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Calcium and rabbit food


Incorrect dietary intake of calcium is often suggested as an underlying cause of disease in rabbits. Too little calcium in the diet calcium is linked with dental disease and feeding high amounts of calcium could contribute to sludgy urine and other urinary tract problems. Feeding a balanced diet that does not have too much or too little calcium is beneficial but it can be hard to know what to feed. 

How do rabbits regulate calcium?

Rabbits are different from humans (and dogs and cats) in the way they regulate calcium. The amount of calcium that is absorbed from the intestine in rabbits is not regulated by how much the body needs as it is in humans. Instead the amount of calcium that is absorbed depends on how much calcium is available in the intestine, so if the diet contains a high level of calcium, a lot will be absorbed. This is advantageous in an animal that might need a lot of calcium. Rabbits can be mated as soon as they have given birth so they may be pregnant and lactating. A breeding female will need plenty of calcium for her continually growing teeth and the foetuses in her uterus as well as the babies she is feeding.  In rabbits, calcium regulation by the kidneys instead of the intestine. They are adapted to excrete excess calcium or retain calcium if the rabbit needs it. The excess calcium forms a sediment in the urine. This is normal.

What is the correct amount of dietary calcium?

Most packets of rabbit food show how much calcium is in the diet. It is shown as a percentage (see example on left). This percentage is based on information from studies about the amount of calcium that a rabbit needs to grow and develop strong bones. These studies have shown that a dietary level of 0.6-1% calcium is required for optimum bone calcification. Although this might seem straightforward, it is not. The amount of calcium that a rabbit needs is not a fixed amount for every rabbit. It depends on several factors :

  • Growing, pregnant or lactating rabbits will need more calcium than a neutered adult pet rabbit. Calcium is required to form new bone in growing rabbits. Calcium in the bones of developing foetuses or unweaned babies comes from the mother. 
  • Rabbit with a healthy teeth need a constant supply of calcium to lay down new dental tissue. All the teeth continually grow at a rate of 2-3mm per week. In rabbits with advanced dental disease, the teeth stop growing so the rabbits need less calcium.
  • Rabbits that are kept indoors and not exposed to sunlight may not have enough vitamin D to help them absorb calcium from the diet. This won't matter if the diet has enough calcium in it but will matter if the diet is low in calcium.

Calcium and sludgy urine

Healthy rabbits are able to clear excess calcium from the body easily. They have evolved to do this and the kidneys are able to excrete large amounts of calcium into the urine where it forms a sediment. Wild rabbits, that are slim and fit, will run around a mix up the sediment with the rest of the urine as they are doing so. In addition to normal urination, wild rabbits will often squirt urine onto other rabbits or objects to mark their territory. They do this because they are not neutered and need to attract a mate and establish their territory. Constantly running around and squirting urine mixes up the sediment in the bladder and expels it. In contrast, neutered pet rabbits tend to sit around all day so any sediment in the bladder will fall to the floor and accumulate. Litter trained, neutered pet rabbits only urinate if they need to. There is no stimulus to spray urine or squirt in around their territory. This is good for the owner of the rabbit but it has the potential for sediment to accumulate in the bladder. If the rabbit also has a problem that makes it painful to urinate (e.g, arthritis, cystitis, urethritis, sore hocks, sore back etc. etc) they will hold on to the urine for as long as possible so the sediment can settle out in the urine. Pain (or bladder atony) can prevent a rabbit emptying it's bladder completely during urination. This means the sediment can build up even more because the part of the bladder that is emptied first during urination holds the urine that has just come from the kidneys. This urine has fluid mixed with the sediment. The amount of water that is passing through the body also plays a part. If the rabbit is also eating a dry diet (e.g.hay and nuggets) and not drinking much water, the urine will be concentrated with more sediment in it. These rabbits are candidates to develop 'sludgy urine' . Although the condition is not directly caused by high dietary calcium, restricting the amount of calcium in the diet as well as increasing their water intake can help with the treatment. Providing water from a bowl rather than a sipper bottle may help with this. It has been shown ( http://www.ncbi.nlm.nih.gov/pubmed/21091544) that rabbits that drink from sipper bottles consume less water.

