Carbohydrates, Glucose & Glycemic Index

Article published at: Oct 26, 2023
Carbohydrates, Glucose & Glycemic Index
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Sep 21


Carbohydrates, also known as sugars or saccharides, are widely prevalent in nature and constitute essential components of food, serving as a source of energy for the organism. They also play structural roles (e.g., starch, glycogen, cellulose) and contribute to flavour characteristics. Carbohydrates are synthesised in plants through the process of photosynthesis and are stored in the form of starch, while in animal organisms, they are stored as glycogen. The term 'carbohydrate' was initially coined because the compounds falling under this category corresponded to the general chemical formula Cx(H2O)x, meaning they originated from hydrated carbon, with hydrogen and oxygen present in the same ratio as in water.

The term 'carbohydrate' persists today even though known compounds such as acetic acid (C2H4O2) or lactic acid (C3H6O3) adhere to the general formula of carbohydrates without possessing their characteristic properties, just as other well-known carbohydrates do not conform to this formula. Thus, the designation 'carbohydrates' is a term for an entire class of compounds.

Carbohydrates are divided into the following groups:

a) Monosaccharides or simple sugars, which are the simplest members of carbohydrates and are not subject to hydrolysis (e.g., glucose, fructose).

b) Oligosaccharides, composed of a small number, typically from two to about ten, monosaccharide units.

c) Polysaccharides, composed of a large number of monosaccharide units, such as amylose, which contains 100-2000 units of the monosaccharide glucose in its molecule. The high-molecular-weight compounds of polysaccharides exhibit differences in their physical and chemical properties compared to the monosaccharides from which they are composed.

The most important polysaccharides in nature include starch (the storage material of plants), glycogen (the storage material of animals), and cellulose (a structural component and supportive material in plants).


Glucose plays a central role in human metabolism as it participates in both glycolysis and fermentation processes, thus making it a valuable element for energy production. The choice of cells to utilise glucose or lipids and ketones (by-products of lipid metabolism in hepatic cells when blood glucose levels are low) as an energy source depends on both the specific functions performed by each cell in our organism and our dietary patterns. Nervous and brain cells prefer glucose and require a certain amount of it to function efficiently.

When the body receives a limited amount of carbohydrates from the diet, glycogen stores in the liver and muscles are quickly mobilised to meet this demand. Once these glycogen reserves start depleting, metabolism initiates a conservation program. All cells that do not have an immediate need for glucose reduce their consumption and, if necessary, cease it altogether. Glucose, which at the given moment becomes a rare and precious resource, is made available exclusively to those cells that have an absolute requirement for it, as they cannot survive without it, primarily the nervous cells.


In the human body, glucose dissolved in the blood, along with the blood itself, functions like a perfect hydraulic system. Cells have the capacity to 'draw in' glucose solution through specialized glucose transporters. This precise mechanism ensures a continuous supply of glucose to cells with high priority, even when blood glucose levels are low. Cells with lower priority for glucose are compelled to resort to secondary energy sources, such as lipids and ketones.


To adequately nourish cells with this life-critical substance, the body distributes glucose through the bloodstream. Our bodies are well-equipped to handle situations of glucose deficiency, signalling hunger as an indication of the need for glucose. However, the body is not as well-prepared for situations of excess, as occurs after a meal. In such cases, our bodies must release significant amounts of insulin, a hormone that facilitates the transport of glucose from the blood to cells, and does so rapidly. However, when cells are already saturated with glucose, high levels of glucose can damage their delicate structures. Consequently, cells attempt to find alternative means of utilising glucose, converting it into fat, a secure and passive form of energy storage. Elevated glucose levels and the subsequent insulin release leading to fat creation are the reasons why insulin is now considered the hormone associated with obesity.

Therefore, glucose not only provides energy and serves as the basis for the synthesis of various substances but also poses risks when present in high concentrations in the blood, potentially causing cellular damage and serious illnesses.

Several tissues and cells are exposed to risks associated with high blood glucose concentrations. These tissues are among the first to absorb glucose when needed, especially during episodes of elevated sugar levels. Specifically, the retina, neurons, and endothelial cells are the primary targets of prolonged hyperglycemia. If blood sugar levels remain consistently high, it can lead to chronic complications associated with diabetes, including retinopathy, nephropathy, and vascular damage. In more severe cases, complications may include blindness, neurological damage, limb amputations, cardiovascular diseases, and even cancer. Glucose, being a primary food source, is also exploited by cancer cells.

