Methionine and cystine

Methionine and cystine are two sulfur amino acids that the calculator shows together — because together they form the pool of sulfur available for many key processes. Methionine must come from the diet (the body won't produce it), cystine can be synthesized from it, but not the other way around — it's a one-way dependency [1]. In a well-composed meat diet they are rarely truly lacking, but a low result in the calculator is a clear signal of protein quality (see Protein).

Methionine is the main donor of methyl groups and a precursor of S-adenosylmethionine (SAMe) — important for the liver, detoxification, and DNA methylation. Cystine is a precursor of glutathione, the most important intracellular antioxidant. Together they supply sulfur for the synthesis of keratin (coat, claws, paw pads). In the dog they are also a substrate for endogenous synthesis of taurine; the cat synthesizes taurine very poorly, so — note — a good supply of sulfur amino acids does not replace separate taurine monitoring in the cat [1].

Species difference: The cat has a higher requirement than the dog because, as an obligate carnivore, it cannot "spare" protein — it constantly breaks it down, and on top of that uses sulfur amino acids for the synthesis of felinine (an amino acid specific to cat urine) [1] [3].

Where it's found: The best sources are red muscle meat (beef, lamb, game), hearts (especially beef and lamb — pure muscle with an excellent profile), eggs (very high concentration per gram of protein), and fish. Poultry, contrary to popular opinion, is not a poor 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 overall 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 calculator [2]. Symptoms: dull, brittle coat, fragile claws, slower wound healing, lowered immunity, slowed growth in puppies and kittens. In the cat the deficiency is more serious — it limits the already inefficient taurine synthesis (risk of DCM, retinal degeneration). The requirement rises during heavy shedding and during convalescence.

Excess: Unlikely from natural raw materials. The problem concerns supplementation with synthetic DL-methionine: it acidifies the urine (dangerous with oxalate urolithiasis), raises homocysteine levels, burdens the liver and kidneys [2]. A meat diet already naturally acidifies the urine, so additional acidification is usually unnecessary. Don't supplement DL-methionine without a diagnosed reason, a calculated dose, and veterinary supervision.

Dependence on cofactors and what to do with a low result

Methionine metabolism requires vitamins B6 (conversion to cysteine), B12, and folates (remethylation of homocysteine) — with regular liver and varied offal these cofactors are usually supplied. With a low bar, first check the proportion of muscle meat to connective tissues, increase the share of meat and hearts, consider adding eggs (in the cat looking at the same time at arginine), make sure there is liver. A supplement is a last resort.

Sources

1. 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
2. Hand MS, Thatcher CD, Remillard RL, Roudebush P, Novotny BJ (2010). Small Animal Clinical Nutrition, 5th Edition. https://www.markmorrisinstitute.org/sacn5.html
3. Pezzali JG, et al. (2024). Minimum methionine requirement in adult cats. J Anim Sci. https://doi.org/10.1093/jas/skad411
4. FEDIAF (2021). Nutritional Guidelines for Complete and Complementary Pet Food for Cats and Dogs. https://europeanpetfood.org/self-regulation/nutritional-guidelines/