How Do Dietary Protein Concentration and Quality Affect Feline Immune Function?

Immune function in healthy adult cats is only marginally affected by dietary protein concentration and quality, according to a study recently published in PLoS ONE.

Dietary proteins and amino acids play important roles in feline immune function. Dietary proteins can trigger allergic reactions in cats. Arginine regulates cytokine production and mediates autoimmune diseases. To date, though, little research has been done to assess how variations in dietary protein levels affect immunologic parameters in cats, particularly healthy cats.

At the Institute of Animal Nutrition in Berlin, 10 healthy adult cats were fed canned food diets with different protein concentrations and qualities. Three diets contained high-quality protein (greater proportion of meat and blood meal), and three contained low-quality protein (greater proportion of collagen-rich material). Protein concentrations were categorized as low (36%—37%), medium (43%–45%), or high (55%–56%). Dietary sunflower oil amounts were adjusted to ensure comparable energy density among the diets.

In a randomized crossover design, cats were individually fed each diet during 6-week feeding periods. At the end of each feeding period, the authors fasted the cats and collected their blood. Blood samples were used for the following analyses:

• Differential blood cell count
• Leukocyte phenotyping of B cells, T-helper cells, T-killer cells, antigen-presenting cells, and myeloid cells
• Lymphocyte proliferative activity
• Monocyte and granulocyte phagocytic activity
• Tumor necrosis factor alpha (TNF-α) and interferon gamma (INF-γ) concentrations

The authors also analyzed the nutrient and amino acid compositions of each diet.

On the differential blood cell count, the authors observed a linear effect of eosinophilic granulocyte percentage. This percentage increased with increasing dietary protein concentration, independent of protein quality. This linear effect suggests immune activation with higher dietary protein intake. Because the highest eosinophilic granulocyte percentage was still within normal range, however, the authors believed the linear effect indicated a general immunostimulating effect with high dietary protein levels. Neither dietary protein concentration nor quality significantly affected percentages of other blood cells (lymphocytes, monocytes, neutrophilic and basophilic granulocytes).

Phenotypic results indicated similar percentages of leukocyte subtypes among diets.

Lymphocyte proliferative activity was assessed by performing a proliferation assay with several mitogens (pokeweed mitogen, concanavalin A, and phytohemagglutinin, M form) and a negative control (cell culture media). Neither dietary protein concentration nor quality significantly affected mitogen-stimulated lymphocyte proliferative activity.

Phagocytic activity was evaluated using PHAGOTEST^TM, a commercial diagnostic kit. Monocyte phagocytic activity was highest with the lowest dietary protein concentrations. The authors believed this increased activity was not due to protein but to another ingredient—sunflower oil—in the diets. Previous studies have reported that conjugated linoleic acids increased macrophage phagocytic activity in dogs, cats, and humans. In the current study, sunflower oil amounts were highest in the diets with the lowest protein concentrations; the higher fatty acid content in these diets could have contributed to increased monocyte phagocytic activity.

Granulocyte phagocytic activity and numbers of phagocytic monocytes and granulocytes were not significantly affected by dietary protein concentration or quality.

Supernatant from the proliferation assay was used to measure TNF-α and INF-γ concentrations. TNF-α and INF-γ supernatant concentrations did not vary significantly according to dietary protein concentration or quality.

For future studies, the authors suggested evaluating the relationship between fatty acids and feline monocyte phagocytic activity, as well as the relationship between dietary protein concentration and blood eosinophil levels.

Dr. Pendergrass received her doctorate in veterinary medicine from the Virginia-Maryland College of Veterinary Medicine. Following veterinary school, she completed a postdoctoral fellowship at Emory University’s Yerkes National Primate Research Center. Dr. Pendergrass is the founder and owner of JPen Communications, LLC.