Stomach lipid digestion

Based on data on PAHs, absorption of PAH components of coal tar products after oral exposure may be positively influenced by the presence of oils and fats in the stomach, and bile in the intestines (Agency for Toxic Substances and Disease Registry 1995). Due to relative water insolubility of PAHs, absorption is enhanced by solubilization in an intermediate phase than can be metabolized during the process of lipid digestion and absorption. Excretion after oral exposure may be detected hours to days after exposure. 

Dietary retinyl esters, retinol, and provitamin A carotenoids are dispersed and emulsified in the stomach during the gastric phase of lipid digestion. They then enter the lumen of the duodenum where extensive hydrolysis of retinyl esters takes place. In infants, the bile salt-stimulated lipase in human milk may play a role in retinyl ester hydrolysis in the duodenum (Fredrikzon et al., 1978). The major retinyl ester hydrolase activity that acts in the intestines is, however, derived from the pancreas. 

Pancreatic juice neutralizes the acidic chyme entering the duodenum from the stomach. Neutralization not only prevents damage to the duodenal mucosa, but also creates a neutral or slightly alkaline environment optimal for the function of pancreatic enzymes. The pancreas also secretes several enzymes involved in the digestion of carbohydrates, proteins, and lipids. 

Fats and other lipids are poorly soluble in water. The larger the accessible surface is—i. e., the better the fat is emulsified—the easier it is for enzymes to hydrolyze it (see p. 270). Due to the special properties of milk, milk fats already reach the gastrointestinal tract in emulsified form. Digestion of them therefore already starts in the oral cavity and stomach, where lipases in the saliva and gastric juice are available. Lipids that are less accessible—e.g., from roast pork—are emulsified in the small intestine by bile salts and bile phospholipids. Only then are they capable of being attacked by pancreatic lipase [4] (see p. 270). 

Enzymes are named for what they do. For example, the enzyme given off by the stomach, which splits proteins as part of the digestion process, is called gastric proteinase. The gastric part of the name refers to the enzyme s origin in the stomach. Proteinase denotes that it splits up protein molecules. The common name for this enzyme is pepsin. Similarly, the enzyme produced by the pancreas that breaks down fats (lipids) is called pancreatic lipase. Its common name is... 

A major question to address in the future is how structure influences the dynamics of digestion and the signaling processes involved. Armand et al. (1996, 1999) investigated the digestion and lipid absorption from emulsions with different droplet sizes in humans. Healthy subjects received intragastrioally a coarse (10 pm) and a fine (0.7 pm) lipid emulsion of identical composition in random order. Gastric and duodenal aspirates as well as triglyceride appearance in the blood were analyzed. They found an increase in droplet size in the stomach however, the fine emulsion retained droplets... 

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