The Main Deficiency of Vitamin B: An In-Depth Guide

January 4, 2026 •

3 min read

According to the National Institutes of Health, vitamin B12 deficiency is estimated to occur in about 6% of people under age 60 and 20% of those over age 60, making it one of the most common deficiencies. This guide explores the reasons behind this and other key deficiencies, addressing what is the main deficiency of vitamin B.

Quick Summary

The most prevalent vitamin B deficiency involves B12, often leading to megaloblastic anemia and neurological problems, especially in older adults and vegans. Folate (B9) deficiency is also common, particularly during pregnancy. This deficiency can result from diet, malabsorption issues like pernicious anemia, or medical conditions.

Key Points

Vitamin B12 is the main deficiency: A deficit in vitamin B12 (cobalamin) is the most prominent and common B vitamin deficiency, particularly in older adults and vegans.
Causes often relate to absorption, not just diet: While diet is a factor, especially for vegans, the most common cause of B12 deficiency is malabsorption due to conditions like pernicious anemia.
B12 and folate (B9) deficiencies are linked to anemia: A lack of either B12 or B9 can lead to megaloblastic anemia, a blood disorder where red blood cells are abnormally large and dysfunctional.
Neurological damage is a serious risk: Untreated B12 deficiency can cause severe and sometimes irreversible neurological issues, including nerve damage, confusion, and memory loss.
Folate deficiency is critical during pregnancy: A lack of vitamin B9 (folate) during pregnancy increases the risk of neural tube defects in the developing fetus.
Diagnosis requires blood testing: Identifying a specific B vitamin deficiency typically involves blood tests to measure circulating levels of the vitamin.
Treatment varies by cause: Treatment ranges from oral supplements for dietary shortfalls to lifelong injections for malabsorption issues like pernicious anemia.

The vitamin B complex comprises eight essential, water-soluble vitamins crucial for cellular metabolism, nerve function, and red blood cell production. Deficiencies in vitamin B12 (cobalamin) and vitamin B9 (folate) are the most widespread and clinically significant, often resulting in a specific type of anemia.

The Prominence of Vitamin B12 and Folate Deficiencies

Deficiencies in B12 and folate are particularly notable due to their prevalence and associated health issues. A lack of either can cause megaloblastic anemia, characterized by abnormally large, immature, and poorly functioning red blood cells.

Vitamin B12 (Cobalamin) Deficiency

Vitamin B12 deficiency is common, especially among older adults and vegans. B12 is mainly found in animal products, and its absorption requires intrinsic factor, a protein produced in the stomach.

Common causes of B12 deficiency:

Pernicious Anemia: An autoimmune condition leading to a lack of intrinsic factor and impaired B12 absorption.
Vegan Diet: Requires B12 supplementation or fortified foods.
Age-Related Atrophic Gastritis: Decreased stomach acid production affects B12 absorption.
Gastrointestinal Surgery and Disorders: Conditions like Crohn’s or Celiac disease can interfere with B12 absorption.
Medications: Certain drugs, including metformin and proton pump inhibitors, can reduce B12 levels.

Vitamin B9 (Folate) Deficiency

Folate deficiency is also significant, often due to low dietary intake, particularly in those with poor nutrition or alcoholism. Folate is vital for cell growth and DNA synthesis and crucial during pregnancy to prevent neural tube defects.

Key risk factors for folate deficiency:

Inadequate Diet: Insufficient intake of folate-rich foods like leafy greens and legumes.
Alcoholism: Can interfere with folate absorption.
Malabsorptive Conditions: Chronic digestive diseases.
Increased Need: Pregnancy and certain medical conditions.

The Spectrum of Deficiencies and Their Manifestations

Deficiencies in other B vitamins can also cause specific health problems. Severe thiamine (B1) deficiency can lead to beriberi, while a lack of niacin (B3) can cause pellagra. Although less common in developed countries due to fortified foods, these deficiencies remain risks for vulnerable populations.

Comparison of Major Vitamin B Deficiencies


Deficiency	Primary Role	Main Causes	Key Symptoms
Vitamin B12	Red blood cell formation, nerve function, DNA synthesis	Pernicious anemia, vegan diet, aging, GI disorders	Fatigue, weakness, pale skin, nerve damage (tingling/numbness), memory issues
Vitamin B9 (Folate)	DNA synthesis, cell growth, red blood cell production	Poor diet, alcoholism, pregnancy, malabsorption	Fatigue, anemia, irritability, mouth sores, poor growth
Vitamin B1 (Thiamine)	Energy metabolism, nerve function	Alcoholism, diet of polished rice	Beriberi (nerve and heart issues), confusion, muscle weakness
Vitamin B3 (Niacin)	Energy conversion, cell processes	Poor diet (corn-heavy), alcoholism, malabsorption	Pellagra (diarrhea, dermatitis, dementia)

Diagnosis and Treatment

Diagnosis involves medical history, physical exam, and blood tests to measure vitamin levels. For B12, intrinsic factor antibodies may be tested for pernicious anemia. Treatment depends on the specific vitamin and cause, including dietary changes, oral supplements, or injections for severe B12 deficiency. Lifelong treatment may be needed for malabsorption issues.

Conclusion

The primary deficiency in the vitamin B complex is most commonly vitamin B12, often due to malabsorption like pernicious anemia, followed by folate deficiency, which is critical during pregnancy. While other B vitamin deficiencies exist, they are less common in developed nations due to fortified foods. A balanced diet helps prevent deficiencies, but those at risk, such as vegans, older adults, and those with chronic digestive disorders, may need supplementation. Early detection and treatment are vital to manage deficiencies and prevent complications, especially neurological damage from chronic B12 deprivation. For more information on vitamin B12 absorption, consult the MSD Manuals.

Frequently Asked Questions

The most common cause of vitamin B12 deficiency is pernicious anemia, an autoimmune condition where the body cannot produce a protein called intrinsic factor, which is necessary for B12 absorption.

Older adults and individuals on a strict vegan diet are most at risk for vitamin B12 deficiency. Other risk factors include a history of gastrointestinal surgery, Crohn’s disease, and long-term use of certain medications.

Vitamin B deficiencies are diagnosed through a blood test that measures the levels of specific B vitamins. For B12, further tests may be done to check for conditions like pernicious anemia.

If left untreated, vitamin B12 deficiency can cause irreversible neurological damage. While supplements can reverse anemia and other symptoms, nerve damage may persist.

Common symptoms include fatigue, weakness, pale or yellow skin, a sore tongue, tingling or numbness in the hands and feet, memory problems, and depression.

Folate is essential for proper cell division and DNA synthesis. A deficiency during pregnancy can increase the risk of serious birth defects, such as neural tube defects, in the unborn baby.

Yes, a poor or restrictive diet is a major cause of deficiencies, especially in the case of folate (B9) and B12 for vegans, as these vitamins are concentrated in animal products and certain plant foods.