For dogs with diarrhea, fat-restricted diets have long been suggested, largely because the digestion and absorption of fats are much more complex than for other nutrients. As a result, fat malabsorption or maldigestion can develop in response to many different conditions, and fermentation of undigested fats in the colon can lead to secretory diarrhea and intestinal inflammation.

However, diets high in fat have been recently suggested for cats with diarrhea, largely because more calories in smaller volumes of food can be achieved. The effects of either a fat-restricted or high-fat diet on chronic diarrhea in cats have not been well-studied. But recently published research seeks to begin an investigation into the role of dietary fat in chronic diarrhea in cats.

Study methods
This study featured 55 client-owned cats with a history of chronic diarrhea that were free of infectious disease and had no evidence of systemic disease affecting the gastrointestinal tract. The cats had a minimum of three episodes of diarrhea per week for at least one month before the study and had continued to experience diarrhea after treatment with a full-spectrum intestinal parasiticide, including treatment for Giardia species. Cats requiring immediate medical care or those that had been treated with long-acting corticosteroids within two weeks of the study were excluded. Throughout the study, participants could not receive corticosteroids, gastrointestinal motility modifiers, oral antibiotics, parasiticides, or other medications.

The cats were randomly assigned to one of two groups for a six-week period. The cats were fed either a high-fat (20% dry basis) or a low-fat (10% dry basis) diet for the duration of the study. Both experimental diets used the same ingredients (adjusted only for fat content) and were formulated to be highly digestible and provide complete nutrition. Owners were unaware of which diet their cats were receiving. The owners agreed to feed only the assigned diet, give no medications, and keep a daily journal recording a fecal score (from watery to firm or dry feces on a 0-100 scale) for each defecation. The daily fecal scores for an entire week were then averaged, and the averages for weeks 1, 3, and 6 were used to evaluate the response.

Study findings
Significant improvements were seen in fecal score for most (78.2%) cats participating in the study, regardless of diet group. This finding suggests that dietary change alone rather than fat content may be more important when treating chronic diarrhea in cats. The protein sources in both diets—soy and turkey—were common to many commercial cat foods and did not represent a novel protein for the cats in the study. All of the cats that responded to the diets did so within one week and reached peak improvement at three weeks. Furthermore, 36% of the cats developed normal stools.

While subtle, an association was also seen between low serum B₁₂ concentrations and fecal scores. The response to the diets was poorer for cats with B₁₂ concentrations below the reference range at baseline, and the degree of improvement was also less. Serum B₁₂ concentrations were not reevaluated at any time, so, as the authors of this study note, it is not known whether the deficiency improved along with the fecal scores or whether these cats would have had a more significant response to the diets had their low B₁₂ concentrations been corrected.

Finally, no definitive diagnosis was made for any of the study participants, so it is not known whether the nonresponders had different causes of their chronic diarrhea than those that had improvement. Based on this new data, dietary change for cats with chronic diarrhea appears to be beneficial and should be considered when treating these cases, but the fat content of the diet seems to be unimportant.

Laflamme DP, Xu H, Long GM. Effects of diets differing in fat content on chronic diarrhea in cats. J Vet Intern Med 2011;25(2):230-235.

Link to abstract: http://onlinelibrary.wiley.com/doi/10.1111/j.1939-1676.2010.0665.x/full