Soy is a versatile food. You can eat soybeans whole or as a processed food product, such as soy milk or tofu. The nutrient content of soy may vary depending on how it is processed before you eat it, and the digestion of soy depends on what nutrients it retains after processing. For example, soy protein isolate may contain no carbohydrates or fats and so would undergo only protein digestion. Whole soybeans, however, contain all three macronutrients: protein, carbohydrate and fat.
Protein Digestion
When the protein from soy reaches your stomach, your acidic gastric juices begin to unfold the protein from its globular shape to a more linear configuration. As this happens, the digestive enzyme pepsin attacks the bonds that link together the amino acids of the protein. The resulting soy protein fragments, or peptides, then move from your stomach to your small intestine. In your small intestine, more digestive enzymes, including trypsin, chymotrypsin, aminopeptidase and carboxypeptidase, break the soy peptides down even further, into smaller peptides and finally to individual amino acids. The lining of your small intestine then absorbs the amino acids from the soy protein into your blood.
Carbohydrate Digestion
The digestion of soy carbohydrate begins with the salivary enzyme amylase, which starts to break food starch down to sugars as you chew. When amylase reaches the acidic environment of your stomach, it becomes inactivated, and the undigested and partially digested carbohydrates from your soy food move into your small intestine. Within the small intestine, additional amylase -- secreted by your pancreas -- continues to digest the large soy starch molecules into maltose, a disaccharide comprising two glucose molecules joined together. Finally, the digestive enzyme maltase cleaves the maltose into individual glucose units before absorption by the cells of your small intestine.
Fat Digestion
The fats in soy consist of triglycerides, molecules with three fatty acids bound to a glycerol backbone. The bulk of soy fat digestion begins in your small intestine, where a fluid called bile is secreted from your gallbladder. Bile functions to emulsify the soy fat, making it more water-soluble. As this happens, the pancreatic enzyme lipase breaks the soy triglyceride into its individual fatty acids and glycerol. The cells lining your small intestine then absorb these products of fat digestion.
Considerations
The water in soy is not digested, but most of it is absorbed by your large intestine. Similarly, vitamins and minerals in soy do not undergo digestion; rather, they attach to other molecules, such as lipids or transporter proteins, for absorption into your bloodstream.
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