Thus, glucose offers a dual potential for energy release, either through glycolysis or fermentation. In our bodies, we have cells and tissues that extract the energy they require from fats and ketones. This is dictated by the fact that our bodies do not possess the capacity to store significant amounts of glucose. In humans, glycogen storage lasts only one to two days, even during periods of inactivity. In high-intensity athletic endeavors, our glycogen stores deplete in approximately 30 minutes.


When glycogen stores are depleted, the only way to produce energy is through the metabolism of fats. The breakdown of fat stores primarily releases fatty acids. Only a small portion of fats, approximately 1/16, produces glycerol, which, in turn, can be converted into glucose. On the contrary, fatty acids are made available for either energy utilization or the formation of ketone bodies (acetoacetic acid, acetone, and beta-hydroxybutyric acid). These ketone bodies serve as an energy source for the brain and cardiac muscles. In situations of glucose deficiency, the brain can activate an energy-saving program that primarily relies on glucose, with the remaining energy coming from ketone compounds.


The Confusion of Concepts

During our university years in Bologna, an exceptional and challenging institution, known as the oldest university in Europe, we attended a seminar where an English Physical Education professor stated, 'Sugar is harmful and impairs performance.' We exchanged bewildered glances! How could sugar be harmful? Sugar provides energy; the brain 'burns glucose,' as do muscles. How could he claim such a thing? He must be crazy! We, the students, couldn't comprehend it. Not immediately, but over time, we came to understand...

Energy and vitality do not come from sugar, specifically sucrose, the white or dark refined crystalline powder we have in our kitchens and use as a sweetening agent in our coffee or tea. The energy that humans need is derived from carbohydrates, not sugar. Carbohydrates, whether simple or complex, constitute all plants, especially fruits, vegetables, grains, and legumes. When consumed sensibly, these carbohydrates have a beneficial impact. However, it is essential to clarify that just because carbohydrates are good, it does not mean that sugar is also beneficial!

Why Does Sugar Harm Us?

The final product of carbohydrate digestion, starting from starch, is glucose, a simple sugar that belongs chemically to the hexoses (six-carbon atoms). However, this simple sugar is beneficial when produced by the body itself, gradually, starting from complex sugars like starch. Fructose, another simple sugar, is found in fruits and vegetables. What happens if, on the contrary, we consume refined sugar? Problems arise!

Someone might argue that there is no difference since both sugar and natural complex sugars like starch ultimately yield glucose in the digestive process. This is true only if we consider digestion as an "in vitro" event, or a laboratory experiment where only the produced calories matter. However, if we refer to the "in vivo" mechanisms and observe the qualitative characteristics of our diet, we will understand that the absorption of refined sugar in the small intestine occurs too rapidly. The body does not tolerate sudden fluctuations in glucose levels (glycemic).

If glycemic drops to 60 mg/dl, the nervous and hormonal systems react to prevent disturbances and cellular damage that could lead to seizures, hypoglycaemic coma, and death. Conversely, if glycemic reaches 600 mg/dl, there is a risk of death from hyperglycaemic coma. The body then activates mechanisms to prevent these dangers. What actually happens?

The pancreas intervenes by playing a peculiar role. In the center of this gland, the islets of Langerhans regulate blood glucose through the production of insulin and glucagon, two hormones with complementary functions. Glucagon is the hormone that increases blood glucose, while insulin decreases it through three different mechanisms:

  1. Storing glucose in muscles for physical activity until replenishment.

  2. Storing excess glucose in the liver as glycogen.

  3. Transforming glucose molecules into fat.

If the increase in blood glucose is slow, the pancreatic response will be limited, and various mechanisms will come into play gradually. Conversely, if the increase is rapid, as in the case of refined sugar and some other high glycemic index foods, the response from the pancreas will be rapid and drastic. It will lead to a sharp increase in insulin, which necessarily floods the bloodstream in an explosive and forceful manner. The effects of this event will be a sudden decrease in blood glucose levels, with the consequent risk of hypoglycaemia, prompting the body to mobilise its sugar stores (glycogen). Thus, a vicious cycle is initiated where individuals constantly feel tired, leading to obesity and type II diabetes. If such a person stops consuming sweets for two weeks, they will immediately feel an increase in energy and significant relief in their body.

It's worth noting that among simple sugars (mainly glucose and fructose), fructose has a slower absorption rate, making it a better choice when obtained directly from fruits and vegetables rather than chemically processed sources.

