Essential Macronutrient- Carbohydrate: The Primary Fuel

Nutrition: Nutrition refers to the biological and physiological process of ingesting, assimilating, and utilizing food to promote growth, metabolism, and repair of tissues.¹  To maintain proper health, a balanced diet of foods that contain the key nutrients must be consumed.^{2, 3}

Nutrients: Nutrients are chemical substances in foods that sustain the basic functions of the body to maintain life.² Nutrients provide energy, build cells and tissues, regulate bodily processes like breathing and are derived from different foods. Broadly, nutrients are classified into macro and micronutrients. Carbohydrates, lipids, and proteins are the macronutrients responsible for providing energy, repairing tissues, and building the body structure. Vitamins and minerals are the micronutrients that are essential for metabolism process. Vitamin A, D, E, K are fat soluble and vitamin B and C are water soluble. Water is the other main component without which no metabolism or other living processes can be maintained.²  

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Macronutrients

The daily energy requirement comes from large quantities of carbohydrates, lipids, and proteins. These are carbon-based compounds that are metabolized to produce the cellular energy adenosine triphosphate (ATP)- the ultimate energy molecule that is made by the cell mitochondria.⁵

Food energies are measured in calories or kilocalories. Individual macronutrient needs vary depending on age, weight, and pre-existing health conditions. Carbohydrates are the main energy source for the body. It is very efficient and is the most preferred energy source of the body. Usually, the body breaks down carbohydrates first for its energy, then the fats and finally the protein only when carbohydrates and fats are depleted. Since protein builds the structure and organs of the body, once protein starts breaking down, the body starts to lose tissue.⁶

Carbohydrates

These are the primary energy source for the brain, nervous system, and muscles. After food intake and digestion, all types of carbohydrates must be broken down (small intestine) or converted (by liver) to the single molecule glucose which gets released in the bloodstream. Once in the blood, it gets taken up by the cells in our tissues to be used as a source of energy for all cellular activities. Excess glucose in the cells is then stored as glycogen particles in the liver and muscles. Liver glycogen can be broken to glucose and released in blood when sugar levels are down in blood stream. However, muscle glycogen can only be used for the muscles. Carbohydrates are not stored in high amounts in the body. The body can store only around 600 gm of glycogen and 80% of it is in skeletal muscle, the rest in liver mainly and some in heart muscle.⁷

Stored glycogen is used during fasting periods like overnight sleep. For each gram of glycogen an additional 3 gm of water is stored.⁸ This is why diets high on carbohydrates also retains excess water and causes weight gain.

Body glycogen depletes after 80 minutes of very strenuous exercise. It is recommended to lower the intensity so that glycogen does not deplete since it takes 48 hours and high carbohydrate diet to store glycogen again and also slowing down will simultaneously increase fat combustion.⁹ 

Finally, when blood glucose and glycogen storage is saturated, excess glucose is converted to fat and stored in adipose tissue.¹⁰

Sedentary men and women require 130 grams of carbohydrate in a day to maintain brain function.¹¹

Role of Hormones:

Glucose level in the blood is controlled by the pancreatic hormone insulin. When blood glucose level is elevated, insulin is released which binds to insulin receptor and activates it. The activated receptor signals the GLUT4 transporters from within the cell to the cell membrane which then pulls the glucose from the blood into the cell where it is either used for energy or stored in the form of glycogen by the glycogenesis process in the liver and muscle. More glucose in blood means more insulin as well. Insulin, along with storing glucose is also the storage hormone of protein and fat and is a satiety hormone that gives the feeling of fullness. If insulin is not released or if it fails to attach to the receptor, glucose will build up in the blood, as the GLUT4 is not brought to the cell membrane. Another pathway of bringing GLUT4 transporters to the cell membrane is muscle contraction. So, in type 2 diabetes exercise is effective as it induces muscle contraction which can bring GLUT4 transporters to the cell membrane to take up excess glucose from the blood.¹⁰

