Cancer
Carcinogenicity is the ability or tendency of a chemical to induce tumors, increase their incidence or malignancy, or shorten the time of tumor occurrence.Adding nitrites to meat has been shown to generate known carcinogens such as nitrosamines; the World Health Organization (WHO) advises that each 50 g (1.8 oz) of "processed meats" eaten a day would raise the risk of getting bowel cancer by 18% over a lifetime. The World Health Organizations review of more than 400 studies concluded, in 2015, that there was sufficient evidence that "processed meats" caused cancer, particularly colon cancer; the WHOs International Agency for Research on Cancer (IARC) classified "processed meats" as carcinogenic to humans (Group 1); "processed meat" meaning meat that has been transformed through salting, curing, fermentation, smoking, or other processes to enhance flavour or improve preservation. ).Nitrosamines can be formed during the curing process used to preserve meats, when sodium nitrite-treated meat is cooked, and also from the reaction of nitrite with secondary amines under acidic conditions (such as occurs in the human stomach). Dietary sources of nitrosamines include US cured meats preserved with sodium nitrite as well as the dried salted fish eaten in Japan. In the 1920s, a significant change in US meat curing practices resulted in a 69% decrease in average nitrite content. This event preceded the beginning of a dramatic decline in gastric cancer mortality. Around 1970, it was found that ascorbic acid (vitamin C), an antioxidant, inhibits nitrosamine formation.
Methylenetetrahydrofolate reductase deficiency is the most common genetic cause of elevated serum levels of homocysteine (hyperhomocysteinemia). It is caused by genetic defects in MTHFR, which is an important enzyme in the methyl cycle.Common variants of MTHFR deficiency are asymptomatic and have only minor effects on disease risk. Severe variants (from nonsense mutations) are vanishingly rare. Symptoms
The common MTHFR deficiencies are usually asymptomatic, although the 677T variant can cause a mildly increased risk of some diseases. For individuals homozygous in the 677T variant, there is a mildly elevated risk of thromboembolism (odds ratio 1.2), and stroke (odds ratio 1.26). There is also an elevated risk of neural tube defects among children of individuals with the C677T polymorphism (odds ratio 1.38).For cardiovascular risk, common MTHFR deficiencies were once thought to be associated but meta-analyses indicate that correlation this was an artifact of publication bias. Causes
MTHFR is the rate-limiting enzyme in the methyl cycle, which includes the conversion of homocysteine into methionine. Defects in variants of MTHFR can therefore lead to hyperhomocysteinemia.There are two common variants of MTHFR deficiency. In the more significant of the two, the individual is homozygous for the 677T polymorphism. This variant in particular is the most common genetic cause of hyperhomocysteinemia. The resulting enzyme is thermolabile and in homozygotes, enzymatic activity is depressed to 35% of its usual level. The second variant is a milder one, caused by a homologous 1298C polymorphism. This leads to 68% of the control values of enzyme activity, and it normally does not lead to low serum folate.