Digestion involves breaking down large food particles into molecules that are small enough for your gut to absorb. Chewing your food begins the process, but the bulk of digestion relies on the action of gastrointestinal fluids that contain digestive enzymes. These enzymes are synthesized and secreted in different parts of your digestive tract, and they are specific for the type of food they act on.
Protein-digesting enzymes break large protein molecules into single amino acids. The first of these enzymes your food protein encounters is pepsin in the gastric juice within your stomach. Unlike most digestive enzymes, pepsin is active in a highly acidic environment, and it breaks the protein into smaller units called polypeptides. When polypeptides move from your stomach to your small intestine, they are subject to action by several enzymes secreted by your pancreas and small intestine. Some, such as trypsin and chymotrypsin are known as endopeptidases and clip the polypeptides into even smaller pieces. Others – the exopeptidases carboxypeptidase and aminopeptidase – snip off amino acids from either end of the polypeptides. The net result of these protein-digesting enzymes is a pool of individual amino acids ready for absorption.
The digestive enzymes that work on your dietary carbohydrates include amylase and a variety of sugar-specific enzymes. Amylase is present in both your saliva and your pancreatic juice, and it works to break large starch molecules into maltose, a sugar consisting of two glucose units bonded together. Your small intestine releases enzymes that cleave disaccharides, or two-sugar molecules, into their single sugar components. For instance, lactase breaks lactose into glucose and galactose, sucrase cleaves sucrose into glucose and fructose while maltase reduces maltose to two individual glucose molecules. The single sugars can then undergo absorption by the cells lining your small intestine.
The fats, or triglycerides, you eat are acted on by a digestive enzyme called lipase, manufactured by your pancreas and secreted into your small intestine. Lipase is a water-soluble enzyme, which does not react with your dietary fat until these lipids are mixed with a fluid, produced in your liver, called bile. Bile has an emulsifying effect on fat, reducing it to smaller and smaller droplets until lipase can mix with it well enough to digest each triglyceride into fatty acids and a monoglyceride molecule. At this point your small intestine can absorb the products of fat digestion.
Some people have trouble digesting certain carbohydrates. For instance, if your small intestine does not make enough lactase to fully digest the lactose in your diet, the undigested sugar can cause unpleasant symptoms, such as bloating, gas and abdominal pain, as it travels through your digestive tract. Other foods that may not be fully digested in your gut can include beans, cabbage, broccoli, whole grains, onions and asparagus. In these cases, supplemental digestive enzymes can help you break down the carbohydrates in the foods that contribute to digestive upset.
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