Your diet provides the nutrients you need to maintain health. Some of these nutrients are too large to be useful to your body without further processing, and your digestive system accomplishes this by breaking them down to a size small enough to enter your bloodstream. Once in your blood, the digested nutrients travel throughout your body to the cells that need them.
The sugars and starches that make up carbohydrates are the main source of energy for your cells. In their simplest form, carbohydrates are monosaccharides, or single sugar units. They can also exist as disaccharides, made up of two monosaccharides, or starches, composed of many monosaccharides – specifically glucose – linked together into a large molecule. The digestive enzyme amylase, secreted from your salivary glands and pancreas, reduces starch molecules to single glucose units, while sugar-specific enzymes from your small intestine break apart disaccharides into individual sugars. Once these carbohydrates are fully digested, the cells lining your small intestine absorb them and transport them to your bloodstream.
Your digestive system breaks down your dietary fats to fatty acids and monoglycerides. Because your digestive juices are water-based, the fats you eat can’t mix well with them until they combine with a fluid called bile. Bile, produced in your liver and released into your small intestine, separates fat particles into tiny droplets that can mix with water. In this way, the digestive enzyme lipase can act on the fat and separate it into its components. The resultant fatty acids and monoglycerides are small enough for your intestine to absorb and pass along to your circulatory system.
Your small intestine cannot absorb a molecule as large as protein. Your gastrointestinal tract therefore breaks it down to its individual building blocks called amino acids. The process begins in your stomach, where hydrochloric acid uncoils the protein molecule and the acid-activated enzyme pepsin cuts it into smaller pieces. The pieces move to your small intestine, where additional digestive enzymes clip off amino acids one by one. Absorptive cells in this part of your gut then take in the single amino acids and carry them to your bloodstream.
Not all the foods you eat undergo digestion. Some – vitamins, minerals and monosaccharides, for instance – are already small enough for absorption by your intestines without further processing. Other food components cannot be digested because you lack the necessary digestive enzymes. Dietary fiber falls into this category, and, even though it doesn’t add a specific nutrient to your diet, it can help improve your digestive health by adding bulk to your waste products. In contrast, the inability to digest a certain nutrient can cause digestive distress, as in the case of lactose. If you don’t produce the enzyme lactase, undigested lactose passes through your system and can lead to stomach cramps or diarrhea.
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