Finally, the sugar we buy for our homes is a refined sweetener that comes from the processing of sugar beets or sugar cane. Its name is sucrose, and it's a plant-derived sugar that has become artificially processed. Furthermore, sucrose is a disaccharide (composed of one glucose and one fructose molecule) that exists in some plants and fruits but is never formed in the digestive process. Indeed, when we consume carbohydrates, we will eventually reach glucose through the breakdown of substances like dextrose and maltose from starch. Sucrose is never formed, but if introduced into the body, it will break down into glucose and fructose due to the enzyme sucrase.

However, a deficiency in this enzyme in the duodenum and the first part of the ileum, which is common in humans, allows a significant amount of undigested sucrose to pass through the small intestine. This can lead to fermentation and discomfort in the body. This condition can be referred to as sugar intolerance, and it generally goes unrecognised, mainly because few dietitians dare to completely prohibit their patients from consuming sugar-containing sweets. Nevertheless, a VegAnic dietitian would dare to do so because they possess deep knowledge of the metabolic and psychological consequences of sugar abuse and would suggest better alternatives as sweeteners.

Similar is the intolerance to lactose, as almost all of us have reduced levels of lactase, which breaks down lactose (milk sugar) into galactose and glucose. Thus, lactose reaches the colon to be metabolised by the intestinal flora, which is ill-equipped for this task, causing gas and toxic metabolites that irritate the intestinal mucosa, leading to diarrheas and abdominal discomfort, among other adverse conditions.


Sugars constitute the primary source of energy for the entire organism, with glucose being the sole active substrate utilized by the brain. However, unlike muscles, the brain lacks the capacity to store glucose. Therefore, its function depends on a continuous and uninterrupted supply of sugars in the bloodstream. Deprivation of glucose for even a few minutes can result in cellular death in brain cells. To mitigate this, the body has a series of endocrine and exocrine glands involved in carbohydrate metabolism, including the salivary glands, pancreas, liver, and thyroid.

The carbohydrates we consume, including starch, first encounter salivary amylase in the mouth, which continues its action in the stomach. When food passes from the stomach to the duodenum, the pancreas secretes pancreatic amylase to refine digestion and convert complex carbohydrates (starch) into glucose, which can be absorbed in the small intestine. At this point, it is essential to discuss the glycemic index.

The glycemic index is a numerical representation of the speed at which blood glucose levels rise after consuming a specific food compared to the rate of increase caused by a reference food, conventionally set as glucose, with a value of 100. The higher the number, the faster blood glucose levels rise.

It is advisable to prefer foods with a low glycemic index (below 60) because a rapid increase in blood glucose levels (fast transport of sugars from the small intestine to the bloodstream) makes it more challenging for the pancreas to regulate insulin secretion. This, in turn, leads to increased difficulties in controlling body weight, cholesterol levels, electrolyte balance, and overall health.

Carbohydrates with a low glycemic index include fruits and vegetables, with some exceptions (such as carrots, potatoes, pumpkin, watermelon, cantaloupe, and grapes). Whole-grain cereals, legumes, and dried fruits also have a low glycemic index. Carbohydrates with a high glycemic index are sugary sweets, soft drinks, white bread, white pasta, and polished rice.

Of course, discussing the glycemic index without considering the glycemic load, or the quantity of carbohydrates consumed throughout the day, would be misleading. Excessive consumption of carbohydrates, even with a low glycemic index, still poses risks for weight gain and other health factors. However, it is easier to lose control when basing our diet on high glycemic index foods, which provide less satiety and nourishment.

If the majority of carbohydrates consumed are derived from cereals, legumes, and vegetables, the absorption of carbohydrates will be slower, resulting in a lower glycemic index.

Now, discussing carbohydrate metabolism, the primary source of energy, without addressing the topic of physical activity, would be incomplete and misleading. These two factors are closely interconnected.

The more sedentary one's lifestyle, the higher the chances of developing insulin resistance, leading to obesity and the development of dyslipidemia and various diseases. In fact, a sedentary lifestyle renders the muscles and liver less responsive to insulin, forcing them to convert excess glucose into fat to prevent prolonged high blood glucose levels, which poses a serious risk to our health.

Physical exercise is, therefore, the most effective means of combating insulin resistance and maintaining a muscular system capable of efficiently utilizing glucose. In essence, it is moderate physical exercise that promotes the harmonious interaction between the musculoskeletal and hormonal systems, helping us avoid serious health problems.

Gerassimos Tsiolis, PhD in Biochemistry
University of Bologna, Italy