Why is my vitamin D and B12 so low?

October 30, 2025 •

4 min read

According to the Cleveland Clinic, inadequate absorption is the most common cause of vitamin B12 deficiency, while insufficient sun exposure is the primary driver for vitamin D deficiency. Understanding why your vitamin D and B12 so low is crucial for identifying the right solution, as the causes can range from simple dietary issues to more complex medical conditions.

Quick Summary

This article explores the multiple factors behind low vitamin D and B12 levels, including dietary choices, malabsorption problems, certain medications, and specific health conditions. It provides a comprehensive breakdown of the reasons for deficiency and outlines potential solutions.

Key Points

Dietary Intake Matters: Vegan and vegetarian diets are often deficient in B12, while limited intake of fortified foods or fatty fish can lower vitamin D levels.
Malabsorption is a Key Cause: Conditions like pernicious anemia, celiac disease, and Crohn's disease interfere with nutrient absorption, often affecting both vitamins.
Sunlight is Essential for Vitamin D: Lack of sun exposure, especially during winter months or for those with darker skin, is a primary cause of vitamin D deficiency.
Medications Can Interfere: Certain drugs, such as acid reducers and diabetes medication like metformin, can lower B12 levels by impeding absorption.
Age Increases Risk: The natural aging process can reduce both B12 absorption due to decreased stomach acid and vitamin D synthesis in the skin.
Obesity is a Factor: Excess body fat can sequester vitamin D, leading to lower circulating levels in the bloodstream.

What Drives Concurrent Vitamin Deficiencies?

Low levels of both vitamin D and B12 can be a sign of a larger issue, as their deficiencies often co-exist. While the two vitamins perform different functions, several factors can affect them simultaneously, from lifestyle choices to underlying medical problems that impede absorption. Pinpointing the root cause is the first step toward effective treatment.

Dietary and Lifestyle Causes

One of the most straightforward explanations for low vitamin D and B12 levels is inadequate intake. Our modern diets and lifestyles can easily contribute to deficiencies in both essential nutrients. These factors include:

Vegan or Vegetarian Diets: Vitamin B12 is found almost exclusively in animal products like meat, eggs, and dairy. Vegans, and to a lesser extent vegetarians, must rely on fortified foods or supplements to meet their needs. While vitamin D is synthesized by the skin through sun exposure, many fortified foods also contain it. If fortified foods are not regularly consumed, deficiency is a real risk for those on plant-based diets.
Inadequate Sunlight Exposure: The primary way our bodies produce vitamin D is through direct skin exposure to sunlight. People who spend most of their time indoors, live in northern latitudes with limited winter sun, or consistently use sunscreen may not produce enough vitamin D naturally.
Age-Related Absorption Issues: As we get older, our ability to absorb nutrients, including vitamin B12, decreases. This is partly due to the gradual thinning of the stomach lining (atrophic gastritis), which can reduce the stomach acid needed to release B12 from food. The skin's efficiency at producing vitamin D also declines with age.

Medical Conditions and Malabsorption

For many, the problem isn't intake but absorption. A variety of gastrointestinal and autoimmune diseases can damage the digestive tract, preventing the body from properly absorbing vital nutrients.

Autoimmune Conditions: Pernicious anemia is an autoimmune disease where the immune system attacks stomach cells, leading to a lack of intrinsic factor—a protein vital for B12 absorption. Conditions like Crohn's disease and celiac disease can cause inflammation and damage to the small intestine, impairing the absorption of both vitamins.
Weight-Loss Surgery: Procedures like gastric bypass can reduce the size of the stomach or bypass parts of the small intestine where B12 and other nutrients are absorbed, leading to malabsorption.
Inflammatory Bowel Disease (IBD): Both Crohn's disease and ulcerative colitis cause chronic inflammation of the digestive tract, interfering with nutrient absorption.

Medication and Other Factors

Certain medications and lifestyle factors can also contribute to lower vitamin levels. For instance, long-term use of certain drugs can interfere with nutrient absorption.

