Can you gain weight from taking vitamin B12? The surprising truth

January 9, 2026 •

4 min read

A 2023 study found that lower vitamin B12 levels are associated with higher body weight in young adults and adolescents. This counters the popular misconception that you can gain weight from taking vitamin B12, as evidence suggests the vitamin's role is far more complex and often misunderstood.

Quick Summary

Taking vitamin B12 does not directly cause weight gain. However, correcting a deficiency can normalize a previously suppressed appetite. Low B12 levels are often linked to slower metabolism and fatigue, which can indirectly contribute to weight increase.

Key Points

No Direct Link: Taking vitamin B12 does not directly cause weight gain in healthy individuals, as it is a water-soluble vitamin and excess is excreted.
Indirect Impact: Weight gain can occur indirectly from a B12 deficiency due to associated fatigue, a sluggish metabolism, and decreased physical activity.
Corrects Appetite: For individuals whose appetite was suppressed by a deficiency, B12 supplementation can restore normal eating patterns, which might be perceived as weight gain.
Supports Metabolism: B12 is a crucial cofactor for enzymes involved in converting food into energy, meaning adequate levels are vital for a healthy metabolism.
Inverse Association: Observational studies frequently find that lower B12 levels are associated with higher BMI and a greater risk of obesity.
Injections are Not a Magic Cure: While B12 injections can boost energy for those with a deficiency, they are not a proven weight-loss solution for healthy people and don't directly cause weight gain.

Understanding the Complex Relationship Between Vitamin B12 and Weight

While the internet is rife with claims connecting vitamin B12 supplements to weight gain, the scientific consensus paints a more nuanced picture. Vitamin B12, or cobalamin, is a crucial water-soluble nutrient involved in a myriad of bodily functions, including DNA synthesis, red blood cell formation, and nervous system function. When it comes to body weight, the effects are rarely straightforward and are almost always tied to an underlying deficiency rather than supplementation in a healthy individual.

The Core Function of Vitamin B12 in Metabolism

At a fundamental level, vitamin B12 plays a vital role in cellular energy production. It acts as a cofactor for enzymes essential for the metabolism of fats and carbohydrates, helping your body convert the food you eat into usable energy. If you have a sufficient intake of B12, this process runs efficiently. However, if you are deficient, your body's ability to convert food into energy can be impaired, leading to a cascade of symptoms that can affect weight:

Fatigue and Lethargy: A classic symptom of B12 deficiency is severe fatigue and a lack of energy. This reduces your motivation for physical activity, leading to a reduction in calorie expenditure and potential weight gain over time.
Sluggish Metabolism: Because B12 is so involved in metabolic pathways, a deficiency can cause these processes to slow down. When your body isn't efficiently burning calories for energy, it may store more fat.
Appetite and Mood: A severe B12 deficiency can sometimes cause a loss of appetite and even nausea. For some individuals, however, fatigue-related mood changes like depression can lead to emotional eating and weight gain. Correcting a deficiency can restore a normal appetite, which might be perceived as weight gain by someone who has been unknowingly under-eating.

The Inverse Relationship: Low B12 and Higher Weight

Contrary to the myth that B12 causes weight gain, a significant body of observational research suggests an inverse relationship between vitamin B12 status and body weight. Studies have consistently shown that people with obesity or a higher Body Mass Index (BMI) tend to have lower serum vitamin B12 levels compared to those with a normal BMI. This is a correlation, not a direct cause, and factors like a nutrient-poor diet often seen with obesity likely play a role. Genetic studies also offer supporting evidence, with research suggesting higher B12 levels are causally associated with a reduced BMI and body fat percentage. This evidence points towards adequate B12 intake potentially protecting against weight gain, rather than causing it.

The Effect of B12 Status on Weight


Feature	B12 Deficiency	Adequate B12 Levels
Energy Levels	Significantly reduced, leading to fatigue and weakness.	Normalized, supporting consistent daily activity and physical exercise.
Metabolism	Slows metabolic processes, hindering the efficient conversion of food into energy.	Supports efficient metabolic functions, including the breakdown of fats and carbohydrates.
Appetite	Can be erratic; some experience appetite loss, while others may experience mood-related cravings.	Normalized appetite regulation, reducing the risk of overeating or unhealthy cravings.
Physical Activity	Often decreases due to low energy and stamina.	Enables and encourages an active lifestyle.
Associated Weight Change	Indirect weight gain from reduced activity and a sluggish metabolism.	No direct link to weight gain; adequate levels may be associated with healthier weight.

What About B12 Injections?

Some weight-loss clinics have promoted B12 injections as a weight-loss tool, but the mechanism is not a magic bullet. For individuals with a deficiency, an injection can quickly restore energy levels. This boost in vitality can lead to increased physical activity and a healthier appetite, which in turn supports weight management goals. However, for a person who is not deficient, excess B12 is simply flushed out of the body since it is water-soluble. There is no solid evidence that B12 injections cause weight gain in healthy individuals.

Conclusion: The Final Verdict on B12 and Weight

In conclusion, the idea that you can gain weight from taking vitamin B12 is a common misconception rooted in misunderstanding its role in the body. The opposite is often true: a deficiency in B12 is more likely to cause indirect weight gain through reduced energy, fatigue, and a slowed metabolism. Correcting a deficiency with supplements or injections can restore normal metabolic function and energy levels, which may help with weight management rather than hinder it. It's crucial to consult a healthcare provider to determine if a B12 supplement is necessary for you. A balanced diet and regular exercise remain the cornerstones of healthy, sustainable weight management. You can find additional information on vitamin B12 from reliable sources like the National Institutes of Health.

Source: National Institutes of Health (NIH) - Vitamin B12 Fact Sheet for Consumers

Frequently Asked Questions

No, B12 injections do not directly cause weight gain. In individuals with a deficiency, the injections can restore energy and normalize a suppressed appetite, which may lead to weight stabilization or gain, but this is a correction to a pre-existing issue, not a side effect of the supplement.

A deficiency in vitamin B12 can cause severe fatigue and reduced energy levels. This leads to decreased physical activity and a potential slowdown of your metabolism, which can indirectly contribute to weight gain over time.

There is no direct evidence that B12 increases appetite in healthy individuals. However, for those with a B12 deficiency that has caused appetite loss, supplementing the vitamin can restore it to a normal, healthy level.

Observational studies have found that individuals with obesity or higher BMI often have lower levels of vitamin B12. This suggests an inverse relationship, though it is a correlation and not necessarily a cause-and-effect relationship.

B12 is a key vitamin that helps your body convert the food you eat, including carbohydrates and fats, into usable energy. It is a cofactor for important metabolic enzymes.

Common symptoms of B12 deficiency include fatigue, weakness, pale skin, sore tongue, memory loss, and a tingling or numbing sensation in the hands and feet. Some people may also experience a loss of appetite.

Certain groups are at a higher risk of B12 deficiency, including older adults, vegans and vegetarians, and people with conditions that impair nutrient absorption, such as Crohn's disease or pernicious anemia.

If you are not deficient, taking extra B12 is unlikely to speed up your metabolism further. Your body will simply excrete the excess, as it's a water-soluble vitamin. For those with a deficiency, restoring levels will help normalize metabolism.