Digestion & absorption

Digestion and absorption of carbohydrates

Dietary carbohydrates are one of the three main macronutrients, alongside proteins and fats. Macronutrients are the nutrients we need in larger quantities that provide us with energy and support the body's functions. Carbohydrates provide 4 kcal/gram, protein provides 4 kcal/gram, whilst fats provide 9 kcal/gram. Alcohol also provides energy in the diet, at 7 kcal/gram (1).

All carbohydrates are made from small building blocks called sugar units or monosaccharides. Carbohydrates are grouped based on their chemical structure and how they are digested and absorbed by the body. Carbohydrates include “simple” carbohydrates and “complex” carbohydrates.

Simple carbohydrates

There are two types of simple carbohydrates. Monosaccharides (mono- = “one”; sacchar- = “sugar”) are the simplest sugars. They include glucose and fructose, which can be found in fruits, vegetables and honey, and galactose which can be found in dairy products. Disaccharides (di- = “two”) are composed of two monosaccharides and include sucrose, lactose, and maltose. Sucrose can be found in sugar beet, sugar cane and some fruits. Lactose is mainly found in dairy products such as cows’ milk, yogurt, and cheese. Maltose can be found in grains and wheat.

Complex carbohydrates

Complex carbohydrates are made up of longer chains of monosaccharides linked together to form oligosaccharides (3 – 9 monosaccharides) or polysaccharides (poly = “many”, > 9 monosaccharides). Starches are a group of complex carbohydrates which are found in a range of plant foods including grains like wheat, rice, barley, oats, rye, corn, and breads and potatoes. Dietary fibres are another group of complex carbohydrates found in plants that cannot be completely broken down by human digestive enzymes. These substances include waxes, lignin, beta-glucans, and some polysaccharides such as cellulose and pectin. Fibre can be found in grains, fruits, vegetables, beans and pulses.

How carbohydrates are broken down in the digestive system

Before the body can use the food that is eaten, it must be broken down into its simplest forms (2). For example, all carbohydrates are broken down to, and absorbed as, monosaccharides.

The breakdown of carbohydrates starts in the mouth. When food enters the mouth, and we start to chew, saliva is released. Amylase, an enzyme in saliva, begins the breakdown of starches. Enzymes are proteins made by the body, that speed up the chemical reactions that take place to breakdown and metabolise foods.

Breakdown continues in the stomach where acid acts upon the food, now referred to as “chyme”, to kill any harmful bacteria. Next, the chyme enters the duodenum, the first part of intestine, where it stimulates the pancreas to release more amylase. Amylase breaks the starches down further into maltose and oligosaccharides. The small intestine then produces additional enzymes, which include maltase, sucrase, and lactase. These enzymes down the remaining carbohydrates (maltose, sucrose and lactose, respectively) into simpler monosaccharides (single sugar units) ready for absorption.

Fibre cannot be digested. Instead of being broken down by intestinal enzymes like other carbohydrates it continues through the digestive system, entering the colon. Here fermentable fibre is broken down by intestinal bacteria before it is passed out of the body in stools. Non-fermentable fibre is passed through the body unchanged.

Digestion and absorption of sugar

Sugar is made up of two monosaccharides, glucose and fructose. Upon eating foods containing sugar, the glucose and fructose units are split apart by sucrase in the small intestine. These simple sugar units are then absorbed into the bloodstream, where they are transported to cells and used for energy or stored as glycogen in the liver and muscles, for later use as energy.

How the body uses carbohydrates for energy and other essential functions

Monosaccharides (e.g., glucose and fructose) are absorbed through the lining of the intestine, into the bloodstream. Fructose is taken up by the liver, where it is converted into glucose, glycogen, or triglycerides. Blood glucose is carefully controlled within the body to ensure that the cells have a constant and sufficient supply to function. When glucose levels increase in the blood, the hormone insulin produced by beta cells found in the pancreas, helps glucose to move from the bloodstream into cells. Insulin also signals the liver to store glucose for later use.

Uses of carbohydrates in the body:

Cells use glucose to produce adenosine triphosphate (ATP), the body’s main energy currency. ATP is important for the functioning of the brain, and for muscles and organs to work properly (3).
The brain uses glucose as its main energy source. Glucose is transported across the blood-brain barrier, where it supports the normal functioning of the brain (4).
Fibre supports digestive health by promoting regular bowel movements and a healthy gut microbiome (5).

