Japanese eel (Anguilla japonica) as a novel protein in cat food improved coat smoothness, hair structure and antioxidant capacity in a 56-day feeding trial conducted in China. Researchers also documented lower blood triglyceride concentrations.
The study, published June 16, 2025, in Frontiers in Veterinary Science, evaluated Japanese eel as a partial replacement for chicken in baked cat food. Scientists from Zhejiang University, the Shanghai Academy of Agricultural Sciences and pet food manufacturers conducted the research.
However, the International Union for Conservation of Nature (IUCN) Red List classifies Japanese eel as Endangered. Commercial aquaculture depends on wild-caught juveniles raised to market size in captivity. While this isn’t the only pressure on wild eels, increased demand could affect the endangered species.
Feeding trial compared three eel inclusion rates
In the experiment, researchers divided 24 healthy adult cats, averaging 3.61 kilograms in body weight, into three groups of eight. Each group received one of three formulations:
- Control: 0% Japanese eel and 68% chicken breast
- Treatment 1: 14% Japanese eel and 54% chicken breast
- Treatment 2: 40% Japanese eel and 28% chicken breast
Eel diets improved coat quality and hair structure
Researchers used scanning electron microscopy to examine the structure of individual hairs. After 56 days, cats receiving the 40% eel diet had significantly higher coat softness and luster scores than control cats.
On a three-point scale, average coat softness reached approximately 3.0 in the 40% eel group, compared with 2.2 in the control group. Hair luster scores were also approximately 3.0 and 2.4, respectively.
Both eel-containing diets produced statistically significant reductions in hair cuticle scale thickness.
Omega-3 fatty acid concentrations increased
Replacing chicken with Japanese eel increased the omega-3 fatty acid content of the experimental diets. Omega-3 fatty acids represented 0.36% of dry matter in the control diet, compared with 0.88% in the 14% eel diet and 0.98% in the 40% eel diet.
After 56 days, both eel-fed groups had significantly higher serum total antioxidant capacity than control cats. The 14% eel group showed the greatest increase.
Blood triglyceride concentrations decreased
Blood triglyceride concentrations were significantly lower among cats receiving eel-containing diets.
| Diet | Serum triglycerides (mmol/L) |
| 0% eel | 0.454 |
| 14% eel | 0.345 |
| 40% eel | 0.321 |
Other serum measurements, including total protein, albumin, total cholesterol and alanine aminotransferase activity, did not differ significantly.
Although cats consuming eel-containing diets experienced numerically lower average daily weight gain, the differences were not statistically significant. Average daily gain was approximately 2.4 grams in the control group and 2.1 grams in the 40% eel group.
Daily food intake and fecal scores also showed no significant differences among the groups. Neither crude protein nor crude fat digestibility differed significantly among treatments.
The researchers concluded that Japanese eel could replace a portion of chicken in adult cat diets while improving several measured nutritional and coat characteristics. They favored the 14% inclusion rate because the higher level generally did not produce additional statistically significant benefits and eel costs more than chicken.
The research received funding from the Shanghai Academy of Agricultural Sciences and Hangzhou PETISALL Biotechnology Co., Ltd. Several authors were employed by PETISALL or PETIDEAL Pet Food.
Japanese eel conservation status raises sourcing questions
Japanese eel is classified as Endangered on the International Union for Conservation of Nature (IUCN) Red List, with a declining wild population. Although commercial aquaculture raises eels to market size, production remains dependent on wild-caught juveniles known as glass eels. However, researchers have not conclusively established the extent to which glass eel fishing contributes to the species' decline. Habitat degradation, pollution and changing ocean conditions are also potential factors.
Expanding commercial use of Japanese eels could create additional demand for a species already classified as Endangered. If that demand were supplied through conventional eel aquaculture, it could increase demand for wild-caught glass eels.
Sources
Yuan, Y., et al. (2025). The effect of Japanese eel as a main ingredient on hair condition, antioxidant ability, apparent total tract digestibility and body weight gain in cat food. Frontiers in Veterinary Science, 12, 1553320.
IUCN. Anguilla japonica — Japanese Eel. The IUCN Red List of Threatened Species.
Japan Fisheries Research and Education Agency. Glass Eel Production Division.
Ijiri, S. (2025). History of the Development of Artificial Maturation and Seedling Production in Japanese Eel: Past, Present and Future. FFI Journal, 230(3), 193–199.
