What will happen if vitamin B12 and vitamin D are low?: A guide to dual vitamin deficiency

October 9, 2025 •

4 min read

According to a 2023 study in Mumbai, 82% of executives showed a vitamin D3 deficiency, while 14% were deficient in B12. Understanding what will happen if vitamin B12 and vitamin D are low? is crucial, as this dual deficiency can lead to a complex array of overlapping and distinct health issues impacting everything from your nerves to your bones.

Quick Summary

Low levels of both vitamin B12 and vitamin D can lead to serious health issues, including exacerbated fatigue, cognitive decline, nerve damage, and bone weakness. While each deficiency presents unique challenges, their combined effect can intensify symptoms. Addressing this dual deficiency through proper nutrition, sunlight, and supplements is key to preventing long-term complications.

Key Points

Overlapping symptoms: Both B12 and D deficiencies commonly cause fatigue and mood changes, which can be more severe when both are low.
Neurological damage risk: Low B12 can lead to irreversible nerve damage, including numbness, tingling, and poor coordination.
Bone health compromises: Vitamin D deficiency seriously impacts calcium absorption, leading to soft bones (osteomalacia) and increased fracture risk.
Cognitive decline: Combined low B12 and D levels are associated with a higher risk of cognitive impairment and dementia, particularly in older adults.
Addressing malabsorption: Dietary and lifestyle changes are not always enough, especially for conditions like pernicious anemia or Crohn's disease, which require medical intervention.
Dietary sources are different: B12 is primarily found in animal products and fortified foods, while D is obtained through sunlight exposure and fatty fish.

The Critical Roles of Vitamin B12 and Vitamin D

Vitamin B12 (cobalamin) and vitamin D are both essential micronutrients, but they play distinct, yet equally vital, roles in the body. B12 is water-soluble and primarily involved in nerve function, DNA synthesis, and red blood cell formation. A lack of B12 can disrupt these fundamental processes, leading to significant neurological and hematological issues.

Vitamin D, a fat-soluble vitamin, functions more like a hormone and is primarily known for its role in calcium absorption and bone health. However, it also plays a critical part in regulating the immune system and influencing cell growth. Low vitamin D levels weaken the immune response and can cause bones to soften.

The Impact of Low Vitamin B12 and Vitamin D: Overlapping and Exacerbated Symptoms

While each vitamin deficiency has unique symptoms, some signs overlap. When both are low, these issues can be more pronounced and severe. The combined deficiency can make diagnosis challenging, as a person may attribute all symptoms to one cause. Fatigue, for instance, is a hallmark of both deficiencies, and having low levels of both can make it overwhelming.

Common Symptoms of Low Vitamin B12:

Fatigue and Weakness: Due to the inability to produce enough healthy red blood cells to carry oxygen.
Neurological Issues: Numbness, tingling sensations (paresthesia), and muscle weakness are common due to B12's role in nerve health.
Cognitive Problems: Confusion, memory loss, and a decline in mental abilities can occur, especially in older adults.
Anemia: Low B12 can lead to megaloblastic anemia, where the body produces abnormally large red blood cells that cannot function correctly.

Common Symptoms of Low Vitamin D:

Bone and Muscle Pain: Weakened bones and muscles can cause widespread pain and aches.
Muscle Weakness and Cramps: Poor calcium absorption affects muscle function.
Mood Changes: There's a documented link between low vitamin D and mood disorders like depression.
Weakened Immune System: The immune system's ability to fight off pathogens is compromised, leading to increased susceptibility to illness.

Exacerbated Dual Deficiency Symptoms:

When both deficiencies are present, the impact is often greater than the sum of its parts. For example, the fatigue from both low B12 and low D can be particularly debilitating. Neurological and cognitive issues may also be worsened. Furthermore, low vitamin D's impact on muscle weakness can be compounded by B12's role in nerve health, affecting coordination and balance.

Long-Term Consequences of Untreated Dual Deficiency

Ignoring these deficiencies can lead to severe and sometimes irreversible long-term health problems.

Irreversible Nerve Damage: Chronic, untreated B12 deficiency can cause permanent neurological damage, including spinal cord degeneration and peripheral neuropathy.
Severe Bone Disorders: Persistent vitamin D deficiency can lead to osteomalacia (softening of bones) in adults and rickets in children. This increases the risk of fractures and bone deformities.
Exacerbated Cognitive Decline: The combined effect of low B12 and low D has been linked to increased risk of cognitive impairment and dementia, particularly in the elderly.
Cardiovascular Risks: Elevated homocysteine levels, a consequence of B12 deficiency, are a risk factor for cardiovascular diseases. Severe anemia from B12 deficiency can also lead to heart and lung complications as the heart works harder to pump oxygen.

