What is classed as short-term fasting?

January 4, 2026 •

4 min read

According to nutrition experts at Mass General Brigham, a fast is considered a short-term fast once it exceeds 12 hours. This is because it marks the point where the body begins to transition its primary fuel source from glucose to stored fat. The duration of what is classed as short-term fasting can vary, encompassing several different intermittent fasting methods.

Quick Summary

This article defines short-term fasting, explores popular methods like 16:8 and 24-hour fasts, and details the metabolic shifts that occur. It also covers the associated benefits and risks.

Key Points

Duration: What is classed as short-term fasting is a period ranging from 12 to 24 hours without food, coinciding with many intermittent fasting protocols.
Metabolic Shift: A core feature of short-term fasting is the 'metabolic switch,' where the body moves from burning glucose for energy to using stored fat.
Common Methods: Popular methods include daily time-restricted eating (e.g., 16/8) and weekly calorie-restricted days (e.g., the 5:2 diet).
Key Benefits: Potential health benefits include improved insulin sensitivity, support for weight management, and triggering cellular repair via autophagy.
Stay Hydrated: During the fasting period, it is essential to drink plenty of calorie-free fluids like water, black coffee, and unsweetened tea.
Not for Everyone: Certain individuals, such as pregnant women, those with a history of eating disorders, or type 1 diabetics, should avoid fasting unless medically supervised.
Ease In: Beginners can start with a less restrictive method, like a 14-hour fast, to help their body adjust to the new eating pattern.

Defining the Duration: What is Classed as Short-Term Fasting?

While the exact definition can vary slightly, a medically and scientifically accepted standard for what is classed as short-term fasting is a period without food ranging from approximately 12 to 24 hours. This window is typically long enough to trigger a metabolic shift away from burning recent food for fuel and towards consuming stored energy, primarily fat. Short-term fasting is most often associated with intermittent fasting (IF), which involves cycling between periods of eating and abstaining from food.

The Metabolic Shift: What Happens During a Short-Term Fast?

When you begin a fast, your body goes through several metabolic stages. In the first few hours after your last meal, you are in the 'fed state,' where your body is busy digesting food and using glucose for energy. After this is used up, the body enters the post-absorptive and then the fasted state. This is when the crucial 'metabolic switch' occurs. Here's a breakdown of the key metabolic changes:

Glucose Depletion: Within about 12 hours, your body's primary storage of glucose, known as glycogen, is largely depleted.
Fat Burning (Ketosis): To maintain energy levels, the body switches to breaking down stored fat for fuel. The liver converts fatty acids into ketones, which can be used by the brain and muscles.
Autophagy Activation: Many short-term fasting protocols are long enough to initiate autophagy, a cellular process of cleaning out and recycling damaged cell components. This is often triggered between 14 and 18 hours of fasting.

Common Short-Term Fasting Protocols

Different short-term fasting methods exist, allowing individuals to choose a schedule that best fits their lifestyle. Some popular options include:

16/8 Method: This time-restricted eating approach involves fasting for 16 hours and limiting all food and calorie-containing beverages to an 8-hour window. Many people find this manageable by simply skipping breakfast.
14/10 Method: A slightly less restrictive version of time-restricted eating, the 14/10 method involves fasting for 14 hours and eating within a 10-hour window. This is often a good starting point for beginners.
Eat Stop Eat (24-hour fast): This involves a full 24-hour fast, once or twice a week, from a meal one day to the same meal the next. For example, finishing dinner at 7 p.m. on Monday and not eating again until 7 p.m. on Tuesday.
The 5:2 Diet: With this method, you eat normally for five days of the week. On the other two non-consecutive days, you significantly restrict your calories to around 500-600.

