What Happens When Vitamin D and B12 are Low?

December 31, 2025 •

4 min read

According to research, deficiencies in Vitamin D and B12 have become a silent epidemic, with many people unaware they are affected. These two essential vitamins play crucial but different roles in the body, and when levels drop, the effects can range from subtle to severe, impacting everything from energy and mood to long-term neurological health.

Quick Summary

This article outlines the distinct and overlapping symptoms associated with low levels of vitamin D and vitamin B12. It details the causes, potential long-term complications affecting the nervous system and bones, and effective treatment options for restoring these vital nutrients.

Key Points

Symptom Overlap: Fatigue, mood swings, and general weakness can be signs of both low vitamin D and low B12, making professional diagnosis essential.
Neurological Risks: Severe and prolonged B12 deficiency can cause irreversible nerve damage, including tingling, numbness, and balance issues.
Bone Health Concerns: Low vitamin D impairs calcium absorption, leading to bone pain, muscle weakness, and increased risk of osteoporosis and fractures.
Anemia Link: B12 deficiency is a primary cause of megaloblastic anemia, characterized by large, ineffective red blood cells that lead to chronic tiredness and weakness.
Vulnerable Groups: Vegans, older adults, and those with gastrointestinal conditions like Crohn's disease or pernicious anemia are at higher risk for one or both deficiencies.
Diagnosis is Crucial: Due to overlapping and sometimes subtle symptoms, blood tests are the most accurate way to diagnose a deficiency in either vitamin.
Treatment Methods: Treatments range from dietary changes and sun exposure for vitamin D to oral supplements or injections for vitamin B12, depending on the cause and severity.

Why are Vitamin D and B12 So Crucial?

Vitamin D and Vitamin B12 are two of the most vital nutrients for overall health, each with specific functions that support the body's major systems. Vitamin D is a fat-soluble vitamin and also a hormone, primarily known for regulating calcium and phosphate absorption, which is critical for strong bones. It also plays a significant role in immune function, regulating mood, and fighting inflammation. Vitamin B12, or cobalamin, is a water-soluble vitamin essential for nerve function, DNA synthesis, and red blood cell production. A deficiency in either can disrupt these fundamental processes and lead to significant health issues if left unaddressed.

The Impact of Low Vitamin D Levels

When vitamin D levels are low, the body's ability to maintain healthy bones and an effective immune system is compromised. The initial symptoms can be subtle and often mistaken for other issues. As the deficiency progresses, more specific signs and more serious conditions can develop.

Bone and Muscle Pain: Low vitamin D impairs the body's ability to absorb calcium, which can lead to bone demineralization, causing aches, pains, and muscle weakness. In severe cases in adults, this can lead to osteomalacia (softening of the bones), while in children, it can cause rickets.
Fatigue and Depressed Mood: Chronic fatigue and unexplained tiredness are common complaints associated with vitamin D deficiency. This is often linked to its role in mood regulation, with studies showing a connection between low levels and an increased risk of depression.
Weakened Immunity: Vitamin D supports immune function, and a deficiency can lead to frequent infections and a compromised ability to fight off illnesses.
Hair Loss: In some cases, low vitamin D levels have been linked to hair thinning or loss.

The Impact of Low Vitamin B12 Levels

Because Vitamin B12 is so vital for nerve health and blood cell formation, a deficiency can manifest through neurological, psychological, and physical symptoms. The body stores large amounts of B12 in the liver, so a deficiency can take years to appear, but symptoms will worsen over time if untreated.

Neurological Problems: One of the most distinctive signs of low B12 is nerve damage, leading to tingling or numbness (paraesthesia) in the hands and feet. This can progress to problems with balance and coordination, muscle weakness, and, in severe cases, nerve and spinal cord degeneration.
Fatigue and Anemia: B12 is essential for producing healthy red blood cells. A deficiency causes megaloblastic anemia, where red blood cells are abnormally large, leading to persistent fatigue, weakness, shortness of breath, and pale skin.
Cognitive and Psychological Effects: Low B12 can cause a variety of psychological and cognitive symptoms, including brain fog, memory issues, irritability, depression, and, in advanced cases, confusion and dementia.
Gastrointestinal Issues: Symptoms like nausea, diarrhea, loss of appetite, and a sore or red tongue can also point to a B12 deficiency.

