Diet and Nutrition

How Does the Body Turn Carbohydrates into Glucose?

When carbohydrates are digested, they are sent into the bloodstream and become sugars called glucose.

How Does the Body Turn Carbohydrates into Glucose?

What are carbohydrates?

The body converts the food we eat into energy. Even though we obtain calories and energy from different food sources, such as carbohydrates, fats, and proteins, our main source of energy is still from carbohydrates. The body converts these carbs into glucose, which is the main sugar in the blood. 

There are many food sources that contain a combination of proteins, fats, and carbohydrates. How much food we consume usually affects how fast the body converts that food into glucose. The following food sources show how fast they affect blood sugar levels:

  • Carbohydrates: The body converts all carbohydrates into glucose, which quickly affects blood sugar levels within 1-2 hours after eating. Carbohydrates include sugar, rice, bread, pasta, potatoes, fruits, and vegetables. 
  • Protein: The consumption of foods rich in protein usually has a very little impact on blood sugar levels because most glucose from these protein sources are stored in the liver and are not released into the bloodstream. Examples of foods high in protein are meat, fish, eggs, nuts, cheese, and other dairy products.
  • Fat: Glucose from fat is slowly absorbed and will not cause an immediate spike in blood sugar levels. Dietary fat sources include olive oil, avocado, salad dressings, and butter. 

Eating a healthy, balanced meal that contains proteins, fats, and carbohydrates can help keep blood sugar levels from rising too quickly. 

Types of Carbohydrates

The two types of carbohydrates are simple and complex carbohydrates. 

1. Simple 

Simple carbohydrates are found in refined sugars, such as white sugar. Candies and lollipops are also simple carbohydrates but do not have important nutrients in them. However, you can get simple carbs from more nutritious foods, such as milk and fruits since these foods also contain fiber, vitamins, and other nutrients like calcium. 

2. Complex 

Complex carbohydrates are also called starches, which include rice, pasta, bread, crackers, and other grain products. Other complex carbs are refined grains, such as white rice and white flour, which have less fiber and nutrients due to processing. However, there are also some unrefined complex carbs that are better to consume because they still contain vitamins, minerals, and fiber, which help promote a healthy digestive system.

Metabolism of Carbohydrates

Carbohydrates are broken down into simple sugars during digestion and are transported all throughout the body through the circulatory system. The digestion of carbohydrates starts in the mouth with the action of the enzyme called salivary amylase. This enzyme acts on starches an enables the absorption of monosaccharides across the walls of the small intestine. The absorbed monosaccharides are then transported to the body's tissues and begin the process of cellular respiration. 

The small intestine tends to only absorb single sugar molecules. For this reason, digestive enzymes break down carbohydrates into three monosaccharides, namely glucose, galactose, and fructose. Monosaccharides are transported to the liver, which turns galactose and fructose into glucose. The liver then sends the produced glucose into the bloodstream, where it is transported to the cells that need energy. When there are excessive glucose levels in the blood, the liver stores glucose into glycogen or fat. 

One study concluded that a diet containing a high amount of carbohydrates will cause an increase in glucose and insulin levels. The cells take up glucose and either convert it into glycogen or break it down so that energy can be produced. The liver and muscles are the main storage areas of glycogen. Whenever energy is needed from these stores, glucose can be obtained.

Glycogen is broken down so that glucose can be obtained. In order to finally produce ATP, a series of reactions take place. These reactions depend on certain factors, such as enough supply of oxygen and glucose. By using different sets of reactions, glucose can still be broken down if the oxygen level is lacking. However, when it comes to energy, the body will turn to other fuel sources when glucose levels are low.

The following are the steps for glucose metabolism:

  • Glycolysis - In this process, glucose basically breaks down to pyruvic acid. Through this process, a couple of ATP molecules are produced.
  • Krebs Cycle - First, pyruvic acid is converted into acetyl-CoA and enters into the Krebs cycle. By series of reactions, fuels break down into water and carbon dioxide, so that more usable energy is generated. For the cycle to continue, the original compound is regenerated. This cycle takes place in the mitochondria, which are double-membrane structures that are small and oval in shape. They are also known as the powerhouse of the cell.
  • Oxidative Phosphorylation - This is the metabolic pathway, wherein cells utilize enzymes for nutrient oxidation, thus releasing energy for the production of adenosine triphosphate (ATP). 

Carbohydrates are efficient when it comes to producing adenosine triphosphate (ATP), which is an energy-providing chemical that drives many cellular processes. For this reason, they remain the body’s main source of energy. The body prefers the breakdown of carbohydrates first, followed by fats. If both carbohydrates and fats are depleted, the body uses proteins for energy.

In one study, people with type 2 diabetes were given a diet low in carbohydrates and more in protein. The study noted the blood glucose levels of people in this diet.

During starvation, the body will break down fat for energy since few carbohydrates are present. In order to provide energy, the body will break down proteins once fat and carbohydrates are depleted.

Metabolic processes that occur in the presence of oxygen is referred to as aerobic respiration. In the combustion of various fuels, oxygen acts as an oxidizing agent. Oxygen-accepting electrons are produced due to special reactions that take place. This causes a release of energy and production of ATP. Moreover, two waste products are excreted from the body, and they are carbon dioxide and water. To carry out the process of aerobic respiration, carbohydrates are the main fuel. Certain reactions cannot take place in the absence of oxygen.

References

Carbohydrates and Blood Sugar. https://www.hsph.harvard.edu/nutritionsource/carbohydrates/carbohydrates-and-blood-sugar/

Carbohydrate Metabolism. https://courses.lumenlearning.com/suny-ap2/chapter/carbohydrate-metabolism-no-content/

Gannon, M.C. and Nuttal, F. Effect of a High-Protein, Low-Carbohydrate Diet on Blood Glucose Control in People With Type 2 DiabetesSep 2004, 53 (9) 2375-2382; DOI: 10.2337/diabetes.53.9.2375

Adamska, Edyta. et. al. Intake of Meals Containing High Levels of Carbohydrates or High Levels of Unsaturated Fatty Acids Induces Postprandial Dysmetabolism in Young Overweight/Obese MenBioMed research international vol. 2015 (2015): 147196. doi: [10.1155/2015/147196]

Key Takeaways

  • Carbohydrates include sugar, rice, bread, pasta, potatoes, fruits, and vegetables. 
  • The body converts all carbohydrates into glucose, which quickly affects blood sugar levels within 1-2 hours after eating.
  • Carbohydrates are efficient when it comes to producing adenosine triphosphate (ATP), which is an energy-providing chemical that drives many cellular processes.