How Your Body Absorbs Fuel

After swallowing your traditional sports drink (that breaks down into glucose, and not fructose); it reaches the stomach before moving down to your intestine. During that journey, the various types of carbohydrate found in the drink are broken down to glucose by your digestive system. Glucose is the main source of energy for the body during exercise. This Glucose must then pass through the intestine wall, by way of Glucose Transporters and into the blood stream to be taken to the working muscles. 

However, the Glucose Transporters only allow glucose to pass through relatively slowly and this results in a bottleneck at the wall of the intestine. It’s thought that this Glucose ‘bottleneck’ is what limits the maximum amount of carbohydrate your body can absorb, from a traditional sports drink, to around 60 grams per hour.

In long distance events, nutrition is critical

Maltodextrin: This is a carbohydrate used in many traditional sports drinks. It’s a common type of carbohydrate that’s broken down to glucose by digestion and passes through the wall of the intestine at a maximum rate of 60gram per hour.

Fructose (fruit sugar): Is a unique carbohydrate that’s not broken down to Glucose by digestion. Fructose passes through the wall of the intestine using a completely different set of Transporters to Glucose (GLUT5). Fructose does not get caught in the Glucose ‘bottleneck’ and it can provide your working muscles with an additional 30gram per hour of carbohydrate. (1)(2)(3)(4)(5)(6) 

A ratio of 2 parts maltodextrin to 1 part fructose has been shown to be the most effective in providing your muscles with carbohydrate. If we consume 60g glucose per hour, then we can provide our working muscles an additional 30g of carbohydrate per hour through fructose. 

As carbohydrate is the primary fuel for endurance sport, the more carbohydrate you have available, the faster and further you will be able to go. A number of independent research studies, are based on 2:1 fructose drinks, and they have clearly demonstrated a substantial performance and endurance advantage when compared to traditional sports drink formulations. (5)

When considering absorption rates, the aim is to balance liver release and muscle absorption at 1g/min (7). Despite fuel being used from both the liver and the muscles, hypoglycemia is one of the first reasons athletes fatigue during exercise which takes place when the liver glycogen (fuel) stores are used up. Without carbohydrate ingestion (no sports drinks or food) to suppress liver glucose production, even when only racing or training at between 70-85% VO2Max, these liver glycogen stores will be depleted after around 2 hours (8).

Each sport brings its own challenges and opportunities

After swallowing food be it in liquid or solid form, the ability for your body to use the ‘food’ is determined by the following four areas:

  1.  Gastric emptying 
  2.  Intestinal absorption 
  3.  Muscle glucose uptake 
  4. Oxidation limit carbohydrate used by muscles.

In most studies the stomach has still been fully emptied with doses of carbohydrate between 70-100g/hr (9)(10)(11)(12).

As described above, the intestinal absorption is balanced at approximately 60g/hr of glucose and a further amount of fructose polymers. This is set to 30g/hr as even a limited amount of fructose (50g/hour) (13) produces gastrointestinal discomfort (14). This is because there is limited capacity to absorb fructose in the intestine so it then travels to the colon where metabolism by bacteria produces chemicals that can induce colonic discomfort. 

Ingested carbohydrate during exercise is burned by the muscles in place of blood glucose derived from the liver, (15) and this rate of use increases up to an intensity of 60%VO2Max (16)(17). Carbohydrate ingestion during exercise does not however increase the rate of glucose output by the liver during exercise (18)(19)(20). It simply substitutes all or part of the glucose that would be released by the liver and any excess is stored as liver glycogen stores. Trained athletes may oxidise more ingested carbohydrate than untrained athletes (21), but only glucose infused straight into the blood stream (i.e injected) allows muscular oxidation rates to be increased further (up to 150g/hour) or

with caffeine

.

For more information on what you need for your event, check out this

blog

.

References

(1) Massicotte et al. 1986;

Massicotte, D. Peronnet, F. , Allah C., Hillaire-Marcel, C., Ledoux M, Brissons G, (1986) Metabolic response to (13C) Glucose and 13C Fructose ingestion during exercise. Journal of Applied Physiology 61, 1180-84 

(2) Massicotte et al. 1989; 

Massicotte, D. Peronnet, F., Hillaire-Marcel, C, Brissons G, Bakkouch, K, Hillaire-Marcel, C, (1989) Oxidation of glucose polymer during exercise: Comparison with glucose and fructose. Journal of applied Physiology 66, 179-183 

(3) Guezennec et al. 1989; 

Guezennec CY, Satabin P, Duforez F, Merino D, Peronnet F, Kozeit J, (1989) Oxidation of Corn Startch glucose, and fructose ingested before exercise. Medicine and Science in Sports and Exercise 21, 45-50 

(4) Jandrain et al 1993; 

Jandrain, B.J., Pallikarakis, N., Normand, S., Pirnay, F., Lacroix, M., Mosora, F., Pachiaudi, C., Gautier, J.F., Scheen, A.J., Riou, J.P., Lefébvre, P.J. (1993). Fructose utilisation during exercise in men: Rapid conversion of ingested fructose to circulating glucose. Journal of Applied Physiology 74, 2146–54. 

