Published yesterday in the Journal of Medical Case Reports is a new paper from GrassrootsHealth. Titled “25-Hydroxyvitamin D variability within-person due to diurnal rhythm and illness: a case report,” the objective of this study was to determine if vitamin D levels fluctuate throughout the day and whether the timing of the test before or after supplementation mattered.

As a case study, the investigation looked at a single individual, with vitamin D samples collected three times a day, roughly 8 hours apart, on four separate days. At each collection time 5 separate samples were collected and the results averaged.

What did the study discover?

This study found that mid-day vitamin D levels were approximately 20% higher than morning levels and approximately 13% higher than evening levels (see figure below). Taking a vitamin D supplement before or after testing did not make a difference. Interestingly, the study participant experienced a cold during the study and it was discovered that her vitamin D levels were about 25% lower on the day immediately before symptom onset compared to other testing days. By the time cold symptoms had passed, two weeks later, the vitamin D levels had returned to match the previous levels.

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How does this information affect vitamin D testing?

For individuals tracking their vitamin D levels, it would be best to collect your blood sample at the same time of day to have a consistent measurement over time. Clinical trials should also keep this in mind when testing participants to reduce error from this variability. Additionally, this case study demonstrates how an acute illness can result in a lower than usual vitamin D level even before symptoms appear.

About the Authors

Christine French joined GrassrootsHealth in 2009. She has been involved in verifying and analyzing vitamin D related data from D*action questionnaires, as well as from other sources, and preparing publications based on that data. With training in science education and public outreach, French has been instrumental in making GrassrootsHealth data accessible and understandable. She earned her bachelor’s degree in biological sciences from UC Santa Barbara and MS in environmental sciences from UC Riverside.

Sharon McDonnell has been working in the field of public health for thirteen years and has been an epidemiologist and biostatistician at GrassrootsHealth for the past six years. At GrassrootsHealth, she has been involved in developing methodology, data analysis, and manuscript preparation for numerous projects concerning vitamin D and health outcomes. She received a B.S. degree in Psychobiology from UCLA and an MPH in Infection Diseases, specializing in epidemiology and biostatistics, from UC Berkeley.

How can I track my vitamin D levels and health?

To help you track your nutrient levels and health outcomes, GrassrootsHealth has created an online tracking system called myData-myAnswers. You can track details about your supplement use and nutrient levels. You can also track many pre-set health conditions, such as colds, or add your own custom trackers. This will help you discover how your nutrient levels impact your health. Check it out today!

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Important Message

NEWS ALERT: States, Governments Acknowledge the Need for Vitamin D to Build the Immune System; Vitamin D*action has achieved a major milestone of ‘Moving Research into PRACTICE’!

EVERYONE needs to know about the benefits of vitamin D... We need your help to do this. Donating $25.00 today will positively affect millions of lives!

We will take this opportunity to use our extensive network to get pertinent information on vitamin D to the new US COVID Panel as well as local and state government representatives around the world.

Donate $25 today to help support this outreach effort (and, let us know if your employer could match the donations made by individuals from now until January 1, 2021!). You can also "text-to-give" by simply texting Daction to 44321.

Donations made through GiveLively are received in full by GrassrootsHealth.

What Does it Take YOU to Get Your D to 40 ng/ml (100 nmol/L)?

Did you know your health could be greatly affected by making sure you have a vitamin D level of at least 40 ng/ml (100 nmol/L)? Help us help you.

STEP 1 - Do you know what your vitamin D level is? If not, be sure to test today to find out.

STEP 2 – Determine your target level. Are you at your target level? Experts recommend a level of at least 40-60 ng/ml (100-150 nmol/L).

STEP 3 – Need to boost your level? Use the D*calculator to see how much vitamin D it may take to reach your target. Opt for the Loading Dose for a quicker boost.

STEP 4 – Optimize how your body absorbs and utilizes vitamin D with co-nutrients and these simple steps.

STEP 5 – Re-Test! This is an important step to make sure you have reached your target level, and to ensure you are not taking too much! Re-testing after 3-4 months is recommended.

STEP 6 – Adjust, Repeat…

Give your immune system the nutrients it needs to support a healthy you and protect yourself from unnecessary diseases, especially COVID-19.


The first Randomized Controlled Trial on vitamin D and COVID-19 has shown a 96% lower risk of ICU admission for those receiving vitamin D (as 25(OH)D to quickly boost vitamin D blood levels) along with the standard treatment, compared to those receiving standard treatment alone.

These results support many previous observational studies showing a relationship between vitamin D levels and intake and COVID-19 severity.

Review the Latest Nutrient Research for COVID-19

GrassrootsHealth Nutrient Research Institute has launched the new Immune Boost project with the use of our myData-myAnswers nutrient health system that nearly 15,000 people are already using for their health. Specific markers that influence immune health are suggested for testing as part of this project including:

  • Vitamin D
  • Omega-3 Index
  • Essential elements magnesium, selenium, and zinc
  • hsCRP

Our goal is to demonstrate how one can use the Nutrient Research Model established by Dr. Robert Heaney to show the effect of vitamin D serum levels of at least 40 ng/ml (100 nmol/L) on risk reduction for all ethnicities in the population. Status and intake of other nutrients will also be analyzed for any type of relationship to immune status and symptom severity. Join the project today!

Please let us know if you're interested in helping sponsor this project.

CLICK HERE for updates and new information about the project.

Through GrassrootsHealth Nutrient Research Institute, you can also test your essential elements magnesium, copper, zinc and selenium, toxins such as lead, mercury and cadmium, as well as your omega-3 levels, inflammation levels and thyroid stimulating hormone (TSH) level. Find out your levels today! Log on to the test selection page (click the link below) to get your tests and see for yourself if your levels can be improved.

Make sure you track your results before and after, about every 6 months!

Click Here to Access the Test Page

How can I track my nutrient intake and levels over time?

To help you track your supplement use and nutrient levels, GrassrootsHealth has created the Personal Health Nutrient Decision System called

For each specific supplement, you can track what days you take it, how much, and many other details.  This will help you know your true supplemental intake and what patterns of use work for you to reach and maintain optimum nutrient levels. Check it out today!