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Low protein
Low protein diets are not as popular with consumers in comparison to high protein diets. Low protein diets are frequently associated with renal care formulas and other prescribed therapeutic diets.

Low protein diets (adding to wiki page "cat food")
Protein content between cat foods can vary anywhere from a low protein content of 26% to a content as high as 50% or above. The National Research Council (NRC) recommends protein content for cat diets is to be around 20% for adult maintenance and 21.3% for growth and reproduction The Association of American Feed Control Officials (AAFCO) recommendations however suggest a higher protein level of 26% for adult maintenance and 30% for growth and reproduction.

Cats are obligate carnivores and require protein in their diet as an important component in energy metabolism and maintenance of lean body muscle. Cats are greatly adapted to a high protein diet since they utilize protein as their main energy source, keeping carbohydrate utilization very minimal. In addition, cats will continue to metabolize protein even through protein deficiency by catabolizing lean body muscle. More specifically cats like most mammals, require the appropriate dietary amino acid balance to supply the essential amino acids, which are used for a multitude of body functions. It is crucial for low protein diets to meet AAFCO recommendations to ensure nutrient requirements are met.

In general, lower protein diets tend to consist of a greater carbohydrate content, which means a potentially higher glycemic response in cats. Glycemic response refers to the response of glucose concentrations in the blood after a meal. Hexokinase is the enzyme cats use for metabolizing glucose because they do not possess the more efficient enzyme glucokinase. Cats are susceptible to high prolonged postprandial glucose and insulin periods with high glucose peaks, resulting from the carbohydrate content of the diet. The degree of glucose peaks and the duration of the elevated glucose levels ultimately depends on the glycemic index of the carbohydrates used in the diet. Typically a diet of low protein is replaced with added carbohydrate content. In some cases, fat can be added to lower protein diets in order to increase the energy density of the food product.

A nutritional balance of amino acids is important in low protein cat diets as cats are highly sensitive to deficiencies in arginine, taurine, methionine, and cysteine. Therefore, supplementation of these essential amino acids is important and commonly found in cat foods. The supplementation of polyunsaturated omega-3 fatty acids is common in products with lower protein as it plays a positive role in protecting the kidneys which is an issue for many cats on a low protein diet. Similarly, mineral composition is important and low protein diets result in a low phosphorus content that benefits the renal care and prevention of kidney disease. .

Protein
AAFCO recommends 26% crude protein for feline diets on a dry matter basis, however dry cat food from the top 3 cat food brands in Canada of 2010 contained 34%, 40%, and 36% protein respectively, on a dry matter basis. High animal protein in cat food has increased in popularity due to consumer demand for natural diets, in which protein content is usually much higher than the AAFCO minimum levels. The effects of these trends results in a higher need for animal protein, which may have detrimental effects on sustainability.

6.4% of the USA's greenhouse gas emission is estimated to come from agriculture, and over 50% of the agricultural greenhouse gas emission comes from livestock. The large amount of livestock and fish protein used in pet food diets has been shown to have a much larger impact on the environment when compared to vegetable protein. Following only human trends, by 2050 approximately 1 billion hectares of land will need to be used for agriculture to sustain the human population.

In 2015, an estimated 77.8 million dogs and 85.6 millions cats were living in the USA. The consumer desire to feed their pets premium foods which advertise healthy and human-grade ingredients coupled with the increasing prevalence of pet ownership is causing increased pressure on the meat industry which could result in increased land usage for raising livestock to meet the growing demand. In a study conducted by Okin in 2017, he suggests that if a quarter of all animal protein used in the food of american pets was human-grade, it would be equivalent to the energy needs of 5 million Americans. Okin uses an estimate of 33% of an animal's energy needs is derived from animal products, however this is conservative in that many diets now have more than 33% of their diet in animal protein alone. Lowering protein levels in feline diets may help to improve the sustainability of both the human and pet food system by decreasing pressure on livestock agriculture and ultimately improving environmental effects.

