okok, on Jul 19 2009, 04:57 PM, said:
Vegan diets are supposed to be low in methionine, but they don't seem to have longer life expectancy. Anyone have any studies, know more?
Vegan diets are not low met, not low enough
to show easily measurable benefits at least. Let me explain (or actually, I'll just generously quote M's response
). When talking about "low met" in the context of life extension per se (max. life span extension á la CR) and in the context of this thread named "methionine restriction
" we are talking very, very low met indeed. This is not just "low" met; it is dangerously
low met and cannot be achieved using any
The MR used in these studies to get a life-extension effect is quite severe -- a 0.17% Met, zero-cysteine diet, vs. a 0.86% Met diet, corresponding roughly to getting a dangerously-low 263 mg of Met+Cys in a 70 kg human (RDA 1330 mg). Moreover, it's not clear how one would attempt to reduce the riskiness of this without also potentially blunting the benefits of restriction: any metabolic defect induced by chronic Met deficiency could be the key to its life-extending effect. Humans given low-Met diets for experimental chemo or because of congenital postload hyperhomocysteinemia wind up eating basically nothing but potatoes,a few vegetables, and a customized super-low-met amino acid shake. With the very low level of evidence supporting its benefits, this was clearly a crazy risk IAC....
Furthermore, any restrictive diet has to be implemented ensuring optimal nutrition (ON). Most standard vegan diets, however, fail to supply many semi-essential nutrients. This problem alone might outweigh all the small benefits of a vegan diet, which is only relatively
low in met and only in some cases. Emphasis mine:
In practice, a high-protein, low-methionine diet is one composed of a lot of non-grain vegetarian protein [!= most vegan diets!]. All legume proteins are good, and lentils are exceptionally noteworthy as being high %protein, low-Calorie, and unusually low-Met even for a legume, per gram of protein and per Calorie. Dairy (aside from whey) is moderate in methionine, and Quorn, despite its eggwhite content, is also pretty acceptable (certainly compared with meat).
Also, notice that perhaps low met is not enough, but it should be also high in Pr (or maybe it shouldn't be depending on what you make of the IGF-1 data), while being low in met to show most benefits. Most vegan diets are rather low in P, though.
Additionally we do not know if there are no risks inherent to real MR in humans:
Finally: some time ago, I looked into the possibility of trying to reap any potential benefits of MR, while avoiding side effects by taking related nutrients (cyst(e)ine, folic acid, betaine, etc) to accomplish some of its other functions. A consideration of many papers on the subject (especially (9,10) and the accumulating evidence of risk from high-dose folic acid and other methylating nutrients) convinced me that, as "engineer's logic" always suggests in such cases, it's a mug's game to attempt to second-guess metabolism.
Most vegan diets are unhealthy in the first place and this may overshadow any benefits derived from their relatively low met content, assuming they are lower in met to begin with.
Edited by kismet, 20 July 2009 - 05:56 PM.