I read somewhere that cysteine is present in onions. But my teacher told me that the cysteine in onions is denatured by the harsh stomach acids. Is this true? If not , are onions viable source of cysteine for a healthy person?
Cysteine is an amino acid. It is true that it is denaturated in the stomach, but this means only its secondary structure ("shape"), but not the chemical bonds, are changed. So, the cysteine from onions will be absorbed as proper cysteine.
I'm not aware of any special health benefits of cysteine in otherwise healthy individuals, though.
Also, vegetables are in general low in protein and hence in cysteine. They are high-protein foods (meat, fish, eggs, nuts) that are high in cysteine.
Note, that cysteine is not the same as N-acetyl cysteine.
- 100 grams of a raw onion contains 0.004 g cysteine. (USDA.gov)
- 100 grams of cooked chicken leg contains 0.321 g cysteine (USDA.gov), so about 80 times as much as one big onion.
(NOTE: There is a typo in USDA.gov: they have wrongly written cystine instead of cysteine)
N-Acetylcysteine (NAC) is a supplement that acts as a "prodrug" for L-cysteine, which means NAC is converted to cysteine in your body.
It is true that cysteine is converted to glutathione and more cysteine would provide more glutathione. But "more glutathione" as such is not already better for health. Further, you would need to know which exact health effect you want to achieve.
My conclusion: Although, onions are viable, they are relatively poor source of cysteine.