Cats and Carbohydrates: Myths and Truths

This article is part of the series in which we explore the science of nutrition. In the previous article of the series, we talked about macronutrients: protein and fat. In today’s article, we are finally covering the last (and most disputed) one: carbohydrates. We will see what they are, what their role is, and some controversial subjects such as the benefits of low-carb diets.

What are carbs?

Carbohydrates are molecules made of carbon, hydrogen, and oxygen. They generally have the formula Cm(H2O)n  which explains their name as “hydrated carbons”. Carbohydrates are the major components of plants.
They can be classified into 4 categories: monosaccharides, disaccharides, oligosaccharides, and polysaccharides.

Monosaccharides

Monosaccharides are also called simple sugars. They contain between 3 and 7 carbons. The most important ones are glucose, fructose (fruit sugar), and galactose. Glucose is the end product of starch digestion, the form of carbohydrate that circulates in the bloodstream and is used by the body’s cells for energy.

Disaccharides

Disaccharides are composed of two monosaccharides. Lactose, the sugar found in milk, is composed of a molecule of glucose and galactose. Sucrose (also called table sugar) is composed of a molecule of glucose and fructose. Maltose is composed of two molecules of glucose; it is not common in food, but it is created during the digestion process.

The monosaccharides and disaccharides are often referred to as “simple sugars”, as they are the ones that can have a sweet taste.

Oligosaccharides

Oligosaccharides are composed of 3 to 10 monosaccharides linked together. They are generally non-digestible and they are classified as prebiotic fibers. We will explore more in detail what prebiotics and probiotics are in a dedicated article. A well-known kind of oligosaccharides are the ones containing fructose: they are called fructooligosaccharides or (FOS). You can often find them in kibbles as a source of prebiotics.

Polysaccharides

Finally, polysaccharides are the biggest group of carbohydrates. They contain more than 10 monosaccharides and up to many thousands. It’s the main form of carbohydrates in plants, used as cell wall material like cellulose or for energy storage like starch. 
To simplify, we are going to talk of only 2 categories of polysaccharides: starches which are digestible carbohydrates, and fibers which are non-digestible carbohydrates.
We covered fibers in a specific article, and we will focus more specifically on starch in this one.
Starch is the energy storage of plants. During digestion, it breaks down into glucose. Examples of ingredients rich in starch are rice, wheat, oat, corn, potatoes, and peas, …

What is the role of carbohydrates?

Glucose has several important roles in the organism: it is the only source of energy for several tissues like the central nervous system, brain, and red blood cells. Glucose is also required for the synthesis of essential body compounds like DNA and RNA, as well as non-essential amino acids.
Carbohydrates, like proteins and fat, are also a source of energy for the body.
However, while glucose is essential for the organism, cats do not have a dietary requirement for carbohydrates. In fact, cats are able–through gluconeogenesis–to synthesize glucose from proteins. That’s why you won’t see a line for carbohydrates in any of the nutrient requirements standards for cats like the NRC, AAFCO, or FEDIAF. [1]
As gluconeogenesis is particularly efficient in cats, even in demanding life stages such as gestation and lactation, cats are able to synthesize sufficient glucose if enough proteins are provided in their diet. [2]

Can cats digest carbs?  

Cats have a limited ability to efficiently use large amounts of carbohydrates. They have only 5% of pancreatic amylase (the enzyme to digest starch). As opposed to dogs, they don’t produce amylase in their saliva. They also lack the liver enzymes to assimilate simple sugars, and the sugar transport system cannot adapt to varying levels of carbohydrates.
Signs of maldigestion like diarrhea and bloating, as well as adverse metabolic effects like hyperglycemia, appear when cats are fed large amounts of carbohydrates (more than 40% of the dry matter). [3]
However, when starches are correctly processed and cooked (like in kibbles) it seems that they are well tolerated in levels up to 35% DM. [3]
Cats also have a low level of the intestinal enzymes sucrase and lactase, so they can’t digest sucrose (table sugar) and lactose. That’s why it’s not recommended to give cats big amounts of milk. It’s estimated that the upper limit is 85 ml of milk for a 4 kg cat. [4]
While we know that cats can digest a certain amount of carbs and use them, it doesn’t necessarily mean that it is what is best for them. There is definitely some controversy among the scientific community concerning the impact of carbohydrates on cats. In fact, there is still a limited amount of research on the mechanism of carbohydrate metabolism and digestion in cats. [5][6][7]

Why do I recommend feeding a low-carb diet to your cat?

While cats can digest starch, I prefer–and recommend–feeding a low-carb diet for healthy cats.

Metabolism

The first reason is simply to respect their metabolic needs and adaptations as obligate carnivores, as mentioned in a recent study from 2017. It has been hypothesized that because of all these metabolic adaptations, feeding a diet rich in carbohydrates could put the cat organism under stress and have negative health effects in the long term. [8][9][10][16][17]

“Commercial foods containing high levels of carbohydrate might not satisfy the metabolic needs of obligate carnivores cats.”

