ingestion of carbohydrate at rates of around 60 – 90 g/hour is recommended. Although certain individuals do experience some serious gastrointestinal issues regardless of the dosage so I would recommend developing an individualised feeding strategy with assistance from a professional performance nutritionist. The optimal carbohydrate mixture would be a combination of glucose and fructose, maltodextrin and fructose, or glucose, sucrose and fructose.

RECOMMENDED PRODUCTS

Existing sports drinks offer such formulations, lets look at some of the options:

* ratio not stated.

Based on the existing products on the market, my recommendation would be High5 EnergySource, this appears to be optimal in regards carbohydrate formulation, and would probably be the most practical in that the sachet can be carried easily and mixed with water easily too. High5 also offer an EnergySource plus option that contains additional caffeine, we will go on to examine the beneficial effects of caffeine in the part two.

You will also notice that Coca-Cola is included on the above list, a number of world leading endurance sports athletes now utilse Coca-Cola during exercise as an ergogenic aid owing to its favourable carbohydrate content, it also contains caffeine (although only in small doses (32mg/330ml)) and offers the obvious advantage of being the most palatable option. Flat Coca-Cola is being used by leading tour de France teams. The Australian Institute of Sport actually create a Coca-Cola slush (ice) drink for their athletes as the body of supportive evidence of pre-cooling strategies continues to grow rapidly (12). Obviously flat Coca-Cola will reduce the gastrointestinal issues that accompany the ingestion of carbonated drinks, which could otherwise impair athletic performance.

CARBOHYDRATE GELS

Please note that some carbohydrate sports gels are promoted during endurance exercise, but owing to the reduced carbohydrate concentrations you would be required to consume around 4 – 5 of these gels every hour to reach the maximum threshold for carbohydrate oxidation. This offers obvious disadvantages. Firstly, you would need to carry a small back pack to transport them in and also you would have to spend a couple of seconds 4 – 5 times each hour opening and consuming the gels.

So for the longer endurance events, over 2-hours I would suggest you consume drinks as opposed to gels. Gels are advantageous during shorter endurance events, or when training to maintain blood glucose concentrations and provide lower doses of single source glucose for oxidation. I’m not aware of any gel that contains the optimal 2:1 ratio of glucose:fructose, any such formulation is likely to be extremely sweet, sticky, sickly and not suitable for consumption during an endurance event.

CONCLUSION

In conclusion a triathlete, or any endurance athlete would benefit from carbohydrate supplementation during exercise. I have provided a recommendation for a product based on the carbohydrate content alone, in part two I will go on to advise which additional ingredients will be of benefit to your performance and again provide recommended products/strategies.

Matt Jones – MSC Nutrition Lead Performance Nutritionist

REFERENCES

1. Gillum, T, L., Dumke, C, L., & Ruby, B, C. (2006). Muscle glycogenolysis and resynthesis in response to a half Ironman triathlon: a case study. International Journal of Sports Physiology & Performance, 1, 408-413.
2. Noakes, T, D. (2000). Physiological models to understand causes of fatigue and the adaptations that predict or enhance athletic performance. Scandinavian Journal of Medicine & Science in Sports, 10, 123-145.
3. Phillips, S, M., Green, H, J., Tarnopolsky, M, A., Heigenhauser, G, J., & Grant, S, M. (1996). Progressive effect of endurance training on metabolic adaptations in working skeletal muscle. American Journal of Physiology Endocrinology & Metabolism, 270, 265-272.
4. Burke, L., & Deakin V. (2006). Clinical Sports Nutrition. (3rd edition). Australia. McGraw-Hill Australia Pty Ltd.
5. Allen, D, G., Lamb, G, D., & Westerblad, H. (2008). Skeletal muscle fatigue: cellular mechanisms. Physiology Reviews, 88, 287-332.
6. Temesi, J., Johnson, N., Raymond, J., Burdon, C., & O’Connor, H. (2011). Carbohydrate ingestion during endurance exercise improves performance in adults. Journal of Nutrition, 141, 890 – 897.
7. Jeukendrup, A, E., Wagenmakers, A, J., Stegen, J, H., Gijsen, A, P., Brouns, F., & Saris, W, H. (1999). Carbohydrate ingestion can completely suppress endogenous glucose production during exercise. American Journal of Physiology, 276, 672-683.
8. Wright, D, A., Sherman, W, M., & Dernbach, A. (1991). Carbohydrate feedings before, during, or in combination improve cycling endurance performance. Journal of Applied Physiology, 71, 1082–1088.
9. Jeukendrup, A. (2010). Carbohydrate and exercise performance: the role of multiple transportable carbohydrates. Current Opinion in Clinical Nutrition & Metabolic Care, 13, 452 – 457.
10. Currell, K., & Jeukendrup, A. (2008). Superior endurance performance with ingestion of multiple transportable carbohydrates. Medicine & Science in Sports & Exercise, 40, 275 – 281.
11. Jentjens, R., Achten, J., & Jeukendrup, A, E. (2004). High rates of exogenous carbohydrate oxidation from multiple transportable carbohydrates ingested during prolonged exercise. Medicine & Science in Sports & Exercise, 36, 1551-1558.
12. Ross, M., Abbiss, C., Laursen, P., Martin, D., & Burke, L. (2013). Precooling methods and their effects on athletic performance: a systematic review and practical applications. Sports Medicine, 43, 207 – 225.