Comparison Table: Short-Term Fasting Methods


Method	Fasting Duration	Eating Window	Typical Frequency	Key Benefit/Feature
16/8	16 hours daily	8 hours daily	Daily or several times per week	Often considered the most sustainable daily approach
14/10	14 hours daily	10 hours daily	Daily	Excellent entry point for new fasters
Eat Stop Eat	24 hours	N/A (single day fast)	1-2 times per week	Allows for normal eating patterns most days, but can be more challenging
5:2 Diet	2 days restricted (500-600 kcal)	5 days normal eating	2 non-consecutive days per week	Flexible schedule, avoids daily fasting

Potential Benefits of Short-Term Fasting

For many healthy individuals, short-term fasting can offer numerous benefits beyond simple calorie reduction, including:

Weight Management: By compressing the eating window, many people naturally consume fewer calories, leading to weight and fat loss.
Improved Insulin Sensitivity: Short-term fasting can help the body regulate blood sugar levels more efficiently, which is important for preventing type 2 diabetes.
Cellular Repair (Autophagy): Fasting triggers the process of autophagy, where the body cleans out and repairs damaged cells, potentially slowing aging and reducing inflammation.
Enhanced Mental Clarity: Many fasters report increased focus and improved cognitive function, which may be linked to the brain becoming more efficient at using ketones for fuel.
Heart Health: Studies have shown that short-term fasting can lead to improvements in blood pressure, cholesterol, and other markers of heart health.

Considerations and Safety Precautions

While generally safe for healthy adults, short-term fasting is not suitable for everyone. Potential side effects can include headaches, fatigue, and irritability, particularly during the adjustment phase. These effects typically subside over time. Certain groups should avoid fasting without strict medical supervision:

Pregnant or breastfeeding women
Individuals with a history of eating disorders
People with type 1 diabetes, as fasting can cause dangerously low blood sugar
Children and teens under 18
Anyone taking prescription medications that must be taken with food

For those who are able to fast, hydration is key. Staying well-hydrated with water and calorie-free beverages like black coffee or tea is essential. The quality of food consumed during eating periods also matters greatly; eating healthy, whole foods is necessary to maximize benefits and avoid nutrient deficiencies.

Conclusion: Making Short-Term Fasting Work for You

What is classed as short-term fasting typically falls within the 12 to 24-hour range, covering many popular intermittent fasting methods. It's a strategy that triggers important metabolic shifts, including fat-burning and cellular repair, which may lead to significant health benefits. The key to successful short-term fasting is finding a method that fits your lifestyle and focusing on high-quality nutrition during your eating periods. Always consult a healthcare provider before beginning any new dietary regimen to ensure it is appropriate for your individual health needs. For those who choose to proceed, starting with a gentler method like 14/10 can help ease your body into the new routine, allowing you to experience the potential rewards of this flexible eating pattern.

[Authoritative Outbound Link]: The New England Journal of Medicine provides an in-depth review of intermittent fasting and its effects on health, aging, and disease

Frequently Asked Questions

Yes, a 12-hour fast is typically the minimum duration considered to be a 'fast' in the scientific community and is often used as an introductory method for beginners.

Yes, a 24-hour fast is an example of short-term fasting, though it is on the longer end of the spectrum for this category. It is often done once or twice a week (known as the 'Eat Stop Eat' method).

Yes, the 16/8 method, which involves fasting for 16 hours and eating during an 8-hour window, is one of the most popular and sustainable forms of short-term intermittent fasting.

No, drinking plain black coffee with no additives like sugar or cream will not break a fast. It has virtually no calories and can even help suppress appetite.

Metabolic switching is the process where your body, after depleting its glucose stores, switches to burning stored fat for energy. This is a key physiological event that occurs during short-term fasting.

Short-term fasting is generally not recommended for those who are pregnant, breastfeeding, under 18, have a history of eating disorders, or have certain medical conditions like type 1 diabetes.

When breaking a fast, it's best to eat nutritious, whole foods. Focus on lean proteins, healthy fats, fruits, vegetables, and whole grains to replenish your body and avoid overeating.