Nutrition Diet: What Is Restricted During Fasting for Health and Weight Loss

October 25, 2025 •

5 min read

According to research published in the New England Journal of Medicine, fasting triggers a metabolic switch in the body, shifting from burning glucose to fat for energy. Understanding what is restricted during fasting is key to effectively initiating this metabolic process for health benefits and weight management.

Quick Summary

During fasting, all calorie-containing foods and most caloric beverages are restricted to initiate metabolic switching. This means abstaining from solid foods, sugary drinks, and supplements with caloric additives while staying hydrated with zero-calorie fluids like water, black coffee, and unsweetened tea.

Key Points

Caloric Restriction: The fundamental rule of fasting is to avoid all caloric intake to prompt your body to burn stored fat for energy.
Beverage Choices: While water, black coffee, and unsweetened tea are generally permitted, sugary drinks, milk, and smoothies should be avoided as they contain calories.
Supplement Awareness: Many common supplements, including protein powders, fish oil, and gummy vitamins, contain calories or additives that will break a fast.
Safe Fast-Breaking: When ending a fast, prioritize easily digestible foods like broth, cooked vegetables, and lean protein to avoid digestive discomfort.
Medical Consultation: Always consult a healthcare professional before starting a fasting regimen, especially if you have pre-existing health conditions.
Religious vs. Health Fasting: Rules vary significantly depending on the purpose. Religious fasts like Ramadan often restrict even water, while intermittent fasting allows zero-calorie fluids.

The Core Principle: Avoiding Caloric Intake

The fundamental principle behind most health-related fasting protocols is to abstain from consuming any calories during the fasting window. The body's primary energy source is glucose from carbohydrates, which is stored in the liver and muscles as glycogen. When you stop eating, your body uses up its glycogen stores. Once these reserves are depleted, typically after 12 hours or more, the body initiates a process called metabolic switching. It then begins to burn stored fat for fuel, which is a key goal for many who fast for weight loss. Consuming any calories during this period, even in small amounts, can trigger an insulin response that stops this process and effectively breaks the fast.

Foods and Drinks That Immediately Break a Fast

To ensure your fast is effective, it is critical to avoid all sources of calories, especially those from carbohydrates and protein, which cause the most significant insulin spikes.

Solid Foods

Any solid food, regardless of its nutritional content, will break a fast. Even a small snack like an apple or a handful of nuts will provide your body with fuel, halting the metabolic switch to fat-burning.

Sugary and Caloric Beverages

Soda and sugary drinks: These are loaded with sugar, which will immediately spike your blood sugar and insulin levels.
Fruit juices and smoothies: While often perceived as healthy, these contain natural sugars that provide a significant number of calories and will break a fast.
Milk and dairy products: The lactose in milk and other dairy items contains sugar and protein, both of which are caloric and will end a fast.
Alcohol: All alcoholic beverages are caloric and must be avoided.

Refined Carbohydrates and Processed Foods

Foods like white bread, pasta, pastries, and processed snacks are high in refined carbs and sugar. These are not only forbidden during the fasting window but are also counterproductive to the healthy eating habits encouraged by fasting.

Supplements to Use with Caution or Avoid

Protein powder: Any protein powder, including whey and collagen, contains calories and will break a fast by triggering an insulin response.
Gummy vitamins: These are often sweetened with added sugars or contain gelatin, a protein source, making them unsuitable for fasting.
Fish oil and MCT oil: Since these contain fat, they technically add calories. While fat has a minimal impact on insulin, it still provides energy that can interrupt the body's deeper fat-burning processes. Some practitioners of "dirty fasting" might tolerate these, but for a strict fast, they should be avoided.

Safe Consumptions During a Fast

Staying hydrated is crucial during fasting. The following zero or very low-calorie beverages are generally permitted:

Water: Plain water, sparkling water, and flavored (zero-calorie) water are essential for staying hydrated.
Black coffee: With only about five calories per cup, black coffee is acceptable for most fasting protocols. It can also help with hunger pangs.
Plain, unsweetened tea: Herbal teas, green tea, and black tea without any additives are safe.
Bone broth: This contains some calories but is often considered acceptable for longer fasts to help replenish electrolytes.
Medications: Prescription medications generally do not break a fast, but you should always consult a doctor, as some require food for optimal absorption or to prevent stomach irritation.