Comparing the Effects of Low Vitamin D vs. Low B12

While both deficiencies can cause fatigue and mood issues, their underlying mechanisms and defining characteristics differ significantly. Understanding these differences can help in pinpointing the likely cause of your symptoms.


Feature	Low Vitamin D	Low Vitamin B12
Primary System Affected	Musculoskeletal and Immune	Nervous and Hematological (Blood)
Defining Symptoms	Bone pain, muscle weakness, frequent infections.	Neurological symptoms (tingling), balance issues, severe anemia.
Key Functions Impaired	Calcium absorption, immune response, mood regulation.	DNA synthesis, red blood cell production, nerve health.
Primary Cause	Lack of sunlight exposure, dietary intake issues.	Malabsorption (e.g., pernicious anemia, stomach issues), vegan diet.
Onset of Symptoms	Can develop over months or years, often subtly.	Can develop slowly over many years due to large liver stores.

The Importance of Addressing Both Deficiencies

Because some symptoms, like fatigue and depression, overlap, it is crucial to consult a healthcare professional for proper diagnosis through blood tests. A deficiency in one vitamin can sometimes exacerbate health issues related to the other. Addressing both low vitamin D and B12 is often necessary for individuals with certain health conditions or lifestyle choices, such as a vegan diet or malabsorption issues.

Treatment plans vary depending on the severity and cause of the deficiency. Vitamin B12 treatment may involve injections for immediate replenishment, followed by oral supplements, especially for those with malabsorption issues. For dietary-related deficiencies, tablets and fortified foods may suffice. Vitamin D deficiency is often treated with supplements and increased sun exposure, which stimulates the body's natural production. A balanced diet is also essential for maintaining adequate levels of both vitamins over the long term.

Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice. Always consult a healthcare provider for diagnosis and treatment plans.

Potential Long-Term Complications

Untreated deficiencies in vitamin D and B12 can lead to serious, and in some cases, irreversible, long-term health problems. Chronic low vitamin D can lead to severe bone demineralization, increasing the risk of fractures and osteoporosis, particularly in older adults. Long-term B12 deficiency can result in permanent neurological damage, such as peripheral neuropathy or spinal cord degeneration. It is also linked to an increased risk of heart conditions, while severe deficiencies in pregnant women can cause serious birth defects. The risk of certain cancers, including stomach and intestinal cancer, is also elevated in those with pernicious anemia, a condition that causes B12 malabsorption.

Conclusion: Prevention is Key

Both low vitamin D and B12 levels can have a significant impact on your health, affecting your energy, mood, and long-term physical well-being. The symptoms can be subtle and easily overlooked, but ignoring them can lead to serious and lasting complications. Regular health check-ups, especially for those in high-risk groups, can help in early detection. A balanced diet, adequate sun exposure, and targeted supplementation, under medical supervision, are the cornerstones of preventing and treating these deficiencies. By understanding the distinct roles of these vitamins and recognizing the warning signs, you can take proactive steps to protect your health and vitality.

Learn more about nutritional deficiencies from reliable sources like the NIH.

Frequently Asked Questions

The most common cause of low vitamin D is insufficient exposure to sunlight, which is necessary for the skin to produce it. Inadequate dietary intake and medical conditions affecting absorption can also be factors.

Individuals following a vegan or strict vegetarian diet, older adults with reduced stomach acid, and people with malabsorption issues like Crohn's disease or pernicious anemia are at higher risk for B12 deficiency.

Yes, both deficiencies can cause fatigue and low energy. Vitamin D supports mitochondrial function for energy, while B12 is critical for red blood cell production, and a shortage of either can lead to tiredness.

Treatment for a vitamin B12 deficiency typically involves supplements, often starting with injections before transitioning to oral tablets. Vitamin D deficiency is usually treated with supplements and increasing controlled sun exposure.

If left untreated, deficiencies can lead to serious health problems. Low vitamin D can cause osteoporosis, while low vitamin B12 can result in permanent nerve damage and cognitive decline.

Yes, mood swings and depression have been linked to both low vitamin D and low B12 levels. Vitamin D is involved in serotonin production, and B12 is essential for brain health.

While some symptoms overlap, tingling in the hands and feet, or other neurological symptoms, are more specifically associated with low B12. Bone and muscle pain without a clear cause is more indicative of a vitamin D deficiency.