Reducing consumption of this single amino acid extends lifespan by 15-44% in every laboratory animal tested to date. This effect is sufficient to completely explain previous results that found dietary restriction of total calories led to lifespan extension. Turns out it wasn't the calories -- it was the methionine.
Methionine restriction reduces visceral fat with concomitant decreases in basal insulin, glucose, and leptin, and increased adiponectin and triiodothyronine. Methionine restriction also prevents age-associated increases in serum lipids.(source)
A new paper published this month by Koziel et al confirms previous methionine restriction results in human cell culture, mice, rats, fruit flies, and yeast. Koziel et al used human cell culture to specifically examine mitochondrial function and showed that oxidative stress is reduced by this dietary intervention. This work supports the traditional free radical theory of aging-by-oxidative stress.
Methionine is an interesting amino acid because it promotes growth, including muscle and bone development. It is essential, and that is the point: restriction appears to promote a healthy stress-response that can confer adaptive resilience to senescence. Other researchers have postulated that methionine restriction causes an increase in autophagy, the process whereby cells break down and recycle unused or damaged cell components. Instead of the free radical theory of aging, this theory would postulate that Methionine restriction induces a starvation-type response where cells begin to recycle their constituents at an increased pace. In this theory, "stress" is a good state to be in, because it puts cells in an active phase of self-repair. Cite: 1 and 2.
* The two other most promising age-defying compounds are rapamyacin (an antibiotic with a plethora of strange and powerful effects throughout the body) and resveratrol.