In the previous article, types of carbohydrates were discussed. We know glucose and fructose are two important sugars from health point of view. Starch is major storage form of the glucose. In this post, the digestion and metabolism of glucose will be discussed.
What Happens in Heathy Metabolic State
When we consume food, the starch in food is converted to glucose, by the action of enzymes present in digestive tract. The digestion starts from the time we put our food in the mouth. If we chew a piece of bread for some time, we will get a sweet taste due to conversion of starch into glucose by salivary amylase, an enzyme present in saliva. The glucose is primarily absorbed in the small intestine and enters the blood stream, raising the blood glucose or sugar level (as commonly called). The brain senses this increased blood sugar levels and instruct beta cells of pancreas to secrete a hormone called INSULIN. Insulin opens gate of cells for entry of glucose. Without insulin, glucose cannot enter cells. In Type-I diabetes mellitus, the beta cells of pancreas are destroyed and there is a lack of insulin. Giving insulin to Type I diabetes patients is essential for normal body functions. The cells take the glucose needed by them, and then close their gates to stop further entry. Usually, 80% of the glucose absorbed is utilized by the peripheral cells and 20% of the glucose goes to the liver for further disposal. After two hours of meal, the blood sugar comes back to normal values (<126 mg/dl). Once blood sugar levels fall, the brain instructs pancreas to stop insulin release. After a meal, the blood sugar should come to normal value within two hours, but Insulin takes more time to come back to premeal or fasting levels. This cycle is repeated with every meal. This state is called as insulin sensitive stage, characterized by normal insulin and normal blood sugar level and no obesity or overweight.
After absorption the cells, , utilize the glucose for their needs. Surplus glucose is converted to GLYCOGEN (storage form of glucose) in the liver (around 250-300 gms), muscles and other cells. Brain cells exclusively require glucose for functioning, followed by muscles. Minor amount of glucose is also required for pentose pathways (responsible for creating basic sugars for RNA and DNA), and hexose pathways. Excess glucose is converted to fat and stored in liver and visceral organs (heart, kidney, and abdomen) as reserve. The fate of absorbed glucose is summarized as under:-
(a) Krebs Cycle (Burning to fuel)
(b) Glycogen formation (storge in liver and muscles)
(c) Hexose Pathways (formation of hexose sugars)
(d) Pentose Pathways (formation of pentose sugars)
(e) Lipogenesis (Conversion to fats)
Table : Blood Values During Insulin Sensitive Stage
| Blood Sugar Test | Normal Value |
| Blood Sugar-Fasting (no food for 8-10 hours, only water) | <100 mg/100 ml of blood |
| Blood Sugar -Post Prandial (2 hours after meal) or PP | <140 mg/100 ml |
| Blood Sugar (random) | <140 mg/100 ml |
| *Insulin- Fasting | < 5 units/100 ml |
| *Insulin- Post Prandial or PP | <55 units/100 ml |
The blood sugar is strictly maintained in the desired range by various hormones. When we don’t eat (during fasting or sleeping), the blood sugar is maintained by converting glycogen to glucose under the influence of another hormone GLUCAGON. Hormone Cortisol also raises blood sugar under stressful conditions of fear, flight and fight responses. The detail of glucose metabolism can be found in the article by Mihir N Nakrani.
Different foods raise the blood sugar differently. Carbs raise the most, and fats the least and proteins in between. The capacity to raise blood sugar is depicted by Glycemic Index (GI) which tells us that how much blood glucose will be raised by a particular food (taking glucose value as 100). Foods rich in carbs specially processed food cause a sharp rise in blood sugar. A comprehensive list of glycemic Index of foods is available at verywellhealth.com
In the next article, the role of glucose in health and diseases will be discussed. Hope you have liked the article. Your comments, criticism and suggestions are welcome.