Soy is widely used in cat food. It is an inexpensive source of protein. While some plants (e.g. soy) may have substantial amount of protein, there are can be downsides of feeding cats foods they are not prone to eating in nature. In case with soy and soy-derivatives, there’s a risk of endocrine disruption due to high amount of isoflavones.
Soy and soy products may cause imbalance of thyroid hormones in cats. This study demonstrated an increase of serum T4 thyroxine after only 3 months of dietary soy consumption. The increase in thyroid hormones can cause an increase in appetite. Interestingly, the study on dogs did not demonstrate similar results. This may be attributed to species differences in soy isoflavone metabolism and excretion.
The fact that cats are obligate carnivores (hypercarnivores) means not only that meat is the basis of their nutrition (70%), but also that their physiology has developed differently from meso- (50%) and hypocarnivores (<30%). Cats are not able to metabolize soy isoflavones as effectively as most other species. [study]. This is simply because cats have never had to deal with consuming substantial amounts of plants and their body didn’t have the need to develop such mechanisms.
All that being said, a small amount of soy protein isolate in cat food doesn’t appear to cause problems for cats. It is a highly digestible protein (not balanced for cats, but the food is balanced in the end as other amino acid sources are added).
Sources
- White HL, Freeman LM, Mahony O, Graham PA, Hao Q, Court MH. Effect of dietary soy on serum thyroid hormone concentrations in healthy adult cats. Am J Vet Res. 2004 May;65(5):586-91. doi: 10.2460/ajvr.2004.65.586. PMID: 15141877.
- Redmon JM, Shrestha B, Cerundolo R, Court MH. Soy isoflavone metabolism in cats compared with other species: urinary metabolite concentrations and glucuronidation by liver microsomes. Xenobiotica. 2016;46(5):406-15. doi: 10.3109/00498254.2015.1086038. Epub 2015 Sep 14. PMID: 26366946; PMCID: PMC4967369.