Acid-Reducing Drugs: Proton pump inhibitors (PPIs) and H2 blockers, used to treat indigestion and acid reflux, reduce stomach acid production. This acid is needed to separate B12 from the food it's bound to, making absorption difficult.
Metformin: This common diabetes medication has been linked to lower B12 levels, especially with long-term use.
Obesity: A higher body mass index can lead to lower vitamin D levels. Fat cells sequester vitamin D, preventing its release into the bloodstream.
Excessive Alcohol Use: Chronic alcohol consumption can damage the digestive system and impair nutrient absorption.

Low Vitamin D vs. Low Vitamin B12: A Comparison of Causes


Cause	Impact on Vitamin D	Impact on Vitamin B12
Dietary Intake	Limited food sources (fatty fish, fortified dairy) and often supplemented.	Found exclusively in animal products; vegan and vegetarian diets are high-risk.
Sun Exposure	Primary source via UVB synthesis in the skin.	No impact; synthesized by bacteria.
Malabsorption	Linked to conditions affecting fat absorption (e.g., celiac, cystic fibrosis).	Linked to lack of intrinsic factor (pernicious anemia) and stomach/bowel issues.
Age	Decreased synthesis in the skin with older age.	Decreased stomach acid production impairs absorption.
Medications	Steroids, seizure meds, and others can affect levels.	Metformin, PPIs, and H2 blockers inhibit absorption.

Seeking a Diagnosis and Taking Action

If you suspect you have low vitamin D and B12, the first step is to consult a healthcare provider. A simple blood test can measure your levels and help determine the cause. Depending on the diagnosis, treatment may involve:

Supplementation: Oral supplements are common for both deficiencies. In some cases of severe B12 malabsorption, injections may be necessary.
Dietary Changes: Adjusting your diet to include more fortified foods, animal products (for B12), and fatty fish (for D) can help.
Increased Sun Exposure: For vitamin D, safe, moderate sun exposure can be beneficial, particularly during summer months.
Addressing Underlying Conditions: For deficiencies caused by malabsorption, managing the root condition (e.g., celiac disease, pernicious anemia) is critical.

Conclusion

Experiencing low vitamin D and B12 levels is a common health concern with diverse underlying causes. While dietary choices and insufficient sun exposure are frequent culprits, malabsorption issues from conditions like celiac disease, pernicious anemia, or gastric surgeries play a significant role. Medications and advanced age can also contribute. Identifying the specific reason for your deficiency with a healthcare provider is essential for creating an effective treatment plan, which may include targeted supplementation, dietary adjustments, and managing any underlying medical conditions. Don't leave your low vitamin levels a mystery; get them checked to ensure your long-term health and vitality. For reliable information on maintaining adequate levels, consider resources like the NIH Office of Dietary Supplements.

Frequently Asked Questions

Yes, fatigue is one of the most common symptoms of both vitamin D and B12 deficiency. Low B12 can lead to anemia, which results in extreme tiredness, while low vitamin D can also cause persistent fatigue and low energy.

To increase your vitamin D, get moderate sun exposure, consume foods like fatty fish (salmon, tuna), egg yolks, and fortified products such as milk and cereal. Supplements may be necessary, especially for those with limited sun exposure.

Vitamin B12 is naturally found in animal products, including meat, fish, poultry, eggs, and dairy. Vegans and vegetarians often rely on fortified foods like nutritional yeast and cereals, as well as supplements.

Pernicious anemia is an autoimmune condition where the body cannot produce intrinsic factor, a protein required to absorb vitamin B12 in the gut. This leads to a severe B12 deficiency that typically requires lifelong injections.

Yes, many types of weight-loss surgery, such as gastric bypass, alter the digestive system and can significantly impair the body's ability to absorb vitamin D and B12. Lifelong monitoring and supplementation are often required.

You should consult your healthcare provider and pharmacist about all medications you take. Drugs such as acid-reducing PPIs and metformin are known to interfere with B12 absorption, while steroids and laxatives can affect vitamin D.

Some symptoms like fatigue and mood issues can overlap. However, tingling or numbness in the hands and feet (pins and needles) is more indicative of a B12 deficiency, while bone pain and muscle weakness are classic signs of low vitamin D.