How Do We Metabolise Lactose?
Article by Emer O’Connor
The understanding of our metabolism can be daunting to some, but the actual breakdown can be straightforward and beneficial to know. The most important carbohydrate is glucose, which is broken down by the body via various processes known as glycolysis, the Kreb’s cycle and oxidative phosphorylation to generate ATP, which the body uses as energy. This article will solely focus on the carbohydrate molecule lactose. Lactose is a type of sugar known as a disaccharide which is composed of two single sugar molecules (monosaccharides), glucose and galactose linked together via a β-1-4 bond.
Where can I find lactose in my diet?
The majority of lactose in our diets comes from milk. The consumption of milk has many beneficial health effects for adults and children. Research suggests that it may help with bone health and diseases such as osteoporosis and is packed with very important nutrients such as calcium, B vitamins, and phosphorus. In the UK, most people who drink milk normally consume cow’s milk however, in certain parts of the world, it can also be consumed from goats, buffalos, sheep and camels. The composition of milk contains five components: water, fat, proteins (casein & whey), lactose, and minerals.
Lactose is synthesised within milk glands that are present in mammals that produce milk. Lactose can be found in most mammalian milk, with the exception of some animals that do not produce lactose-based milk, such as sea lions. To break down lactose in our bodies, humans need to use a natural enzyme called lactase, so that the sugars can be absorbed into the small intestine. One of the widely common digestive problems associated with the intake of this carbohydrate is lactose intolerance. This intolerance is caused by the body not making enough lactase resulting in people not being able to fully digest lactose.
Digestion of lactose
Food must be broken down into basic nutrient components before the body can use the food that is eaten. The starting point begins in the mouth with the mechanical breakdown of carbohydrates. The body is able to digest lactose using lactase. This will eventually break down lactose into two separate sugar molecules, glucose and galactose. When carbohydrates have broken down into their single sugar units, they can then be transported to the intestinal cells ready to be absorbed by the small intestine.
Absorption of lactose
Most of the ingested lactose is broken down into its monosaccharide (single sugar) products, and are finally absorbed in the small intestine. The majority of the ingested lactose will be absorbed in the form of glucose and galactose which results from digestion. The digestion of lactose requires the intestinal enzymes lactase, from this structure glucose directly enters the glycolysis pathway and is largely used for energy. On the other hand, galactose requires a unique universal metabolic pathway for its breakdown and potential detoxification but has importance in the role of neurological and immunological processes. The metabolic pathways that break down glucose and galactose in the body are complex, but they are often interlinked and have the same end product.
Galactose metabolism
Galactose metabolism begins when it’s been absorbed as it progresses into the bloodstream and is then transported into the liver. While the majority of galactose load remains in the liver, some small fractions are sent directly to the brain and also in suckling mammals to the mammary glands to reform lactose. Galactose is metabolised by a pathway known as Leloir and contains four enzymes that help to break down galactose to eventually create energy for the body to use.
Glucose metabolism
Glucose metabolism involves multiple processes, including glycolysis, which is the starting point for the breakdown of carbohydrates. Glucose enters the liver via transporters in the cell membranes from the bloodstream where it can be broken down further. The first related pathway is glycolysis. Glycolysis occurs in almost every living cell and can be described as the sequence of reactions that convert glucose into pyruvate to eventually yield ATP, which the body uses as energy.
The Kreb’s Cycle
The Krebs Cycle, commonly known as Citric Acid Cycle, can organise a number of substances which can be derived from the by-products of carbohydrates. The Krebs cycle and electron transport chain (ETC), which is discussed later on, work together so that they can generate energy in the form of ATP. Glycolysis can generate ATP directly but also high energy electrons that can be carried from inside cells by a molecule known as NADH to the ETC in the mitochondria, which is often called the ‘powerhouse’ of cells due its role in energy production.
The Electron Transport Chain
Energy rich compounds that are by-products of glycolysis and The Krebs Cycle can then be used to generate energy in the electron transport chain. This transport chain is a link of a series of electron carriers which are organised into complexes which work with coenzymes and other proteins to help the process of creating ATP (energy).
In summary
The outlined information of digestion, absorption and metabolism gives a detailed description of the importance of lactose within the human body. Milk has remained a beneficial and contributing factor within human health and shows a complex yet interesting cycle of how the human body facilitates and utilises one the main carbohydrate components, lactose. Following the glycolytic and Leloir pathways, glucose and galactose are important sugars and play a vital role in our absorption and metabolism of lactose.
This blog was written by Emer O’Connor, a current MSc student in Food, Nutrition & Health at University College Dublin. She studied her undergrad in Economics at University College Cork however, surrounded by family with health related careers, and a passion for nutrition and aiding people to live a healthier life, she decided to pursue her career in nutrition. Emer aspires to become a registered nutritionist in the future and particularly branch into clinical nutrition. Keep up with latest info on her LinkedIn account.
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