Yeah. Apparently it’s a thing.

It is even enough of a thing that a group of scientists from Sao Paulo State University in Brazil decided to study it. More precisely, they studied a few dietary components that might contribute to unpleasant smells produced by members of this adorable (but sometimes stinky) breed.

Is flatulence a problem?

First. All dogs fart (even Stanley, my fussy Toller, who insists he does not). Second, fecal matter always has some unpleasant odor, though this can vary enormously. So, if you are a dog owner who is hoping to find the dietary Holy Grail that will completely prevent gas and make your dog’s poop smell like lilacs, better adjust your expectations. Or instead, get a stuffed dog. By all accounts, they are fart-free.

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What’s that Smell?

In healthy dogs, flatulence and smelly feces are normal and occur as a result of several things. Intestinal gas can originate from swallowed air, diffusion of gas from the dog’s bloodstream, and most significantly, production of gas via bacterial fermentation of undigested food particles in the large intestine. It is these latter gases that provide the unpleasant odors that owners may object to. (Not to put too fine a point on this, but the same process occurs in humans).

Interestingly, up to 99 percent of the gases that a given dog produces are completely odorless. However, that final 1 percent….. uh oh. These gases pack quite a punch. They include hydrogen sulfide (rotten eggs), methanethiol (dirty socks), and dimethyl sulfide (boiled cabbage). No wonder owners of French Bulldogs do not allow them on the couch. Additional compounds that contribute to odor, called volatile organic compounds (VOCs), are found in the feces.

Does Diet Influence Odor?

You bet it does. Dietary protein, carbohydrates and fiber can all influence the types of gas and VOCs that are produced and excreted. However, it is the end products of protein fermentation that are most odiferous. Dietary protein that is incompletely digested in the small intestine moves to the large intestine where bacterial fermentation occurs, resulting in the production of ammonia, amine groups, indole, phenol, and several other compounds – all of which contribute to unpleasant smells in feces. In foods, the protein source, its degree of processing, and level digestibility are important factors in determining how much gas and odors are generated when fed to dogs. (Note: There are additional, and potentially more serious reasons to be concerned about these fermentative end products. However, for our purposes in this essay, we are focusing on odors only).

Why French Bulldogs?

Well, although I could find only anecdotal evidence, it appears that French Bulldogs have a reputation for being quite flatulent. This is so much “a thing” that French Bulldog information websites warn potential adopters about this trait ahead of time. Two explanations that are provided are that the breed is known to have sensitive stomachs (whatever that means) and that because of their brachycephalic facial structure, individuals tend to swallow air while eating. Regardless, this breed is known (at least anecdotally) to be gassy and the Brazil researchers found a group of healthy adults to study.

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The Study

A group of eight adult French Bulldogs were enrolled in the study. The researchers fed them four different protein source combinations, included in a dry, extruded dog food. These were: (1) poultry meal; (2) wheat gluten meal; (3) a blend of poultry meal and wheat gluten meal; and (4) a blend of poultry meal, wheat gluten meal and a hydrolyzed protein. The study design involved feeding each food for 28 days prior to assessments and allowed each dog to be fed all four foods. Measurements included fecal VOC quantification, and sensory (smell) analysis of fecal odor by trained human evaluators (yeah, you read that right).

Results: All of the dogs farted and produced smelly feces. No surprise there (at least to French Bulldog owners). The researchers also detected several dietary influences:

• VOCs: A group of 78 different compounds were detected in the dogs’ feces. Of these, short chain fatty acids and two important VOCs, indole and phenol, were present in highest concentrations. Phenol and indole are produced from undigested protein and are also responsible for the infamous and quite unpleasant “poop smell” that anyone who has wielded a poop bag will recognize. Short-chain fatty acids are equally unpleasant – they have the odor of vomit.
• Feces quality: No adverse effects were observed; all of the dogs produced normal and well-formed feces when fed each diet.
• Protein source: Feeding poultry meal lead to a high level of phenol in feces while feeding wheat gluten resulted in high levels of indole. The human evaluators (68 different people) reported significant differences in fecal odor among the four diets. Specifically, when dogs were fed foods containing poultry meal alone, fecal odor was strongest and most unpleasant. Adding wheat gluten protein lead to reduced fecal odor.

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Take Away for Dog Folks

Regardless of the breed of dog that you live with, this research (plus previous studies discussed in the paper) provides evidence that the source and quality of dietary protein significantly influences the production of odiferous compounds in dogs’ feces. Most importantly, undigested protein that reaches the large intestine and is broken down by bacteria produces compounds that contribute to highly unpleasant odors. While this study did not directly measure French Bulldog farts, it can be assumed that at least some of the odors found in their feces contributed to their gaseous emissions as well (though certainly, that is a topic in need of further study). We also know from previous work that the amount of undigested protein that ends up in the large intestine is influenced by both the total level of dietary protein as well as its digestibility.

What we do not know, and what has not (yet) been studied, is the influence of thermal processing on a protein source’s tendency to produce indoles, phenols and other odiferous compounds in a dog’s flatulence and feces. The current paper, along with previous work, studied only dry extruded foods containing rendered animal protein meals. These meals originate from various types of animal by-products and are subject to handling and heat treatment that can damage dietary protein. It would be interesting to know, for example, if there are differences in the production of phenols, indoles, and other VOCs when chicken (or another protein source) is fed to dogs in raw, freeze-dried, dehydrated or gently cooked form. In other words, are the smelly poops reported in this study caused by the protein source itself (as the authors suggest) or do various processing effects associated with rendering and extrusion have an influence?

Regardless, feeding a high quality food (protein source) appears to be not only important for our dog’s health and wellness, but possibly also for our own comfort in terms of how pleasant it is to be around our dogs and how unpleasant picking up in the yard might be.

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Cited Study: Urrego MIG, Pedreira RS, Santos K, et al. Dietary protein sources and their effects on fecal odour and the composition of volatile organic compounds in faeces of French Bulldogs. Journal of Animal Physiology and Animal Nutrition 2021; 105:65-75.

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