Why Would Vitamin D2 Be Low? Understanding the Causes of Ergocalciferol Deficiency

January 12, 2026 •

4 min read

According to the National Health and Nutrition Examination Survey, a significant portion of the population suffers from vitamin D insufficiency. For those who follow plant-based diets or rely on certain fortified foods, a primary concern is understanding why would vitamin D2 be low, which is often linked to dietary or absorption problems.

Quick Summary

Low vitamin D2 levels are often caused by insufficient dietary intake, malabsorption disorders, or problems with liver and kidney function. Certain medications can also interfere with the body's vitamin D metabolism, contributing to a deficiency.

Key Points

Limited Dietary Intake: Vegan diets relying solely on natural sources may not provide sufficient vitamin D2, necessitating careful meal planning or fortification.
Malabsorption Disorders: Digestive conditions like celiac disease, Crohn's disease, and cystic fibrosis can prevent the proper absorption of fat-soluble vitamin D2.
Organ Dysfunction: Impaired liver and kidney function can disrupt the conversion of vitamin D2 into its active, usable form within the body.
Medication Interference: Certain drugs, such as anticonvulsants and steroids, can increase the rate at which vitamin D is broken down, leading to lower levels.
Surgical History: Individuals who have undergone gastric bypass or other weight-loss surgeries are at higher risk for low vitamin D2 due to reduced intestinal absorption.
D2 vs. D3 Considerations: While D3 from sun exposure is often the main source for many, those with minimal sun exposure or plant-based diets depend heavily on D2, which has a potentially different metabolism.

What is Vitamin D2?

Vitamin D is a fat-soluble vitamin crucial for bone health, immune function, and calcium absorption. It exists in two primary forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). While D3 is produced in the skin upon sun exposure and found in animal products, D2 comes from plant sources like fungi (mushrooms) and fortified foods. For individuals with limited sun exposure or those following a vegan or plant-based diet, ergocalciferol (D2) often becomes a significant source of vitamin D. A low level of this form can have various underlying causes.

Inadequate Dietary Intake

For many, the most straightforward reason for low vitamin D2 is not consuming enough of it. Unlike vitamin D3, which is abundant in fatty fish and egg yolks, D2 has fewer natural sources. This can pose a challenge for vegans, vegetarians, and individuals with food allergies or intolerances to fortified products.

Reliance on a Plant-Based Diet: Natural sources of vitamin D2 for plant-based eaters are limited to UV-exposed mushrooms and some fortified foods. A diet that excludes or lacks these items can easily become deficient. The bioavailability of D2 from mushrooms can vary, and without careful planning, nutritional gaps can occur.
Lack of Fortified Foods: In many countries, foods like milk, cereals, and orange juice are fortified with vitamin D. However, those who avoid these products due to personal choice or allergy (e.g., lactose intolerance) miss out on a key dietary source. Not all plant-based milks or juices are fortified, so it is essential to check labels carefully.

Malabsorption Conditions

Since vitamin D is a fat-soluble vitamin, its absorption in the gut is tied to the body's ability to digest and absorb fats. Medical conditions that interfere with fat absorption can therefore lead to low vitamin D2 levels, regardless of dietary intake.

Celiac Disease: This autoimmune disorder damages the lining of the small intestine, impairing the absorption of many nutrients, including fat-soluble vitamins like D2. Symptoms can include digestive issues, fatigue, and bone pain.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease and ulcerative colitis cause chronic inflammation of the digestive tract. This inflammation can reduce the efficiency of vitamin D absorption and lead to lower levels.
Cystic Fibrosis: This genetic disorder causes the production of thick, sticky mucus that can block ducts in the pancreas, preventing digestive enzymes from reaching the intestine. This results in poor fat digestion and malabsorption of vitamin D.
Gastric Bypass Surgery: Procedures that alter or bypass parts of the small intestine, such as gastric bypass surgery, can significantly reduce the surface area available for nutrient absorption, including vitamin D. Patients often require lifelong supplementation.

