American Society For Nutrition

Utilizing Multiple Intestinal Sugar Transporters for Endurance Exercise

Utilizing Multiple Intestinal Sugar Transporters for Endurance Exercise

Excellence in Nutrition Research and Practice
Posted on 01/22/2011 at 12:51:29 AM by Student Blogger
By: Jon C.

First let me begin by introducing myself:  I am Jon C., a doctoral student studying biochemical human nutrition at Michigan State University.  My research focuses primarily on studying the effects of caloric restriction on natural killer (NK) cell biology and antiviral immunity.  However, I hope to begin this series of blogs discussing another topic which always results in lengthy discourse between me and other nutrition professionals; ultrarunning. 

    I am an endurance junky.  I love endurance sports, particularly running, but also have done my share of long distance biking and swimming.  I generally run between 60 and 100 miles a week depending on the events I am training for and my responsibilities to school and my lab.  In the last two years I have participated in 9 ultramarathons, including two 50 mile races, but have found I am an average runner in this fast growing sport.  An ultramarathon is defined as a run that is longer than a marathon (26.2 miles).  Generally events range from 50 kilometers (~31 miles) all the way up to runs greater than 100 miles.  One of the most interesting facets of this type of sport is that it heavily involves nutrition and metabolism before, during and after running.  Indeed, I have learned a good deal from my mistakes involving fluid intake, fuel choices, and electrolyte replacement during races and after races; I hope to cover many of these topics in future blogs.  Perhaps the best way to introduce the nutritional considerations that occur during these types of activities is to use my own experience.   
Last weekend a friend and I went on a 6 hour, 35 mile run on snowy trails in the Great Lakes area.  I wear a heart monitor to measure how hard I am working and have found during a run such as this I use upwards of 4000 calories (turned out to be 4021 calories).  Any exercise requiring beyond 3000 calories and done at a reasonable pace requires additional carbohydrates due to limited glycogen stores (1).    During training runs I aim to consume 200 calories/hour to sustain my running, but at race pace I try to take in upwards of 300 calories/hour.

    My preferred source of fuel during long runs are gel packets which in my opinion offer the advantage of allowing easy tracking of energy intake to avoid the dreaded bonk (exhaustion) and have been shown to be as effective as sports drink when used properly (2).  Sports nutrition companies have formulated these products very specifically, yet include simple instructions that allow these products to be used with little to no knowledge of the nutritional paradigms behind them.  Carbohydrate gel products often include the simple instruction, “consume 1-2 packets for every hour of exercise,” but offer little to no explanation as to why you should use this amount.  They typically contain 100-120 calories, mostly in the form of carbohydrate (20-25 grams carbohydrate) and some contain fats and amino acids. 

During this run I consumed 10 gel packets, and about 300 calories from a concentrated sports drink.  These fuels are mostly maltodextrin, a glucose polymer that is readily digested during strenuous exercise (3); however they also contain some fructose.  I hadn't fully understood the purpose of including fructose in these gel packets, however, recent literature suggests concept of combining multiple sugars in carbohydrate gels or drinks may be beneficial for performance (4).  It is estimated that the maximal glucose absorption rate in the gut is between .8-1.7 grams/min (4), mainly through sodium-dependent glucose cotransporters (SGLT1) (5) while intake exceeding 3 grams/min can lead to gastric distress (3).  Fructose on the other hand, is absorbed mainly via GLUT-5 facilitative transporters, resulting in minimal competition with glucose for absorption (6). Thus, it is thought that maximal exogenous carbohydrate absorption can be achieved by utilizing a combination of glucose and fructose in solution. 

These studies have several ramifications for runners.  Most importantly, they indicate that by incorporating fructose into fuel choices during events that demand exogenous carbohydrate consumption may allow the runner to increase the number of calories consumed compared to maltodextrin alone.  These studies also report that the use of a combination of glucose and fructose results in reduced gastric distress (1, 4).  For many people participating in endurance events the goal is to simply finish the event, and often this is dependent on the ability of the athlete to continue to consume fuel.  While there are many variables that contribute to the gastric distress, this research gives insight into a way to both increase fuel intake and maintain a happy gastrointestinal tract.  

Works Cited

1.    Jeukendrup AE, Moseley L, Mainwaring GI, Samuels S, Perry S, Mann CH. Exogenous carbohydrate oxidation during ultraendurance exercise. J Appl Physiol. 2006 Apr;100:1134-41.
2.    Pfeiffer B, Stellingwerff T, Zaltas E, Jeukendrup AE. CHO oxidation from a CHO gel compared with a drink during exercise. Med Sci Sports Exerc.  Nov;42:2038-45.
3.    Jeukendrup AE, Jentjens R. Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research. Sports Med. 2000 Jun;29:407-24.
4.    O'Brien WJ, Rowlands DS. Fructose-maltodextrin ratio in a carbohydrate-electrolyte solution differentially affects exogenous carbohydrate oxidation rate, gut comfort, and performance. Am J Physiol Gastrointest Liver Physiol.  Jan;300:G181-9.
5.    Ferraris RP, Diamond J. Regulation of intestinal sugar transport. Physiol Rev. 1997 Jan;77:257-302.
6.    Shi X, Schedl HP, Summers RM, Lambert GP, Chang RT, Xia T, Gisolfi CV. Fructose transport mechanisms in humans. Gastroenterology. 1997 Oct;113:1171-9.

1 Comment

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