Hey Catherine,

You bring up a good point on vitamin D3 therapy.  There are a lot of folks that get goosey when they see a suggested vitamin D3 dose of 10,000 IU/day and they worry about toxicity.  That sounds like a lot but in reality, it isn't. 

Vitamin D3 is the only free vitamin (actually it's a prehormone or hormone precursor) and our skin can generate 15,000 IU of vitamin D3 when exposed to the UV-B in direct mid-day sunlight while wearing a bathing suit without sunblock in as little at 10 minutes.

Vitamin D3 is also one of the safest nutrients we can take.  I've looked over a lot of literature on vitamin D3 and have yet to find a confirmed case of death due to vitamin D3 overdose...  You can't say that for verapamil...  Moreover, the health risks of a vitamin D3 deficiency are far greater than any potential risk of vitamin D3 toxicity.

Part of the problem stems from the use of the unit of measure for vitamin D3 and other vitamins, the International Unit (IU).  An IU is a measure of potency based on the biological activity or effect. If you look at the ratio of mass or weight to IU you'll see that 10,000 IU equals 250
micrograms (µg or mcg) of vitamin D3 (cholecalciferol).   

A microgram is a unit of mass equal to one millionth (1/1,000,000) of a gram. The photo below shows just what 10,000 IU (250 µg) of vitamin D3 looks like compared with two 1200 mg liquid softgel Omega 3 Fish Oil capsules.

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Note the size of the two small vitamin D3 liquid softgel capsules, (5,000 IU each).

All this brings us to vitamin D3 toxicity, yours and many others area of concern.  Vitamin D3 by itself isn't toxic...  Its toxicity arises from its effect on calcium homeostasis...  another good word to understand...  Homeostasis is like a biological thermostat or control mechanism.  Our bodies need to maintain the serum level of calcium in a very narrow range to function properly.  The normal range is 9–10.5 mg/dL or 2.2–2.6 mmol/L and our bodies have a number of biological control mechanisms to keep it in this range. 

If the serum concentration of calcium gets too high or too low, we have all kinds of problems...  One of the biggest problems is our nerves fail to function properly so there goes the heart, brain and just about every other major organ in the body...

What happens if we get too much vitamin D3 is it overloads the calcium homeostasis control mechanisms, our serum levels of calcium rise above normal and we become hypercalcemic...    Hypercalcemia is the primary clinical measurement of vitamin D3 toxicity.

Now for the big question...  How much vitamin D3 can we safely take and still not worry about vitamin D3 toxicity?  The important thing to understand is it isn't so much the dose of vitamin D3, but rather how effectively the body metabolizes it into 25(OH)D3 and then into 1,25(OH)2D3, the active metabolite that pushes calcium into the bloodstream.

As 25(OH)D is the most frequently used lab test for vitamin D3 status, it's the best measure to tell if you are getting close to a toxic level.  Studies to date indicate 200 ng/mL, (500 nmol/L) is the lower threshold for vitamin D3 intoxication.

The best and most current answer on how much vitamin D3 you can take on a sustained basis comes from a study conducted by Garland et al, attached below.  They concluded "Universal intake of up to 40,000 IU vitamin D per day is unlikely to result in vitamin D toxicity."

The following graph illustrates this finding. 

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This data comes from an ongoing D*action Project, where people pay for a 25(OH)D lab test every six months after starting vitamin D3 at various doses.

As you can see, sustained doses of vitamin D3 at 10,000 IU/day and 20,000 IU/day are very safe as neither results in a serum concentration of 25(OH)D near the 200 ng/mL toxicity threshold. Even doses as high as 40,000 IU/day appear safe as this study concluded.

I spoke with the folks running the D*action program and over 400 people were dosing at 10,000 IU/day and many of them have been participating for as long as two years with 25(OH)D lab tests every six months.  None of them have come anywhere close to toxic levels of 25(OH)D.

Hope this helps.

Take care,

V/R, Batch