What is Not Allowed During Intermittent Fasting?

November 18, 2025 •

5 min read

According to a 2022 study published in Frontiers in Public Health, intermittent fasting is an eating pattern that alternates between free-feeding and fasting periods. Understanding what is not allowed during intermittent fasting is crucial, as consuming the wrong items can halt your progress and negate the metabolic benefits of the fasting state.

Quick Summary

An overview of forbidden foods, beverages, and habits to avoid during the fasting window. A breakdown of how calories, certain macronutrients, and even some zero-calorie items can interrupt the fasting process, including an explanation of why processed foods and sugary drinks are particularly detrimental.

Key Points

Caloric Intake Breaks a Fast: Any amount of calories, no matter how small, will technically break a fast and halt the metabolic benefits, including fat burning.
Forbidden Foods: Strictly avoid processed snacks, sugary desserts, refined grains, most fruits, and condiments with added sugar during your fasting window.
Safe vs. Unsafe Drinks: Stick to zero-calorie beverages like water, black coffee, and plain herbal tea. Avoid all sugary drinks, alcohol, milks, and caloric creamers.
Check Your Supplements: Gummy vitamins, protein powders, and many other supplements contain calories or ingredients that can break a fast. Opt for non-caloric versions or take them with your meals.
Mind Your Eating Window: Overeating unhealthy junk food during your feeding window can counteract the benefits of fasting. Focus on balanced, nutrient-dense meals.
Consult a Professional: Certain individuals, including pregnant women, those with diabetes, or a history of eating disorders, should avoid intermittent fasting or consult a doctor first.

Understanding the Core Principle: What Actually Breaks a Fast?

At its heart, intermittent fasting is a cycle of eating and not eating, designed to keep insulin levels low for a sustained period. The primary goal is to shift the body's energy source from glucose (sugar) to ketones, a byproduct of fat breakdown. The cardinal rule is simple: any caloric intake will, technically, break a fast. While some approaches allow for a small number of calories (often under 50), a strict fast permits only non-caloric beverages. The moment you consume calories, your body's insulin levels rise, and the fat-burning state is disrupted. This applies to more than just food; it includes sugary drinks, most supplements, and even certain 'healthy' fats if consumed in large enough quantities.

The Forbidden List: Foods to Avoid During Your Fasting Window

While the specific foods you eat during your feeding window can vary, the fasting period has a much stricter set of guidelines. Avoid any foods that contain calories, no matter how small the amount. This includes seemingly healthy items that might be tempting to snack on during a hunger pang.

Snack Chips and Processed Foods: These are typically high in unhealthy fats, sugar, and sodium, and offer little nutritional value. They provide a quick calorie hit that will immediately break your fast and spike insulin.
Sugary Desserts and Candy: Loaded with refined sugar, these will cause a rapid spike in blood sugar and insulin, completely counteracting the metabolic goals of fasting.
Baked Goods and Refined Grains: Foods like bread, pastries, and pasta are high in refined carbohydrates. They are quickly digested and trigger an insulin response, making them a no-go.
Most Fruits: While healthy, fruits contain sugar (fructose) and calories that will break your fast. Save them for your eating window.
Sweets and Condiments with Added Sugar: Items like ketchup, barbecue sauce, and sugary cereals contain hidden calories and are detrimental to maintaining a fasted state.
Excessive Fats or Oils: Although some proponents of a "dirty fast" consume small amounts of healthy fats like MCT oil, a stricter fast requires avoiding all caloric intake, including these fats, to prevent disrupting autophagy.

Drinks that Break a Fast: The Dehydration Trap

Staying hydrated is critical during intermittent fasting, but what you drink matters immensely. Water is always acceptable, but many common beverages will break your fast, and some can cause further issues.

Sugary Drinks: This includes soda, fruit juice, and sweetened iced teas. They are loaded with calories and sugar, and will immediately halt your fast.
Alcohol: Any form of alcohol contains calories and is strictly prohibited during the fasting window. It can also cause dehydration and disrupt sleep.
Milks and Creamers: Adding milk, cream, or other dairy/dairy-alternative products to coffee or tea introduces calories and protein, which can trigger an insulin response. A splash of milk is debated, but for a strict fast, it is not allowed.
Flavored Waters and Enhanced Beverages: Check the labels carefully. Many flavored waters contain hidden sugars or artificial sweeteners that can still provoke an insulin response.
Protein Shakes and Smoothies: These are explicitly designed to be caloric and will absolutely break a fast. Save them for your eating window.