Causes of Deficiency and Prevention Strategies

Deficiencies can stem from dietary issues, malabsorption problems, or other medical conditions. For vitamin B12, since it's primarily found in animal products, vegetarians and vegans are at a higher risk. Malabsorption issues like pernicious anemia, Crohn's disease, and certain gastric surgeries also prevent proper absorption. For vitamin D, causes include limited sun exposure, darker skin, obesity, and certain medical conditions.

Prevention involves a combination of dietary adjustments and supplementation. For B12, consuming fortified foods or supplements is essential for those with dietary restrictions or absorption issues. For vitamin D, safe sun exposure, dietary intake of fortified foods and fatty fish, and supplementation are crucial.

Comparison of Vitamin B12 and D Deficiency Symptoms


Feature	Low Vitamin B12	Low Vitamin D	Dual Deficiency Impact
Energy Level	Extreme fatigue, weakness, lethargy	Fatigue, general malaise	Exacerbated and more severe fatigue
Neurological Health	Numbness, tingling, poor coordination, memory loss	Mood changes like depression	Increased severity of cognitive issues and nerve damage
Musculoskeletal	Muscle weakness, tongue inflammation	Bone pain, muscle aches, cramps, weakness	Combined muscle and bone pain, higher risk of fracture
Mental Health	Depression, irritability, confusion	Mood changes, depression	Increased severity of psychological problems
Anemia Risk	High risk of megaloblastic anemia	No direct link to this type of anemia	High risk of megaloblastic anemia from B12, possibly compounded by overall health decline

Addressing Low Vitamin Levels Through Diet and Supplementation

To correct these deficiencies, a multi-pronged approach is necessary. For many, increasing dietary intake of vitamin-rich foods is the first step. For vitamin B12, this includes meat, fish, eggs, and fortified cereals. For vitamin D, options include fatty fish, dairy products, and sun exposure.

However, in cases of severe deficiency or malabsorption issues, dietary changes alone may not be enough. In such situations, healthcare providers may recommend supplements. These can range from oral tablets to intramuscular injections, especially for severe B12 malabsorption issues like pernicious anemia. Always consult a healthcare professional before starting supplementation to determine the correct dosage and form.

Conclusion

Understanding what will happen if vitamin B12 and vitamin D are low highlights the critical need for proactive nutritional management. While the symptoms can vary and overlap, addressing both deficiencies is vital for protecting against long-term complications affecting neurological function, bone strength, and mental health. With proper diagnosis and a comprehensive treatment plan incorporating diet, sunlight, and professional guidance, most people can effectively restore their vitamin levels and improve their overall health.

For more information on the critical role of Vitamin B12 in maintaining neurological health, you can refer to the detailed review from Frontiers in Pharmacology(https://www.frontiersin.org/journals/pharmacology/articles/10.3389/fphar.2019.00406/full).

Frequently Asked Questions

Early signs can include extreme fatigue, muscle weakness, and mood changes such as depression. B12 deficiency may also cause a sore tongue or numbness, while low vitamin D can cause bone and muscle pain.

Yes, tingling in the hands and feet, known as paresthesia, is a common symptom of vitamin B12 deficiency, caused by nerve damage. Low vitamin D is not typically associated with this symptom directly.

Diagnosis is typically made through blood tests that measure the levels of each vitamin. Your healthcare provider will also consider your symptoms, medical history, and risk factors.

Combining supplements can be effective, but the treatment plan depends on the cause and severity of the deficiency. For severe B12 malabsorption, injections may be necessary. Always consult a doctor to determine the right dosage and treatment approach.

For vitamin B12, good sources include meat, fish, dairy, and fortified cereals. For vitamin D, fatty fish like salmon, fortified milk, and safe sun exposure are key.

Recovery time varies depending on the severity and underlying cause. With consistent and proper treatment, many symptoms can improve within weeks or months, though some long-term neurological damage from B12 deficiency may be irreversible.

Older adults, individuals with digestive disorders like Crohn's disease, those who have had gastric surgery, people with limited sun exposure, and vegans/vegetarians are all at higher risk.

Yes, a vitamin B12 deficiency is a known cause of megaloblastic anemia, which results from the production of abnormally large red blood cells.