I didn't really answer it, because calcium bloodlevels are a bitdifferent ball of wax. Angela mayhave more to say on it, she has a much better grasp of this stuff thanme.
Here's the 'standard' article alot of experts derived their opinions from, although it was written in1998, I'm sure there have been more studies, but I don't know ofanydrastic changes of opinion. (Red urineandthe Vitamin D aspects I think have had a little morework).
I highlighted a few bits in navy, but not too carefully.
[align=left]CALCIUM METABOLISM IN RABBITS
by Leah Postman,DVM
[size=+1]C[/size]alcium is a common mineral element which isfundamental to many normal bodily functions. Of the 15 known essentialmineral nutrients, calcium is the one found in largest amounts inanimals, and therefore is required in the largest amounts. Almost all(99%) of the total body calcium is found within the bones and teeth.While most mammals make one or two sets of teeth to last a lifetime,rabbits and horses continually form and wear down their teeththroughout their life span. For rabbits, continual tooth eruption is afactor in long-term calcium requirements. The other 1% of body calciumsupports critical metabolic functions, including nerve impulseconduction, muscle contraction, heart rate and contraction, and bloodclotting.
Most mammalian species follow a similar pattern of calcium metabolism.While rabbits do not follow the typical mammalian prototype, I willdescribe it for comparison purposes. After calcium is eaten, it isabsorbed by the small blood vessels supplying the intestines, into thebloodstream. The level of calcium in the blood is closely controlled bytwo hormones: parathyroid hormone and calcitonin. These two hormonesact in tandem to keep calcium levels within the blood optimallybalanced. Parathyroid hormone (parathormone, PTH) is produced by theparathyroid glands: tiny glands located adjacent to the thyroid glands.When the parathyroid detects that the level of calcium in the blood isgetting too low, it secretes extra PTH. This increased level of PTH inthe blood acts on cells in the kidney to stimulate formation of anactive form of Vitamin D. This activated Vitamin D in turn acts oncells in the intestine to increase calcium binding capacity. In thisway, more of the ingested calcium is picked up by the intestines, andthe low blood calcium is corrected by increased intestinal absorption.This intricate loop acts to protect animals from the potentially deadlyconsequences of
low blood calcium -- heart attacks,seizures, and muscular tetany.
Just as too little calcium in the blood can be dangerous, so can toomuch. To protect against elevations in the blood calcium level, thethyroid glands produce calcitonin. The thyroid produces more calcitoninwhen it senses an increase in circulating calcium. This increased levelof calcitonin acts on cells within the bones to increase absorption ofcalcium into the bones. The calcium within bones not only providesstructural integrity, but serves as a reserve supply for the body,should nutritional sources of calcium become scarce. Just as PTHprotects animals from the consequences of LOW circulating levels ofcalcium, calcitonin protects animals from the consequences of
HIGH circulating levels of calcium-- seizures,mineralization of tissues and blood vessels.
Rabbits metabolize calcium very differently from other animals. Unlikeother mammals, rabbits' blood calcium levels fluctuate widely,dependent upon the level of calcium in their diet.
Rabbitsalso have very complete intestinal absorption, and thereby end up withblood calcium levels that are generally higher than othermammals. Although Vitamin D mediates intestinal absorption ofcalcium in most mammals, intestinal calcium absorption in rabbits isapparently independent of Vitamin D. Only rarely, in conjunction withthe changes in calcium metabolism associated with birth and the highcalcium requirements of lactation, do their calcium levels drop to adangerous level, resulting in tetany. On a practical level, for thespayed house bunny, this is not a concern.
What is aconcern however, is the upper end of the blood calcium range.
In other mammals, the thyroid-produced hormone calcitonin acts to lowerblood calcium levels, to keep them below a ceiling value. This type ofcontrol does not seem to exist in rabbits.
Their bloodcalcium levels become elevated in direct proportion to the calciumconsumed in their diet. To make the bunny calcium situationeven more of a conundrum,
rabbits are even more unusualbecause they, being inherently prone to elevations in blood calcium,filter excess calcium through their kidneys, and excrete it throughtheir urine. Whereas most mammals can rid their bodies ofexcess calcium efficiently and without adverse effects through bile andintestinal secretion, bunnies apparently cannot. Studies on pika,jackrabbits and mountain hare have demonstrated that the wild lagomorphrelatives of domestic rabbits share this pattern of urinary excretionof calcium.
The kidney's serve as the body's filtration system -- to discard wasteproducts, to conserve normal levels of electrolytes within the body,and to regulate water balance. To accomplish this, incoming blood isjuxtaposed with a network of fine tubules, across which filtrationoccurs. The delicate tissues within the kidneys are ideally suited tofiltering soluble substances --ions, glucose, proteins. But once thekidneys have reached their (limited) capacity for reabsorption, calciumprecipitates within the urine, and is excreted as insoluble crystallinesalts. This is what is in that cloudy white urine ("sludge"), or grittypuddles that you may have cleaned. It has been theorized -- not provenyet -- that the red pigment often seen in rabbit urine, although notblood, is a blood pigment indicating damage to the kidneys' tubules,most likely by calcium salts. Some feel that "red urine" is caused byharmless plant pigments, and is in no way a problem.
