Small Intestine

The small intestine has three distinct compartments: the duodenum (closest to the stomach), jejunum (middle), and ileum (closest to the large intestine). The liquid mass of consumed food formed by the stomach is passed into the small intestine for additional digestive processing and, for the first time in the GI tract, absorption into the blood and lymph. The pyloric valve separates the stomach from the small intestine. The small portion of the small intestine proximal to the pyloric valve is the primary absorption site for the bivalent minerals, including iron, calcium, magnesium, and zinc (all of which are important to athletes).

Bivalent minerals are competitively absorbed because the site of absorption is relatively small. Therefore, an excessively high intake of one bivalent mineral may occupy the entire absorption site and make the absorption of other bivalent minerals difficult. The principle of nutrient balance is critical (i.e., more than enough is not better than just enough). For instance, female athletes are often appropriately concerned about iron status, but a frequent high-dose intake of iron may mitigate the absorption of calcium, magnesium, and zinc to create a series of other nutrition problems. Put simply, the strategy for bivalent mineral intake is crucial for optimizing nutrition health and, because these minerals are so closely tied to muscle function and bone health, athletic performance.

A short distance from the proximal duodenum where bivalent minerals are absorbed, the bile duct and pancreatic duct enter the small intestine.⁴ The pancreas is stimulated by secretin, which travels up the pancreatic duct to cause the release of pancreatic juice into the duodenum. Because of its large volume (between 20 and 27 ounces daily, or 600 and 800 milliliters) and high pH (~8.0), pancreatic juice neutralizes the acidity of the food mass that has left the stomach. The pancreas also releases several digestive enzymes, including

• pancreatic amylase to digest starch into dextrins and maltose;
• pancreatic proteases to digest larger proteins into smaller polypeptides; and
• pancreatic lipase to digest fats into monoglycerides, individual fatty acids, and glycerol.

Of course, the pancreas also produces powerful hormones (insulin by the beta cells and glucagon by the alpha cells). Insulin will be discussed fully in following chapters.

The liver manufactures bile, which is stored in the gallbladder until it is required. Cholecystokinin, which is produced in the small intestine, travels up the bile duct to stimulate the gallbladder to release its stored bile into the small intestine. Bile is a powerful emulsifying agent that helps in the digestion of fats.⁵ The daily bile volume is between 17 and 37 ounces (500 and 1,100 milliliters), helping to explain our efficiency at digesting and absorbing fats.⁶

The mucosal (border) cells of the small intestine (mainly in the duodenum) produce enzymes that break down disaccharides into their component monosaccharides. Specifically, these disaccharidases do the following:

• Sucrase breaks down sucrose to glucose and fructose.
• Maltase breaks down maltose to two molecules of glucose.
• Lactase breaks down lactose to glucose and galactose.

These seemingly minor digestive enzymes are important for athletes to consider, particularly as they relate to the composition of sports beverages. For instance, a sports beverage deriving 100 percent of its energy from pure glucose (the ultimate energy source for cells) would induce delayed gastric emptying and, once absorbed, would produce a sudden, high, and relatively short-lived rise in blood sugar (glucose). On the other hand, an isocaloric sports beverage containing a combination of sucrose and glucose would have some inherent advantages in maintaining blood sugar for a longer period of time. The lower concentration of glucose in the beverage would not significantly delay gastric emptying, so there would be a more rapid infusion of glucose into the blood without achieving such a high peak. The sucrose would be digested into its component glucose and fructose, a process that takes some time. The glucose from this breakdown would follow the glucose already absorbed, and the fructose would be converted to glucose in the liver (still more time required) before infusion into the blood. The end result is a lower glucose peak but a much more sustained infusion of glucose into the blood, helping the athlete feel energized for a longer time.

Nutrients are absorbed mainly in the duodenum and jejunum, but some absorption also takes place in the ileum and the large intestine:

• Minerals are mainly absorbed in the proximal duodenum.
• Monosaccharides and water-soluble vitamins are mainly absorbed in the jejunum.
• Fat-soluble vitamins, amino acids, fats, vitamin B₁₂, ⁷ and bile salts are absorbed mainly in the ileum.

The interior surface of the small intestine is composed of microvilli that dramatically enlarge its absorptive surface, accounting for an extraordinary efficiency in absorbing consumed energy substrates: 98 percent of all digestible carbohydrate is absorbed; 95 percent of all fat is absorbed; and 92 percent of all protein is absorbed.