The human body requires macronutrients such as fats, carbohydrates, and proteins to build and maintain itself, as well as sufficient quantities of vitamins, minerals, and water to make the biochemical processes of life possible. The digestive system is a specialized gropu of organs that work together to extract these nutrients from the foods we eat.
Carbohydrate digestion begins in the mouth, where salivary amylase begins the process of breaking starches down into simpler sugars. The mechanical action of chewing also helps with digestion by breaking down the cellular structure of the foodstuff and increasing the surface area over which digestive enzymes can access the food.
The stomach is a highly acidic pouch whose main purpose is to grind food into a paste called chyme. The hydrochloric acid in the gastric juices helps break down proteins in the food into structures that can be absorbed in the intestines later on. The stomach then releases the partially-digested food into the small intestine in small amounts, allowing the intestinal juices to continue the digestion and absorption of the nutrients.
The majority of nutrient absorption happens in the small intestine, which is a convoluted tube with corrugated walls. Those walls are covered with tiny finger-like projections called villi. This internal structure creates an immense amount of surface area over which nutrients can be absorbed by the body. At the beginning of the small intestine, juices from the pancreas and liver are added to the chyme to reduce the acidity of the food that has come out of the stomach. Bile is also added, which aids in the digestion of fats and lipids. As the food passes through the small intestine, enzymes break the nutrients into molecules that can pass through the walls of the villi and into the bloodstream.
By the time food reaches the end of the small intestine, most of its nutritional elements have been removed. What remains is passed into the large intestine, where excess water is reabsorbed by the body, along with some micronutrients that still remain. The remaining undigested food is passed along the large intestine to the rectum to be excreted as feces. (Sizer & Whitney, 2011)
While the body requires fats in order to function, different types of fats are metabolised and used differently. Some, such as some plant oils like olive or vegetable oil, provide more benefit to the body because of how they bind and travel in the bloodstream. These unsaturated fats are less likely to contribute to high cholesterol and heart disease. Other fats, such as animal fats or mechanically-altered plant oils called trans fats, are more likely to result in heart disease.
Fiber plays a vital role in proper digestion and digestive health. A high-fiber diet can reduce the amount of cholesterol absorbed in the gastrointestinal tract, reducing the level of cholesterol in the blood overall, and helping reduce the risk of heart disease. (Chandalia, M., et al, 2000) Fiber contributes to colon health by providing bulk to feces, as well as food to the microorganisms that live in a healthy large intestine. Fiber also helps produce feelings of satiety, which helps reduce overall calorie intake and helps maintain a healthy weight.
References:
Chandalia, M., et al (2000). Beneficial Effects of High Dietary Fiber Intake In Patients With Type 2 Diabetes Mellitus. New England Journal of Medicine. Retrieved from http://www.nejm.org/doi/pdf/10.1056/NEJM200005113421903
Sizer, F. & Whitney, E. (2011). Nutrition Concepts & Controversies, 12th Edition. Belmont, CA: Wadsworth.
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