Your digestive system moves food through your GI tract and breaks it into smaller parts that your body absorbs as macronutrients, vitamins and minerals. Although you eat and excrete every day, the digestive process and how your body absorbs nutrients are more complicated than you might think.
As your body digests the food you eat, it utilizes vitamins, minerals and macronutrients at different points along the digestive tract. This absorption process usually takes between three and six hours after eating.
The Digestive System Process
The digestive system consists of the gastrointestinal (GI) tract, the liver, the pancreas and the gallbladder. Food enters the mouth and passes through the body to the anus via the hollow organs of the GI tract, which consists of the mouth, esophagus, stomach, small intestine and large intestine. The organs work with a combination of nerves, hormones, bacteria and blood to complete the complex process of digestion.
According to the National Institute of Diabetes and Digestive and Kidney Diseases, digestion is important for breaking down food into nutrients, which your body uses for energy, growth and cell repair. In order for your blood to absorb vitamins and minerals and carry them to cells throughout your body, food and drink must be converted into smaller molecules. The nutrients include:
- Carbohydrates: This macronutrient is grouped into two categories: simple carbohydrates, which include sugars found in foods such as fruit, vegetables and milk, and complex carbohydrates, which consists of starches and fiber found in whole grain breads and cereals, starchy vegetables and legumes.
- Protein: A macronutrient found in foods such as meat, eggs and beans, which your body digests into smaller molecules called amino acids.
- Fats: A macronutrient that's categorized into healthy fats, which consist of oils such as corn, canola, olive, safflower, soybean and sunflower, and less-healthy fats that are found in butter shortening and snack foods.
- Vitamins: Micronutrients that are classified by the fluid in which they dissolve; water-soluble vitamins include all B vitamins and vitamin C, while fat-soluble vitamins include vitamins A, D, E and K.
- Minerals: Micronutrients that are elements in food that your body needs to function normally. Some minerals that are essential for health include calcium, potassium, magnesium and iron.
Absorption of Nutrients
What you eat isn't as important as the nutrients you absorb. The amount of time it takes to digest food and absorb nutrients varies from person to person, particularly when it comes to transit times through different sections of their GI tracts. The time required for food to move through the digestive tube is also dependent on a variety of different factors, including the composition of the meal, gender, reproductive status and psychological factors.
In healthy humans, roughly 50 percent of stomach contents empty in two and a half to three hours and totally empty in four to five hours after a standard meal, according to a review published in the February 2018 issue of Nutrients. The body requires another two and a half to three hours before 50 percent of the contents of the small intestine empty and 30 to 40 hours for the remaining components to transit through the colon.
The absorption process usually begins three to six hours after eating. In general, carbohydrates digest the fastest, followed by protein absorption and then fats. The exact time of digestion is dependent on the food you ate, the complexity of the molecule, the nature of the nutrient and the order of the breakdown that occurs in the digestive system.
Digesting Dietary Fiber
Dietary fiber, found mainly in fruits, vegetables, whole grains and legumes, is probably best known for relieving symptoms of constipation. Also known as roughage or bulk, fiber includes the parts of plant foods your body can't digest or absorb. Instead of being digested like carbohydrates, proteins and fats, fiber passes through your stomach, small intestine and colon relatively intact.
An American Journal of Clinical Nutrition March 1989 study, which is still widely referenced today, measured the apparent nutrient absorption and upper GI transit with fiber-containing enteral feedings. The study tested two amounts of fiber that participants consumed along with a liquid meal containing zinc.
Both resulted in lengthened transit time, but only the higher-fiber meal significantly lowering serum zinc concentrations. This study demonstrates that dietary fiber can decrease apparent nutrient absorption and increase transit time in the upper GI tract without affecting apparent glucose absorption.
- National Institute of Diabetes and Digestive and Kidney Diseases: "Your Digestive System & How it Works"
- American Journal of Clinical Nutrition: "Apparent Nutrient Absorption and Upper Gastrointestinal Transit With Fiber-Containing Enteral Feedings"
- Nutrients: "Gastrointestinal Transit Time, Glucose Homeostasis and Metabolic Health: Modulation by Dietary Fibers"
- MedlinePlus: "Complex Carbohydrates"
- MedlinePlus: "Simple Carbohydrates"
- UCSB Science Line: "Can You Please Tell Me the Digestive Timeline for Protein, Fat, and Starch?"