Written by Laura Atkins and Susan Smith Ph.D.

Biology of calcium
Why calcium is an issue in rabbits
Why calcium is not an issue in rabbits

I. Biology of Calcium

Calcium is required for proper health and functioning of any animal. It is involved in muscle contraction, transmission of impulses along nerves, blood coagulation, neurotransmitter synthesis, and, of course, is one of the structural components of bone. Too little calcium results in rickets (called osteomalacia in older animals), which is basically softening of the bone. Severe deficiencies can produce muscle tremors (tetany).

Most animals, including humans, have various hormones such as vitamin D (calcitrol), parathyroid hormone (PTH) calcitonin and estrogen that all play a part in maintaining a consistent level of calcium in the blood. In mammals other than rabbits, calcium absorption from the intestine is primarily controlled by vitamin D, estrogen and PTH. Other minerals, such as phosphate and magnesium also play a role in calcium absorption.

Serum calcium levels are regulated by a number of factors, including breaking down bone (to increase blood calcium levels) and building up bone (to decrease blood calcium levels), excretion in the urine and feces (to decrease levels) and absorption through the intestine (to increase levels). The balance and interactions between all these systems is tightly controlled to maintain (in humans) a serum calcium level between 4.4 and 5.3 mg/dl.

II. Why calcium is an issue in rabbits

Rabbits are different in that they appear to absorb most of the free calcium in their diet. The calcium level in their blood is not regulated well, in fact blood levels increase in proportion to the calcium in their diet. They also primarily excrete calcium through the urine, unlike other mammals which primarily excrete calcium through the bile and ultimately through the feces. Rabbits also excrete a highly alkaline (basic) urine which, combined with a high calcium level, can form calcium carbonate crystals ("bladder sludge") or stones.

Bladder sludge and stones are the reason many veterinarians and rabbit people believe that calcium should be limited in rabbits. In some cases, they are probably correct. The form of calcium in rabbit pellets is highly absorbable, as is the calcium in alfalfa. This is primarily free calcium and the bunny intestine acts like a sponge and sucks it all in. Other forms of calcium, such as calcium oxalate, are not as absorbable (in some cases unabsorbable).

So, how much calcium is absorbed is dependent on what kind of calcium is in the diet as well as how much is present.

III. Why calcium is not an issue in rabbits

In terms of what we routinely feed bunnies, that is: hay, pellets, and vegetables, not all are created equal in calcium amount and type. Pellets contain calcium from alfalfa hay and calcium from sources like limestone. This type of calcium is in a very absorbable form and is something that is a good idea to watch. But, there are other reasons to limit pellets, so we assume there is not an overwhelming amount of calcium in them. For a bunny with problems, feeding a low calcium (typically timothy based) pellet is not a bad idea.

Alfalfa hay contains a high level of potassium as well as calcium. One of the texts I have suggests that the high level of potassium in alfalfa hay may contribute the formation to sludge or urine crystals. Certainly there is some evidence that the other minerals, such as potassium, magnesium, phosphate and even sodium, play a role in sludge formation in rabbits.

That leaves vegetables, and their calcium content. For the majority of vegetables, water levels are between 80 and 95% of the total weight of the plant. Many times when calcium levels are being reported, they are reported as a percentage of *dry weight*, which does not take into account the weight of water. So the actual amount of calcium being fed may be much less when you look at what you're actually feeding, not the dry weight of the food. Many vegetables that have significant calcium levels also have significant oxalate levels. Calcium bound to oxalate (called, oddly enough, calcium oxalate) is not readily absorbable by the rabbit. Any calcium that is not absorbed cannot contribute to sludge.

Needless to say, I don't pay attention to calcium content in veggies. What I do try and do is rotate veggies frequently to give the bunnies a balanced diet. I do have a list of the calcium content of selected vegetables. If you are interested in oxalate levels of various vegetables I have a chart of that, too. To round out the group, there is also a list of calcium content of selected hays. In general, feeding a "high calcium" veggie is not going to harm your bunny. As always, you want to monitor bunny's output. If feeding one veggie or another consistently results in sandy looking urine, then you may want to decrease the amount you feed, or remove it from their diet entirely.

^I am looking for confirmation of this elsewhere, however, it was an un-footnoted statement so I cannot look at the original research. This does agree with other research I have seen that indicates other minerals and ions (sodium, potassium, magnesium among them) change the level of calcium excreted. In other words, I dont disbelieve the statement, but it is so close to what Ive been arguing I want to make sure I have all my facts in order before I start using it.