At a glance
Fasting keeps insulin levels low, allowing the body to burn stored fat and activate cellular repair processes. While any amount of calories technically breaks a fast, different foods have varying effects on insulin levels and, therefore, the body’s metabolic response. Understanding which foods, drinks, and supplements have minimal metabolic impact can help you achieve sustained fasting success.
How many calories break a fast? While technically, any caloric intake interrupts a strict fast, the metabolic impact of food largely depends on how strongly it stimulates insulin release.
Learn how different macronutrients influence insulin levels, and which options may help support satiety, nutrient balance, and weight loss goals.
What does it mean to break a fast?
A traditional fast involves eliminating all foods and consuming only water to keep insulin levels low. This allows the body to enter ketosis, a metabolic state in which stored fat is used for energy.
Research published in Nutrients links nutritional ketosis to several health benefits, including improved appetite control, weight loss, enhanced cognitive performance, and a lower risk of cardiometabolic disease.
Prolonged fasts, typically lasting 18 to 48 hours, also stimulate autophagy, a cellular renewal process in which the body recycles damaged proteins and cellular organelles. Autophagy may reduce oxidative stress and help support metabolic health, cellular repair, and healthy aging.
Any food or beverage that raises insulin levels can break a fast, as elevated insulin signals the body to shift back into a fed state. Calories from food or drinks stimulate insulin release and digestion, which interrupts the biological processes associated with fasting.
Watch the video below to learn more about meeting your energy needs while fasting.
How many calories break a fast?
Technically, any calorie intake will break the fasting state.
However, the metabolic impact depends less on calorie count alone and more on how strongly a food stimulates insulin.
Carbohydrates and protein tend to trigger the greatest insulin response, as these macronutrients raise blood sugar levels, which can disrupt fat burning and other processes associated with fasting.
While fats also contain calories, they have minimal impact on insulin levels compared to carbohydrates and protein. Because of this, some nutritious fats may be enjoyed during the fasting window to help curb hunger and extend fasting periods while maintaining low insulin levels.
However, strict fasting protocols typically avoid all calorie-containing foods and beverages until the eating window to maximize the physiological benefits of fasting.
Calorie intake vs. nutrient intake
Calories come from foods that contain carbohydrates, protein, or fat, which can raise blood glucose levels and stimulate insulin release to varying degrees.
Increases in glucose and insulin signal the body to shift into a fed state, initiating digestion and interrupting fasting-related metabolic processes.
However, supporting nutrient balance during fasting doesn’t have to require meaningful calorie intake. Many nutritional supplements contain very few calories while providing essential vitamins, minerals, and phytonutrients that support normal physiological function.
A study published in Current Opinion in Clinical Nutrition and Metabolic Care found that individuals who follow intermittent fasting plans, such as alternate-day fasting, are at increased risk of nutritional deficiencies.
The authors summarize that fasting may contribute to, “….reductions in the intake of key micronutrients such as calcium, magnesium, potassium, folate, vitamin C, and various B vitamins.”
For this reason, it’s often recommended to incorporate supportive nutrients into your fasting routine to help maintain healthy micronutrient levels.
Fasting-friendly nutrient sources include:
- Sea salt
- Sugar-free electrolyte powders
- Wheat grass juice powder
- Nutritional yeast
- Trace minerals
- Vitamin D
What you should avoid while fasting
Bone broth is often considered a fasting-friendly beverage, but it’s rich in calories, protein, and amino acids. Although it’s a good choice for supporting gut health, nutritional balance, and collagen intake, it’s best avoided during the fasting period.
Similarly, collagen peptides, protein supplements, and branched-chain amino acids (BCAAs) can break your fast by activating glucose-related metabolic pathways.
Additionally, sugary, calorie-dense beverages such as coconut water, sodas, fruit juices, sugar-sweetened coffee or tea, and energy drinks can quickly break your fast.
It’s also recommended to avoid artificial sweeteners, such as maltitol, saccharin, aspartame, and sucralose. Though they’re zero calories, these sweeteners are still high on the glycemic index, meaning they have significant effects on blood sugar and insulin levels.
What foods and drinks can you have while fasting?
Although it’s not permitted to consume calories on a strict water fast, beginners and those looking to incorporate fasting into their daily routines may include certain foods or drinks that help manage cravings and make it easier to extend the fasting window.
Here are five foods and beverages that won’t break a fast.
1. Lemon water
Water with fresh lemon juice can help maintain hydration and electrolyte balance. Because lemon juice contains virtually zero calories or carbohydrates, it won’t disrupt the fasting state.
Drinking lemon water may also help you lose weight by suppressing appetite and supporting balanced blood sugar and insulin levels.
“While lemon water doesn’t directly help you lose weight, certain phytonutrients in lemon water help support lower blood sugar and insulin levels, which helps reduce fat storage,” explains Dr. Berg.
2. Black coffee
When consumed without sugar, milk, or flavored creamers, moderate amounts of coffee can help boost energy and mental focus without significantly affecting insulin levels.
For those who don’t enjoy black coffee, bulletproof coffee may also be a good option during fasting.
Bulletproof coffee is prepared by blending black coffee with healthy fats such as grass-fed butter, coconut oil, whole cream, or medium-chain triglyceride (MCT) oil, which can help promote satiety, support fat metabolism, and extend the fasting window.
3. Non-starchy vegetables
Non-starchy vegetables with a high fiber content may be acceptable in small amounts, particularly during extended fasts.
Because fiber has minimal impact on blood sugar and insulin demand, small portions of vegetables such as leafy greens, celery, or raw cruciferous vegetables may be consumed during the fasting window with little metabolic disruption.
These foods are also rich sources of nutrients and beneficial plant compounds, including glucosinolates and sulforaphane, which support the body’s antioxidant defenses.
Additionally, fibrous, low-carb vegetables can serve as a gentle way to transition out of a prolonged fast, as introducing large amounts of food too quickly may increase the risk of refeeding syndrome in some individuals.
4. Tea and herbal infusions
Unsweetened green tea and herbal teas, such as peppermint tea and chamomile, contain little to no calories.
They’re also rich in antioxidants and bioactive plant compounds that may boost the effects of fasting. Many herbal teas contain polyphenols and catechins that help combat oxidative stress and support cellular repair processes.
Green tea, in particular, is a rich source of epigallocatechin gallate (EGCG).
Research published in Nutrients found that EGCG is associated with many health benefits, including promoting autophagy, which may help reduce the risk of neurodegenerative disease, cancer, premature aging, and obesity.
5. Natural sugar alternatives
Unlike cane sugar and artificial sweeteners, certain sugar alcohols and plant-derived sweeteners can be used in small amounts during fasting.
Erythritol, stevia, and monk fruit are low-glycemic sweeteners that generally don’t stimulate changes in blood sugar or insulin levels when used to sweeten fasting-friendly beverages such as coffee or tea.
Key takeaways
- Compared to fat, carbohydrates and protein stimulate the strongest insulin response, thereby breaking the metabolic fasting state.
- Although food will break your fast, many nutritional supplements, such as electrolyte powders, nutritional yeast, and wheatgrass juice powders, are safe and recommended during fasting.
- Avoid bone broth, collagen and protein supplements, sugary beverages, and artificial sweeteners, as they trigger insulin responses, thereby breaking the fast.
- Lemon water, herbal teas, and coffee sweetened with natural sugar alternatives are fasting-friendly and may offer additional health benefits.
