Should You Take B12 While Fasting?

December 17, 2025 •

4 min read

Over one-third of UK vegetarian and vegan women were found to have a B12 deficiency in one study. For those engaging in intermittent or extended fasting, managing nutrient intake is a critical consideration, raising the question: should you take B12 while fasting?

Quick Summary

Taking pure B12 supplements during fasting is generally acceptable as it is a water-soluble vitamin without calories. However, absorption can be more efficient when taken with food due to stomach acid and intrinsic factor. The optimal timing depends on the specific fasting protocol and the supplement's form to maximize benefits and avoid stomach discomfort.

Key Points

Water-Soluble Nature: B12 is water-soluble, meaning it generally won't break a fast if taken in a pure, calorie-free form.
Absorption Timing: While a pure supplement can be taken anytime, absorption of B12 is often more efficient with food due to the intrinsic factor mechanism.
Choose the Right Form: Avoid gummies and sweetened supplements, as the added sugars will break a fast. Stick to pure pills, capsules, or unsweetened liquids.
Stomach Sensitivity: Some people may experience stomach discomfort taking B vitamins on an empty stomach; a large glass of water can help.
Consider Fasting Goals: For strict water-only or autophagy-focused fasts, some may choose to avoid all supplements, while those focused on weight loss can safely take pure B12.
Vegan/Vegetarian Needs: Individuals on plant-based diets are at higher risk of B12 deficiency and may benefit from consistent supplementation, which can be done during intermittent fasting periods.

Understanding Vitamin B12 and Fasting

Vitamin B12, or cobalamin, is a crucial water-soluble vitamin essential for energy production, nerve function, and red blood cell formation. Fasting, whether intermittent or extended, involves abstaining from food for a period, which alters the body's metabolic state. The primary concern for many fasters is whether supplements will break their fast by triggering an insulin response or introducing calories. Being a water-soluble vitamin, B12 does not require food for its basic function, but its absorption process is complex and can be influenced by the presence of food.

Pure B12 supplements, typically in tablet, capsule, or liquid form, contain no calories and therefore won't break a standard fast focused on calorie restriction. However, additives like sugar or artificial sweeteners found in gummies or chewables can interrupt the fasted state by provoking an insulin response. For those practicing strict water-only fasts for purposes like autophagy, even a calorie-free supplement may be considered a break from the purest form of fasting.

The B12 Absorption Process Explained

B12 absorption is a multi-step process that is more efficient with food. In its natural state within food, B12 is bound to protein and must be released by stomach acid and enzymes. The free B12 then binds to a protein called intrinsic factor, which is essential for absorption in the small intestine. Taking a B12 supplement on an empty stomach bypasses some of this initial digestive work, but intrinsic factor secretion is often stimulated by the presence of food. This is why some sources suggest B12 absorption is better with a meal, even though a pure supplement can technically be taken during a fast.

Considerations for Different Fasting Types

Intermittent Fasting (e.g., 16:8): For those on intermittent fasting protocols, the simplest approach is to take your B12 supplement during your eating window. This ensures maximum absorption and eliminates any concerns about breaking your fast. If you choose to take it during your fasting window, opt for a pure, unsweetened pill or capsule with a large glass of water to avoid stomach upset.
Extended Fasting (e.g., 24+ hours): During longer fasts, replenishing water-soluble vitamins like B12 becomes more important to prevent deficiencies, especially if you have an existing risk factor. Taking a pure B12 supplement with water is advisable to support energy levels and metabolic functions, as the body does not store large amounts of water-soluble vitamins.
Water-Only Fasting: For the most rigorous fasts where only water is consumed, any supplement intake is generally avoided. This is based on the principle that the body recycles existing nutrient stores during a short, medically supervised fast. However, this is a highly specific and strict practice, and most fasters incorporate pure supplements without issue.

Potential Risks and Benefits

Risks of Taking B12 on an Empty Stomach:

Nausea/Stomach Discomfort: Some individuals report stomach upset when taking B vitamins on an empty stomach. Taking the supplement with a large glass of water can help mitigate this.
Suboptimal Absorption: While free B12 doesn't require food to be absorbed, the absence of food can mean a less efficient intrinsic factor response, potentially leading to reduced absorption.

Benefits of Taking B12 While Fasting (Intermittent):

Energy Support: B vitamins are crucial for converting food to energy. Taking B12 during your fasting window, especially in the morning, may help support energy levels and combat the fatigue sometimes associated with fasting.
Convenience: For people who prefer a simple routine, taking supplements at a fixed time each day, regardless of their eating schedule, can be more convenient.
Preventing Deficiencies: For individuals at high risk of B12 deficiency, such as vegans, vegetarians, or those with digestive issues, consistent supplementation is vital. Taking B12 during fasting ensures they don't miss a daily dose.

Comparison of B12 Supplement Forms During Fasting


Supplement Form	Compatibility with Fasting	Considerations
Capsule/Pill	High (if pure)	No calories or sugar. May cause stomach upset for some when taken on an empty stomach.
Sublingual Liquid/Drops	High (if unsweetened)	Absorbed directly under the tongue, bypassing the stomach. Ensure it contains no sugar or flavorings that could break the fast.
Gummy/Chewable	Low	Contains calories, sugar, and additives that will break a fast and spike insulin. Should only be taken during the eating window.
B12 Injection	High (for medical purposes)	Delivered directly into the muscle, so it doesn't affect the GI tract. Typically for medically confirmed deficiencies; not a standard fasting supplement.

Summary of Best Practices

For the average person practicing intermittent fasting, taking a pure B12 supplement is generally safe and won't break the fast, as long as it's free of sugars and other caloric ingredients. However, those concerned about maximizing absorption or sensitive to stomach upset may prefer to take their B12 with a meal during their eating window. For extended fasts or specific health concerns, it's crucial to consult with a healthcare provider. Individuals with confirmed B12 deficiency may also find injections to be a more effective option, especially when fasting.

In conclusion, the decision to take B12 while fasting hinges on your specific fasting goals, the supplement's form, and your body's individual response. By understanding the nuances of B12 absorption and the various forms of supplementation, you can make an informed choice that supports your overall health and fasting regimen.

Frequently Asked Questions

No, a pure B12 supplement in capsule, pill, or unsweetened liquid form will not break your fast, as it contains no calories or sugar.

Absorption of B12, particularly from food, is a complex process involving stomach acid and intrinsic factor, which are stimulated by eating. Some evidence suggests absorption can be more efficient with food, though a pure supplement is still absorbed to some extent without it.

No, B12 gummies and other sweetened forms of the vitamin should not be taken while fasting. They contain sugars and additives that will trigger an insulin response and break your fast.

If you experience stomach discomfort, take your B12 supplement with a large glass of water. Alternatively, you can save your supplement for your eating window when you consume a meal.

Most vitamins and minerals, including B12, do not interfere with autophagy. In fact, some studies suggest certain vitamins may support the process, but more research is needed.

Yes, as B12 is predominantly found in animal products, vegans and vegetarians are at higher risk of deficiency. They should continue regular supplementation to prevent deficiencies, even while fasting.

For extended fasts, taking a pure B12 supplement is important to prevent deficiencies, as the body doesn't store water-soluble vitamins. Injections are also an option for those with confirmed deficiencies.