Salaun, F et al. “Impact of macronutrient composition and palatability in wet diets on food selection in cats.” Journal of animal physiology and animal nutrition (2017)

Natural diets

Diets low in carbohydrates are also the natural diets of wild and feral cats, which are virtually identical to domestic cats. Their daily energy intake is composed of 52% protein, 46% fat, and 2% as carbohydrates. It’s also important to note that domestic cats remain very close to their wild cat ancestors: they mostly differ in appearance and behavior. Cats still have the metabolism and physiology of wild cats. [8][10]
The amount of plant material in wild cats' diets is negligible. However, they could ingest some starch as part of the stomach of the whole prey they eat. This is however unsure as it also has been observed that cats tend to remove the digestive tract of the prey they eat. [32]

Cat preferences and carbohydrate ceiling 

When cats are given the choice, they will select food with low carbohydrates, or that amounts to about 10% of their daily metabolizable energy. However, it’s important to bear in mind that in some of the studies the cats couldn’t select a carbohydrate intake lower than ~10% of their daily metabolizable energy, which may skew the results. [9][5][11][12]
It’s interesting to note that cats can be deceived into choosing a high carbohydrates diet when palatability enhancers are used and when animal proteins are replaced with wheat gluten and pea proteins. [13]
In these studies a carbohydrates ceiling was observed: cats preferred not to eat enough rather than eat more carbohydrates than this ceiling of ~ 3 g of carbohydrates per kg of cat per day. Feeding a diet high in carbohydrates and low in protein could lead to insufficient protein and energy intake.

Gut microbiome

Another thing to take into consideration is the effect of different macronutrient proportions on the gut microbiome. This is an area of research that is still in its infancy, but it seems that a high protein diet could promote healthy microbiomes. [14][15][20]

Diseases 

This last point is controversial, but several studies have hinted that a high carbohydrate diet could cause insulin resistance and obesity. [16][17][18][31][32]
At the same time, it’s now well established that low-carbohydrate diets can help reverse insulin resistance and diabetes, as well as being useful for both weight loss and preventing obesity. [6][8][18][19]
Two studies hypothesized that as a high carbohydrate diet raises the urinary pH, it could cause struvite crystals and even blockage. [1][22]

For all these reasons, while cats can tolerate and digest high levels of carbohydrates, I prefer to feed them low carbohydrates diets (less than 5 or 10% of the metabolizable energy).

Note

When considering the amount of carbohydrates in a diet, it’s also important to consider the type of starch as they won’t all have the same effect on blood glucose. Starch with a high glycemic index (like rice) will raise the blood glucose level higher than starch with a lower glycemic index (like barley or sorghum). [23]

Can some carbohydrates have benefits?

While diets low in carbohydrates are recommended, it doesn’t mean that some specific carbohydrates can’t be beneficial. In fact, the carbohydrates family is wide and can’t be summarized exclusively to starches.

Fibers are part of the carbohydrates and while not being essential (meaning that they are not required to keep your cat alive), they can have benefits, especially for the gastrointestinal health of your cats. We explored more in detail the benefits of fibers, and how much you need to add to your cat’s diet, in the article dedicated to fibers.

Vegetables are another example of plant food that got a bad reputation because they are plant matter and contain starch. However, while on a dry matter basis vegetables are high in fibers and starch when fed as is, they mainly contain water. Adding vegetables can have some benefits as they are a source of phytonutrients. These compounds are interesting as they can have antioxidant properties (like beta-carotene), lower the risks of cancer (like flavonoids), anti-inflammatory, … [21][24][27][28]
As they contain lots of water, vegetables like zucchini are helpful to increase the water intake, which can have benefits for cats having recurrent urinary issues. [30]

Finally, in some very specific disease cases, like late-stage chronic kidney diseases, adding starch to the diet can be a way to reduce the phosphorus content of the diet while still providing enough energy.

Why do kibbles contain moderate to high levels of starch?

Knowing the metabolic adaptations of cats for low carbohydrates, it’s legitimate to try to understand why kibbles contain moderate to high levels of starch.

The first reason is very practical: starch is required for the extrusion process of kibbles. When cooked, starch gelatinize and it is the glue that holds the kibbles together. [2][3]

The second reason is economical: starch is a cheaper source of energy than animal proteins. Providing energy with carbohydrates is said to “spare” proteins. In fact, as we have seen in the article about energy, the first thing the body will do with the macronutrients is to fulfill its energy needs. In the case of the proteins that means the amino acids will first be used to provide energy before being used for their metabolic roles like tissue repairs. So, by providing energy as carbohydrates–which don’t have unique metabolic roles–amino acids can be used exclusively for their metabolic roles and so are needed in lower quantities. [2][3]

If you feed kibbles, you need to make some calculations to determine the amount of carbohydrates in the food. In fact, manufacturers don’t need to indicate the amount of carbohydrates on the label. You can calculate an approximate value using this formula:

Carbohydrates = 100% - protein % - fat % - crude fiber % - mineral content (ash) % - moisture %

This is an estimation, as this number will include both the starch and soluble fibers, while insoluble fibers are in the crude fiber percentage. This estimated number is often called NFE for Nitrogen Free Extract.

Note on grain-free kibbles

It has been a few years since grain-free kibbles have been popular while at the same time carbohydrates have been heavily demonized. However, it’s important to note that grain-free doesn’t mean without carbohydrates: in fact, grains are replaced by other sources of starch like legumes or potatoes. As we have seen, a source of starch is mandatory for the extrusion process. The sources of starch used to replace grain are often less digestible and can create maldigestion like bloating, gas, or diarrhea. [25][26]
So, if you want to feed a lower amount of carbohydrates, grain-free kibbles aren’t necessarily a good idea. The best way is simply to verify the starch content of the kibbles you are feeding.

To summarize, as there are many types of carbohydrates, we cannot only consider starches. Contrary to popular belief, cats can digest starches when cooked, but they indeed can’t tolerate high amounts of starch. The metabolism of cats is adapted to a low carbohydrates diet. Nevertheless, some specific carbohydrates such as fiber can have health benefits.

Sources

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[32] Leyhausen P (1979) Cat behavior. The predatory and social behavior of domestic and wild cats. Garland STPM Press, New York