(5) Adopo et al. 1994; 

Adopo, E., Péronnet, F., Massicotte, D., Brisson, G.R., Hillaire-Marcel, C. (1994). Respective oxidation of exogenous glucose and fructose given in the same drink during exercise. Journal of Applied Physiology 76, 1014–19. 

(6) Burelle et al. 1997; 

Burelle, Y., Péronnet, F., Massicotte, D., Brisson, G.R., Hillaire-Marcel, C. (1997). Oxidation of 13C-glucose and 13C-fructose ingested as a preexercise meal: Effect of carbohydrate ingestion during exercise. International Journal of Sport Nutrition 7, 117–27. 

(7) Coggan and Coyle 1988; 

Coggan, A.R., Coyle, E.F. (1988). Effect of carbohydrate feedings during high-intensity exercise. Journal of Applied Physiology 65, 1703–9. 

(8) Noakes, 2003; 

Noakes T, (1985,2003) Lore of Running, USA, Oxford University Press 

(9) Hawley, Dennis, et al 1992; 

Hawley, J.A., Dennis, S.C., Noakes, T.D. (1992a). Oxidation of carbohydrate ingested during prolonged endurance exercise. Sports Medicine 14, 27–42. 

(10) Hawley, Dennis, et al 1992; 

Hawley, J.A., Dennis, S.C., Nowitz, A., Brouns, F., Noakes, T.D. (1992b). Exogenous carbohydrate oxidation from maltose and glucose ingested during prolonged exercise. European Journal of Applied Physiology 64, 523–27. 

(11) Wagenmakers et al. 1993,

Wagenmakers AJ1, Brouns F, Saris WH, Halliday D. (1993). 

Oxidation rates of orally ingested carbohydrates during prolonged exercise in men. 

Journal of Applied Physiolog

y 75(6):2774-80.

(12) Saris et al. 1993; 

Saris, W.H.M., Goodpaster, B.H., Jeukendrup, A.E., Brouns, F., Halliday, D., Wagemakers, A.J.M. (1993). Exogenous carbohydrate oxidation from different carbohydrate sources during exercise. Journal of Applied Physiology 75, 2168–72. 

(13) Peronnet et al. 1997; 

Péronnet, F., Burelle, Y., Massicotte, D., Lavoie, C., Hillaire- Marcel, C. (1997). Respective oxidation of 13C-labelled lactate and glucose ingested simultaneously during exercise. Journal of Applied Physiology 82, 440–46. 

(14) Murray, Paul et al. 1989; 

Murray, R., Paul, G.L. Siefert, J.G., Eddy, D.E., Halaby, G.A. (1989). The effects of glucose, fructose, and sucrose ingestion during exercise. Medicine and Science in Sports and Exercise 21, 275–82. 

(15) Bosch et al. 1994; 

Bosch, A.N., Dennis, S.C., Noakes, T.D. (1994). Influence of carbohydrate ingestion on fuel substrate turnover and oxidation during prolonged exercise. Journal of Applied Physiology 76, 2364–72. 

(16) Pirnay et al 1982; 

Pirnay, F., Crielaard, J.M., Pallikarakis, N., Lacroix, M., Mosora, F., Krzentowski, G., Luyckx, A.S., Lefébvre, P.J.(1982). Fate of exogenous glucose during exercise of different intensities in humans. Journal of Applied Physiology 53, 1620–24. 

(17) Pirnay et al 1995; 

Pirnay, F., Scheen, A.J., Gautier, J.F., Lacroix, M., Mosora, F., Lefébvre, P.J. (1995). Exogenous glucose oxidation during exercise in relation to the power output. International Journal of Sports Medicine 16, 456–60. 

(18) J.A. Hawley et al 1994b; 

Hawley, J.A., Bosch, A.N., Weltan, S.M., Dennis, S.C., Noakes, T.D. (1994b). Glucose kinetics during prolonged exercise in hyperglycaemic and euglycaemic subjects. ingestion or glucose infusion on fuel substrate kinetics during prolonged exercise. Pflügers Archives 426, 378–86. 

(19) Jeukendraup, Raben et al. 1999; 

Jeukendrup, A.E., Raben, A., Gijsen, A., Stegen, J.H., Brouns, F., Saris, W.H. Wagenmakers, A.J. (1999). Glucose kinetics during prolonged exercise in highly trained human subjects: Effect of glucose ingestion. Journal of Physiology 515, 579–89. 

(20) Jeukendraup, Wagenmakers, et al. 1999; 

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, E672–83. 

(21) Burelle et al. 1999; 

Burelle, Y., Péronnet, F., Charpentier, S., Lavoie, C., Hillaire-Marcel, C., Massicotte, D. (1999). Oxidation of an oral [13C] glucose load at rest and prolonged exercise in trained and sedentary subjects. Journal of Applied Physiology 86, 52–60.

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