Both a vitamin D deficiency (such as one caused by a disruption of the gut microbiome) and an oversupply that affects intestinal motility have been linked to constipation. A deficiency is present when levels fall below 30 nmol/L, while an oversupply begins at values above 125 nmol/L.

In this article, we explain how vitamin D may be associated with digestive problems and what optimal vitamin D levels should look like. In Germany, an estimated 33–52% of adults and 40–80% of children have an insufficient vitamin D level, with values varying depending on sunlight exposure and the season (Bundesinstitut für Risikobewertung, 2024). The risk of deficiency increases particularly during the autumn and winter months, when there is less sunlight. Studies also show that people in Germany spend over 90% of their time indoors (Zeit, 2023).

Table of Contents

• What is vitamin D and why is it important?
• How does a vitamin D deficiency affect digestion?
• How does a vitamin D oversupply affect digestion?
• What other symptoms occur with vitamin D deficiency or excess?
• What causes vitamin D deficiency or oversupply?
• Prevention & tips for everyday life
• Summary

What is vitamin D and why is it important?

Vitamin D is a fat-soluble vitamin that exists in two main forms: vitamin D2 (ergocalciferol) and vitamin D3 (cholecalciferol). It is unique in that our bodies can produce it naturally through the skin when exposed to sunlight. In small amounts, it is also found in foods such as fatty fish, eggs, and fortified products. Vitamin D plays a critical role in several bodily functions.

Important functions of vitamin D include:

• Regulation of calcium and phosphate metabolism for healthy bones and teeth
• Supporting the immune system in fighting infections
• Promoting muscle function
• Supporting cell division and renewal
• Maintaining cardiovascular health

Sources: Mayo Clinic, 2023; NIH, 2024

Vitamin D levels and how to interpret them

Vitamin D is measured using two units: ng/ml (nanograms per milliliter) and nmol/l (nanomoles per liter). These units are based on different measurement systems used in various countries. The table below provides general guidelines for vitamin D levels. However, individual needs may vary.

Recommendations:

• In the case of low vitamin D levels, it is recommended to increase sun exposure, take supplements if advised by a doctor, and adjust the diet to include vitamin D-rich foods such as fatty fish and eggs.
• In the case of high vitamin D levels, supplement intake should be reduced or discontinued, and both calcium and vitamin D levels should be monitored by a physician.

In the case of abnormal levels, regular testing may be helpful to ensure optimal supply.

How does a vitamin D deficiency affect digestion?

A vitamin D deficiency can influence digestion via the microbiome. The microbiome consists of trillions of microorganisms in the intestine and is central to digestion and the immune system.

Studies have shown that vitamin D influences the composition of the microbiome. High vitamin D levels promote beneficial bacterial species such as Akkermansia and Bifidobacterium, which are important for healthy gut function, and reduce harmful bacteria. Vitamin D also appears to strengthen the intestinal barrier, which protects the body from harmful bacteria. A deficiency can weaken this barrier, increasing the risk of inflammation and contributing to digestive issues like constipation. Microbiome changes caused by a vitamin D deficiency can slow intestinal motility and impair nutrient absorption.

Sources: Cleveland Clinic, 2024; Yamamoto & Jørgensen, 2020; Luthold et al., 2017; Singh et al., 2020

How does a vitamin D oversupply affect digestion?

Excess almost exclusively arises from excessive intake of supplements.

Elevated vitamin D levels can lead to hypercalcemia, as too much vitamin D increases calcium absorption in the intestines, resulting in elevated calcium levels in the blood. This can impair intestinal motility by disrupting nerve and muscle activity in the intestines, which may lead to constipation. Hypercalcemia can also interfere with water absorption in the intestines, making bowel movements more difficult and increasing the likelihood of constipation (Mayo Clinic, 2022).

What other symptoms occur with a vitamin D deficiency or excess?

In addition to changes in digestion, there are other symptoms associated with a vitamin D deficiency or excess:

Vitamin D deficiency:

• Fatigue, exhaustion
• Depressive mood
• Muscle weakness or pain
• Weakened immune system, frequent infections

Vitamin D excess:

• Nausea, vomiting
• Constipation or diarrhea
• Loss of appetite and weight loss
• Excessive thirst and frequent urination
• Confusion and disorientation

Source: Cleveland Clinic, 2022

What causes vitamin D deficiency or oversupply?

Both a deficiency in vitamin D and an oversupply (hypervitaminosis D) can lead to health issues. The following risk factors are key to the development of vitamin D deficiency or hypervitaminosis D.

Sources: NIH, 2024; Cleveland Clinic, 2023

Prevention & tips for everyday life

• Sun: 10–20 minutes of daily sun exposure on face and arms may be sufficient in summer months.
• Diet: Regularly include vitamin D-rich foods (salmon, herring, eggs, fortified products).
• Monitoring: If uncertain or taking supplements, check levels regularly.

With Homed-IQ’s vitamin D test, you can conveniently measure your blood vitamin D level at home and determine whether you may be suffering from a deficiency. In addition to vitamin D, the vitamin deficiency test also allows you to test for other important vitamins such as B12 and folic acid (B9). Test results are evaluated in ISO-certified laboratories that are also used by medical practices and clinics.

Summary

Vitamin D plays a crucial role in the health of bones, muscles, and the immune system, as well as in maintaining a healthy digestive function. Both a deficiency and an oversupply—though the latter is rare—can be associated with constipation. Understanding risk factors and conducting regular tests can help prevent such issues.

References

Cleveland Clinic. (2024, April 16). Dysbiosis: What it is, symptoms, causes, treatment & diet. Retrieved September 10, 2024, from https://my.clevelandclinic.org/health/diseases/dysbiosis#symptoms-and-causes

Luthold, R. V. (2017). Gut microbiota interactions with the immunomodulatory role of vitamin D in normal individuals. Metabolism, 69, 76–86. https://doi.org/10.1016/j.metabol.2017.01.011

Frontiers. (2019). Relationships between vitamin D, gut microbiome, and systemic autoimmunity. Frontiers in Immunology. Retrieved September 10, 2024, from https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2019.03141/full

Singh, P. (2020). The potential role of vitamin D supplementation as a gut microbiota modifier in healthy individuals. Scientific Reports, 10, 21641. https://doi.org/10.1038/s41598-020-77806-4

Mayo Clinic. (n.d.). Vitamin D. Retrieved September 10, 2024, from https://www.mayoclinic.org/drugs-supplements-vitamin-d/art-20363792

Cleveland Clinic. (n.d.). Vitamin D deficiency: Causes, symptoms & treatment. Retrieved September 10, 2024, from https://my.clevelandclinic.org/health/diseases/15050-vitamin-d-vitamin-d-deficiency

NIH Office of Dietary Supplements. (2024, July 26). Vitamin D – Health professional fact sheet. Retrieved September 10, 2024, from https://ods.od.nih.gov/factsheets/VitaminD-HealthProfessional/

Mayo Clinic. (n.d.). Vitamin D toxicity: What if you get too much? Retrieved September 10, 2024, from https://www.mayoclinic.org/healthy-lifestyle/nutrition-and-healthy-eating/expert-answers/vitamin-d-toxicity/faq-20058108

Cleveland Clinic. (2023, February 21). Vitamin D toxicity: What it is, causes, symptoms & treatment. Retrieved September 10, 2024, from https://my.clevelandclinic.org/health/diseases/24750-vitamin-d-toxicity-hypervitaminosis-d