Calcium in Captivity

What does Vitamin D actually do in the body? Well it has a number of functions, but one of the most important for our reptiles at home is regulating calcium and phosphorus levels.  Active Vitamin D (calcitriol) increases how much calcium and phosphorus an animal absorbs in their intestines, helps regulate resorption of calcium from the bones, and also can reduce how much calcium is excreted through the kidneys.  Another big player in the Calcium/Phosphorus regulation game is parathyroid hormone.  Parathyroid hormone (secreted by the parathyroid glands, located in the neck) increases active vitamin D (calcitriol) by telling the kidneys to convert more of the early form of vitamin D (calcidiol) into calcitriol.  Increasing calcium levels (and calcium:phosphorus ratios) and calcitriol levels in turn suppress parathyroid hormone secretion, keeping the body in balance.  These basic feedback loops are crucial to helping maintain metabolic regulation in the body, and in this case keeping the correct levels of calcium and phosphorus in the bloodstream.  Because of these feedback loops, how much calcium, phosphorus, and UV light (and so vitamin D) our reptiles receive in their enclosures greatly impact their parathyroid hormone levels.  

So what happens if this feedback loop stops working? Or if there aren’t enough inputs (especially calcium) into this system?  Nutritional secondary hyperparathyroidism (NSHP, also called Metabolic Bone Disease, or MBD) is one of the most common health issues we see in our captive reptiles.  NSHP occurs when our reptiles receive insufficient UV light and heat (which help make vitamin D), insufficient dietary calcium, or excess dietary phosphorus, which lead to increased parathyroid hormone secretion.  The parathyroid hormone then increases vitamin D levels, which in turn increase resorption of calcium from the bones in order to increase calcium levels in the blood.  But if too much calcium is taken from the bones, the bones become soft and can break or become rubbery (hence the term ‘rubber jaw’ as another name for NSHP).  If calcium levels get extremely low (despite taking it from the bones), our reptiles can start to have muscle twitching and even seizures.  Because of this, it is very important we balance our reptile’s calcium and phosphorus intake, and make sure they have appropriate UV lighting.  While a calcium deficiency is the most common nutritional issue we see in our captive reptiles, it is possible to give too much as well! This can lead to a condition termed ‘metastatic mineralization,’ where calcium starts to deposit in organs and soft tissue structures, causing damage and dysfunction.  

So how do we make sure our reptiles are getting the right amount of calcium and phosphorus in their diets?  Studies looking at wild reptiles have shown that most reptiles eat a diet with much more calcium than phosphorus.  Ratios of dietary calcium to phosphorus can vary by species (and age), but generally a 2:1 ratio (calcium:phosphorus) is recommended as a minimum.  For young, rapidly growing reptiles with lots of bone growth, a ratio as high as 7:1 may be needed.  

So where do calcium and phosphorus come from in the diet?  Most dietary phosphorus comes from protein.  Because proteins are relatively abundant in most types of food, it’s easy to get enough phosphorus for our reptiles; too easy, in fact, because we want to make sure they are getting more calcium! Calcium is very abundant for our reptiles that eat whole prey including bones, so our snakes and larger lizards typically do not have too many concerns.  But what about the smaller reptiles eating insects? Do insects contain calcium?  Unfortunately the answer is...not much.  Most insect shells are made up of chitin, a complex sugar (sort of similar to cellulose, like cardboard), and do not carry much calcium.  Additionally, most commercially bought insects are nutritionally depleted, so the calcium our reptiles get by eating a newly bought cricket or mealworm is pretty minimal.  We can compensate for this by feeding the insects a high-calcium insect food for a few days prior to feeding them to our reptiles (‘gut-loading’), and by dusting the insects with a calcium supplement when we feed them to our reptiles.  Some calcium supplements contain phosphorus as well, but remember we already are feeding our reptiles plenty of phosphorus, so we want to avoid these (look for supplements that are phosphorus free or at least have a very high calcium to phosphorus ratio).  Why do we gut-load AND dust our insects? Gut-loading alone can only increase the calcium:phosphorus ratio so much (the insects can only keep so much calcium in their bodies, one study showed gut-loaded crickets maxing out around 1.7:1, less than the 2:1 minimum we need), so the dusting helps bump the calcium:phosphorus ratio up to a more ideal range.

So what about our vegetarian reptiles? In the wild, our vegetarian reptiles (such as tortoises) eat a lot of grass hays, which are low in protein (low in phosphorus).  This means that, despite not eating any crunchy bones, they can still ingest a high calcium to phosphorus ratio diet.  Fresh veggies and greens, on the other hand, tend to have more protein (more phosphorus) and so reduce the calcium to phosphorus ratio.  Even ‘high calcium’ darker greens still have a lower calcium to phosphorus ratio than we prefer, so our vegetarian reptiles will also benefit from calcium dusting their food!

-Dr. Cosimini

Dietary Manipulation of the Calcium Content of Feed Crickets - Mary E. Allen and Olav T. Oftedal

Journal of Zoo and Wildlife Medicine

Vol. 20, No. 1, Nutrition Issue (Mar., 1989), pp. 26-33 

Nutritional Quality of Natural Foods of Juvenile and Adult Desert Tortoises (Gopherus agassizii): Calcium, Phosphorus, and Magnesium Digestibility
Hazard et al

Journal of herpetology

Vol. 44 (March 2010), pp. 135-147