Macronutrients: A Brief Overview

Macronutrients: A Brief Overview Leave a comment

Carbohydrates, fat and protein provide the required energy to maintain the body’s functions during physical activity and at rest. Moreover, these macronutrients sustain the body’s functional and structural integrity. 


Carbohydrates are composed of the elements Carbon, Hydrogen and Oxygen hence the name Carbohydrate. This macronutrient is abundant in foods such as fruits and grains. Once carbohydrates have been ingested, they are metabolized in the mouth, the stomach and the intestines into glucose. 

Dietary carbohydrates are divided into 3 major classes: monosaccharides, oligosaccharides and finally polysaccharides. Monosaccharides, or simple sugars, are the simplest form of carbohydrate and cannot be broken down to smaller units. The most abundant and nutritionally important monosaccharide is glucose. Oligosaccharides are composed of short-chains of monosaccharides linked together, for example a disaccharide (the most abundant oligosaccharide) such as sucrose which consists of one glucose molecule and one fructose molecule. Polysaccharides are monosaccharides units linked together in long chains of hundreds to even thousands of units and are commonly referred to as complex carbs which are further divided into two forms: fiber and starch.

Fiber is an indigestible form of carbohydrate. Dietary fiber is further divided into two distinct forms: insoluble fiber and soluble fiber. Insoluble fiber, also known as cellulose provides numerous health benefits such as absorbing and even removing toxins providing for great digestive health. Soluble fiber is able to dissolve in water forming a gel or paste (within the digestive tract) which slows the rate at which food normally passes through the small intestine. The slowing down in turn increases the rate of absorption of nutrients from the food. Dietary fiber offers numerous health benefits such as the normalization of blood cholesterol levels, lowering of blood glucose levels and increased laxation. Research has proven that specific kinds of fiber has been known to bind to cholesterol and prevent it from being absorbed by the body, leading to a decreased risk of cardiovascular disease.

Common Sources of Dietary Fiber

Soluble Fiber: oats, vegetables, fruits, legumes, beans and peas.

Insoluble Fiber: bran, whole grains, wheat, rice and rye.

Function of Carbohydrates

  • The foremost function of carbohydrates is to provide energy to the body’s cells
  • Promote fat metabolism
  • Spare muscle protein from being broken down
  • Metabolism of Carbohydrates

The use of carbohydrates by cells depends on their absorption into the blood from the GI tract. This is usually restricted to single monosaccharide units. Disaccharides and Polysaccharides must first be hydrolyzed or broken into individual monosaccharides before their absorption is possible. Once absorbed, the monosaccharides are then transported into the muscles and tissues throughout the body where they are metabolized further in order to generate energy. When glucose is not needed for immediate energy use, it is stored in the liver and the muscle as glycogen by forming branched chains. The body’s capability storing glycogen is limited and once the glycogen stores have been fully replenished, any excess glucose is metabolized and converted into fat.

Sources of Carbohydrates

Aside from lactose and a modest amount of glycogen, plants are the main dietary source of carbohydrates.


A variety of water-insoluble chemical compounds are referred to as lipids which include all fats and oils within the human diet. Similar to carbohydrates and protein, fats are also made up of carbon, hydrogen and oxygen, however the ratio of oxygen to carbon and hydrogen is lower in lipids and therefore lipids provide for a more concentrated source of energy. 

The type of fat within the diet plays an important role in general health and is linked to numerous chronic diseases. There are 3 main types of fatty acids which differ in their molecular bonds and number of hydrogen atoms. Fats can be saturated, monounsaturated (containing one carbon-carbon double bond) or polyunsaturated (containing two or more carbon-carbon double bonds).

The body needs to acquire two types of polyunsaturated fatty acids from the diet because it cannot produce them: Linoleic acids (Omega-6 fatty acids) and Linolenic acid (Omega-3 fatty acid). Insufficient dietary intake of these fatty acids results in symptoms such as scaly skin, reduced growth and even dermatitis. Numerous studies have also shown that diets high in Omega-3 fatty acids for example in countries where larger amounts of fatty fish are consumed have a decreased risk of cardiovascular disease. 

Cholesterol is yet another group of lipids which is required in the formation of essential compounds within the body which includes vitamin D, steroid hormones and bile salts. This molecule is also an integral part of cell membranes and myelin sheath (provides insulation around nerve fibres). Cholesterol, however can be synthesized by the liver and therefore not dietary essential. Low density lipoproteins are considered “bad cholesterol” as they can transport cholesterol to the cells of the body, whereas high density lipoprotein is considered “good cholesterol” because it transports cholesterol away from cells and back to the liver to be excreted. 

Triglycerides provide the majority of energy that is derived from dietary lipids. 

Trans-fatty acids behave much like saturated fatty acids within the body and are found in partially hydrogenated vegetable oils such as shortening and margarine. Lower amounts are also found in meats and dairy products. Both trans-fats and saturated fats, even in small amounts, increase the risk of heart disease by increasing blood levels of low-density lipoprotein cholesterol. Therefore it is recommended to keep dietary intake as low as possible and to consume a nutritionally adequate diet. Trans fats and saturated fats can promote diabetes, obesity, heart disease and also certain types of cancer. This is due to the fact that they heighten levels of low-density lipoprotein, cause inflammation in arteries and lower good cholesterol levels or high-density lipoproteins. 