Man in shopping aisle looking at 2 drink bottles

Understanding fructose

Fructose can be found as a monosaccharide in fruits, vegetables, and honey or bound together with glucose as a disaccharide (i.e., sucrose). Sucrose is found naturally in plants, including some fruits and vegetables, and is extracted to produce table sugar. Fructose can also be found in high fructose corn syrup, a sweetening agent used in some manufactured foods and drinks. Fructose is one of the three most common dietary monosaccharides, along with glucose and galactose. Fructose tastes slightly sweeter, compared to glucose. Fructose and glucose share the same chemical makeup (C6H12O6) however they differ in how they are metabolised, absorbed and used in the body (6). Fructose is a “ketone sugar”, while glucose is an “aldehyde sugar”, meaning they are metabolised differently. Unlike glucose, which is metabolised by all cells in the body, fructose is mainly metabolised in the liver. Fructose is taken up by the liver, where it is converted into glucose, glycogen, or triglycerides.

Factors affecting carbohydrate absorption

The rate at which carbohydrates are digested and absorbed is affected by a range of factors (see table below for some examples). These factors affect how the body responds to carbohydrates.

Type of carbohydrate consumed

Simple sugars, such as glucose and fructose, are absorbed more quickly than complex carbohydrates, which need more time to be digested and broken down into simpler forms.

Presence of dietary fibre

Soluble fibre can slow down the absorption of glucose.

Consuming carbohydrates with fats and/or protein

Eating carbohydrates with fats and/or protein (such as a jacket potato with beans) can slow down the absorption of carbohydrates. This results in a less rapid increase in blood sugar levels and a prolonged release of energy.

Processing and cooking methods

Cooking can increase the accessibility of carbohydrates for digestion. This means that they are absorbed more quickly. If cooked and then cooled (such as cold pasta), increases in blood glucose is more gradual.

Individual variations in digestive enzymes

Different people have different concentrations of digestive enzymes in the intestines. Certain enzymes increase or decrease how fast carbohydrates are digested and absorbed.

Gut microbiota

Gut microbiota are the living organisms in the gut that help with the fermentation of otherwise undigested food. The type and number of organisms in the gut affects how the more complex fermentable carbohydrates are broken down and absorbed.

Glycaemic index (GI)

GI is a ranked measure of how quickly or slowly carbohydrates are digested and increase blood glucose levels over a set period (usually 2 hours). Carbohydrates can be grouped into low, medium, and high GI categories. Low GI carbohydrates (less than 55) are digested more slowly. They include beans, milk and oats. Sucrose has a medium GI (around 65). GI High GI carbohydrates (greater than 70) include white bread and short grain rice. Size, texture and ripeness of foods can alter GI. For example, an unripe banana may have a GI of 30, whilst a ripe banana may have a GI of 51 (7).

Glycaemic load (GL)

GL relates to the GI and quantity of carbohydrates eaten. A high GL is over 20, whilst a low GL is under 10. Foods can have a high GL, despite having a low GI, if eaten in large amounts. For example, although pasta has a low GI, a large serving has a high GL, and can lead to a rapid increase in blood glucose (7).

References

British Nutrition Foundation. Energy balance and weight. https://www.nutrition.org.uk/health-conditions/overweight-obesity-and-weight-loss/energy-balance-and-weight/
Digestion and Absorption of Carbohydrates. Lane Community College; 2021. https://med.libretexts.org/@go/page/39963
Dunn J, Grider MH. Physiology, Adenosine Triphosphate. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2023 Jan-. https://www.ncbi.nlm.nih.gov/books/NBK553175/
Mergenthaler, P., Lindauer, U., Dienel, G. A., & Meisel, A. (2013). Sugar for the brain: the role of glucose in physiological and pathological brain function. Trends in neurosciences, 36(10), 587–597.
NHS. Good Foods to Help Your Digestion. https://www.nhs.uk/live-well/eat-well/digestive-health/good-foods-to-help-your-digestion/
Merino, B., Fernández-Díaz, C. M., Cózar-Castellano, I., & Perdomo, G. (2019). Intestinal Fructose and Glucose Metabolism in Health and Disease. Nutrients, 12(1), 94.
Oregan State University. Glycaemic Index and Glycaemic Load. https://lpi.oregonstate.edu/mic/food-beverages/glycemic-index-glycemic-load