Vitamin D supplements- dosage information
12-02-2010, 12:17 PM
Vitamin D supplements- dosage information
Edited and updated 1-15-2012 (edited for grammar and clarity 7/19/2011, corrected erroneous formula 1-15-2012)
We often see the comment that we should be careful with vitamin D supplements to avoid overdoses. I found a reference in Mader's "Reptile Medicine and Surgery", p. 1069, that gives us a guideline for the dosage. (Please note that Mader is clear about this being a guideline, not the result of careful research!) The target range is probably around 5-10 International Units (IU) per kilogram of tortoise a day. That would be roughly 10-20 IU per pound for those who do not understand that metric is the superior system for this sort of thing . (Note- for the purposes of this article, figure that a kilogram, or 1000 grams is about 2 pounds, and 100 grams is about 3.5 ounces)
The danger zone is about 500 IU/kg a day. Overdoses can have nasty consequences, but to hit the danger zone, you would have to work at it a little. The main symptom of an overdose is calcification of soft tissues- which is hard to see, especially in the early stages.
I cannot figure the dosage you get from the sun because there are far too many variables, but if your tortoise has free access to sunlight strong enough to give you a good tan, then you don't need to worry about this. If you live so far north that it is hard to tan in the winter, you may need to supplement with either UVB light or vitamins. NOTE: You cannot overdose on vitamin D created by the sun or UVB lighting. While there are risks from too-strong UV on animals (skin cancer, eye damage, etc.), vitamin D overdose is not one of them!)
UVB light (from bulbs) is a HUGE question mark right now- what is the best way to offer general UVA for vision and behaviors, and UVB-295nm range for D3 fabrication? Heck if I know. I would suggest that if you are using UVB lighting in your set-up (and you should, for several reasons), then you may STILL want to offer some supplemental vitamin D at the lower dosage end.
Vitamin D is critically important in animals for many reasons- it is necessary to properly metabolize calcium in the cells (no vitamin D, no calcium in the cells) but it also helps with bone density, a strong immune system, fighting cancer, affects nerves and muscle development and function, and helps with growth and regulation. UVB-produced D also helps regulate the sleep and seasonal patterns, reproduction, and so forth. It also affects things as diverse as the pineal gland, which affects many of the bodily systems.
Vitamin D is present in many foods, but be aware that some studies have suggested that dietary D does not work as well as 'solar D'- or maybe not at all. In any case, it probably does not do exactly the same thing as 'solar D' does.
IU's of Vitamin D in 100 grams of food-
- Cod liver oil- 8840 IU (or about 74 IU per drop, but may also be high in mercury)
- Fresh mushrooms- 14 IU
- Fresh mushrooms exposed to UV (i.e. wild-picked)- 500 IU (note that this is vitamin D2, a less potent or effective version of the vitamin. It may only count as about 1/4th to 1/3rd as useful)
- Mazuri Tortoise Diet- 299 IU
- Canned tuna in oil- 235 IU
- Beef liver- 15 IU
- Whole egg- about 20 IU
- One a Day Vitamin Tablets- 400 IU per tablet, or about 10-40 IU in a pinch of a crushed tablet (a common serving size for tortoises)
- Reptical with D3- 13.7 IU per gram of powder
Comments about 'forest-dwelling tortoises get all the D they need in their diet' are misleading- there have not been any clinical studies done to support this, and comparing the doses of D in common foods with what they eat in the wild makes it unlikely to be true as a regular thing.
Some vitamins are washed out of the body daily and need constant replenishment- B, C, K, etc. Vitamins A, D, and E are stored in the fat and used as needed. Since they are needed every day by a growing or active tortoise, we do not want the fat reserves to get too low, so need to help ensure an on-going dosage. (Note that the flip side of this is that the tortoise needs a healthy amount of body fat to store energy in as well! A good argument for making sure they get enough food.) Hibernating tortoises need to store enough vitamin D to withstand the winter, so it is very possible that at least some species can absorb enough from being outside to last the winter indoors. (I am not sure I would count on this for non-hibernating species.)
As you think about vitamin D supplements, remember that 'wet' is better than 'dry'. Vitamin D in powder or dry tablet form is less effective than it is in actual food or in a oil base, like gel-caps. Some sources even suggest that 'dry' D does not count. The supplements in Mazuri, cat food, etc. are in a oily base, but they will still loose effectiveness over time, one reason for expiration dates on them.
I think the most convenient version would be liquid vitamin D, which comes in 500 IU to 5000 IU per 5 drops strengths. This lets you drizzle it fairly easily over the meal.
For the dosing, I would aim for-
- 7ish IU per kilogram (or, about 0.007ish IU per gram) per day for tortoises without supplemental UVB.
- 5ish IU per kilogram (or, about 0.005ish per gram) per day for tortoises with UVB lighting.
- no supplemental D for tortoises under natural sunlight, especially nearer the Equator.
- The doses can be offered daily, every few days, or weekly, whatever is convenient as long as they get the dose they would have gotten if it was offered every day.
A good balance of calcium and D (and, of course, some other stuff) are vital to a tortoises health, especially in the early years. There is no reason to be afraid to offer supplemental calcium and vitamin D to give our tortoises the best chances for success. Note, however, that if you offer additional calcium, you MUST ensure there is enough vitamin D to process it! 'Unprocessed' calcium is rather dangerous to have in the system.
Like any good thing, you can go too far. If you are giving a tiny tortoise an entire high-dose (2000 IU) D gel cap every day, you will be overdoing it. Short of that, we should be OK.
Vitamin D in Wikipedia
Dr. Douglas Mader's "Reptile Medicine and Surgery", p. 1069, etc.
and a note of thanks to EgyptianDan for the inspiration!
Mark, in Nebraska. Librarian of the Tortoise Library, a resource for tortoise keepers (with a slight focus on forest or omnivorous species).