During digestion, foods are broken down into their smallest components and sent throughout your body so that you stay healthy and have the energy you need to get you through the day. The foods you eat are not in a form your body can use. They need to be broken down into smaller molecules so that you can absorb and use their nutrients. Among the three macronutrients, carbohydrates are digested the fastest, protein takes longer and fat takes the longest amount of time.
Your digestive system is a series of long, twisting tubes through which food travels. Your mouth, esophagus, stomach and small and large intestines make up the digestive tract. As soon as you start chewing food, you start digesting it through an enzyme your saliva produces. Your stomach and small intestine produce digestive juices, or acids and enzymes, to continue to break down the fat and protein in what you've eaten. Your body needs more time to break down fat because fat molecules are bigger than protein molecules.
Proteins in food are large molecules called long-chain amino acids. Digestion of long-chain amino acids starts in your stomach where the enzyme pepsin is released. Pepsin breaks down protein into molecules that are small enough to travel to the small intestine. Your pancreas then sends various proteases, or enzymes made to break down proteins. Proteases attack the proteins sent from your stomach and break them down into amino acids. Amino acids then enter your bloodstream. Protein absorption rates vary by person and protein source. You can absorb 8 to 10 grams of whey protein per hour, while it takes up to four hours to digest two eggs, which contain about 12 grams of protein.
Fats require special digestive action before they can be absorbed. The end products of digestion need to be water-soluble because they are carried in your blood or lymph. Fats are not water-soluble until they undergo certain changes. Your pancreas secretes and sends the enzyme lipase to your stomach to begin fat digestion. Bile produced by your liver then dissolves fat into tiny droplets in the small intestine. At this point, fats are split into fatty acids and glycerol, which are absorbed by the intestine. They then recombine with a little bit of protein to form chylomircrons, which enter circulation and are absorbed. Fats take up to six hours for complete digestion and storage in your fat cells compared to protein, which can be absorbed in as little as one hour.
Protein is found in foods such as meat, fish, poultry, eggs, dairy products and beans. Protein is needed to build and repair body tissue and maintain cells. It is also used as a last-resort source of energy. Between 10 and 35 percent of your daily calories should be from protein sources. Fat is found in various foods and listed differently on nutrition labels depending on its molecular structure. Fat molecules are an excellent source of energy for your body. They also serve to protect your organs and absorb other nutrients, such as vitamins and minerals. You should limit your fat intake to between 20 and 35 percent of your total calories.
- National Digestive Disease Information Clearinghouse: Your Digestive System and How It Works
- Colorado State University: Exocrine Secretions of the Pancreas
- Precision Nutrition: Research Review: Protein Supplements -- Are You Absorbing Them?
- Nutrient Timing for Peak Performance; Heidi Skolnik
- Centers for Disease Control and Prevention: Protein
- Centers for Disease Control and Prevention: Dietary Fat
Michelle Fisk began writing professionally in 2011. She has been published in the "Physician and Sports Medicine Journal." Her expertise lies in the fields of exercise physiology and nutrition. Fisk holds a Master of Science in kinesiology from Marywood University.