In the pursuit of creating a healthy, well-rounded diet, many of us will consume foods from a wide variety of food groups. The energy we absorb and consequently store from the foods we eat will depend on various factors, including the types and amounts of food we consume, daily exercise regimes, and how our bodies handle nutrients (carbohydrates, fats, proteins, vitamins, minerals and water) through digestion and absorption. 

Before our bodies can make use of the food we eat, it must first break it into smaller particles for our bodies to harness the nutrients within. This starts in the mouth, where mastication (the act of chewing food) turns food into a bolus (a mass of chewed food just before swallowing). 

Enzymes in our saliva begin a chemical reaction of breaking down foods in preparation for digestion and absorption by the body – this is also why our ‘mouths water’ from the smell of food when hungry. This is the body's response in preparation for food consumption.

Macromolecule or macronutrient?

Macromolecules and macronutrients have different definitions – while they are commonly used interchangeably, they are technically not the same thing. 

Macromolecule is a broad term referring to any very large molecule. They can be organic compounds like carbohydrates and natural fibres (e.g. cotton), or synthetic compounds like plastics, synthetic fibres and adhesives. The four major biological macromolecules are carbohydrates, lipids, proteins and nucleic acids.

Macronutrient refers specifically to the macromolecules that provide dietary energy to the body. The main macronutrients are carbohydrates, fats and proteins. Macronutrient is the term used when discussing human nutrition.

What are Macronutrients?

Before we explore the process of how macronutrients are broken down in the body, it is important to understand what macronutrients are. Macronutrients are the essential molecules that provide dietary energy to the body. They can be broken down into the following three nutrients.

Carbohydrate
Starch and sugars fall into the category of carbohydrates. Starch and sugars are made up of building blocks called monosaccharides (a monosaccharide is typically a single sugar unit). Starch is made up of multiple monosaccharides, whereas most sugars are made up of only one or two monosaccharides. 

Protein
One of the most varied forms of macronutrients we consume is protein. Proteins come in a wide range of forms and can be found in many different foods such as lean meats, fish, eggs, dairy products, nuts and seeds, oats, vegetables and legumes. 

Fat
Fat can take many forms in the foods we eat. Fats can be grouped into saturated fats, trans fats, monounsaturated fats and polyunsaturated fats. Monounsaturated fats can be found in foods like olives, avocados, almonds, pecans, pumpkin and sesame seeds, etc. 

Polyunsaturated fats are found in foods such as corn and flaxseed oils, walnuts, fish and canola oil. Saturated fats are found mostly in animal products like beef or cheese, but can also be found in some plant-based foods like coconut and palm oils. 

What about Fibre and Alcohol?
Fibre and alcohol can be considered macronutrients, however they are generally not included in the definition. This is because fibre is indigestible and does not provide a source of energy. Alcohol does provide energy, but it cannot be stored and is not essential for survival.

What happens to macronutrients when they are digested?

The body breaks down food into the various macronutrients using mechanical and chemical digestion processes in different parts of our body. 

Our mouths chew food into smaller fragments while enzymes in our saliva start to chemically break down foods. 

Our stomachs use a mechanical process called peristaltic mixing to churn food while adding gastric juices to assist in the chemical digestion of proteins. 

Mechanical digestion continues through both the small and large intestines, along with chemical digestion processes that are further explained below. 

How does the body break down carbohydrates?

The digestion process for carbohydrates starts in the mouth where an enzyme called salivary amylase begins to break down food starches into disaccharides. Salivary amylase is deactivated by stomach acid, but can continue to break down carbohydrate in the stomach if it is trapped within a bolus of food not in contact with the acid. 

Once carbohydrates travel past the stomach, they venture into the duodenum (a part of the small intestine) where they mix with digestive secretions from the pancreas, liver, and gallbladder. An enzyme called pancreatic amylase continues to break down any remaining complex carbohydrates into disaccharides. Other enzymes called maltases, sucrases, and lactases break down disaccharides into monosaccharides (or single sugar units).

For further information about the breakdown and use of carbohydrates in the body, see

Digestion, absorption and transport of carbohydrates

What happens to disaccharides during digestion?

How does the body break down protein?

Digestion of protein begins in the stomach. Food is mixed with an enzyme called pepsin which helps proteins break down into chains of amino acids called peptides. Gastric acid also helps to partially break up proteins to allow pepsin better access.

Once this food moves into the small intestine, a range of other enzymes (trypsin, elastase, and chymotrypsin) are released from the pancreas and start to reduce peptides into amino acids and di- and tri-peptides (small chains of amino acids). These are then absorbed into the bloodstream via transporters located in the wall of the small intestine.

How does the body break down fats?

Fats start their digestion process in the mouth via the enzyme called lingual lipase. Digestion continues in the stomach with the aid of lingual lipase and gastric lipase. 

The majority of fat digestion occurs in the small intestine. Bile is released into the duodenum which helps in the digestion of fats via emulsification. Emulsification break down fats into smaller globules so they can be more evenly distributed through the small intestine. This also makes the fats more accessible to pancreatic lipase, which is the enzyme that further digests fats into free fatty acids. Free fatty acids are absorbed though the intestinal wall via diffusion to be transported around the body, or are taken up and incorporated into intestinal cells.

Some fats will also pass undigested into the large intestine where they are broken down by gut bacteria into short-chain fatty acids. 

How does the body absorb the macronutrients?

As the different macronutrients travel through our digestive system and are broken down into smaller components, our body will absorb these nutrients at different stages of the path food travels through our body. 

The body absorbs monosaccharides (the break down product of carbohydrates) in the small intestine. Monosaccharides are absorbed across the intestinal epithelium to be transported through the bloodstream and used by different cells in the body. 

Once proteins have been broken down into peptides and amino acids, they are absorbed into the bloodstream through transporters located in the wall of the small intestine. 

Lipids are also digested in the small intestine with the help of bile salts, and pass through the intestinal wall mostly by diffusion.

REFERENCES

• NHMRC. Nutrient Reference Values for Australia and New Zealand: Macronutrient Balance. Accessed 14 April 2021 at: https://www.nrv.gov.au/chronic-disease/macronutrient-balance
• Goodman BE. (2010). Insights into digestion and absorption of major nutrients in humans. Adv Physiol Educ, 34:44-53.
• Clemens RA, Jones JM, Kern M, et al. (2016). Functionality of sugars in foods and health. Comprehensive Reviews in Food Science and Food Safety, 15(3): 433-470.
• Mann J and Truswell AS. (2017). Essentials of Human Nutrition, 5th Edition. Oxford:Oxford University Press.