Metabolism of the main nutrients Part II — Carbohydrates
From “fattener” to “bad guy sugar” — carbohydrates are said to have all kinds of negative connotations, but they’re not all that bad. As is so often the case, the amount and composition of carbohydrates determine what they do in our bodies. Carbohydrates (“sugar”) provide the body with quick energy. Together with protein and fats, they are among the essential components of our diet. Depending on the food, carbohydrates cause blood sugar to rise at different rates, which also determines how quickly we become hungry again. If the body does not get enough carbohydrates — for example in the case of a low-carb diet — it has to fall back on its own nutrient depots and energy production from proteins and fats.
6 facts-to-go: briefly & concisely summarized for you!
- Carbohydrates are indispensable for supplying energy to our body cells.
- The level of this rise in blood glucose triggered by a particular food is known as the glycemic index (GI).
- The higher the glycemic index of a food, the greater the rise in blood glucose levels.
- The human body’s metabolic rate declines by nearly 2 percent per decade after age 30.
- The most dramatic loss of muscle mass and resulting decline in metabolic rate occurs between the ages of 50 and 70.
- Since stress activates the release of cortisol and cortisol slows metabolism, avoiding stress or at least practicing relaxation techniques can also help.
What role do carbohydrates play as an energy source?
Carbohydrates are indispensable for the energy supply of our body cells. They make it possible for certain metabolic processes to take place in the first place. Colloquially, carbohydrates are referred to as “sugars” and consist of various organic building blocks such as carbon, hydrogen and oxygen atoms. The carbohydrate family includes both monosaccharides and polysaccharides. Glucose and fructose are examples of simple sugars, while starch, glycogen, and cellulose are examples of polysaccharides. The polysaccharides are also known as polysaccharides and are made up of several simple sugar molecules. Polysaccharides serve as energy stores (e.g., starch and glycogen) and as structural components (e.g., chitin in insects and cellulose in plants).
Simple carbohydrates such as fructose or glucose are immediately absorbed into the bloodstream. They cause a rapid rise in blood glucose levels, but they also drop again quickly. These fluctuations make us hungry. Foods with complex carbohydrates (= polysaccharides, starches), on the other hand, saturate more and last longer because they cause the blood glucose level to rise more slowly.
How are carbohydrates utilized?
During digestion, carbohydrates are broken down into simple, soluble sugars that can be transported through the intestinal wall into the bloodstream to be distributed throughout the body. However, the digestion of carbohydrates already begins in the mouth: when chewing the food, the body releases the digestive enzyme amylase with the saliva. This initially breaks down the polysaccharides into disaccharides. This cleavage is briefly interrupted by the gastric acid in the stomach and continued again in the small intestine by the digestive enzymes of the pancreas. Double sugars are finally split by enzymes in the small intestine wall to single sugars (glucose, fructose…). These smallest units of carbohydrates now pass through the intestinal vessels into the blood and to the liver, where hepatocytes (liver cells) either “release” the glucose back through the circulatory system or store excess glucose as glycogen. Cells throughout the body can take up the free glucose from the blood under the influence of insulin and convert it into energy.
Through a series of reactions, some of the energy contained in the glucose is transferred to ADP to form ATP. This process — glycolysis — thus aims to generate ATP, which is essential for cellular respiration. Glycolysis can be divided into two phases: the energy-consuming phase and the energy-generating phase. The first phase is the energy-consuming phase, so for each molecule of glucose, two ATP molecules are needed to start the reaction. However, at the end of the reaction, four ATP are produced, resulting in a net gain of two ATP energy molecules.
How can the body produce its own glucose when it needs it?
Under conditions of low glucose, such as fasting, starvation, or low-carbohydrate diets, glucose can be synthesized from lactate, pyruvate, glycerol, alanine, or glutamate. This process, called gluconeogenesis, is almost the opposite of glycolysis and is used to generate glucose molecules for glucose-dependent organs, such as the brain, when glucose levels fall below normal. Gluconeogenesis is the synthesis of new glucose molecules from pyruvate, lactate, glycerol, or the amino acids alanine or glutamine. This process occurs primarily in the liver, i.e., under conditions of fasting, starvation, and low-carbohydrate diets. So the question arises, why is the body making something that it has just laboriously broken down?
