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.

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.

 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.

Dietary calcium and grass

The amount of grass that a rabbit would need to eat to meet its daily allowance is 463g. 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.