The folly of an RDA for vitamin D

Tom is a 50-year old, 198-lb white male. At the start, his 25-hydroxy vitamin D level was 28.8 ng/ml in July. Tom supplements vitamin D, 2000 units per day, in gelcap form. Six months later in January (winter), Tom's 25-hydroxy vitamin D level: 67.4 ng/ml.

Jerry is another 50-year old white male with similar build and weight. Jerry's starting summer 25-hydroxy vitamin D level: 26.4 ng/ml. Jerry takes 12,000 units vitamin D per day, also in gelcap form. In winter, six months later, Jerry's 25-hydroxy vitamin D level: 63.2 ng/ml.

Two men, similar builds, similar body weight, both Caucasian, similar starting levels of 25-hydroxy vitamin D. Yet they have markedly different needs for vitamin D dose to achieve a similar level of 25-hydroxy vitamin D. Why?

It's unlikely to be due to variation in vitamin D supplement preparations, since I monitor vitamin D levels at least every 6 months and, even with changes in preparations, dose needs remain fairly constant.

The differences in this situation are likely genetically-determined. To my knowledge, however, the precise means by which genetic variation accounts for it has not been worked out.

This highlights the folly of specifying a one-size-fits-all Recommended Daily Allowance (RDA) for vitamin D. The variation in need can be incredible. While needs are partly determined by body size and proportion body fat (the bigger you are, the more you need), I've also seen 105 lb women require 14,000 units and 320-lb men require 1000 units to achieve the same level of 25-hydroxy vitamin D.

An RDA for everyone? Ridiculous. Vitamin D is an individual issue that must be addressed on a person-by-person basis.

Comments (26) -

  • terrence

    1/9/2011 1:11:23 AM |

    Is the folly of an RDA for vitamin D at least partly because it is a hormone?

    I am also sure that different people absorb vitamins and hormones differently; and a person probably absorbs them  differently at different times, as well. Another reason an RDA won't work very well, if at all.

  • Stephen

    1/9/2011 1:32:15 AM |

    Any undiagnosed kidney problems/disease?

    I've read that something like 25% of people have undiagnosed kidney disease and that this can impact the conversion rate of D to the active form.

  • Martin Levac

    1/9/2011 3:10:14 AM |

    I posit that the main factor that determines the resulting 25-hydroxy vitamin D level is dietary fat intake.

  • Anonymous

    1/9/2011 3:19:20 AM |

    How do I find out how much I need?

  • Anonymous

    1/9/2011 3:33:25 AM |

    After 2 years of every 6 months blood tests, I've settled on 10,000iu/day.

    Found great price for 10,000iu gelcaps here:

    Caution...they tried to improperly charge me sales tax.

  • Andrew

    1/9/2011 3:57:16 AM |

    there are heaps of vitamin D isonomers, you don't know what proportions of the different isonomers and toxisterols are in the supplements and also they are quite fragile and really need a preservative like sodium sulfite which has become unfashionable

    this problem of what vitamin D actually is has zero research done on it, let alone what is in supplements!

    it's at least 20 years away before a reasonably informed RDA can be given

  • Might-o'chondri-AL

    1/9/2011 4:37:53 AM |

    Genetics does have it's opportunity to alter things. The unactivated D's  binding receptor has to co-function with the retinoid-X-receptors and get to the D response element of our mutable genes to start transcription of activated D.

    There are 8 D pathways and several known vitamin D receptor gene variations. The receptor variants show pronounced association with different population lineages. The level of circulating (measureable) activated D is affected - and then too the 1/2 life of active D is not a long cycle even in ideal metabolism.

    In the kidney making active D, the 1,25(OH)2D3 type, needs the enzyme "CYP27B1" to respond to the parathyroid hormone.

    Curiously the same enzyme in our macrophages induces synthesis of active D there (outside our kidneys). Certain noxious bacteria (not the parathyroid) in our system trigger Toll-like receptors that start this cascade. This too is a geneticly varied immune function.

