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"Fueling Your Body: Demystifying the Science Behind Carbs, Protein, and Fat"

Our body relies on three primary macronutrients to carry out daily functions. These are Carbohydrates, Proteins, and Fats. During digestion these macronutrients are broken down into simpler compounds that can be absorbed and used by our body. Carbohydrates are broken down into simpler sugar molecules called glucose which is the primary energy source. On the other hand, Fats are transformed into fatty acids while proteins into their simpler building blocks called amino acids. Amino acids can also be used by the body as an energy source; however, their primary function is to help the body in the development of muscles, making of hormones, enzymes, and other important proteins.

Understanding how these macronutrients are metabolized and used to perform essential bodily functions is crucial for maintaining a healthy body. In this blog we will discuss the science behind the metabolism of these macronutrients.


Carbohydrates Metabolism

Carbohydrates are regarded as the primary source of energy for our body. The digestion of carbohydrates starts in the mouth by special enzymes called amylase and continues in the small intestine with other enzymes like sucrase, lactase, and maltase that completely break down complex carbohydrates into simple glucose molecules. These glucose molecules are absorbed in the intestine and released into bloodstream from where they are transported to every cell of the body. Once inside the cell, these glucose molecules are then used to produce energy after passing through a series of chemical reactions, while the excess glucose is stored in liver as glycogen.

How is blood sugar level controlled?

The blood can accommodate up to 3-6 grams of glucose, similarly the liver and muscles also has a limited capacity of glycogen storage ranging around 500 grams. When the level of glucose increases in the blood usually after a meal our brain signals the pancreas to secrete a hormone called insulin which travels in our body through bloodstream. Insulin stimulates the cells to take up oxygen from the blood thus decreasing the levels of blood glucose. Insulin also triggers the liver and muscles to store excess glucose in the form of glycogen.

As the blood glucose level drops from a threshold the pancreas stops releasing insulin and starts releasing another hormone called glucagon. The glucagon signals the liver to start breaking the stored glycogen and convert it back to glucose. The glucose enters the bloodstream thus increasing blood glucose levels. This balanced hormonal signaling is very important for the body to maintain optimum levels of blood glucose.

In type 2 diabetes, the body develops insulin resistance, which means that the cells no longer respond properly to insulin. As a result, blood glucose levels stay elevated, resulting in hyperglycemia.

Protein Metabolism

Proteins are another class of macronutrients used by the body for muscles development, cell growth, repairing tissues and producing hormones, enzymes, and other important molecules. The breakdown of dietary proteins starts in the stomach by the help hydrochloric acid and proteases, which converts large proteins into smaller chains of amino acids. The breakdown continues in the small intestine where these short amino acid chains are finally broken into amino acids by the help of pancreatic enzymes such as trypsin and chymotrypsin. The amino acids are then absorbed by the body through small intestines and released into the blood from where they are carried into cells and liver where most of the body’s proteins are synthesized.

What happens to excess protein?

The liver has a limited capacity of processing about 7-9 grams of proteins per hour. However, this depends upon individual’s health, state of the liver and type of proteins consumed. If amino acids are in excess the liver converts them into glucose by a process called gluconeogenesis. In order to get rid of the excess amino acids the liver decomposes them by a process called deamination which involves the removal of amino groups from these amino acids. After the removal of amino group, the amino acids are converted into ammonia and then into urea which after kidney filtration is excreted from the body in the form of urine.

Fat Metabolism

Fat is another important macronutrient required by the body for energy production, hormones production, and providing insulation and cushion to vital organs of the body. When we eat fat, it is broken down into fatty acids and glycerol, which can be used as energy by cells all over the body. This process of fats breakdown is called lipolysis.

The metabolism of fats initiates in the small intestine. When fat-containing foods enter the small intestine, they cause the release of hormones that stimulate the secretion of pancreatic digestive enzymes such as lipase. Lipase degrades lipids into fatty acids and glycerol, which are subsequently taken into the circulation and distributed to cells all throughout the body.

How is fat used by the body for energy?

As mentioned before, our body uses carbohydrates as a primary source of energy. But when carbohydrates are scarce, usually during fasting or after an intense exercise, the body starts using fats for its energy production. This is the reason why diets with low carbohydrates are preferred for weight loss, as they force the body to use fats for its energy instead of carbohydrates. Furthermore, fats release more energy per gram than carbohydrates making them a more efficient energy source.

When the body requires energy and the carbohydrates stores are depleted, it degrades fats into fatty acids and glycerol. These components are taken to the liver and transformed into the molecule acetyl-CoA by a process called beta-oxidation. The Acetyl-CoA enters the Krebs cycle and generates energy in the form of ATP (Adenosine Triphosphate), also known as the energy currency of the body.

People often blame carbohydrates and fats for numerous health problems which is absurd. Both carbohydrates and fats are essential components of diet and efficient sources of energy. It’s their overconsumption coupled with a less active lifestyle that leads to health problems. When a person consumes more carbohydrates and fats than their body requires and fails to convert them into energy, the body stores the surplus calories in different areas such as the abdomen, beneath the skin, and in blood vessels.

What factors affect the rate of metabolism?

There are numerous factors that affect the rate of metabolism. Some of these factors include:

Age: Aging can slow down the rate at which your body burns calories due to multiple factors such as losing muscle mass, less exercise, and hormonal changes.

Genetics: The genetic makeup of a person can significantly affect the rate of metabolism.

For instance, some individuals may inherit variations in genes that result in a quicker metabolism, enabling them to burn calories rapidly and maintain a healthy weight. On the other side, some genetic variations can slow down the rate of metabolism which makes it more difficult burn calories.

Hormones: Hormones serve as messengers, regulating the rate at which our body burns calories. Thyroid and testosterone are just two hormones are heavily involved in metabolic regulation.

Diet: Your metabolic rate is directly affected by the quantity of calories you eat each day. When you consume more calories than your body requires, your metabolism speeds up to burn off the extra. When you ingest less calories than you need, your metabolism slows down to preserve energy.

Exercise: Exercise makes our body burn calories faster thus boosting the rate of metabolism. Physical activity promotes muscle growth, which increases metabolic rate since muscular tissue takes more energy to sustain itself.


Understanding how our body breaks down carbohydrates, proteins, and fats is critical for overall health and fitness goals. Carbohydrates offer energy to the body and perform other important functions, but consuming too much of it may contribute to weight gain and other health issues. Protein is necessary for tissue growth and repair, but eating too much may lead to excess glucose and fat accumulation. Similarly, fat is necessary for hormone synthesis and energy generation, and the body may utilize it when carbohydrate supplies are exhausted. The overconsumption of fats, on the other hand, may lead to health problems such as heart disease and obesity. Therefore, maintaining a balanced and diverse diet that supplies all vital nutrients in the proper quantities to promote good health and performance is very important. In addition to a balanced diet regular physical exercise can support and enhance overall well-being by helping to maintain a healthy body weight, encouraging the growth and development of muscle mass, and enhancing physiological homeostasis in general.


David Bauder David J. Bauder, PA-C David Bauder, PA-C, is a certified physician assistant and the assistant medical director at Weight Loss and Vitality in Manassas and Alexandria, Virginia, Washington, DC; and Gaithersburg, MD. He enjoys helping patients optimize their physical and mental health to improve their overall well-being. He earned his physician assistant degree from the University of Texas Health Science Center at San Antonio. Afterward, he gained admission into the reputable graduate program for physician assistant studies at the University of Nebraska Health Science Center in Omaha. David has over 26 years of experience working as a physician assistant. He’s practiced in podiatry, family medicine, emergency medicine, general surgery, urgent care, and functional medicine.

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