What Are Macronutrients?

Lucy Bell-Young

by Lucy Bell-Young

22nd September 2021


A variety of foods rich in macronutrients

Macronutrients are molecularly large nutrients essential for the healthy and normal functioning of an organism. They can only be derived from food. Humans need three major macronutrients: proteins, fats, and carbohydrates.

These macronutrients need to be broken down through metabolic processes in order to be utilised as either energy sources or as source molecular materials to synthesise cellular and tissue structures, as well as regulatory biochemicals, such as hormones.

What Are the Three Macronutrients?

Macronutrients are needed for two main purposes: as a source of energy and as a source of raw materials to build and maintain a biological organism. All macronutrients can be metabolised as a source of energy. These are the three macronutrients:

  1. Carbohydrates

Carbohydrates in the form of sugars are much easier for our bodies to break down than other macronutrients (but they’re not as healthy). Complex carbohydrates, like whole grains and starchy vegetables, need to be digested first and broken down into glucose in order to be utilised during cellular respiration.

The glucose is then broken down through the process of glycolysis, the final product of which is called  2 pyruvate. The entire process involves complex steps that can be divided into two main parts: the energy requiring process and energy releasing process.

  • First part of glycolysis:
Illustration showing the first stage of glycolysis
  • Second part of glycolysis:
Illustration showing the second stage of glycolysis

Glycolysis is a multi-step metabolic pathway that occurs in the cytoplasm of animals, plants, and microorganisms. Excess glucose is converted into glycogen, which is mainly stored in the liver and serves as a readily convertible energy reserve.

Various sources of carbohydrates are available in the daily diet. These include things like bread, potatoes, oats, pasta, and even sugary soft drinks. Based on the National Diet and Nutrition survey in the UK, British adults have the average daily intake of 252 grams of carbohydrates for men, and 198 grams for women. These represent 47.5% and 48.3% of food energy intake for men and women, respectively. Each gram of carbohydrate has an average of four calories of energy.

A calorie is the amount of energy needed to increase the temperature of one gram of water by 1°C. This is the equivalent of 4.1868 joules. According to the survey, the following are the main sources of carbohydrates in British adults’ diets:

  1. Fat

Fats are more useful as a source of reserved energy, and provide about nine calories of energy per gram. When there is excess carbohydrate intake, some of the carbohydrates are converted into fats. In other mammals, having fat reserves during their winter hibernation is an evolutionary adaptation that is very useful for survival during periods of food scarcity.

Similar to carbohydrates, fats – or triglycerides – must first be digested and broken down into simpler constituents, called monoglyceride units, to be utilised by the body. This is done with the help of lipase enzymes. In humans, fat digestion starts in our mouths. While the triglycerides are partially broken down by lingual lipases, cholesterol remains intact.

In the stomach, gastric lipase and mechanical digestion begin. Lipids are absorbed in the small intestine and are broken down more by the pancreas. Further mechanical digestion then occurs, separating the triglycerides into fatty acid units. 

The fatty acids are absorbed in the stomach while most fat is absorbed in the small intestine. They then mix with the blood and are carried throughout the body for various purposes. Some are deposited in the adipose tissues while others are further processed to create other biological molecules, like hormones. Lipid metabolism synthesises new molecules needed by the body. Lipids are very important in terms of maintaining the structural integrity of the cellular membranes.

Fatty acids can also be used by the body through lipid catabolism, as shown in the illustration below. This occurs in the mitochondria of the cell, wherein the lipids are oxidised to release energy:

  1. Protein

Dietary proteins, such as those found in meat (e.g. beef, pork, poultry, and fish), are essential in building muscle mass and other body tissues. To be used by the body, we digest proteins and break them down into amino acids.

The brain, muscles, blood, skin, internal organs, antibodies, enzymes, and hormones like insulin are all made from proteins. Therefore, structurally and functionally, proteins are the most essential macronutrients.

From the formation of embryos to the physiological maintenance of an adult body, protein synthesis is directed by the genetic codes found in our DNA. The genetic switches turn on and off depending on the need. The instructions from the DNA are transcribed by the mRNA that travels from inside the nucleus into the cytoplasm. It then attaches to a ribosome to be translated into proteins. The polypeptides that form proteins are synthesised in the ribosomes.

The illustration below shows the process of protein synthesis:

Examples of Macronutrients

An adult requires an average of around 2,000 kcal (8,400kJ) of energy per day, though this may vary depending on age, sex, body build, health status, and intensity of activity. For example, physically active individuals may require at least an additional 500 kcal of energy per day.

In terms of macronutrients, an adult require around the following amount of nutrients per day to function normally:

  • Total fat: less than 70g
  • Saturates: less than 20g
  • Carbohydrate: 260g
  • Total sugars: 90g
  • Protein: 50g
  • Salt: less than 6g

There are three categories of carbohydrates: sugar, starch, and fiber. They can be derived from various dietary sources. In fact, many foods and beverages combine all three types of carbohydrates. The simplest carbohydrates are sugars, which include lactose from milk and sucrose from various beverages and baked products.

Starches are complex carbohydrates that can be derived from whole grains, pasta, root crops (e.g. potatoes), and noodles. Fiber is another complex type of carbohydrate. It’s good for digestion and regular bowel movements, and can be derived from fruits, vegetables, nuts, and beans.

Dietary fats are essential not only for cell membranes or energy reserves, but also for many other functions, such as insulation for the nerves and synthesis of hormones. Fats come in various forms, including saturated fats, unsaturated fats, trans fats, and monounsaturated fats. They’re necessary in the absorption of the fat-soluble vitamins A, D, E, and K.

Dietary proteins are mainly derived from meat, poultry, fish, dairy products, nuts, and beans. They contain the 20 essential amino acids our bodies need for various functions and biochemical synthesis.


What Are Macronutrients Measured in?

Macronutrients are usually measured in grams and are based on the recommended daily dietary allowance, also called the dietary reference intake (DRI). They’re also measured in terms of the energy they provide per gram. For example, the DRI for fats is 70g per day. This is equivalent to 630 calories per day since fats have nine calories of energy per gram.

Which Macronutrient is Digested the Quickest?

Carbohydrates, particularly simple sugars, are the macronutrients that are easiest to digest because their molecular bonds are relatively weak compared to fats and proteins. While carbohydrates are broken down into glucose in the small intestine, proteins are broken down into amino acids, and fats into fatty acids and glycerol.

What’s the Difference Between Macronutrients & Micronutrients? 

Macronutrients are typically large molecules that are needed by the body in large amounts. Mmicronutrients are usually smaller and simpler molecules needed by the body in trace amounts. Micronutrients mainly function as coenzymes, and take the form of minerals and vitamins. Fruits and vegetables are rich in micronutrients.

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