Methionine and cystine are two sulfur amino acids that the calculator shows together - because together they create a pool of sulfur available for many key processes. Methionine must come from the diet (the body will not produce it), cystine can be synthesized from it, but not the other way around - it is a one-way relationship [1]. In a well-balanced meat diet, they are rarely really missing, but a low result in the calculator is a clear signal of poor protein quality (see Protein).
Methionine is the primary methyl donor and precursor of S-adenosylmethionine (SAMe) - important for liver, detoxification and DNA methylation. Cystine is a precursor of glutathione, the most important intracellular antioxidant. Hence, deficiencies cause oxidative stress. Together they provide sulfur for the synthesis of keratin (hair, claws, pads). In dogs, they are also a substrate of endogenous taurine synthesis; the cat synthesizes taurine very poorly, so - note - a good supply of sulfur amino acids does not replace separate taurine control in the cat [1].
Species difference: A cat has a higher demand than a dog because, as an obligate carnivore, it cannot “save” protein - it constantly breaks it down, and also uses sulfur amino acids for the synthesis of felin (an amino acid specific to cat urine) [1] [3].
What it’s found in: The best sources are red muscle meat (beef, lamb, venison), hearts (especially beef and lamb - clean muscle with an excellent profile), eggs (very high concentration per gram of protein) and fish. Poultry, contrary to popular belief, is not a weak source - the problem is monotony, not the species. Poor sources are collagen tissues: stomachs, lungs, skins, udders, tendons, cartilage - they can be an addition, but not a protein base.
Deficiency: Rare in a diet with an adequate amount of muscle meat. A low result with high total protein is an almost certain signal that the diet is dominated by collagen protein at the expense of muscle - this is one of the most diagnostic parameters in the entire [2] calculator. Symptoms: dull, brittle fur, brittle claws, slower wound healing, reduced immunity, slow growth in puppies and kittens. In cats, the deficiency is more serious - it limits the already inefficient synthesis of taurine (risk of DCM, retinal degeneration). The demand increases during intensive shedding and convalescence.
Excess: Unlikely to be made from natural raw materials. The problem concerns supplementation with synthetic DL-methionine: it acidifies the urine (dangerous in case of oxalate urolithiasis), increases the level of homocysteine, burdens the liver and kidneys, and may also disturb the concentration of other [2] amino acids. A meat diet already naturally acidifies the urine, so additional acidification is usually unnecessary. Do not supplement DL-methionine without a diagnosed reason, calculated dose and veterinary supervision.
Cofactor dependency and what to do if your score is low
Methionine metabolism requires vitamins B6 (conversion to cysteine), B12 and folate (homocysteine remethylation) - with regular liver and a variety of organ meats these cofactors are usually supplied. With a low bar, first check the ratio of muscle meat to connective tissue, increase the proportion of meat and hearts, consider adding eggs (in a cat, also looking at arginine), make sure there is liver. The supplement is a last resort.
Sources
- National Research Council (NRC) (2006). Nutrient Requirements of Dogs and Cats, Chapter 4 - Proteins and Amino Acids. National Academies Press. https://nap.nationalacademies.org/catalog/10668/nutrient-requirements-of-dogs-and-cats
- Hand MS, Thatcher CD, Remillard RL, Roudebush P, Novotny BJ (2010). Small Animal Clinical Nutrition, 5th Edition. https://www.markmorrisinstitute.org/sacn5.html
- Pezzali JG, et al. (2024). Minimum methionine requirement in adult cats. J Anim Sci. https://doi.org/10.1093/jas/skad411
- FEDIAF (2021). Nutritional Guidelines for Complete and Complementary Pet Food for Cats and Dogs. https://europeanpetfood.org/self-regulation/nutritional-guidelines/