  • Paul

    1/9/2011 5:46:37 AM |

    The following is a comment on the Vitamin D Council's website where Dr. John Cannell discusses cofactors required for proper vitamin D metabolism:

    "Vitamin D has co-factors that the body needs in order to utilize vitamin D properly. They are:

    •vitamin K2
    •a tiny amount of vitamin A

    Magnesium is the most important of these co-factors. In fact, it is common for rising vitamin D levels to exacerbate an underlying magnesium deficiency. If one is having problems supplementing with vitamin D, a magnesium deficiency could be the reason why."

  • ben

    1/9/2011 5:58:29 AM |

    My Vitamin D3, 25-OH, levels were 29 ng/ml so I took 4000 IU a day for a year and it rose to 39 ng/ml  (+10ng/ml change).

    I'm shooting for 50ng/ml, so I'll do another year at 4000iu, at which point I think 2000iu/day will maintain that level.

  • Tony

    1/9/2011 11:18:10 AM |

    Don't forget differences in nutrition.  Do they eat the same food? How much grains, fish, meat, and so on?

    Paul mentions Vitamin K2 as a cofactor. These Vitamins don't work in isolation, but are part of the grand metabolism show.

  • Marc

    1/9/2011 3:26:49 PM |

    Dr. Davis, what do most "conventional" Md's deam "toxic levels of vit d"?

    Reason I ask, my girflriend has been supplementing with vit d. 2-4000 iu per day with some breaks here and there for the last 6 months.
    Recent blood test she was told by her MD that vit d levels are way to high. We have been waiting for a week to have them give us the reading....hopefully they will call next week.

    My levels are 63 ngl and I'm curious if many Md's would find that level too high.
    Your feedback is very much appreciated and thank you for al you do.


  • qualia

    1/9/2011 5:05:36 PM |

    @Martin Levac: vitamin D3 is not dependent on fat as a tranporter (D2 is however).

    another reason for bad absorption could be undiagnosed (or silent) celiac. the absorpion of D mainly happens on the tips of the villi, and if they're damanged, absorption is massively disturbed.

    another reason could be chronic infection, which can use up a lot of D in the concerned tissues/cells for fighting it, or increased degradation of 25-OH-D in the liver.

    another question would be if one man's 25ng, is the same as another man's 25ng. could be that inividual levels in the blood are actually not really compareable at all due to genetics, and that the second man's 25ng is actually more like 100ng for him, and therefore the liver desperately tries to bring down the level.

  • Ken

    1/9/2011 5:30:03 PM |

    What is the name of the test for 25-hydroxy vitamin D level? I found a lab that will do this test: Vitamin D 25 Hyrdroxy LC-MS  (Vitamin D 25-Hydroxy, D2 + D3 ).

  • Kevin

    1/9/2011 6:17:54 PM |

    Your sample size is small to warrant this conclusion, "Vitamin D is an individual issue that must be addressed on a person-by-person basis," no?

  • Chris Masterjohn

    1/9/2011 6:44:56 PM |

    Dr. Davis,

    Whether or not you agree with the specific value of the RDA, interindividual variation in requirement does not in any way invalidate the concept of the RDA, because the RDA is not meant to be a one-size-fits-all recommendation.

    On the contrary, the RDA incorporates the concept of a distribution in requirements, and attempts to cover the needs of most people.  The IOM is pretty explicit about that.  

    You could certainly argue that the current RDA is not sufficient to meet the needs of most people, but that's another issue.