Low protein diets and relation to the kidneys
Therapeutic diets with low protein have been adopted by a number of big pet food manufacturers with the goal of supporting renal health for cats with chronic kidney disease (CKD). Chronic kidney disease or CKD is a highly prevalent condition in the feline population and is most common in the aging and older domesticated feline. It has been shown that the lifespan of cats experiencing CKD can be extended by as much as 2 years when receiving therapeutic diets rather than regular maintenance diets. The normal functions of the kidneys are to control a normal fluid balance and to excrete waste products from the blood into the urine. One of the biggest concerns associated to CKD is to reduce the amount of stress put on the kidneys to process and excrete nitrogenous wastes from the body. Low protein diets are a good choice for cats experiencing CKD as it lowers the amount of nitrogenous waste in the body helping to decease the strain put on the kidneys. Nitrogenous wastes are produced by the body when amino acids obtained from the digestion of protein are metabolized to make energy. The exact level of protein that is needed for therapeutic CKD diets is unclear, as sufficient protein in needed for maintenance while still offering a lower nitrogen level for the kidneys.

Low protein diets can be formulated as a wet or dry food with the the main difference between them being the moisture content. These diets are formulated specifically to support the kidneys and urinary tract and also to help prevent the progression of CKD. There are certain dietary parameters that these diets follow on top of low protein such as low phosphorus, high energy density, a higher fat content and inclusion of omega-3 fatty acids to aid the kidneys in different ways. Formulating the diet to have low phosphorus is very important as restricting the amount in the diet has been proven to decrease the progression of CKD. The reason that plasma phosphorus concentration should be kept low for low protein therapeutic diets is because phosphorus can be deposited into soft tissues and become mineralized which can progress kidney damage. Formulating these diets with higher amounts of fat is important to make sure the food is palatable in the absence of protein and promote an increased caloric intake. The higher fat content will also spare the use of protein for energy and help decrease stress on kidney. This inclusion of higher fat content also increase the energy density of the food which is important for cats with decreased appetites. Omega-3 fatty acids are included in therapeutic diets because of their anti-inflammatory properties to aid the diseased kidneys.

Low protein diets and relation to the liver
Low protein diets should not be fed to cats with the liver condition known as hepatic encephalopathy because severe protein restriction can be detrimental to animals with this condition. Cats with this condition should be fed a diet with high quality protein sources that have adequate amounts of the amino acids taurine and arginine.

Benefits to low protein diets (adding to wiki page "cat food")
Feeding a lower protein content can help lower the amount of calcium being excreted in the feces and urine. It has been observed that urinary calcium concentrations and fecal calcium excretion is related to increasing protein concentration in the diet. Increasing calcium in the urine and feces can indicate increased calcium mobilization (for example of bone) within a cat instead of increased intestinal calcium absorption. Lower protein fed to cats can also decrease renal oxalate excretion, and therefore, decrease renal oxalate and calcium concentrations to support the decrease in urinary calcium concentrations, which contributes to the decrease in urinary relative saturations with calcium oxalate and can decrease the probability of a cat developing calcium oxalate stones.

A lower amount of harmful bacteria is accumulated with a lower protein diet which helps to increase the amount of positive bacteria present in the gut.

Consequences to low protein diets (adding to wiki page "Cat food")
In low protein diets, if the protein source isn't a high quality protein such as an animal based protein, cats, especially kittens have been shown to develop retinal degeneration due to a taurine deficiency, an essential amino acid for cats that is derived from animal protein.

Since cats have such a high protein requirement, lower protein content, and thus lower amino acid concentrations, in the diet have been linked to health defects such as lack of growth, decreased food intake, muscle atrophy, hypoalbuminemia, skin alterations, and more. Cats on lower protein diets also have increased probability of not only losing weight, but also losing lean body mass.

Low protein diets that are high in carbohydrates have been found to decrease glucose tolerance in cats. With a decreased glucose tolerance, clinical observations have confirmed that cats consuming large proportions of metabolizable energy in the form of carbohydrates rather than protein, have increased chance of developing hyperglycaemia, hyperinsulinemia, insulin resistance, and obesity.

Cats have a high priority for gluconeogenesis in order to provide energy for tissues like the brain. With this high priority for gluconeogenesis, the rate at which it operates doesn't change despite having a low protein diet. Despite now being able to down-regulate gluconeogenesis, cats are able to regulate their amino acid catabolism in response to a low protein diet, probably by increasing the synthesis of enzyme proteins without altering the catalytic rate. This collectively can make a cat "wasteful" of amino acids supplied, when supplied with a low protein diet

Low protein diets have been shown to lower the levels of eosinophilic granulocytes within cats, which impacts the overall function of the immune system.