We do know that calcium stones are a significant problemfor pet rabbits. These stones can be found anywhere within the urinarytract -- kidneys, ureters, bladder, urethra. Depending on theirlocation, they can be painful and often require surgical removal. Andthey can be very difficult to remove. We also know that urinaryexcretion of calcium correlates directly with the level of calcium inthe diet. In other words, the more calcium that the bunnies eat, themore their kidneys process and eliminate. Given the potentiallyserious, and all-too-common problems associated with calcium excess, itmakes sense to provide no more calcium than what is needed formaintenance of skeletal and neuromuscular needs.
There are two standard sources for determining rabbits' nutritionalrequirements: the National Research Council (NRC, 1977) and FrancoisLebas (Lebas, 1980), both referenced in Cheeke's _Rabbit Feeding andNutrition_, 1987. The NRC did not determine the maintenance requirementfor calcium, but sets 0.4% as the growth requirement; maintenanceshould be considerably less. Lebas determined the maintenancerequirement to be 0.6%.
Based on this data, currentrecommendations are for a calcium content of 0.4% to 0.6% (dry matterbasis). Almost all of the commercial pellets currently availableexceed this, some by almost double.
All of these pellets share a common trait -- they are formulatedprimarily from alfalfa meal. And alfalfa meal, like the alfalfa hayfrom which it is made, has a very high calcium content -- 1.5%. So itis a challenge to make a pellet that starts with alfalfa -- inherentlyhigh in calcium, and dilute the calcium content by 60%. There is reallyonly one pellet that avoids this formulation dilemma. Oxbow HayCompany's Bunny Basics/T starts out with Timothy meal, not alfalfameal, and thereby produces a pellet with 0.4-0.8% calcium.
The other potentially significant source of calcium in rabbits' dietsis fresh greens. Although many people worry about the greens noted forhigh calcium content -- kale, spinach, collards -- I do not.
All fresh greens are about 70% to 85% water, which greatlydilutes all nutrients. To eliminate the potential to misrepresentnutrient content due to variability in water content, it is essentialto compare calcium content on a dry matter basis, which is how thenutrient requirements are expressed on labels and in tables. Forexample, kale, which many recommend avoiding by virtue of its calciumcontent, has 1.6% calcium on a dry weight basis. In other words,dehydrated kale has roughly three times the NRC recommended calciumcontent. However, the fresh bunch of kale is mostly water, so thecalcium content drops dramatically, to 0.24% when fed fresh. If youwere to feed a medium-sized (5 pound) bunny 1/4 cup (about 60 grams) ofa pellet meeting NRC recommendations, he would consume about 0.30 gramsof calcium. To obtain a similar quantity of calcium from kale, the bunwould have to eat over 4 ounces (130 grams) of kale. That is a sizablepile of kale -- about as much as you could stuff in a half gallon milkcarton. Similarly, dandelion greens, have a calcium level of 1.6% on adry matter basis. But since our bunnies eat them fresh, they are onlygetting 0.20% calcium. Again, we see that even for greens with arelatively high calcium content, the calcium level of the leaves thebun is munching is actually quite low.
So, what does all this mean in real life? It mean's that to protect arabbit's kidneys from calcium overload, we must minimize the calcium wefeed them. Alfalfa -- both hay and alfalfa meal derived pellets -- isthe biggest source of calcium overload for domestic bunnies. Minimizingthe alfalfa in rabbits' diets will automatically reduce their calciumlevels. While some veterinarians (notably, Dr. Susan Brown) mayrecommend eliminating pellets entirely from the house rabbit's diet, Ido not. I think a balanced pellet, fed in limited quantities, can helpeven out the fluctuations inherent to hay -- soil content, day length,first cutting vs. second cutting, etc.
From my point of view, there is one preferred pellet -- Oxbow BunnyBasics/T. Oxbow has been the only manufacturer to date to realize thatbasing a pellet on Timothy hay just fundamentally makes more sense.Although there are a few alfalfa meal based pellets with a reasonablylow calcium content, they do not have the fiber content of BunnyBasics/T (29%), so on the fiber count they don't stack up.
I think Bunny Basics/T is the best thing to happen to bunnies sinceFarmer McGregor retired. Of course, good quality grass hay should bethe bulk of your rabbit's diet, with fresh greens for variety, addedfluids and vitamins. I believe that this combination of feeds offersthe best available complete nutritional profile, as well as taking intoaccount a rabbit's idiosyncratic calcium metabolism, and minimizing therisk of associated urinary tract disease.
(c) 1998,[/align]