  • Daniel A. Clinton, RN, BSN

    1/9/2011 11:58:25 PM |

    Great post. I completely agree. Vitamin D is simply too complicated to be addressed in just one number. Most everyone's level of understanding is so superficial that it doesn't extend beyond that one number. When a number like that is created, it takes on more power than it should because most who access it don't know its faults and limitations.
    I know someone who concluded that his bizarre behavior after drinking two bottles of apple juice was from "Too much Vitamin C." He had drank two bottles, each bottle had 2 servings, and each serving had 100% the daily value of Vitamin C. Beyond the giant sugar bolus, in his mind, he had just taken 4x the recommended amount of Vitamin C. Based on his level of understanding, he concluded he may have ingested a toxic amount of Vitamin C. He needed a scapegoat and he found one.
    More damage is done by attempting to dumb down Vitamin D to one number than any benefit an RDA creates. It's simply too complex.  Frankly, it's pretty pathethic that Vitamin D deficiency remains rampant. It says an awful lot about our healthcare system that such a huge percent of the population remains Vitamin D deficient while taking far sketchier prescription drugs for the host of conditions associated with Vitamin D deficiency.

  • Peter

    1/10/2011 1:51:14 AM |

    For people with money, insurance, and education, jit makes sense to look at each person's individual needs.  For everybody else, RDA sounds to me like a reasonable idea.

  • Davide Palmer

    1/10/2011 3:59:10 AM |

    Dr. Davis,

    Would blood calcium levels be an accurate indicator of sufficient Vitamin D intake?

  • reikime

    1/10/2011 4:59:53 AM |


    We were thinking along the same lines... undiagnosed malabsorptive disorders could be responsible for alot of low levels in spite of supplementation.

    Celiac alone affects approx. 1-133! throw in fructose malabsorbtion, UC etc. and no wonder we have an epidemic of low D levels.

  • Martin Levac

    1/10/2011 8:19:08 AM |


    Do you mean to say that vitamin D3 is not fat soluble? Everywhere else it says D3 is fat soluble. Do you know something the rest of the world doesn't?

  • Travis Culp

    1/10/2011 8:43:34 PM |

    I wonder if I may be overdoing it with 5000IU for about half the year (Oregon). I eat natto and 3 cups of steamed spinach per day, so I should be ingesting all of the cofactors in substantial amounts.
    I wonder if it would be more efficient to go to an endocrinologist in order to get this and a proper lipid panel done.

  • Dr. William Davis

    1/10/2011 11:02:15 PM |

    If the IOM has achieved any good at all, it is to further stoke constructive discussion around vitamin D.

    I am quite impressed with the level of comments here. Compare that to the conversations we were having just 2 or 3 years ago. We've come a long way.

    Vitamin D remains on my list of "most incredible health effects ever seen."

  • Carlos

    1/13/2011 5:23:07 AM |

    A belated thank you for all your articles on Vitamin D. I read about the importance of taking Vitamin D in many books but was never willing to go through the hassle of getting my levels checked.

    Well, reading your blog several months convinced me to give it a shot. Turns out that it took 10,000 IU a day just to get me to 54 ng/ml. I am now on 14,000 IU a day to see if I can get into the 60 to 80 ng/ml range.

    I have thus far managed to get through this winter without contracting a cold when half the people I work with are taking turns being out sick with one they've spread amongst themselves. By adding D and CLO (and making dietary changes), my total cholesterol dropped from 215 to 169, my HDL went up from 44 to 50, and my VLDL dropped from 24 to 9. My triglycerides dropped from 143 to 53. Given that I'm only halfway through my weight loss I expect greater improvements yet. Thanks again.

  • liposculpture guide

    1/13/2011 11:11:46 AM |

    This is great, brilliant and knowledgeable post. I agree with your conclusions and will eagerly look forward to your next updates. Just saying thanks will not just be enough, for the wonderful clarity in your writing.

  • Anonymous

    1/21/2011 5:23:28 PM |

    Here is "The Peoples Chemist" Vitamin D link:

  • George

    2/7/2011 9:29:30 PM |

    Dr. Davis, I have had great lipid results, overall health benefits with going to a lower carb, paleo diet as well as supplementing with vitamin D getting to the the mid 50's mg/dl range from the low 30's on 8000 mg vitamin D for the last 2 years. Not sure if related, but one downside has been the 3 occurences of kidney stones in last 18 months. It seems the current recommendations for the stones implicates higher protein diets and increased vitamin D. Have you run into this with any of your patients, what is available to address this?