Impaired Liver and Kidney Function

Once absorbed, vitamin D2 is not yet in its active form. It requires a two-step conversion process involving both the liver and the kidneys. Dysfunction in either of these organs can disrupt this process and lead to a deficiency.

Liver Disease: The liver is responsible for the first conversion step, turning vitamin D into 25-hydroxyvitamin D [25(OH)D]. In chronic liver conditions like cirrhosis, this conversion may be defective, leading to low levels of the active form.
Kidney Disease: The kidneys perform the second conversion step, activating 25(OH)D into 1,25-dihydroxyvitamin D [1,25(OH)2D], the form the body can use. Chronic kidney disease can impair this function, severely affecting vitamin D levels and bone health.

Medications and Genetic Factors

Certain medications can increase the body's breakdown of vitamin D, while rare genetic conditions can cause resistance to the vitamin's effects.

Specific Medications: Anticonvulsants (like phenytoin and phenobarbital), glucocorticoids (steroids like prednisone), and some cholesterol-lowering drugs can accelerate the metabolism of vitamin D, causing a drop in serum levels.
End-Organ Resistance: In rare cases, inherited disorders can cause a resistance to vitamin D at the cellular level, preventing the body from utilizing it effectively even if intake and metabolism are normal.

Comparison of Vitamin D2 and Vitamin D3 Factors


Factor	Vitamin D2 (Ergocalciferol)	Vitamin D3 (Cholecalciferol)
Primary Source	Plant-based, primarily from UV-exposed mushrooms and fortified foods.	Sun exposure on skin, fatty fish, egg yolks, animal products, and supplements.
Absorption	Absorbed well in the small intestine, especially with fat, similar to D3.	Absorbed well in the small intestine, often considered slightly more effective at raising levels long-term.
Metabolism	Metabolized in the liver and kidneys, but some studies suggest it may be cleared faster than D3.	Also metabolized in the liver and kidneys. Some evidence indicates it may be more potent at sustaining blood levels.
Dietary Challenge	Limited natural sources pose a risk for vegans and those avoiding fortified products.	Less of a concern for omnivores, but still requires adequate intake and sunlight.

Conclusion

Low vitamin D2 levels are not a single-cause issue but a multi-faceted problem often involving a combination of dietary choices, compromised digestive function, impaired organ health, and pharmaceutical interference. While dietary changes, like incorporating more mushrooms or fortified foods, are a first step, those with underlying health issues or taking specific medications should consult a healthcare professional. For many, a supplement may be the most effective route to correcting a deficiency, though dosage should always be medically advised. Addressing a low vitamin D2 level is essential for maintaining proper bone density and supporting overall systemic health.

For more detailed information on vitamin D metabolism and absorption, consult the NIH Office of Dietary Supplements fact sheet on vitamin D.

Frequently Asked Questions

The most common causes include insufficient dietary intake, malabsorption issues from conditions like celiac or Crohn's disease, impaired liver or kidney function, and the use of certain medications that accelerate vitamin D breakdown.

A vegan diet can lead to low vitamin D2 because few natural food sources contain it. Unless a person actively consumes UV-exposed mushrooms or relies on fortified plant-based milks and cereals, their dietary intake may be inadequate.

Conditions such as celiac disease, inflammatory bowel disease (Crohn's, ulcerative colitis), and cystic fibrosis can all cause malabsorption of fat-soluble vitamins, including D2.

Yes, chronic liver disease can cause low vitamin D levels because the liver is responsible for the crucial first step in converting vitamin D into its active, usable form.

Yes, medications such as anticonvulsants (phenytoin, phenobarbital), glucocorticoids, and some cholesterol-lowering drugs can increase the body's metabolism of vitamin D, potentially leading to a deficiency.

Many people with low vitamin D are asymptomatic, but severe deficiency can cause symptoms like bone pain, muscle weakness, and fatigue. A blood test ordered by a healthcare provider is the only way to confirm a deficiency.

The primary difference lies in their source: D2 (ergocalciferol) comes from plant sources like mushrooms and fortified foods, while D3 (cholecalciferol) comes from sun exposure and animal products. D3 is sometimes considered more effective at raising and sustaining blood levels.