N-acetylcysteine doesn't appear suitable for life extension approaches focusing on methionine restriction. For the benefit of brosci, I'd add that all ketogenic diet research in animals uses protein and methionine restricted chow, which in my opinion is the principal cause of any observed benefits.
Elshorbagy et al, 2011. Cysteine supplementation reverses methionine restriction effects on rat adiposity: significance of stearoyl-coenzyme A desaturase. J lipid research, 52(1), pp.104-112.
Methionine-restricted (MR) lowered weight gain and at-pad mass/body-weight% (FM/BW%) despite higher food intake/weight than control-fed (CF), and lowered serum cysteine. Hepatic Scd1 expression was decreased, with decreased serum SCD1 activity indices (calculated from serum fatty acid profile), decreased serum insulin, leptin and triglycerides, and higher adiponectin. Cysteine supplementation (MR+Cys) essentially reversed all these phenotypes and raised serum cysteine but not methionine to CF levels. Adding extra cysteine to control diet (CF+Cys) increased serum taurine but did not affect serum cysteine, lipids, proteins, or total weight gain. FM/BW% and serum leptin were modestly decreased. Our results indicate that anti-obesity effects of MR are caused by low cysteine and that dietary sulfur amino acid composition contributes to SCD1 regulation.
Perrone et al, 2012. Genomic and metabolic responses to methionine-restricted and methionine-restricted, cysteine-supplemented diets in Fischer 344 rat inguinal adipose tissue, liver and quadriceps muscle. Lifestyle Genomics, 5(3), pp.132-157.
Methionine restriction diet with cysteine (MRC) reversed most gene and metabolite changes induced by methionine restriction (MR) in inguinal adipose tissue, but drove the expression of Elovl6, Lpin1, Pc, and Pdk1 below control fed (CF) levels. In liver, MR decreased levels of a number of long-chain fatty acids, glycerol and glycerol-3-phosphate corresponding with the gene expression data. Although MR increased the expression of genes associated with carbohydrate metabolism, levels of glycolytic intermediates were below CF levels. MR, however, stimulated gluconeogenesis and ketogenesis in liver tissue. As previously reported, sulfur amino acids derived from methionine were decreased in liver by MR, but homocysteine levels were elevated. Increased liver homocysteine levels by MR were associated with decreased cystathionine β-synthase (CBS) protein levels and lowered vitamin B6 and 5-methyltetrahydrofolate (5MeTHF) content. Finally, MR upregulated fibroblast growth factor 21 (FGF21) gene and protein levels in both liver and adipose tissues. MRC reversed some of MR’s effects in liver and upregulated the transcription of genes associated with inflammation and carcinogenesis such as Cxcl16, Cdh17, Mmp12, Mybl1, and Cav1 among others.
Gomez et al, 2015. Cysteine dietary supplementation reverses the decrease in mitochondrial ROS production at complex I induced by methionine restriction. J bioenergetics biomembranes, 47(3), pp.199-208.
The results obtained in liver showed that cysteine supplementation reverses the decrease in mitochondrial ROS generation induced by methionine restriction at complex I.
Dong et al, 2018. Disease prevention and delayed aging by dietary sulfur amino acid restriction: translational implications. Ann NY Acad Sci, 1418(1), pp.44-55.
Dietary sulfur amino acid restriction (SAAR) is characterized by chronic restrictions of methionine and cysteine but not calories and is associated with reductions in body weight, adiposity and oxidative stress, and metabolic changes in adipose tissue and liver resulting in enhanced insulin sensitivity and energy expenditure. SAAR-induced changes in blood biomarkers include reductions in insulin, insulin-like growth factor-1, glucose, and leptin and increases in adiponectin and fibroblast growth factor 21. On the basis of these preclinical data, SAAR may also have similar benefits in humans.