Nutrition for rabbits

Calcium
Calcium is the most abundant mineral in the body and forms bone and teeth in combination with phosphorus. Rabbits need a constant supply of calcium for their teeth that continually grow at a rate of 2 mm per week.

Note that calcium metabolism is unique in rabbits. Most mammals other than rabbits use vitamin D or calcitriol, parathyroid hormone, and calcitonin to maintain a consistent serum calcium level and calcium absorption from the intestine. However, in rabbits, most dietary calcium is absorbed and excess calcium is excreted through the kidneys in their urine. As a result, blood calcium levels vary substantially with the dietary calcium content.

Rabbits that select cereals and legumes from muesli mixes will be a low calcium diet. Poor quality hay can be deficient in calcium and/or vitamin D.

Calcium deficiency can be a contributory factor to poor tooth and bone quality and dental disease in rabbits.

Excessive calcium in rabbits is excreted in the urine in the form of calcium carbonate, which gives the urine a thick creamy appearance. Urinary calcium levels are related to dietary calcium intake, and large amounts of calcium carbonate in the urine can predispose rabbits to sludgy urine and cystitis. Because of this observation that calcium is readily absorbed in the gut, calcium absorption in the rabbit may not require the action of vitamin D unless dietary calcium levels are restricted.

A minimum of 0.22% is required to support normal growth, but a level of 0.44% is required for bone calcification. A level of 0.5-1.0% or 5-10 g/kg in feed is recommended for pet rabbits.

It is also desirable to maintain calcium to phosphorus ratios between 1:1 and 2:1 in favor of calcium. Imbalances in the ratio have been implicated in dental problems.

The following are some more resources on calcium for rabbit.
 * Dr. Cayla Iske, PhD. (2021). Do Rabbits Need Calcium?
 * Sharon Redrobe, BSc (Hons), BVetMed CertLAS, DZooMed MRCVS RCVS Specialist in Zoo and Wildlife Medicine. (2010). Calcium Homeostasis in the Rabbit
 * Dan Johnson, DVM, DABVP. (2009). Rabbit calcium metabolism, "bladder sludge," and urolithiasis (Proceedings)
 * Frances Harcourt-Brown. Calcium and rabbit food
 * Christine Eckermann-Ross. (2007). Hormonal Regulation and Calcium Metabolism in the Rabbit (Paid access required)

Fat
Fat plays a vital role in a rabbit's diet. It provides not only a valuable source of energy for a rabbit's fast metabolism but is also necessary for the production of motilin, a hormone that is required for gut movement. Fat reduces the intestinal absorption of calcium, which is useful for rabbits with kidney or bladder conditions, and is needed for the absorption of some drugs and the vitamins A, D, E, and K. Fats are important for the production of many hormones and are involved in the control of blood pressure and cholesterol levels. Fats also provide other benefits such as helping to prevent arthritis, naturally reducing inflammation, adding luster and gloss to a rabbit's coat, improving skin tone, and helping to reduce shedding. Additionally, fat helps make food more palatable to rabbits, which can be critical with older, ill, or anorexic rabbits.

Fats are highly digestible by rabbits. Unsaturated fatty acids from plant oils are the most digestible, plant fats still bound in structural plant material are a little less digestible, and saturated fats from animal sources are the least digestible. Most of the breakdown and absorption of fat occurs in the rabbit's small intestine.

A deficiency of fat in rabbit diets can lead to the following:
 * gut motility problems
 * poor coats and loss of hair
 * deficiencies of fat-soluble vitamins
 * immune deficiencies
 * slower repair after injury
 * retarded growth in young rabbits
 * brain and nerve problems with a severe deficiency

Excessive fat levels in a rabbit's diet can lead to the following problems:
 * obesity
 * harm to the immune system
 * increased risk of hepatic lipidosis if a rabbit becomes anorexic
 * atherosclerosis or deposition of fat in the arteries, particularly in magnesium-deficient diets

Lucille Moore recommends the following fat levels in a rabbit's diet:
 * 2-5% for the general rabbit
 * 3-6% for pregnant or lactating does
 * 4-8% for long-haired, wool-producing rabbits (e.g. American Fuzzy Lop, Jersey Wooly, Angora)

Rabbit-safe foods that are high in unsaturated fats include oils such as safflower and nuts such as walnuts or almonds.

Fiber
Dietary fiber, also spelled as fibre, can be separated into indigestible fiber that passes straight through the digestive system without entering the cecum and fermentable or digestible fiber that is directed into the cecum and provides a substrate for the cecal microflora. :28

Indigestible fiber is important for the following reasons. :28
 * Stimulate gut motility that moves digested foods and fluids into the cecum for fermentation.
 * Provide forage material to prevent boredom and behavior problems such as barbering.
 * Provide dental exercise and optimal dental wear.
 * Stimulate the appetite and ingestion of cecals.

The amount of indigestible fiber in the diet is very important, as too little results in GI stasis and other digestive orders while too much results in malnutrition. :55

Indigestible fiber is excreted as separate, round, hard fecal droppings.

Digestible fiber is important for the following reasons. :28
 * Provide a substrate for cecal microflora.
 * Provide optimal cecal pH and volatile fatty acid production.
 * Prevent proliferation of pathogenic (harmful) bacteria in the cecum.
 * Increase fiber content of cecotrophs so they are of firm consistency.