In between meals, the blood glucose level drops. This is the time when the pancreas produces the hormone glucagon that regulates the blood glucose level. Glucagon stimulates the glycogenolysis process in the liver to break down glycogen to glucose which is then released in the blood. Liver glycogen is limited in supply and depletes within 8 hours. Since the brain utilizes glucose as the main fuel and does not store glucose, it is important to maintain blood glucose level even when glycogen is depleted. This is when the liver stimulates gluconeogenesis process in which glucose is produced from non-carbohydrate sources like fats or amino acids from muscle tissues. Chronic under consumption of carbohydrates can affect muscle mass to maintain blood glucose levels. During the time of stress epinephrine and cortisol from adrenal gland regulates blood glucose level. Epinephrine breaks down glycogen from both liver (blood level) and muscle (movement action) to keep the body ready to flight and cortisol is released during stress that breaks muscles and uses the amino acids to make glucose in the liver. Continuous stress elevates cortisol at all times which induces chronic conditions. Both epinephrine and glucagon are responsible for glycogenolysis to release glucose into the blood.¹⁰

Lowering of insulin and increase of glucagon is the signal for body to start burning the stored energy- first glycogen and then fat. It is important to remember that the body will not use fats while insulin levels are high. High carb diet will always keep the insulin high and make fat burning difficult. Lack of sleep, more precisely lack of deepest state of sleep can lower insulin sensitivity and increase obesity. When people on carb restricted diet (hoping to burn more fat) sleep less than 5 and half hours a night, end up losing more body lean muscle than fat. One of the reasons can be the hormone ghrelin.¹²

Ghrelin is a hunger hormone released primarily by stomach acts on the brain when food intake is chronically low. Ghrelin level goes down after meals, but it has another property of encouraging fat retention and triggering glucose production by the liver. Usually after meals ghrelin goes down.¹³ Book page 79. The short sleepers have higher level of ghrelin that encourages fat retention and glucose production by the liver and in cases of people on a carbohydrate restricted diet, the ghrelin will make liver produce this glucose from amino acids which means loss of muscle. If the carb restricted dieters changed their sleep habits and take a 7–8-hour sleep, the pattern changes to more fat burning.¹²

Leptin is a hormone released from fat tissue. Its action is opposite of ghrelin, it promotes satiety and inhibits appetite. High body fat with high energy increases leptin level. When leptin is high, we are not hungry but when low it signals food intake. However, in obese patients, there is leptin resistance. In this condition, despite having more adipose tissue which makes a lot of leptins, the brain becomes resistant to leptin and the person is in a state of constant hunger.^12,14

Sleep: Adequate sleep is necessary for the above-mentioned hormones. Sleep deprivation leads to the following problems:

Cortisol- Increases	Increases cortisol which increases fat storage and reduces insulin sensitivity.
Growth Hormone- Decreases	Suppresses growth hormone which is needed for tissue repair, glucose regulation and fat burning.
Leptin- Decreases	Leptin level is reduced which will trigger over- eating.
Ghrelin- Increases	Ghrelin level is increased which increases appetite and cravings for sweet, salty, starchy- calorie dense food.
Orexins- Increases	Neuropeptide hormones from hypothalamus become more active, increases food intake and inhibits growth hormone.

Carbohydrate Sources & Types:

Grains, cereals, potatoes, starchy vegetables, bread, milk, nuts, legumes, and fruits are the main sources of carbohydrate. Carbohydrates are divided into monosaccharide or simple sugar (glucose, fructose, galactose), disaccharide (sucrose, lactose, maltose), and polysaccharides which are the complex carbohydrates derived from both plants (starch like cellulose, amylose, and amylopectin) and animals (glycogen).¹⁵ Polysaccharides are further types as digestible (starch, glycogen), semi-digestible (inulin, raffinose) and indigestible (fibers like- cellulose, pectin).¹³

High glycaemic index (GI) foods that cause sugar spikes are- refined carbs (white rice, white processed flour, white sugar, white bread), cereals, candy, sugar, bagels etc. Low GI foods are whole grains, legumes, vegetables, etc.¹⁶

