How long does it take to digest food?

Digestion is a complex process which can last in total between 30 and 120 hours depending on the individual, age, type of food ingested and any potential diseases (1). But let’s first take a closer look at the duration of each stage, and then the point at which the most common digestive issues usually occur following a meal.

The 3 key stages of digestion and their duration

First stage of digestion in the stomach

After briefly passing through the mouth and oesophagus, food reaches the stomach, the first dilated part of the digestive tract. There it is converted into a kind of pulpy mass called chyme, as a result of powerful muscle contractions and the acidity of gastric juice.

The lower part of the stomach then acts as a filter, allowing only the smallest particles and liquids to pass through to the small intestine. Everything else continues to be crushed and kneaded until it reaches the required size.

Liquids usually remain in the stomach for less than 30 minutes unless they are complex (smoothies, protein milks …), and cooked vegetables for just 30-45 minutes, as opposed to 5 hours+ for a portion of meat. Generally speaking, solid and complex foods take between 3 and 7 hours to pass through the stomach (1).

A key stage in the small intestine

When it reaches the small intestine, the main site of digestion, the crushed food is only partially digested. The fats have not yet undergone any transformation. Under the effects of new muscle contractions, the pulp spends 3 to 6 hours travelling to the end of the small intestine.

The process of chemical digestion of food intensifies as it progresses: numerous secretions, such as bile and pancreatic juice, break down the chyme, while the nutrients released cross the intestinal barrier to reach the bloodstream.

Final stage of digestion: the colon

The undigested material then passes into the large intestine which includes the colon. It has two main roles: to absorb water from the remaining residues in order to compact them into stools, and to store them prior to their elimination via the anus. However, it is home to a fascinating ecosystem, populated with billions of micro-organisms and capable of re-processing previously indigestible compounds, such as dietary fibre. This ecosystem, thought to weigh less than a kilogram, is the gut microbiota, with roles in nutrient absorption and protection of the digestive system which are becoming increasingly well-understood.

This final, longer stage lasts around 30 hours on average, but can last a few days. A distinctive feature in humans is that the colon does not contract during the night – it ‘sleeps’ at the same time as the brain (2).

The most common digestive problems and their onset in relation to a meal

Food poisoning

In the case of food poisoning, the food involved is usually contaminated by bacteria, the most common of which include salmonella, E. coli and listeria.

The effects of food poisoning can be felt very quickly: usually after just a few hours, or sometimes even less than an hour after eating the offending food. The length of time it takes for symptoms to appear varies according to which particular contaminated food was eaten, and how much. For example, food poisoning by Clostridium perfringens causes diarrhoea very quickly, as soon as the bacteria reaches the small intestine and releases its toxin.

Flatulence

What about flatulence? Around an hour and 40 minutes after ingestion, the first undigested residues start to reach the colon, triggering the production of gas due to action by the gut microbiota.

This can last for 4 to 6 hours, or even longer when the meal is particularly rich in animal protein. It is often accompanied by discomfort and gurgling noises.