Digestive stations part 2 — how carbohydrates, fats and proteins are digested
Around 95% of all food consumed is metabolized in healthy people. Carbohydrates, proteins, fats, and alcohol are transformed in the body, oxidized, and burned or stored for the production of energy. What mechanisms are behind the digestion of individual nutrients is our topic today.
Sweet fuel
In the body, carbohydrates are fuel, a source of energy, and a component of all cells. They are used to form numerous substances, including glycoproteins and lipids, fatty acids, non-essential amino acids and amino sugars (e.g. glucosamine). As giant sugars (polysaccharides) produced in the body, such as chondroitin sulfate and hyaluronic acid, they are essential for the stability and elasticity of connective tissue, cartilage and bone.
Even though the body can also obtain its energy from fats and proteins, the DGE (German Nutrition Society) believes that at least 25% of the daily diet should consist of carbohydrates. At 40 to 50%, this actual proportion is far too high today.
In the process of digestion, polysaccharides (multiple sugars) are broken down into monosaccharides (simple sugars), as only these can be absorbed by the intestines. They are then absorbed into the cells of the intestinal mucosa. Glucose and galactose are the most rapidly absorbed, followed by fructose. All other simple sugars and sugar substitutes (e.g. xylitol, sorbitol) are absorbed rather slowly into the blood via the intestinal mucosa. Via the blood, the simple carbohydrates reach the liver. There, fructose and galactose are converted into glucose — the “basic sugar”. If there is an excess of fructose, this can, among other things, inhibit energy metabolism as well as protein synthesis — more on this in a moment.
These fats
Fats are found in both plant and animal foods, improve the taste of food and, above all, provide the body with energy. In times of meager nutrition, stored fat serves as a productive energy reserve. The sad fact is that too much fat, especially too much saturated fat, is consumed in Western industrialized nations. The fat content of the diet averages around 40 percent, 10 percent more than the recommended 30 percent. As a result of too much fat consumption and too little physical activity, obesity and its secondary diseases are currently the biggest health problem.
The DGE recommends limiting the fat content of the diet to about 30 to 35 percent and consuming as many unsaturated fatty acids as possible. At 9 to 9.3 calories, the calorific value of fat is twice as high as that of proteins and carbohydrates. With a calorie-excess diet — whether too high in fat or carbohydrates — the fat cells that are already present first increase in size to accommodate all the excess fatty acids. Once these are exhausted, new fat cells are formed to store even more fat. When weight is lost, these newly formed fat cells empty again, but remain for years.
Fats from food also contain other substances, such as phospholipids, cholesterol, fat-soluble vitamins, etc. The enzymes in the mouth prepare the fats for digestion, and in the stomach they are broken down. Short-chain fatty acids can be absorbed directly into the venous blood of the stomach, other fats are emulsified and broken down with the help of enzymes. Spherical lipoproteins are formed for transport into the lymph and blood, the transport form for triglycerides, cholesterol, fat-soluble vitamins and phospholipids.
Cholesterol contained in animal foods is broken down and absorbed in the intestine. The absorption capacity is limited to 3 grams per day, the rest is simply excreted. A rough rule of thumb for acceptable cholesterol levels is 200 + age. Elevated cholesterol can be reduced by eating a diet low in fat and cholesterol, increasing physical activity, and not smoking.
From proteins to machines
Proteins are among the basic building blocks of the body and are made up of long chains of amino acids. The protein supply, and especially the supply of essential amino acids, is largely dependent on diet and should be about 0.8 grams of protein per kilogram of body weight for healthy adults. For a body weight of 70 kilograms, this is about 64 grams of protein daily.
Animal foods in general provide a high-quality composition of proteins that are relatively similar to the body’s own and provide essential amino acids in a ratio that is readily usable by the body. Animal proteins are absorbed more quickly, while plant proteins are absorbed somewhat more slowly. The digestion process begins in the stomach, where dietary proteins are broken down into (poly- and oligo-)peptides with the help of hydrochloric acid, pepsin and enzymes. In the small intestine, these peptides are further cleaved by enzymes (trypsin and chymotrypsin), resulting in di- and tri-peptides and free amino acids that enter the bloodstream.
The liver ensures the best possible balance of amino acids in the blood level and thus the supply of organs and tissues with all amino acids that are currently needed. The skeletal muscles have the largest reservoir of all amino acids and serve to produce energy (gluconeogenesis) in the absence of carbohydrates or fats.
Why is versatility in nutrition so important?
No nutrient alone can ensure the supply of the organism. It requires a balanced and varied diet that takes into account the essential nutritional components.
Text sources:
(1) https://www.oege.at/index.php/bildung-information/nahrungsinhaltsstoffe/kohlenhydrate
(2) Jaminet, Paul et al.: Perfect Health Diet, 2018, DOI: 10.1055/b-0038–150087, II Der Zusammenhang zwischen Nahrung und Nährstoffen
(3) https://www.dge.de/wissenschaft/referenzwerte/fett/?L=0
(5) Rehner, Daniel: Biochemie der Ernährung, 2010, DOI: https://doi-org.emedien.ub.uni-muenchen.de/10.1007/978-3-8274-2217-0
Image sources:
(6) https://www.nahrungsmittel-intoleranz.com/fructoseintoleranz-das-passiert-im-koerper/