Some soluble fibers are water soluble that help reduce cholesterol levels in the blood. Fibers attach to bile salts in the small intestine and eliminate it from the body. Since cholesterol makes bile salts, more fiber will remove more bile salts, thus remove more cholesterol.²⁵ Oats, oat bran, dried beans and peas, nuts, barley, flax, chia, oranges, bananas, blueberries, apples, artichokes, tomatoes, carrots are rich in soluble fibres.²⁶

Amylose is a long chain polysaccharide, made up of linearly placed glucose molecules and is known as resistant starch due to its resistance to enzymatic hydrolysis.¹⁸ These are partially digested and absorbed in the small intestine. Slower digestion makes it a low glycaemic index carb¹⁹  which means they break down into glucose molecules very slowly and help avoid sugar spikes in blood and is the ideal choice of carbohydrate source for type 2 diabetics. Furthermore, as it resists digestion in the small intestine, it ends up in the colon where both amylose and cellulose (vegetables or dietary fibers) ferment to feed the gut microbiome.²⁰

While getting fermented by the gut microbiota in the colon, short chain fatty acids (SCFAs) like acetate, propionate, and butyrate are formed which are important bio-products from the breakdown of the non-soluble amylose and cellulose. These SCFAs have protective effects in the human body through regulation of inflammation, reduction of cancer precursors, protection of gut barrier function, reduction of oxidative stress, and mental wellbeing. ^21,22 

Sources of resistant starch: Cooked and cooled starch which includes- potatoes, rice, oats, pasta, sushi rice, plantain (cooked or uncooked).²⁷

Amylopectin is also a polysaccharide made up of branched chain of glucose molecules, however it is shorter chain than amylose. It has faster digestibility and is easily broken down to glucose in the small intestine to provide energy to body.¹⁸

It is sweeter than amylose and is a high glycaemic index complex carbohydrate. Amylopectin accounts for 70-80% of complex starch compared to 20-30% of amylose.¹⁸

Difference Between Amylose and Amylopectin

Characteristics	Amylose	Amylopectin
Structure	Linear	Branched
Chain Length	Long	Shorter with branches
Proportion in Starch	20-30%	70-80%
Digestibility	Slower	Faster
Solubility in Water	Lower	Higher
Dissolved in Hot water	Forms a gel	Does not form a gel
Glycemic Index	Lower	Higher
Sweetness	Less sweet	Sweeter
Energy Release	Sustained	Quick
Enzyme Activity	More resistant	More easily degraded
Present In	Rice, potatoes, corn	Rice, potatoes, corn

One gram of digestible carbohydrates yields 4 Calories of energy for the cell to process the cellular activity. It is the essential nutrient for proper functioning of the nervous system, heart, and kidneys.²³

Carbohydrate Metabolism Pathways:

1. Glycogenesis- glucose stored as glycogen.
2. Glycogenolysis- breakdown of glycogen to glucose 6-phosphate. Glucagon and epinephrine stimulate glycogenolysis.
3. Glycolysis- Glucose 6-phosphate is converted to pyruvate.
4. Krebs Cycle and Electron Transport Chain- pyruvate is turned to Acetyl CoA. Carb, fat, protein all can be broken down to Acetyl CoA which is the main entry molecule into Krebs cycle. Electron Transport Chain produces ATP molecules. Both take place in the mitochondria of cells.
5. Gluconeogenesis- glucose synthesis from pyruvate, lactate, amino acids, or glycerol from triglycerides. Mostly happens in liver and some in kidney.²⁸

Carbohydrates/glucose are the most easily available and is the main source of energy in the body. Consuming adequate carbohydrates preserves muscle mass. However, the modern diet is excessively dependent on refined carbs that are leading to the epidemic of diabetes, insulin resistance and other metabolic syndromes. More awareness generation is required to add fibers, whole grain food for slow release of glucose in the blood stream, boost gut health, ease constipation, improve insulin resistance, and feel full longer.²⁴

Author of this article

Dr. Maliha Mannan Ahmed, MBBS (BMC), MBA (ULAB), Masters in Healthcare Leadership (Brown University, USA) and Level 1 Certification on Precision Nutrition. 

The Executive Editor of The Coronal.