Certain key organs, including the brain, can only use glucose as an energy source; therefore, it is important for the body to maintain a minimal blood glucose concentration. When the blood glucose concentration falls below this certain point, new glucose is synthesized by the liver to raise the blood concentration to normal.
Pyruvate — the end product of glycolysis — is a common starting material for gluconeogenesis. First, pyruvate is converted to oxaloacetate, which itself is converted again. From this step on, gluconeogenesis is effectively the reverse of glycolysis. Finally, glucose itself is formed through a series of reactions. In gluconeogenesis (compared to glycolysis), the enzyme hexokinase is replaced by glucose-6-phosphatase and the enzyme phosphofructokinase-1 is replaced by fructose-1,6-bisphosphatase. This helps the cell to regulate glycolysis and gluconeogenesis independently.
What role does the glycemic index play?
After the intake of carbohydrate-rich meals, there is a relatively rapid and steep rise in blood glucose within one hour. The level of this rise in blood glucose triggered by a particular food is known as the glycemic index (GI). The higher the glycemic index of a food, the greater the rise in blood glucose. Dextrose (glucose) triggers the highest blood glucose levels and is used as a reference value (100 GI).
Foods with a low glycemic index are particularly beneficial because they prevent a strong “up” and “down” of the blood glucose level, allow the blood glucose to rise slowly and thus saturate for longer. The body needs less insulin to supply the cells with energy. If less insulin is used, the risk of developing diabetes mellitus also decreases.
Foods with a low glycemic index include:
- Types of bread: whole-grain bread, whole-grain toast bread, whole-grain pastries
- Muesli: without added sugar, cereal flakes such as oatmeal, bran
- Whole grain rice
- Whole grain pasta, grain products made from whole grain flour, millet, quinoa, polenta
- Potatoes: homemade mashed potatoes, baked potatoes, boiled potatoes
- Vegetables: fresh or frozen, raw or gently cooked
Carbohydrates such as rice, potatoes, bread, vegetables alone are not “fatteners”. Unless they become calorie bombs in combination with sugar and fat through appropriate preparation (fatty spreads, breaded vegetables, deep-fried French fries…). In general, dried fruit and sugary soft drinks and foods should only be enjoyed in moderation. This also applies to products with fructose. One gram of fructose provides just as many calories as one gram of white sugar and, in excess, has also been shown in studies to lead to more rapid fatty degeneration of the liver!
Speaking of calories: In addition to immediate consumption in the body’s cells, the energy supplied is stored in “depots” in the liver and muscles in the form of glycogen, as already mentioned. But these stores are limited. What goes beyond that is converted into adipose tissue.
How does sugar metabolism change with age?
The human body’s metabolic rate declines by nearly 2 percent per decade after age 30. Changes in body composition, including reduced lean muscle mass, are primarily responsible for this decline. The most dramatic loss of muscle mass and resulting decline in metabolic rate occurs between the ages of 50 and 70. Loss of muscle mass equates to decreased strength, which tends to prevent seniors from engaging in sufficient physical activity. This leads to a positive feedback system where the reduced, physical activity leads to even more muscle loss, which further reduces metabolism.
There are some things that can be done to prevent an overall decrease in metabolism and counteract the nature of this decrease. These include: eating breakfast, eating small meals frequently, eating plenty of lean protein, drinking water to stay hydrated, exercising (including strength training), and getting enough sleep. These measures can help prevent a drop in energy levels and curb the urge to increase calorie consumption through excessive snacking. While these strategies are not a guarantee to maintain metabolism, they help prevent muscle loss and can increase energy levels.
Some experts also recommend avoiding sugar, which can lead to excessive fat storage. Spicy foods and green tea may also be beneficial. Since stress activates the release of cortisol and cortisol slows metabolism — especially sugar metabolism — avoiding stress or at least practicing relaxation techniques can also help.
Text-Sources:
(1) Burgsteins Handbuch Mikronährstoffe, M. Zimmermann, Haug Verlag, 11. Auflage, Stuttgart, 2007
(2) Mikronährstoffe, U. Gröber, Wissenschaftliche Verlagsgesellschaft Stuttgart, 3. Auflage, Stuttgart, 2011
(3) Ernährungsmedizin, K. Widhalm, Deutscher Ärzte Verlag, 3. Auflage, Köln, 2009
(4) Sluijs et al.: Dietary glycemic index, glycemic load and the risk of type 2 diabetes. In: Journal Nutrition 2013; 143:93–99
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