Grass, good-quality hay, wild plants, and garden weeds are ideal sources of digestible and indigestible fiber. :55

Protein
Growing rabbits have better health when the ratio of digestible fiber to crude protein is increased over 1.3. Generally, this means digestible fiber should be over 20% and crude protein below 16%.

Lucille Moore recommends the following protein levels in a rabbit's diet:
 * 12-16% for pet rabbits
 * 17-20% for long-haired rabbits and larger breeds (e.g. Flemish, Checkered Giant)
 * 16-20% for pregnant does
 * 18-21% for lactating does
 * 12-14% for young rabbits between 3-9 weeks of age

A dietary deficiency in protein in rabbits can lead to the following symptoms:
 * poor tissue regeneration
 * restricted absorption of micronutrients
 * changes in appearance and amount of hair - often seen as reddish and thin fur in malnourished rabbits
 * reduced body ability to eliminate some drugs and their metabolites

A methionine deficiency in rabbits can cause the following symptoms:
 * creatinuria or an increased concentration of creatine in the urine
 * muscle degeneration
 * weight loss
 * paralysis
 * death

A lysine deficiency in rabbits can cause the following symptoms:
 * reduced growth
 * weight loss

Excessive protein in a rabbit's diet can cause the following issues:
 * strain on liver and kidneys
 * increased urine production
 * alteration of microflora and increased pH in the cecum
 * increased urea production
 * reduced gastrointestinal motility

Vitamin A
Vitamin A, or retinol, is a fat-soluble, organic compound that is necessary for vision, bone development, maintenance of tissue integrity, reproduction, and immunological response. Vitamin A also plays an important role in combatting infection and has been termed the 'anti-infective vitamin'.

Rabbits housed indoors or in hutches and fed on cereal mixtures and poor-quality hay are candidates for vitamin A deficiency if they do not eat the parts that contain the vitamin and mineral supplement. Deficient animals are susceptible to disease and infection, and a high incidence of enteritis occurs in vitamin A-deficient rabbits.

Recommended vitamin A levels are the following:
 * 6,000 IU/kg for growing rabbits
 * 10,000 IU/kg for breeding does

The National Research Council recommends the addition of no more than 16,000 IU as a safe upper level. A deficiency may occur less than 2000 IU/kg. Toxicity can occur in feeds of 190,000 IU or higher.

Fresh green foods and grass are good sources of vitamin A.

Vitamin D
Vitamin D is a fat-soluble vitamin that is also a hormone that plays an important role in calcium and phosphorus metabolism. Vitamin D also plays a role in insulin secretion and glucose tolerance in the rabbit and helps maintain the transplacental calcium gradient in pregnant does.

Sunlight is required for vitamin D synthesis in rabbits, and rickets can be induced in growing rabbits with insufficient dietary vitamin D by keeping them in the dark or under artificial light. Some owners may choose to set up artificial UVB lights indoors for their rabbits if they do not get significant amounts of natural sunlight outdoors. There are no long term negative effects of doing so, but vitamin D can also be absorbed orally in rabbits and is usually readily found in pellets and sun-dried vegetation.

The main function of vitamin D is to maintain serum calcium levels within the normal range. High quantities of vitamin D causes bone resorption and raise blood calcium levels. Low quantities of vitamin D reduce intestinal absorption and renal conservation of calcium and results in a drop in blood calcium. Osteomalacia (softening of the bones) can result from both deficiency and excess of vitamin D. Unlike other animals, vitamin D is not required for the intestinal absorption of calcium as it appears to be passive and efficient, but vitamin D does increase the intestinal absorption of calcium and is required if dietary levels are low.

In rabbits, vitamin D also plays an important role in phosphorus metabolism, and a vitamin D deficiency results in a reduction in intestinal absorption of phosphorus.

Vitamin D deficiency may be a contributory factor in the development of dental disease in rabbits due to inadequate calcium absorption when dietary calcium levels are low. One study suggests that a serum 25-hydroxyvitamin D concentration of 17 ng/mL serves as the threshold for vitamin D deficiency in rabbits.

A level of 800-1200 IU/kg of feed is recommended for adult pet rabbits, and 600-1000 IU/kg for growing or breeding rabbits. Diets with less than 300 IU/kg are considered deficient. Dietary levels as low as 2300-3000 IU/kg appear to be toxic. :67

The following are some more resources about vitamin D in rabbits.
 * Bright Eyes Sanctuary. The Truth About Vitamin D: is it the answer to rabbit dental disease?
 * Vetlexicon. Hypovitaminosis D
 * Vetlexicon. Hypervitaminosis D
 * Johanna Mäkitaipale et al. (2019). Diet is a main source of vitamin D in Finnish pet rabbits (Oryctolagus cuniculus)

Vitamin E
Vitamin E is a fat-soluble vitamin that acts synergistically with selenium and prevents oxidative damage by peroxides. Vitamin E is also involved in blood clotting, stability of membrane structure and maintenance of immunity.

Vitamin E deficiency classically results in nutritional muscular dystrophy, damage to the heart muscles, exudative diathesis, liver disorders, increased incidence of lactation problems, and reproductive failure.

A dietary level of 1-70 mg/kg of body weight or 17-50 mg/kg of feed is suggested for pet rabbits.

Green forages and cereals are good sources of vitamin E. Young grass contains more vitamin E than mature forage. Leaves contain 20 to 30 times as much vitamin E as stems, and as much as 90% can be lost during haymaking.

Below are some articles with more information about the importance of vitamin E for your rabbits.
 * Nikki White, Flop Bunny Syndrome