Carbohydrates Overview - Nutrition
What are Carbohydrates?
Carbohydrates (saccharides) are a group of carbon-based molecules, that are naturally found as monosaccharides, disaccharides, or polysaccharides. Carbohydrates are a common source of energy, however they are not considered an essential nutrient in dogs or cats. Functionally carbohydrates are classified as absorbable, digestible, fermentable or non-fermentable. When consumed in food, absorbable and digestible carbohydrates typically provide approximately 4 kcal of energy per gram, whereas fermentable and non-fermentable carbohydrates provide little to no usable energy.
Absorbable carbohydrates include monosaccharides, such as the glucose, fructose, and galactose; the sugar alcohols mannitol, sorbitol and xylitol; disaccharides, such as lactose (glucose + galactose), sucrose (glucose + fructose) and maltose (glucose + glucose); and the polysaccharides such as starch, glycogen (from animal muscle and liver) and amylose and amylopectin from plants.
Fermentable and non-fermentable polysaccharides include resistant starches (formed from absorbable starch during the cooking process); non-structural short-chain oligosaccharides, such as fructooligosaccharides (FOS); and structural non-starch polysaccharides from plant cell walls, (commonly referred to as dietary fibre) such as cellulose, lignin and hemicellulose.
Why are they important?
Dietary carbohydrates are not considered essential nutrients for dogs and cats, but all animals have a metabolic requirement for glucose. Additionally, the by-products of bacterial fermentation of oligosaccharides and polysaccharides (short-chain fatty acids) support optimal intestinal function and health.
Roles in the Body
- Energy Production: All cells in the body have a requirement for glucose and it must be obtained regularly either from the diet or synthesised through hepatic gluconeogenesis. Glucose absorbed from the diet can be used directly in intermediate metabolism (ATP production) or to synthesise glycogen and fatty acids. In the absence of dietary starches or sugars, hepatic gluconeogenesis can support maintenance of normal blood glucose levels, gluconeogenic amino acids and glycerol in dogs and cats.
- Intestinal Health: Non-absorbable carbohydrates (oligosaccharide and polysaccharide dietary fibres) are resistant to degradation by mammalian enzymes. These carbohydrates are more commonly referred to as dietary fibres and can be divided into two broader categories depending on whether they can be further metabolized (fermented) by intestinal bacteria or not.
- Fermentable, non-absorbable carbohydrates can be utilized as an energy substrate by bacteria in the large intestine. By-products of bacterial fermentation include CO, H2, methane, and the short-chain fatty acids acetate, propionate and butyrate, which can support optimal colonocyte function and intestinal health. Diffusion of acetate and proprionate across the colonic mucosa facilitates water reabsorption. Butyrate is the preferred energy substrate of colonocytes.
- Non-fermentable, non-absorbable carbohydrates include structural components of plant cell walls such as cellulose, lignin and bran. These forms of carbohydrates resist degradation by animal or bacterial enzymes and pass through the intestinal tract intact.
Cats and Carbohydrates:
Cats are able to digest and absorb dietary sugars and starches well but have low glucokinase activity in the liver and do not adapt carbohydrate metabolism to dietary intake. There has been controversy over the role of dietary carbohydrate in development of obesity and diabetes mellitus in cats but carbohydrate intake has not been shown to be a risk factor in development of obesity, hyperglycaemia or diabetes mellitus in otherwise healthy adult cats.
Consequences of Deficiency
Puppies, especially small and toy breeds, may be unable to maintain blood glucose concentrations from hepatic gluconeogenesis alone and can become hypoglycaemic with low intake of dietary carbohydrates. Hepatic gluconeogenesis may also be inadequate to meet glucose demands during late gestation and lactation unless increased intake of gluconeogenic amino acids are provided in the diet. There are no clinical signs of feeding a carbohydrate-free diet in otherwise healthy adult dogs, though some dogs with recurrent idiopathic colitis may benefit from feeding higher fibre diets.
There are no reports of clinical signs relating to feeding carbohydrate-free diet to cats at any life-stage. Adult cats and growing kittens are able to maintain blood glucose concentrations via hepatic gluconeogenesis.
The sugar-alcohol, xylitol (used as a low glycaemic index sweetener in many human foods) is toxic to dogs and cats and ingestion can lead to severe hypoglycaemia, liver failure and death. No toxicity has been associated with high intake of other carbohydrates in otherwise healthy dogs and cats, though in animals with pre-existing diabetes mellitus increased intake of sugars and starches can contribute to post-prandial hyperglycaemia and increase insulin requirements.
Excessive intake of non-absorbable carbohydrates (both fermentable and non-fermentable dietary fibres) can increase stool bulk and slow gastrointestinal transit time. This may potentially result in constipation in healthy dogs and cats or worsening dysmotility in animals with underlying intestinal disease.
Undigested disaccharides and starches can act as osmotic agents drawing water into the intestinal lumen and can be fermented by intestinal bacteria. Maldigestion and malabsorption of dietary carbohydrate (specifically starch) may be a feature of feline inflammatory bowel disease.
Dietary carbohydrates (both absorbable and non-absorbable) are primarily found in fruits, vegetables, cereal grains, and pulses (i.e. legumes). Animal sourced carbohydrates (glycogen) are also found in muscle and liver.
Diagnosing Carbohydrate Deficiency
If there is low to absent carbohydrate intake with insufficient hepatic gluconeogenesis, clinical signs of hypoglycemia can occur (such as lethargy and seizures). Blood glucose concentrations below the normal laboratory reference interval will be seen on serum biochemistry profiles.
Animals consuming inadequate dietary fibre may exhibit signs of colitis (e.g. tenesmus, hematochezia, mucousy loose stool, increased frequency of defecation), that resolves with addition of fibre to the diet.
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Date reviewed: 22 May 2015
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