The word “microbiome” is all the rage among dog researchers and dog folks these days. In most instances when you hear this term, it refers to a dog’s gut microbiome. However, another important microbiome system in the dog’s body is the skin. Similar to the gut, the skin’s bacterial makeup is comprised of multiple species, varies significantly among dogs, and is important for our dogs’ health. Changes in a dog’s skin microbiome have been observed with certain skin diseases and in response to environmental allergens. What is not known however, is how the food that a dog eats may influence the skin microbiome. For example, does feeding a fresh-cooked, moderately processed food result in different bacterial species in the skin compared with feeding extruded, dry dog foods? This is the question that a group of researchers recently asked (1).

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The Study

The primary objective of this pilot study was to examine the effects of feeding either a fresh-cooked dog food (i.e. moderately processed) versus feeding dry, extruded dog foods (i.e. highly processed) on the composition of the skin microbiome. A group of 7 healthy adult dogs living as pets in homes completed the study. All of the dogs were first fed a complete and balanced fresh-cooked food (Freshpet) for 30 days. Following a 4-day transition period, the dogs were switched to an extruded dry dog food (brands were not identified) and fed that food exclusively for 30 days. Skin samples for bacterial analysis were collected at the end of each 30-day feeding period.

Results

The fresh-cooked food was higher in moisture and in the macronutrients protein and fat, when compared with the dry extruded foods. Although total dietary fiber was similar between the products, the fresh-cooked food’s fiber was composed of a significantly higher proportion of soluble (fermentable) fiber, when compared with the dry foods.

The researchers found that the type of food that was fed significantly influenced the make up of the skin microbiome:

• Microbiome Diversity: When dogs were fed the fresh-cooked food for 30 days, their skin microbiome’s alpha diversity was significantly higher compared with when they were fed a dry food for 30 days. Alpha diversity is a measure of both the quantity and the number of different bacterial species in a sample. A numerical value is calculated that reflects overall bacterial heterogeneity. (Generally speaking, high alpha diversity in a microbiome sample is considered to be desirable).
• Species Affected: When dogs were fed the fresh-cooked food, total Staphylococcus species increased in number while Porphyromonas and Corynebacterium decreased. The significance of these specific changes is not fully understood.
• Conclusions: The authors conclude that switching dogs from a fresh-cooked food (the first 30-day period) to a dry, extruded food (second 30-day period) led to a decrease in skin microbiome diversity. However, because multiple dry dog foods were fed and because time (season) was not controlled (see below), it is not certain which dietary or environmental conditions may have influenced these changes.

Limitations

It is important to emphasize that this was a small (pilot) study with several limitations. Perhaps most glaring was the lack of control for an order effect. Typically, a study of this nature would employ a switch-back experimental design. It is pretty simple – half of the dogs are randomly assigned to each food type (fresh-cooked or dry) for the first 30-day period and then switched to the alternate food during the second 30-day period. (Note: I am truly baffled why this was not done…..). This design would have allowed the researchers to control for time- and order-related changes in the skin microbiome that might occur and that would not be attributable to diet. When we are talking about skin, season (time) is an important consideration. Because this was not done – all of the dogs were first fed the fresh-cooked food and then all were switched to the dry foods – the researchers really cannot say for certain that the food alone was the cause of the higher skin microbiome diversity during the first 30-day period. (Personally, while I imagine food was an influence, this gaff is a disappointment and detracts from how far we can go with this study).

The study was also very small – only seven dogs completed the entire trial. For this reason, this should be considered to be a pilot study or perhaps a Proof of Concept trial. A take-away is that this study suggests that the type of processing used in dog foods may influence skin microbiome diversity (and ultimately, skin health). As a proof of concept, it leads to that universal platitude – more studies are needed! (And indeed, there are more coming soon!)

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Cited Paper: Leverett K, Manjarin R, Laird E, et al. Fresh food consumption increases microbiome diversity and promotes changes in bacterial composition on the skin of pet dogs compared to dry foods. Animals 12, 1881; 2022.

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