Dietary calcium calculations

Nutritional studies have shown that a dietary level of 0.6-1% of calcium is required for optimal bone calcification and this percentage is used in most diets for pet rabbits. However it does not provide a basis for calculating the amount of calcium that a rabbit actually eats. For example, if the rabbit ate very little of a high calcium food it would not be ingesting much calcium. Conversely, if it ate lots of a low calcium diet, it could ingest a lot of calcium. The amount of water in the food plays a big part in the amount that a rabbit can actually eat. Knowing the water content of a food item is important because published dietary calcium levels reflect the amount of calcium in the dry matter i.e. after all the water has been extracted. They do not reflect the amount of calcium in the food in the form it is offered to the rabbit. 

Recommended daily allowance of calcium for rabbit

An alternative way of looking at dietary calcium is to think about the 'recommended daily allowance'  (i.e. the amount of any particular food item that a rabbit would need to eat to meet its requirement) instead of a percentage of the diet. Exact calculations of RDA are difficult because rabbits have different requirements according to their weight, age and reproductive status. However a rough estimate can be extrapolated from nutritional data for commercial rabbits and the RDA is about 500mg for a 2.5kg rabbit that is not growing, pregnant or lactating. Calculating the amount of individual food items that a rabbit would need to eat in order to obtain this amount of calcium gives some surprising results that are illustrated here. Water content plays a major part in the quantity of food that provides 500mg. Some of the results are summarised in the table below.

Food

Dry matter

Calcium content (of dry matter)

Weight that would provide 500mg

Approximate amount that the rabbit would have to eat

Alfalfa hay

82%

1.5%

41g

1 large handful

Apples

20%

0.12%

2.083kg

15 apples

Bananas

25%

0.06%

3.333kg

20 bananas

Broccoli

10%

0.48%

1.042kg

2 large florets

Carrots

12%

0.37%

1.126kg

12 carrots

Carrot tops

17%

1.94%

152g

Tops from 1.5 bunches

Dandelions

15%

1.36%

245g

2 large plants

Grass (approx figs)

20%

0.54%

463g

6 large handfuls

Hay (approx figs)

85%

0.52%

113g

4 large handfuls

Kale

15%

1.6%

208g

1 supermarket bag (weigh 200g)

Lettuce (variable)

5%

0.86%

1.163kg

Approximately 2 lettuces

Flaked maize (corn)

22%

0.04%

5.682kg

7 ½ boxes breakfast cereal (weigh 750g)

Spring greens

22%

1.45

157g

¾ average supermarket bag (weigh 200g)

Watercress

5%

1.2%

833g

10 supermarket packets  ( weigh 80g)

Parsley

12%

1.38%

302g

13 small packs of parsley from supermarket  (weigh 23g)

Commercial foods

Average nuggets/pellets/

90%

0.9%

62g

Small handful

Low calcium nuggets

90%

0.5%

111g

Large handful

 It can be seen from this table that it would be easy to ingest a high amount of calcium by eating a lot of 'low calcium' nuggets or pellets but almost impossible to ingest too much calcium from 'high calcium veggies', such as watercress or kale.

Feeding rabbits with urinary tract disease

The bag holds 463g grass, which is approximately the amount that a rabbit would need to eat to meet the recommended daily allowance of calcium. It is a large volume so it is unlikely that a rabbit could eat so much grass that its calcium intake would be excessiveThe amount of grass that would provide the recommended daily allowance (RDA) of calcium also contains 350mls water, which is an important consideration, especially for rabbits with urinary tract disease. The amount of grass that a rabbit would need to eat to meet its daily allowance is 465g. This amount is shown in the bag on the left. Although it may seem a lot, this amount is probably correct for a wild rabbit that spends its day grazing. 

Most of this grass is water. By weighing the amount of grass that was left after it was dried, it can be seen that it contained about 350mls water  i.e 4 glassfuls (see right). This water would be beneficial in keeping the contents of the intestines soft and hydrated and diluting the urine. This is why grass and other leafy green plants vegetables are better for rabbits with sludgy urine (and other urinary tract problems) than hay and nuggets.

Providing plenty of water in the diet and avoiding excessive amounts of calcium in the diet are important for rabbits with urinary tract disease. A diet sheet suggesting  a suitable diet can be downloaded.