What is considered a thin liquid?

November 2, 2025 •

3 min read

According to the International Dysphagia Diet Standardisation Initiative (IDDSI), a thin liquid is categorized as flowing with a viscosity similar to water. This fundamental understanding of liquid consistency is crucial, especially in clinical settings related to swallowing disorders like dysphagia.

Quick Summary

A thin liquid has a water-like consistency, flowing easily through a cup or straw without resistance. This category includes water, juice, coffee, and items that melt into a thin state, such as ice cream or gelatin, posing a risk for individuals with dysphagia.

Key Points

Viscosity: Thin liquids have a low viscosity, meaning they flow easily and quickly, similar to water.
IDDSI Level 0: The International Dysphagia Diet Standardisation Initiative (IDDSI) defines thin liquids as Level 0 within its framework for classifying fluid consistency.
Swallowing Risk: For individuals with dysphagia, thin liquids can be a choking or aspiration risk because they move too quickly for compromised swallowing muscles.
Common Examples: Water, coffee, tea, fruit juice without pulp, milk, and broth are all considered thin liquids.
Melting Foods: Items like ice cream and gelatin can be hazardous for individuals with dysphagia because they melt into a thin liquid upon consumption.
Testing: The IDDSI Flow Test, using a 10ml syringe, is a standard method to confirm if a liquid is indeed thin and flows completely in 10 seconds.

Understanding Liquid Viscosity

Viscosity is a scientific term that describes a liquid's resistance to flow. A thin liquid, or one with low viscosity, flows quickly and with little effort. Water is the classic example, but many other common beverages also fit this profile. In contrast, liquids with high viscosity, like honey or molasses, are much thicker and flow slowly. This characteristic is a primary way to differentiate between various fluid consistencies, from thin to extremely thick.

IDDSI Level 0: The Standard for Thin Liquids

The International Dysphagia Diet Standardisation Initiative (IDDSI) has created a global framework to classify food and drink consistencies for people with swallowing difficulties (dysphagia). Under this system, thin liquids are categorized as Level 0. This is the least viscous level and includes any fluid that flows freely and quickly, just like water.

For individuals without swallowing issues, Level 0 liquids are safe and can be drunk easily from any cup or with any straw. However, for those with dysphagia, this fast flow can be difficult to control, increasing the risk of aspiration, where fluid enters the airway and lungs.

Common Examples of Thin Liquids

Many beverages commonly consumed every day are considered thin liquids under the IDDSI framework. These include:

Water (tap, bottled, sparkling)
Coffee and tea (without thickeners or added cream)
Juices (e.g., apple, cranberry) without pulp
Milk (all types)
Soda and other carbonated drinks
Broth and bouillon
Some nutritional supplements, like Ensure Clear

The Case of Melting Foods

An important consideration for a diet based on liquid consistency is that some solid or semi-solid foods melt into thin liquids. For individuals managing dysphagia, this means seemingly safe foods can become hazardous upon melting. Examples include:

Ice cream
Sherbet
Gelatin (Jell-O)
Popsicles
Ice cubes

Thin vs. Thick Liquids: A Comparison

Understanding the contrast between thin and thicker liquids is vital, particularly when a speech-language pathologist recommends a modified diet. The following table compares thin liquids (IDDSI Level 0) with two common thickened levels.


Feature	Thin Liquid (Level 0)	Nectar-Thick Liquid (Level 2)	Honey-Thick Liquid (Level 3)
Viscosity	Flows like water	Pours quickly from a spoon, slower than thin liquids	Drips slowly in dollops from a spoon
Flow Rate	Very fast	Faster than honey-thick, but requires more effort than thin	Slow-moving
Appearance	Clear or translucent	Visibly thicker, like fruit nectar or a cream soup	Dense and thick, resembling liquid honey
Examples	Water, juice, black coffee	Some nectars, thicker milkshakes, some smoothies	Pureed soups, thick milkshakes
Utensils	Any cup or straw	Sipped from a cup, may require effort with a straw	Eaten with a spoon, can be drunk from a cup

The Importance of Liquid Consistency in Dysphagia

For people with dysphagia, the muscles and coordination required for swallowing may be compromised. Thin liquids are notoriously difficult for these individuals because of their fast flow, which can surprise the swallowing muscles and lead to aspiration. Thickeners are often added to thin liquids to slow their movement, providing more time for the person to control the fluid safely. This modification can significantly reduce the risk of serious complications like aspiration pneumonia.

How to Test for Thin Liquid Consistency

Healthcare professionals often use a standardized method, the IDDSI Flow Test, to confirm a liquid's consistency. It's a precise way to ensure safety for individuals with dysphagia. Here’s a simplified version:

Gather equipment: Use a 10ml syringe and a timer.
Fill the syringe: Remove the plunger and cover the tip of the syringe. Fill it with 10ml of the liquid to be tested.
Release and time: Simultaneously start the timer and release the tip to allow the liquid to flow out for exactly 10 seconds.
Check the residue: For a Level 0 thin liquid, the syringe should be completely empty with no liquid remaining after 10 seconds.

For more detailed instructions and visual demonstrations, the official IDDSI website provides helpful resources. You can find videos on how to perform the flow test correctly on the IDDSI website.

Conclusion: The Final Word on Thin Liquids

What is considered a thin liquid boils down to its water-like viscosity and fast flow. While an everyday reality for most people, this fluid consistency presents a genuine risk for those with dysphagia. The IDDSI framework provides a clear, standardized way to identify and manage thin liquids, ensuring the safety and nutritional well-being of individuals with swallowing difficulties. Understanding this classification, recognizing common examples, and knowing when to use thickening agents is crucial for patients, caregivers, and healthcare providers alike.

Frequently Asked Questions

Thin liquids are dangerous for people with dysphagia because their low viscosity and fast flow are hard for weakened swallowing muscles to control. This increases the risk of the liquid entering the airway instead of the esophagus, a process called aspiration.

The International Dysphagia Diet Standardisation Initiative (IDDSI) is a global standard for classifying liquid and food textures based on thickness and consistency. This framework helps healthcare professionals, caregivers, and individuals with dysphagia to speak a common language and ensure safety.

Most clear juices like apple or cranberry juice are considered thin liquids. However, some pulpy juices or pre-packaged nectars may be slightly thicker and fall into a different IDDSI level. Any juice that melts from a solid (like juicy fruit) is also considered a thin liquid when consumed.

You only need to thicken liquids if you have been medically advised to do so by a healthcare professional, such as a speech-language pathologist, due to a swallowing disorder. Never change the consistency of your liquids without professional guidance.

While the IDDSI Flow Test with a syringe is the most accurate method, a simpler visual cue is to observe the flow. A thin liquid will pour and flow like water from any container, leaving little to no residue on a fork.

No, if you have been advised to avoid thin liquids, you should also avoid foods that melt into a thin liquid in your mouth, such as ice cream, popsicles, and gelatin. The melted liquid can still be aspirated.

The main difference is viscosity. A nectar-thick liquid (IDDSI Level 2) is thicker than water and pours more slowly off a spoon, similar to fruit nectar. A thin liquid (IDDSI Level 0) has a water-like consistency and flows very quickly.