Function of Fat

  • Contains nearly two times the energy as glucose and are the most energy-dense macronutrient which provides many of the body’s tissues and organs with energy.
  • Crucial in the transmission of nerve signals which generate muscle contraction.
  • Cell membranes are composed of fatty acids called phospholipids.
  • Protects and cushions vital organs while insulating them from the thermal stress of colder environments.
  • Serves as a transporter of vitamins A, D, E and K.
  • Helps to delay the onset of hunger due to fat emptying more slowly from the stomach, therefore diets containing some fat are usually more successful than low or no fat.

Metabolism of Fat

Fats are metabolized into short, medium and long chain fatty acids and glycerol. Triglycerides are the storage forms of fat and are mainly found in muscle, the liver and adipose tissue. 

Sources of fat

Monosaturated: Peanut, olive and canola oils, nuts and avocados. (Daily dietary amount: 10-15% of daily caloric intake)

Polyunsaturated: Omega-3 – Canola oil, walnuts, flax seed, tuna, salmon, sardines, mackerel and herring.

Omega-6 – Nuts, seeds, corn, soy, safflower and sunflower oils.

(Daily dietary amount: Up to 10% of daily caloric intake)

Saturated: Coconut, palm and palm kernel oils, meat, poultry, cheese, butter, cream, whole milk and processed foods such as crackers, cookies, chips and baked goods.

(Daily dietary amount: Up to 10% of daily caloric intake).

Trans fat: crackers, pastries, packaged cookies, margarine, shortening, fried foods, low levels in natural meat, poultry and dairy products. (Daily dietary amount: None or as little as possible)


Proteins are an essential macronutrient because of their constituent amino acids. The body must synthesize its own variety of protein and molecules containing nitrogen to make life possible. Proteins are the major structural component of all body cells and the amino acids are the basic building blocks of proteins. The building blocks of protein are 20 amino acids that can be consumed from animal and plant sources. 9 are considered essential because they cannot be synthesized by human enzymes. The remaining 11 are considered as non-essential because they can be synthesized endogenously.

Essential amino acids: Valine, Leucine, Isoleucine, Phenylalanine, Lysine, Histidine, Methionine, Tryptophan and Threonine.

Nonessential amino acids: Proline, Glycine, Tyrosine, Arginine, Alanine, Asparagine, Aspartic acid, Cysteine, Glutamic acid, Glutamine and Serine.

Function of Protein

  • Produce enzymes which are required for many chemical reactions within the body such as blood coagulation, digestion, absorption contractibility and excitability of muscle tissue.
  • Product antibodies for the immune system.
  • Component of peptide hormones such as insulin, glucagon, parathyroid hormone, thyroid hormone, growth hormone, antidiuretic hormone and adrenocorticotropic hormone.
  • Component of transport proteins such as hemoglobin, albumin, transferrin, retinol-binding protein, prealbumin and ceruloplasmin. 
  • Used as a source of fun when muscle glycogen levels are low or depleted such as during intense physical exercise.

Sources of Complete Protein

Fish, Meat, Poultry, Milk and dairy products, rice and beans, whole wheat bread with peanut butter, macaroni and cheese, yogurt and granola, peanuts and sunflower seeds, corn and peas or beans, lentils and bread, oatmeal and milk, beans and cheese burrito, whole eggs.

Metabolism of Protein

Protein that is ingested serves as a source of the essential amino acids and are also the primary source for the requirements of additional nitrogen in the synthesis of the nonessential amino acids and nitrogen-containing molecules. Dietary protein is combined with endogenous protein from the gastrointestinal secretions within the gut and is digested and absorbed as amino acids. The amino acids then lose the nitrogen molecule in the liver, referred to as deamination, to form urea which is then excreted from the body. A new amino acid is formed from the deaminated amino acid. In muscle tissue, enzymes facilitate the removal of nitrogen from specific amino acids and pass it on to other compounds in the reactions of transamination. An amino group then shifts from a donor amino acid to an acceptor acid (known as a keto-acid) and becomes the new amino acid.

In the case of inadequate protein intake, protein synthesis cannot keep up with protein breakdown and the body’s proteins for example in muscle tissue are broken down in order to meet the requirements for amino acids. The results are slowing of tissue repair and a decrease muscle size and strength which results in decreased physical performance. It is important to consume adequate amounts of protein daily due to the fact that unlike carbs and fat, which the body can store as glycogen and triglycerides for later use, amino acids cannot be stored by the body.

Sources of Protein

A common method of classifying protein is by their biological value or BV. The BV is calculated by the amount and proportion of essential amino acids the particular protein source provides. Protein from animal sources such as meats, eggs, fish and dairy products have high BV protein or complete proteins in that they contain all 9 essential amino acids. Plant protein sources such as nuts, legumes seeds and grains do not contain sufficient amounts of some essential amino acids and are considered partially complete or intermediate BV protein. Other plant protein sources such as fruits and vegetables which are lacking essential amino acids are considered incomplete proteins or low BV protein. 

It is of primary importance to consume different sources of protein to ensure proper consumption of all essential amino acids. Animal sources and soy contains all the essential amino acids while by combining various plant proteins complete proteins can be made. 


Fats, carbohydrates and proteins are the body’s macronutrients and can substitute for one another (to an extent) in order to meet the energy needs of the body. Certain ranges of dietary intake for each are set based on evidence that consuming above or below a certain level may be associated with nutrient inadequacy and an increased risk of developing chronic diseases including diabetes, cancer, obesity and/or heart disease.

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