Sugars - Nutrition

What are Sugars?

Sugars are short chain soluble carbohydrates, which include the monosaccharides glucose, galactose, and fructose and the disaccharides sucrose (combination of glucose and fructose), lactose (combination of glucose and galactose) and maltose (combination of glucose and glucose). The sugar alcohols mannitol, sorbitol, and xylitol are commonly included in this group, though are not used in foods intended for dogs and cats.

Why are they Important?

Dogs have taste receptors and a flavour preference for sugars,1 while cats lack lingual sweet receptors 2 and tend not to select preferentially for or against sugars in the diet. The monosaccharide sugars glucose, galactose, and fructose are readily transported across the small intestinal mucosa. Glucose itself can be transported directly into cells for further metabolism to form ATP, can also be used to form glycogen in liver or muscle3, or for lipid synthesis.4 Sugar alcohols exert an osmotic affect in food and in the body.5 Sorbitol and xylitol are primarily used as sweeteners or thickening agents in foods intended for human consumption and are not used in foods for dogs and cats. Mannitol is used therapeutically in veterinary medicine as an osmotic agent to decrease swelling in body compartments such as the brain or interstitium.

Roles in the Body

All body tissues have a requirement for glucose and it must be obtained regularly from either the diet or synthesised through hepatic gluconeogenesis. Once absorbed through the intestinal epithelium, glucose enters the portal circulation and can be undergo intercellular transport for intermediate metabolism and formation of ATP. Glucose, galactose and fructose can also be used to synthesise additional molecules, such as glycogen and fatty acid.3,4 In the absence of dietary starches and sugar, or during periods of starvation, hepatic gluconeogenesis can support maintenance of normal blood glucose concentrations in adult animals as long as adequate amounts of gluconeogenic amino acids are present.6

Consequences of Sugar Deficiency

Dogs:

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 absorbable carbohydrates.7 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.6 There are no clinical signs of feeding a sugar-free diet in otherwise healthy adult dogs.

Cats:

There are no clinical signs of feeding sugar-free diet to cats at any life-stage. Adult cats and growing kittens are able to maintain blood glucose concentrations through hepatic gluconeogenesis.8

Toxicity

The sugar alcohol, xylitol, is toxic to dogs and cats and ingestion can lead to hypoglycaemia, liver failure and death9 and should not be fed to dogs and cats. No toxicity has been associated with high dietary intake of other sugars in otherwise healthy dogs and cats, though cats do not appear to regulate fructose and galactose excretion and will readily become fructos- or galactosuricuric if these monosaccharides are given directly in the diet.10 Increased feeding of sugars in dogs and cats with pre-existing diabetes mellitus can contribute to post-prandial hyperglycaemia and increase insulin requirements.11,12

Dietary Sources

Naturally occurring sugars are found in fruits, cereal grains, and pulses (i.e., legumes). Sucrose may be added to foods as sweeteners or improve texture. Sorbitol is not commonly found in commercial pet foods, but may be included in commercial pet treats or human foods to improve texture.

Diagnosing Sugar Deficiency

If low to absent total carbohydrate intake with insufficient hepatic gluconeogenesis, clinical signs of hypoglycemia can occur (e.g., lethargy, depression and seizures). Blood sugar concentrations below the normal laboratory reference interval (< 5 mmol/L) may be noted on serum biochemistry profiles.