Carbohydrates are not only a source of energy
by Petra Kavradzhieva
January 23, 2019
Carbohydrates are one of the three groups of macronutrients which are essential to sustain an optimal energy level and to maintain a healthy lifestyle. First of all: What exactly is health? According to WHO- World Health Organisation, health is “a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity”.So health is not just the lack of a certain disease, but a harmonious collaboration between all systems in the body.
In order for this collaboration to function properly, one needs to consume all three groups of macronutrients and a great diversity of micronutrients. In this article, I am going to explain why carbohydrates are healthy and necessary for our body.
Firstly, which are the exact sources of carbohydrates? Bread, pasta, and fruits can easily come up to your mind and not only they are included, but also dairy products, high-fiber foods, such as lentils, and potatoes, which are rich in starch. Fruits contain mainly fructose; dairy, especially yogurt and milk, contain lactose.
Medical significance and normal blood glucose concentration
The medical significance of glucose is something very important, to begin with. Of great importance is the blood glucose concentration. The normal concentration ranges from 2,8 mmol/l to 6,1 mmol/l. The optimal concentration is between 4,5 mmol/l to 5,5 mmol/l. Just below 3 mmol/l is typically marked as slight hypoglycemia and we usually call this “hunger”, which is pretty normal and there is nothing to worry about. After eating, the blood glucose can rise up to 11 mmol/l, which is also quite normal, and this is called postprandial hyperglycemia. People who take blood glucose tests should do this in the morning without having eaten absolutely anything. A level over 7 mmol/l is something that should be taken into consideration, especially if you are suspicious of diabetes. Another test is the measurement of the level of glycated hemoglobin (HbA1c). This test measures the level of conjunction between glucose and hemoglobin that has already got out of the destroyed erythrocytes. The erythrocytes have a lifespan of around 120 days and after the breakdown of their cellular membrane hemoglobin gets out of the cells and enters the bloodstream. If the blood sugar is constantly and significantly raised, a lot of glucose molecules are going to be conjugated with hemoglobin and form glycated hemoglobin. This test can determine whether or not one has diabetes.
Since we made the introduction to the medical significance of glucose, it is time for the explanation of the significance of glucose for the overall metabolism of the human body. There is a great deal of myths circulating around the internet, so in this article, it is important that the glucose metabolism, or at least the basics of it, is clarified.
The myth about carbohydrates and weight gain.
It is widely believed that carbohydrates lead to weight gain and obesity. Anything which is consumed excessively can lead to weight gain. Each person has a specific daily caloric requirement to fulfill. If your limit is 2000 kcal a day and you exceed it by 300 kcal you are simply going to store fat in your fat cells and it does not matter which macronutrient you overconsume. Some of you might say that since they have stopped eating bread and other foods rich in carbs, they have lost weight. With the restriction of many kinds of foods, it is possible to restrict the overall energy intake. It is about caloric and energy restriction, not carbohydrate, fat or protein one.
Glucose is metabolized mainly through a process called glycolysis, which can be anaerobic, without oxygen, or aerobic, with the inclusion of oxygen. Aerobic glycolysis is, suffice it to say, Krebs’ cycle.
Glucose and the brain
The brain is extremely sensitive towards blood glucose concentration. It has a glucose transporter, called GLUT3, which has a really high affinity for glucose even if the concentration in the blood is low. The brain receives a constant amount of glucose, which is around 80 mg/min, and the consumption does not change substantially even during workouts. The brain is ensured with a great blood circle, called Circle of Willis, which mainly involves the internal carotid artery, and this is a defensive mechanism so that there is always enough blood for this important organ. We can make simple calculations in order to find out how much glucose our brain needs throughout the 24 h cycle. 80 m/min multiplied by 60 minutes gives 4800 mg per hour. 4 800 mg multiplied by 24 gives 115 200 mg throughout the day and this makes 115 g of glucose. If you multiply it by 4, because 1 g of glucose is 4 kcal, you have approximately 460 kcal. So 460 kcal just for the brain! Neurons use only glucose as a source of energy. Glucose is converted to pyruvate and then enters the Krebs’ cycle. Pyruvate is converted to Acetyl coenzymeAthrough a process called oxygenated decarboxylation. Acetyl-coA interacts with oxaloacetate to form citric acid and throughout the cycle carbon dioxide and water are exuded, then the cycle begins all over again with the same metabolites. Of course, this happens in all other cells, with the exception of erythrocytes, which have only anaerobic glycolysis, and muscle cells when working under anaerobic conditions. Glial cells, the cleaners, and helpers of the neurons convert glucose to pyruvate and then pyruvate to lactate. After that, these cells provide the brain with lactate, which is converted to pyruvate and then pyruvate enters again Krebs’ cycle.
Pentose Phosphate Pathway
Glucose takes part in the so-called Pentose Phosphate Pathway. The pentose phosphate pathway is an alternative route for the metabolism of glucose. It does not lead to the formation of ATP- the universal energy provider- therefore it does not lead to energy formation, but this way has two very important functions. Firstly, NADPH is formed, which is a coenzyme that takes part in anabolic processes, such as synthesis of fatty acids and steroid hormones. Secondly, it leads to the synthesis of ribose, which is a pentose monosaccharide. Ribose is included in the structure of the ribonucleotides of RNA and from ribose, the organism can create deoxyribose, which is in the structure of deoxyribonucleotides of DNA. So this is important for the preservation of the genetic material, synthesis of proteins and many other functions.
Carbs and vitamins
Another role that carbohydrates overall have, not just glucose, apart from being an energy source, is to introduce more vitamins and minerals. Many carbohydrate-rich foods contain trace minerals and irreplaceable vitamins. You might say that 5 cups of veggies do the work but what about lentils? They are an excellent source of B vitamins. with the exception of Vitamin B12. Lentils are especially high in folate, Vit B9. Lack of folate during pregnancy can lead to fetus malformations, especially when it comes to the formation of the neural tube, which begins around the 18-20th day of the pregnancy.
Now you know the importance of glucose metabolism and the functions of this one simple monosaccharide in a healthy organism. Of course, there are great differences between one healthy individual and one who was health problems. This article was for healthy ones and in the future, there is going to be another article for glucose metabolism but in individuals who have health problems. Wait for part 2, in which I explain more about fructose and lactose significance.
Sources: https://www.hsph.harvard.edu/nutritionsource/carbohydrates/ ; Haper’s Biochemistry ; Medical Biochemistry lectures( Biochemistry in Internet) by Mecial university of Sofia