CYCLING PERFORMANCE TIPS

The Athlete and the GI Tract

• Mouth - This is where digestion begins. Of most importance is the mechanical breakdown of food . Of lesser significance is the addition of digestive enzymes in saliva.

• Esophagus - The esophagus is a muscle lined conduit which transports ingested foods and fluids to the stomach. It has specialized muscle at the upper end (the Upper Esophageal Sphincter or UES) and lower end (Lower Esophageal Sphincter or LES) which separate the esophagus from the posterior pharynx and stomach. Both must relax at the appropriate time to allow food to pass from the posterior pharynx, through the esophagus, and into the stomach. Primary contractions of the esophagus itself are not necessary (in the upright position) for food to move into the stomach (gravity alone does the job) but secondary contractions help to clear refluxed acid back into the stomach.

• Stomach - Here mechanical transformation is completed and significant enzymatic breakdown of food occurs. There is no absorption. Emptying is controlled by hormonal and enteric neurologic feedback to assure that food has been mechanically prepared for absorption by the:

• Small intestine (duodenum, jejunum, and ileum) - This is where enzymatic breakdown is completed and absorption of nutrients occurs. The majority of absorption occurs in the upper half of the small intestine.

• Colon (ascending, transverse, descending, sigmoid, and rectum) - The colon serves as a storage area where dehydration of the residual from processed food can occur. Although there is absorption of water, there is no significant absorption of nutrients.

I. The Esophagus (UES, body, & LES)

The esophagus transports food from the mouth to the stomach. It is a smooth muscle lined tube with three physiologically important sections.

• The upper several centimeters is composed of specialized smooth muscle and is referred to as the Upper Esophageal Sphincter (UES). It is located at the base of the posterior pharynx (at the same level as the larynx).
• The middle 20 centimeters is the Body.
• The lower 3 or 4 centimeters is the Lower Esophageal Sphincter (LES). The LES is formed by a combination of
  1. specially adapted smooth muscle and
  2. the mechanical "pinching" effect of the diaphragm on the esophagus.

With swallowing:

1. the UES, which
   • keeps air out of the esophagus with respiration and
   • prevents regurgitation of material from the esophagus into the laryngeal area
   relaxes.
2. The bolus of liquid or food is then propelled by a muscular contraction wave (primary peristalsis) through the length of the esophagus (although gravity does the trick if the esophageal muscle is weakened by disease)
3. the LES (which prevents reflux of stomach contents) relaxes as the bolus reaches it
4. the bolus of food moves into the stomach.

II. The Stomach

• enzymatic digestion
• and additional mechanical disruption

Four factors affect the rate of gastric emptying:

• Solid versus liquid - liquids are emptied from the stomach more quickly than solids.

• Fat content of the food - fat slows the digestive process and will delay emptying of the stomach.

• Sugar concentration (osmolality) - especially with liquids, a sugar content of more than 10% will significantly delay stomach emptying.

• Degree of physical activity level - gastric emptying is slowed by vigorous activity, especially above 70% VO2 max.

III. The Small Intestine (duodenum, jejunum, and ileum)

Fluid is also absorbed from the small intestine. It has been estimated that 10 liters of fluid are secreted by the stomach and small intestine in the digestive process, yet less than 2 liters are delivered to the colon at the ileocecal valve level.

IV. The Colon (ascending, transverse, descending, sigmoid, and rectum)

Next we will review the changes in GI physiology with exercise. These changes will help us to understand why various symptoms develop and also explain why certain nutritional strategies can

• minimize symptoms
• and support more effective fluid and Caloric replacement

When designing a nutritional program to supplement the body's energy stores for an athletic event, the rate of digestion and absorption of foods must be taken into account. The time needed for the stomach to start the digestive process, empty its contents into the small intestine, and have the food components absorbed into the bloodstream will directly affect how quickly any food and fluid will be available to the muscle to provide the supplemental Calories for exercise.