Dark chocolate, like cacao beans, has relatively little protein content (8% protein by wt, ~6% by kcal). Unsweetened cocoa powder is 20% protein by wt, with a (G+S)/(M+C) ratio of 3.9 - comparable to beans.
Methionine restriction does increase autophagy in yeast (at least), one study found elevated autophagy neccesary for its lifespan benefits in yeast.
Ruckenstuhl C et al. 2014. Lifespan extension by methionine restriction requires autophagy-dependent vacuolar acidification.PLoS Genet, 10(5), e1004347.
There are a number of pathways that may mediate the lifespan benefits of methionine restriction. Upregulating UCP1, increasing uncoupled respiration and reducing ROS production at mitochondrial complex I, increasing endogenous H2S production, and inducing FGF21/retrograde response are all candidates. Key mechanistic papers include:
Caro P et al. 2009. Forty percent methionine restriction decreases mitochondrial oxygen radical production and leak at complex I during forward electron flow and lowers oxidative damage to proteins and mitochondrial DNA in rat kidney and brain mitochondria. Rejuvenation research, 12(6), 421-434.
Caro P et al. 2009. Effect of 40% restriction of dietary amino acids (except methionine) on mitochondrial oxidative stress and biogenesis, AIF and SIRT1 in rat liver. Biogerontology, 10(5), 579-592.
Hasek BE et al. 2010. Dietary methionine restriction enhances metabolic flexibility and increases uncoupled respiration in both fed and fasted states. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology, 299(3), R728-R739.
Lees EK et al. 2014. Methionine restriction restores a younger metabolic phenotype in adult mice with alterations in fibroblast growth factor 21. Aging cell, 13(5), 817-827.
Hine C & Mitchell JR. 2014. Calorie restriction and methionine restriction in control of endogenous hydrogen sulfide production by the transsulfuration pathway. Experimental gerontology.
Hine C et al. 2015. Endogenous hydrogen sulfide production is essential for dietary restriction benefits. Cell, 160(1), 132-144.
Wanders D et al. 2015. UCP1 is an essential mediator of the effects of methionine restriction on energy balance but not insulin sensitivity. The FASEB Journal, fj-14.
The two Caro papers and two Hine papers indicate some pretty unique effects of sulfur amino acid (methionine/cysteine) restriction.
Methionine/cysteine restriction is interesting because it appears to account for all the lifespan benefits of protein restriction, and hence most but not all of the benefits of caloric restriction. Moreover, with a lower protein, lower grain vegan or near vegan diet, its possible to approach a methionine restricted state, though actual restriction would require substantial amounts of very low-protein foods like cassava, wine or added oils.
McCarty MF et al. 2009. The low-methionine content of vegan diets may make methionine restriction feasible as a life extension strategy. Medical hypotheses, 72(2), 125-128.
I should also add that methionine restricted diets have been known since the 1940s to largely eliminate spontaneous hepatomas in rats (though its less successful at preventing chemical carcinogenesis or other cancers), and methionine restriction is one of several AA restriction dietary regimens with marked effects on cancer progression (PKU diets have also been tried). This early study I discovered last night showed just how dramatic the effect might be:
Silverstone H, & Tannenbaum A. 1951. Proportion of dietary protein and the formation of spontaneous hepatomas in the mouse. Cancer research, 11(6), 442-446.
B6, B9, and B12 all play important roles in the methionine tranmethylation cycle (which generates SAM for required methylations, and recycles the byproduct homocysteine back into methionine. Ensuring one's replete in those B vitamins (and betaine/choline as well) can remove bottlenecks that would otherwise cause elevation of homocysteine (which used be considered a cardiovascular risk). The enzymes which clear surpluses in the total Met/SAM/SAH/Hcy pool are glycine N-methyltransferase (GNMT), the primary methyltransferase regulating SAM/SAH balance, and cystathionine β-synthase (CBS), the first step on the transulfuration pathway from homocysteine towards cysteine and glutathione synthesis. GNMT and glutathione synthesis consume glycine, while CBS consumes serine (which equilibrates with glycine through a reversible reaction), so by increasing available glycine, we potentially preempt the negative effects of excess free methionine by keeping the pool at a minimal level.
B6, it should be noted, is a cofactor for CBS. B9, however inhibits GNMT activity in some studies. I think there are too many benefits from high folate, and high folate containing foods, to worry about this. Excess glycine works just fine to alleviate experimental hyperhomocysteinaemia induced by excess dietary methionine, and extended mouse lifespan in that to date unreplicated Brind et al 2011 abstract, even when diets are folate replete.
Edited by Darryl, 13 July 2015 - 05:18 AM.