Nutrition has implications in all aspects of skin metabolism including cell renewal, wound healing, immune reactions and combating chronic inflammation. There are a number of ways, therefore, in which nutrition can have an impact on dermatology. Nutrition must not result in undesirable effects through deficiencies, imbalances or allergies. It must contribute to improvements in the quality and appearance of the coat, and nutrition must assist and enhance the skin’s barrier effect providing better prevention against dermatoses and dermatitis.
The importance of proteins
The cutaneous tissue (skin, subcutaneous tissue and coat) forms the largest organ of the body: it represents 12 to 24 percent of the weight of a dog. Moreover, it undergoes an intense renewal process: the epidermis is replaced approximately every 22 days in dogs, and the annual production of hair varies (depending on the breed) between 60 and 180g per kg of weight. These few figures show the intense level of skin and coat requirements, both in terms of their functions and structure.
Proteins represent about 95 percent of the hair structure in cats and dogs. It is reported that 25 to 30 percent of daily protein intake is systemically used solely for skin and coat renewal requirements. Any protein deficit quickly results in obvious effects on the coat: hair diameter and the size of the pilous bulb reduces, hairs become brittle, pilous follicles become dormant and the skin dies.
A protein deficit also affects the skin in the form of keratinisation anomalies: the skin becomes thin and less supple and healing of wounds is adversely affected. Sores and decubitus ulcers are observed as possible consequences of protein deficiencies. The protein deficit is also expressed as immune incompetence and as much greater sensitivity to skin injuries and infections.
The minimum recommended protein intake is 18 percent DM for an adult dog and 25 percent DM for an adult cat. However, the requirement becomes 25 to 30 percent DM for dogs and 30 to 45 percent DM for cats if skin and coat maintenance is of prime importance.
The quantity of protein is not the only factor to be taken into consideration; proteins must also be easily digestible and provide all the essential amino acids required by the cutaneous metabolism. A single limiting amino acid in the diet is all that is needed to disturb protein synthesis and for the dermatological signs described above to appear.
Amino acids and the structure of the skin and coat
Keratin is an essential constituent of the stratum corneum of the epidermis and of hair. It is essentially composed of the sulphur-containing amino acids methionine and cysteine. Sulphur-containing amino acids are most abundant in proteins from an animal source.
Amino acids and coat colour
The quality of the protein intake can affect the colour of the coat. The synthesis of melanins, which are responsible for the colour of the coat (dark eumelanins and yellow and orange pheomelanins), essentially depends upon a sufficient precursor amino acid intake. An insufficient intake of aromatic amino acids (phenylalanine and tyrosine) can adversely affect the synthesis of eumelanin: a black cat or dog can acquire areas of brown lustre and tawny colours become lighter in hue.
Amino acids and cutaneous metabolism
Special attention must also be paid to levels of glutamine and arginine in the diet. Glutamine is the preferred energysource substrate for rapidly regenerating cells, such as immune cells and fibroblasts. Arginine plays an important role in immunity regulation. It also encourages the synthesis of collagen and acts as a precursor of proline. Finally, the production of nitric acid from arginine stimulates the expression of the vascular endothelial growth factor.
A high level of animal protein in the diet ensures a high intake of sulphur-containing amino acids, essential for proper regeneration of the cells of the skin and of the coat. A high protein intake also helps to reduce the risk of any possible deficit of tyrosine and of amino acids which are necessary for the synthesis of melanin coat pigments.
A full reference list is available on request