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High dietary methionine intake increases the risk of acute coronary events in middle-aged men. Virtanen JK, Voutilainen S, Rissanen TH, Happonen P, Mursu J, Laukkanen JA, Poulsen H, Lakka TA, Salonen JT. Nutr Metab Cardiovasc Dis. 2006 Mar;16(2):113-20. Epub 2005 Nov 2. PMID: 16487911 doi:10.1016/j.numecd.2005.05.005 Conclusions The main finding of this study is that long-term, moderately high dietary methionine intake may increase the risk of acute coronary events in middle-aged Finnish men free of prior CHD. More prospective research is needed to confirm the role of dietary methionine in the development of CVD, and whether its effects are independent of homocysteine.

"ScienceDaily (Dec. 17, 2009) - A diet rich in methionine, an amino acid typically found in red meats, fish, beans, eggs, garlic, lentils, onions, yogurt and seeds, can possibly increase the risk of developing Alzheimer's disease, according to a study by Temple researchers. "When methionine reaches too high a level, our body tries to protect itself by transforming it into a particular amino acid called homocysteine," said lead researcher Domenico Praticò, an associate professor of pharmacology in the School of Medicine. "The data from previous studies show -- even in humans -- when the level of homocysteine in the blood is high, there is a higher risk of developing dementia. We hypothesized that high levels of homocysteine in an animal model of Alzheimer's would accelerate the disease." Using a seven-month old mouse model of the disease, they fed one group an eight-month diet of regular food and another group a diet high in methionine. The mice were then tested at 15 months of age -- the equivalent of a 70-year-old human.

A positive association of lumbar spine bone mineral density with dietary protein is suppressed by a negative association with protein sulfur. Thorpe M, Mojtahedi MC, Chapman-Novakofski K, McAuley E, Evans EM. J Nutr. 2008 Jan;138(1):80-5. PMID: 18156408 Results suggest that protein intake is positively associated with aBMD, but benefit at the LS is offset by a negative impact of the protein sulfur acid load. If validated experimentally, these findings harmonize conflicting theories on the role of dietary protein in bone health.

Amino acids have been termed to be protein's building blocks, whereas some amino acids types could be manufactured naturally by the body like lysine, tryptophan, methionine, leucine, valine, threonine, phenhylalanine and isoleucine. There are a few that one has to derive from supplements and food sources.