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How is the Vitamin D Deficiency Threshold Determined?

Vitamin D Deficiency: 20 ng/mL?

There isn't a definitive standard for the optimal concentration yet. So why is a level below 20 ng/mL (50 nmol/L) considered vitamin D deficiency? The reason lies in the consistent decrease of parathyroid hormone (PTH) levels in the blood, necessary for bone health, ranging from 30 to 40 ng/mL (75 to 100 nmol/L). Additionally, calcium absorption levels increase up to 65% within the range of 20 to 32 ng/mL (50 to 80 nmol/L). The optimal level of vitamin D for bone health is determined to maintain minimal parathyroid hormone while maximizing calcium absorption.

vitamin D deficiency threshhold is generally 20 ng/mL

Although the threshold varies in different studies, most research suggests a range between 20 ng/mL and 30 ng/mL. Studies observing calcium absorption rates indicate that 25(OH)D concentrations between 20 to 30 ng/mL optimize absorption (1)

What are the guidelines for clinical testing?

So, how should experiments on the effectiveness of Vitamin D be designed based on the common threshold of 20 ng/mL? While conventional Randomized Control Trials (RCTs) can be conducted for testing, RCTs are generally used to evaluate the effects of drugs (substances that may be absent or present in trace amounts in the body), posing challenges in assessing the effects of nutrients. In that case, how should experiments be conducted?

check the clinical testing guideline for nutrition

The following is an excerpt from Dr. Robert P. Heaney, MD's (1927-2016) guidelines published in Nutrition Reviews in January 2014, titled "Guidelines for optimizing design and analysis of clinical studies of nutrient effects," focusing on research related to musculoskeletal and bone health. (2)

In this guideline, it explains the sequence to follow when conducting research based on nutrients. Please note that additional explanations have been added for better understanding.

When conducting individual clinical studies to understand the effects of nutrients, certain rules are essential to ensure accurate and meaningful results:

Baseline Nutrient Assessment

- It is crucial to measure the baseline nutrient status of participants.

- Basal nutrient status should be used as an inclusion criterion for study participation.

- The initial nutrient status must be recorded in the trial report.

Significant Nutrient Intervention

- Interventions, such as changes in nutrient exposure or intake, must be substantial enough to influence nutrient status significantly.

- These interventions should be quantified through appropriate analyses to gauge their impact accurately.

Monitoring Nutrient Changes

- Changes in nutrient status among participants in the study must be measured diligently.

- These changes should be meticulously documented in the trial report to provide a comprehensive overview.

Focused Hypothesis

- The hypothesis under investigation should focus on changes in nutrient status, not merely alterations in diet.

- The aim is to determine whether a shift in nutrient status produces the desired effect, making the hypothesis precise and targeted.

Optimization of Co-nutrient Status

- Co-nutrient status must be optimized to ensure that the tested nutrient is the sole nutrition-related limiting factor influencing the response.

- This optimization is crucial for isolating the effects of the specific nutrient under examination.

By adhering to these principles, researchers can conduct thorough and reliable clinical studies, laying the foundation for a better understanding of the impact of nutrients on human health.

Example for the test design

For instance, when designing experiments related to this, it would be more meaningful to compare the difference before and after Vitamin D consumption between the group with Vitamin D deficiency and the normal group, rather than conducting experiments on the condition of patients after high-dose Vitamin D intake and a placebo in regular patients. Additionally, it is valid to measure the actual difference in serum concentration between the two groups rather than focusing on the dosage.

right comparison test will lead to right and effective research result

The reason behind this approach is that dosage can be influenced by various factors such as individual physiology, existing health conditions, and dietary habits, leading to varying absorption rates among individuals. This variability in absorption rates could potentially impact the statistical outcomes.


In summary, the article addresses the lack of a definitive standard for optimal vitamin D concentration and explores why levels below 20 ng/mL are considered deficient. It emphasizes the importance of precise experimental designs in studying nutrient effects, particularly focusing on musculoskeletal and bone health. The guidelines outlined by Dr. Robert P. Heaney stress the significance of baseline nutrient assessment, substantial nutrient interventions, diligent monitoring of nutrient changes, focused hypotheses, and optimization of co-nutrient status. The article advocates for comparing nutrient status before and after consumption, rather than focusing solely on dosage, to account for individual variations in absorption rates. By adhering to these principles, researchers can conduct meaningful and reliable clinical studies, contributing to a better understanding of the impact of nutrients on human health.


1. Rosen, C. J. (2011). Vitamin D Insufficiency. New England Journal of Medicine, 364(3), 248–254.

2. Heaney RP. Guidelines for optimizing design and analysis of clinical studies of nutrient effects. Nutr Rev. 2014 Jan;72(1):48-54.

This content is intended for educational purposes only and is not a substitute for professional medical advice. If you have any questions or concerns about your health, please consult with a healthcare professional.

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