Vitamin D: How Supplements Affect Serum 25(OH)D Levels

Sunlight helps make vitamin D.
Some sun on unprotected skin, can be a good thing.
A few years ago a dietician friend sent me an interview with Dr. Michael Holick about vitamin D deficiency. I was stunned by the long reach of this vitamin on human health and physiology. In the past 5–7 years interest in vitamin D deficiency and reports on its connection to various chronic conditions in humans, has skyrocketed.

Generally any health trends with skyrocketing interests, especially those that involve taking supplements, fall off my radar instantly.

In the case of vitamin D, however, I’m a believer but want more information. So when the C. F. Garland et al. report in Anticancer Research (1), “Vitamin D Supplement Doses and Serum 25-Hydroxyvitamin D in the Range Associated with Cancer Prevention” appeared, I thought it worth a read.

First the back story then some study results:

GrassrootsHealth (GRH) is a nonprofit, community service organization “dedicated to promoting public awareness about vitamin D”. GRH has developed a database of information on supplemental vitamin D intake by a self-selected population, and links the supplemental intake information to serum levels of 25(OH)D. Many of the individuals participating in the database have vitamin D intakes in and above the ranges used today for cancer prevention and therapy, ranges reported by Garland et al. (2,3).

Always a skeptic, my review of the GRH website found it to contain links to research by and information about many vitamin D heavyweights, Bruce Hollis, Michael Holick, Cedric Garland and others. GRH supports and publishes on their web site current vitamin D research, as well as video and podcast links to talks by vitamin D experts, on cancer and many other deficiency linked topics.

For this study volunteers were solicited at a seminar hosted by GrassrootsHealth as well as by extensive internet recruitment. A total of 3,667 volunteers, mean age 51 years (+/- 13.4 years) were recruited. According to GRH, the volunteers included both genders and a wide variety of nationalities and levels of health.

Participants were provided with a blood spot test kit for 25(OH)D (alpha-1,25-hydroxyvitamin D). The kit was manufactured by ZRT Laboratories, Beaverton, OR, USA). The user placed a drop of blood onto the test, then returned the test kit for analysis of their blood 25(OH)D level. Tests were repeated every 6 months.

The same volunteers were asked to participate in an on-line health questionnaire every 6 months over a 5-year period. The goal of the questionnaire was to determine health outcomes associated with various serum 25(OH)D levels.

Once participants logged in to find their serum 25(OH)D levels, they were allowed to choose their course of action, to take supplemental vitamin D, to not take supplemental vitamin D or to increase or decrease the amount of vitamin D taken. With their test results, participants received information about normal reference ranges, potential toxicity levels and suggested serum 25(OH)D concentrations of 40-60 ng/ml (nanograms per milliliter of blood).

The most pertinent information from this study is that: 1) 10,000 IU/day of vitamin D can be taken without reaching toxic serum levels, 200ng/ml; 2) 97.5% of the general population would need to take 9,600 IU/day of vitamin D to achieve serum levels of 40ng/ml, which is the low end of the range believed to be cancer protective; 3) a rule of thumb is that per 1,000 IU/day, serum levels of 25(OH)D rise 10ng/ml when starting at 10ng/ml. If the starting serum level is 30ng/ml, 1,000IU/day push levels up just 8ng/ml and if starting serum levels are 50ng/ml, 1,000 IU/day pushes levels up just 5ng/ml.

A picture is worth a thousand words and GrassrootsHealth has a great diagram of 25(OH)D serum level plotted against vitamin D intake by the participants of this study, on their homepage.

Also I highly recommend taking a look at their disease incidence prevention chart by Dr. Cedric Garland, which provides information on what serum 25(OH)D levels are linked with prevention of a variety of cancers, Type 1 diabetes, and MS.

I am not a member of this organization, nor have I been a member in the past. I have not participated in the database. But you can…see GrassrootsHealth for how-tos.

But I am happy that it’s March and day length is increasing, meaning we’re moving into the vitamin D manufacturing and storage season. Get outdoors for 10 minutes of unprotected skin-sun exposure around noon as an excellent starting point for supplementing your serum levels of vitamin D.

ResearchBlogging.orgCedric F. Garland, Christine B. French, Leo L. Baggerly, Robert P. Heaney (2011). Vitamin D Supplement Doses and Serum 25-Hydroxyvitamin D in the Range Associated with Cancer Prevention Anticancer Research, 31, 607-612 : 21378345
2. Garland CF et al. (2009). Vitamin D for cancer prevention: global perspective. Ann Epidemiol 19: 468-483.
3. Garland CF et al. (2007). Vitamin D and prevention of breast cancer: Pooled analysis. J Steroid Biochem Molec Biol 103 708-711.

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Kari Kenefick

Kari has been a science writer/editor for Promega since 1996. Prior to that she enjoyed working in veterinary microbiology/immunology, and has an M.S. in Bacteriology, U of WI-Madison. Favorite topics include infectious disease, inflammation, aging, exercise, nutrition and personality traits. When not writing, she enjoys training her dogs in agility and obedience. About the practice of writing, as we say for cell-based assays, "add-mix-measure".


  1. You failed to mention that the human body produces 15,000 to 20,000 iu of vitamin D in 15 minutes exposure (>40% of skin exposed) and the use of sunscreen and skin cancer scares has created a vitmin D deficiency epidemic.

  2. Thanks for your comment. And yes, I did fail to include primary information on vitamin D, including the fact that the human body makes vitamin D when exposed to sunlight. But there is excellent vitamin D information available online, such as this WebMd link:
    with topics like how vitamin D may cut heart disease risk, or how it may lower a baby’s MS risk. It is said that every cell in the human body has a vitamin D receptor…so stay tuned…lots more information coming. -kbrk

  3. The problem is the gap between population level data and the individual (specificity).

    D exposure/uptake/utilization varies by: age, skin color, latitude, season, city dwelling, lifestyle, diet, obesity, genes, medications, diseases, cofactors, etc. Add-in homeostasis, response curves, and substrate starvation…. therefore dose-to-serum level ratios change over time.

    Therefore, the gold standard for the individual would be: test, dose,retest, adjust, repeat.

    1. Well said, James! Thank you for the comment. I know that testing and dosing here in Madison, WI is becoming the standard of care. A dietician contact in the eastern US tells me that the same thing is happening there. Perhaps we’ll see a study of treated persons, broken down by age and skin color in the next few years to show effects of vitamin D supplementation.

  4. Thanks for reading and commenting, Florencia! Now that the long days of sunshine are winding down here in the northern hemisphere, so too are our chances to make vitamin D by sunlight. I need to refill my vitamin D3 supply for winter. Please keep in touch and yes, we need to read/post more on vitamin D. -Kari

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