Why is vitamin D not getting absorbed? Understanding the root causes

December 12, 2025 •

2 min read

According to the National Institutes of Health, approximately 41% of U.S. adults have a vitamin D deficiency. While many people focus on sunlight and diet, a common and often overlooked issue is the body's inability to properly absorb this crucial fat-soluble nutrient, a problem known as malabsorption.

Quick Summary

This article investigates the complex medical, genetic, and lifestyle reasons behind poor vitamin D uptake, including gastrointestinal disorders, obesity, liver and kidney disease, medication interference, and genetic influences, to help readers understand potential root causes.

Key Points

Gut Health is Key: Many gastrointestinal disorders, including Celiac and Crohn's disease, damage the intestinal lining and prevent proper vitamin D absorption.
Obesity's Effect: Excess body fat can trap vitamin D, reducing its availability in the bloodstream despite adequate intake or synthesis.
Organ Function is Critical: The liver and kidneys are essential for converting inactive vitamin D into its usable, active form. Dysfunction in these organs can lead to deficiency.
Medication Impact: Certain drugs, such as steroids and some cholesterol or seizure medications, can interfere with vitamin D metabolism or absorption.
Genetic Predisposition: Inherited genetic variations can influence how efficiently your body synthesizes, transports, and metabolizes vitamin D.
Take Vitamin D with Fat: For better absorption, consume vitamin D supplements with a meal containing healthy fats like nuts, seeds, or avocado.
Adequate Magnesium: Your body requires magnesium to convert vitamin D into its active form, so ensuring sufficient magnesium intake is important.

The Intestinal Link: When Your Gut Can’t Absorb

Vitamin D is a fat-soluble vitamin, requiring adequate fat absorption for proper uptake in the small intestine. Various gastrointestinal disorders and procedures can impair this process.

Malabsorption syndromes that hinder vitamin D absorption:

Celiac Disease: Gluten damages the small intestinal lining, reducing nutrient absorption.
Crohn’s Disease and Ulcerative Colitis: Inflammatory bowel diseases that decrease the gut's absorptive surface area.
Cystic Fibrosis: Blocks pancreatic enzymes needed for fat digestion.
Chronic Pancreatic Insufficiency: Impairs the pancreas's ability to produce digestive enzymes.
Gastric Bypass Surgery: Reduces the small intestine's absorptive area.

Body Composition: The Role of Fat Cells

Individuals with higher body mass index may have lower circulating vitamin D. This is because excess body fat can sequester vitamin D, reducing its bioavailability. Higher doses may be needed for obese individuals.

Organ Function: The Liver and Kidneys

Vitamin D requires conversion in the liver and kidneys to become active.

Liver Disease: Impairs conversion to 25-hydroxyvitamin D (calcidiol).
Kidney Disease: Impairs conversion to the active form, 1,25-dihydroxyvitamin D (calcitriol).

Medications That Interfere with Vitamin D

Some medications accelerate vitamin D breakdown or hinder absorption.

Medications known to impact vitamin D levels:

Steroids: Increase breakdown and interfere with calcium absorption.
Anticonvulsants: Accelerate vitamin D metabolism.
Cholesterol-Lowering Drugs: Can bind to fat and fat-soluble vitamins, preventing absorption.

Genetic Variations: The Missing Piece of the Puzzle

Genetic polymorphisms can influence vitamin D synthesis, transport, and metabolism.

Optimizing Your Absorption: Practical Strategies

Take it with fat: Improves absorption of fat-soluble vitamin D.
Consider your timing: Taking with the largest meal may optimize absorption.
Include Magnesium: Crucial cofactor for vitamin D metabolism.
Treat Underlying Conditions: Managing GI or organ issues is key.

Comparison Table: Malabsorption Syndromes and Their Impact


Condition	Mechanism of Malabsorption	Effect on Vitamin D Absorption	Required Intervention
Celiac Disease	Damage to intestinal lining by gluten	Significantly reduced	Strict gluten-free diet to heal intestinal villi
Cystic Fibrosis	Pancreatic enzyme insufficiency	Severely limited due to poor fat digestion	Pancreatic enzyme replacement therapy
Gastric Bypass	Bypassing a portion of the small intestine	Reduced absorptive surface area	Lifelong vitamin D supplementation, often high dose
Crohn's Disease	Chronic inflammation of the digestive tract	Reduced due to damaged and inflamed intestinal lining	Medical management of inflammation and potential high-dose supplementation

Conclusion: A Multifaceted Problem

Poor vitamin D absorption has multiple causes, including medical conditions, medications, genetics, and lifestyle. Addressing it requires a comprehensive approach, potentially involving medical testing and managing underlying issues. Consult a healthcare provider if you suspect poor absorption. For more information on vitamin D, consult the NIH Office of Dietary Supplements.

Frequently Asked Questions

Poor vitamin D absorption can be caused by conditions affecting the small intestine and pancreas, such as Celiac disease, Crohn's disease, cystic fibrosis, and chronic pancreatic insufficiency. Additionally, gastric bypass surgery can reduce the absorptive area.

Obesity does not prevent synthesis, but excess body fat can sequester vitamin D, meaning it is trapped in the fat tissue and is not released into the bloodstream for use. This leads to lower circulating levels, and obese individuals may require higher doses to reach sufficiency.

Yes, chronic liver and kidney diseases can severely impact vitamin D levels. The liver performs the first step of converting vitamin D to calcidiol, and the kidneys perform the second conversion to the active form, calcitriol. Impaired function in either organ disrupts this process.

Several medications can affect vitamin D absorption or metabolism. These include steroids (prednisone), certain seizure-preventing drugs (phenobarbital), and cholesterol-lowering medications like bile acid sequestrants and lipase inhibitors.

Symptoms of vitamin D deficiency can be vague and appear gradually. They include fatigue, bone pain, muscle weakness, mood changes like depression, and a greater susceptibility to illness.

Yes, genetic factors can significantly influence how your body handles vitamin D. Polymorphisms in genes that regulate synthesis, transport, and metabolism can affect your circulating vitamin D concentrations and response to supplementation.

To improve absorption from supplements, take them with a meal containing healthy fats. Consider pairing with foods like avocado, nuts, or fatty fish. Ensuring adequate magnesium intake is also beneficial, as it is a cofactor for vitamin D metabolism.