Fasting Types and Their Dietary Restrictions

Fasting is not a one-size-fits-all approach. Different methods have varying rules on what is restricted during fasting. Here is a comparison of some popular methods:


Fasting Method	Fasting Period	Typical Restrictions	Permitted During Fast	Who is it for?
Time-Restricted Eating (e.g., 16:8)	Fast for 16 hours, eat within an 8-hour window (e.g., 12 p.m. to 8 p.m.).	All caloric intake during the 16-hour fasting period.	Water, black coffee, and unsweetened tea.	Beginners, weight management, and metabolic health.
5:2 Fasting	Eat normally for 5 days of the week and restrict calories to 500-600 on two non-consecutive days.	All caloric intake for most of the two fasting days, with a small meal or two allowed.	Water, black coffee, and unsweetened tea. Limited calories on fast days.	Individuals seeking gradual weight loss and metabolic improvement.
Water Fasting	Abstaining from all food for a specific period (24 to 72 hours).	All food and drinks except water.	Water only. Sometimes mineralized water is recommended for longer fasts.	Experienced fasters or those under medical supervision. Not for beginners.
Religious Fasting (e.g., Ramadan)	Varies by religion. Muslims abstain from food, water, and other actions from dawn until sunset.	All food and drinks during daylight hours.	None during the fasting period. Meals are consumed before dawn and after sunset.	Those observing for spiritual reasons.

Safely Ending Your Fast

How you break your fast is just as important as the fast itself, especially for longer periods. Introducing food too quickly can cause digestive distress.

Tips for breaking a fast:

Start with small portions of easily digestible foods.
Opt for hydrating and nutrient-dense options like broths, soups, or simple smoothies.
Eat lean proteins, cooked vegetables, and healthy fats. Eggs, avocados, and fish are good choices.
Avoid heavy, fried, or sugary foods when breaking your fast, as these can cause discomfort and a rapid blood sugar spike.

The Importance of Consultation

Before beginning any fasting regimen, it is highly recommended to consult with a healthcare provider, especially if you have pre-existing health conditions like diabetes, are pregnant or breastfeeding, or have a history of eating disorders. A medical professional can help you determine the safest and most effective approach for your individual health needs and goals.

Conclusion

The question of what is restricted during fasting has a clear answer rooted in the body's metabolic function: avoiding all calories is the key to entering and maintaining a true fasting state. By restricting solid foods, most beverages with calories, and certain supplements during designated periods, individuals can maximize the fat-burning and cellular-repairing benefits of fasting. Staying properly hydrated with water, black coffee, or unsweetened tea is essential, while consuming nutritious, easily digestible foods when breaking the fast is crucial for a safe and effective dietary approach. Always seek professional medical advice to ensure fasting is appropriate for your specific health situation.

Frequently Asked Questions

Any food or beverage containing calories, particularly carbohydrates and proteins, will break a fast by triggering an insulin response and stopping the body's shift into a fat-burning state.

No, adding milk or creamer to your coffee adds calories and protein, which will break your fast. You should only drink black coffee or unsweetened tea during your fasting window.

Yes, many supplements can break a fast. Oil-based supplements like fish oil contain calories, and gummy vitamins often contain sugar. For optimal absorption, fat-soluble vitamins should also be taken with food.

Bone broth contains minimal calories but is a gray area. Some 'clean' fasters avoid it, while others use it during longer fasts to maintain electrolytes. For a strict fast, it's best to stick to zero-calorie options.

Break your fast gently with small portions of easily digestible, hydrating foods. Good choices include broth, soup, cooked vegetables, or a small portion of lean protein.

While zero-calorie sweeteners generally don't break a fast by adding calories, their effect on insulin response is still debated. Some people find that they can increase cravings, so it's often best to avoid them for the cleanest fast.

Religious fasting, such as during Ramadan, often involves complete abstinence from both food and water during specific hours. Intermittent fasting, for health reasons, typically permits zero-calorie liquids like water and black coffee throughout the fasting period.