Does hydrolysed fish protein calm dogs? What the evidence says

Zylkene Plus combines hydrolysed casein with white fish muscle hydrolysate. That raises two questions. Does the fish ingredient calm dogs on its own? And does adding it to casein make the supplement more effective? The published studies give us little reason to answer yes to either. Zylkene Plus product information

The fish hydrolysate trial didn’t show a reduction in overall fear

In 2015, Landsberg and colleagues gave 45 beagles one of two doses of fish hydrolysate or a placebo and played them a recording of thunder. The title describes a modification of the dogs’ response to noise. It stops short of saying the supplement reduced anxiety. The results show why that distinction matters. Landsberg et al., 2015

There was no significant difference between groups in overall fear and anxiety scores, distance travelled or passive fear scores. The change in post-thunder cortisol also did not differ significantly between groups. The significant cortisol results highlighted by the authors came from comparing treated dogs with their own baseline. The trial included a placebo group precisely so the researchers could test whether the treated dogs improved more than dogs given placebo - and there was no statistical difference.

One statistical test found that active fear scores varied across groups, sessions and phases of the thunder test. The paper did not report a direct comparison showing that either fish dose outperformed placebo on that score. The graphs are small and pixelated, which makes the markers and error bars hard to read and limits what readers can interpret from them, however they appeared to show more inactivity in the groups that were given fish. Landsberg et al., 2015, Results

The same thunder test gave movement opposite meanings

In the original paper validating the thunder test, the researchers interpreted more inactivity during thunder as a sign of fear, including freezing. Diazepam reduced that inactivity, and the researchers took that as evidence that the dogs were less fearful. In the fish hydrolysate paper, two of the same researchers interpreted less activity as evidence of a calming effect, possibly to align with the results found in the fish treated groups (above). Araujo et al., 2013 · Landsberg et al., 2015

Does adding fish hydrolysate to casein help?

The 2023 Zylkene Plus study reported improved separation-related behaviour scores over 30 days. Everyone knew the dogs were receiving the supplement, and the study had neither a placebo group nor a group that received casein alone. A separate placebo-controlled study tested fish hydrolysate with melon concentrate. Neither study can tell us what the fish ingredient contributed. Porcheron et al., 2023 · Titeux et al., 2021

I don’t see convincing evidence that hydrolysed fish protein calms dogs on its own. The combination studies give me no reliable basis to conclude that it does anything extra when mixed with hydrolysed casein. 

Overall, the research suggests that when choosing a supplement containing alpha-casozepine, you need to be careful that you aren't paying extra for added ingredients that are not providing any additional benefit.