Comparison of Fasting vs. Non-Fasting Beverages


Feature	Fasting-Safe Beverages	Non-Fasting (Forbidden) Beverages
Calories	Zero or near-zero	Contains calories
Sugar	No added sugars	Contains natural or added sugars
Insulin Impact	Minimal to none	Can cause an insulin spike
Hydration	Excellent for hydration	Can lead to dehydration (alcohol)
Examples	Water, black coffee, herbal tea	Soda, juice, milk, sweetened coffee

Habits and Supplements to Scrutinize

It's not just about what you eat or drink; other factors can also compromise your fast. Being mindful of these can help you avoid common mistakes.

Taking Caloric Supplements: Any supplement containing calories, sugar, or protein can break your fast. This includes gummy vitamins, protein powders, and many collagen peptides. If your goal is autophagy, even BCAAs can be problematic.
Ignoring Nutrient Density (During Eating Window): While technically not breaking the fast, consuming unhealthy, processed foods during your feeding window can undermine your progress. Overeating junk food can negate the metabolic benefits gained during fasting. Focus on nutrient-dense whole foods instead.
Using Artificial Sweeteners: The impact of zero-calorie sweeteners like aspartame or sucralose is debated. Some studies suggest they can still trigger an insulin response or affect gut microbiota, potentially interfering with fasting's benefits. It's best to use them with caution or avoid them during the fasting window.
Smoking: Nutritionist Anjali Mukerjee recommends avoiding smoking during intermittent fasting, as it can be counterproductive to the overall goal of improving digestion and health.
Medication Taken with Food: Individuals on medication that must be taken with food should not attempt intermittent fasting without consulting a healthcare provider.

Who Should Avoid Intermittent Fasting?

While effective for many, intermittent fasting is not suitable for everyone. Certain individuals should avoid or be extremely cautious with fasting to prevent negative health consequences.

Pregnant or Breastfeeding Women: These individuals have higher caloric needs to support the baby's development and milk production.
Individuals with a History of Eating Disorders: Fasting can trigger or worsen unhealthy food behaviors.
People with Type 1 Diabetes: Fasting can cause dangerous fluctuations in blood sugar levels for those with diabetes, especially those on insulin.
Underweight or Malnourished Individuals: This group needs consistent caloric intake to maintain or gain weight and should not engage in restrictive diets.
Children and Teenagers: Individuals who are still growing should not participate in fasting routines.

Conclusion

For intermittent fasting to be effective, adhering to strict rules during the fasting window is essential. Any caloric intake—whether from sugary drinks, processed snacks, or even supplements—will disrupt the metabolic state of fasting. While beverages like water, black coffee, and plain herbal tea are permitted, anything that can raise insulin levels, even if seemingly harmless, should be avoided. Furthermore, the eating window should be filled with nutrient-dense, whole foods to maximize health benefits. Always consult a healthcare professional before starting an intermittent fasting regimen, particularly if you have underlying health conditions or fall into one of the categories for whom it is not recommended.

Frequently Asked Questions

Adding milk, cream, or any caloric creamer to your coffee will break your fast. For a strict fast, only black coffee is allowed. A small splash may be negligible to some, but will still stimulate an insulin response.

The impact is debated, but many experts advise against it. While zero-calorie, artificial sweeteners can trigger an insulin response in some individuals or affect the gut microbiome, which can interfere with the metabolic goals of fasting.

Bone broth technically contains calories and will break a strict fast. However, some modified fasting approaches allow a small amount to help with electrolytes and hunger, but this depends on your specific goals.

No. Both fruit and fruit juice contain sugar and calories, which will raise your insulin levels and immediately break your fast. They should be consumed only during your eating window.

Chewing gum, even the sugar-free kind, can trigger a cephalic-phase insulin response. The act of chewing and the sweetness can trick your body into preparing for food, potentially breaking your fast. It's best to avoid it.

Intense exercise is not recommended during the fasting window for some individuals, as it can lead to fatigue, hindered recovery, and low energy levels. Moderate exercise is typically safer, and listening to your body is key.

When breaking your fast, start with easily digestible, nutrient-dense foods. Good options include cooked vegetables, healthy fats like avocado, lean protein, or fermented foods like yogurt. Avoid large, heavy meals, especially those high in processed carbs and sugar.