Các bệnh lý liên quan vitamin B
Starting in the mid-1980s, numerous studies noted a link between high levels of homocysteine (an amino acid found in the blood) and an increased risk of car- diovascular disease. Many people with high homo- cysteine levels are de cient in vitamins B6, B12, and folic acid. Supplements of these vitamins can reduce homocysteine levels within weeks. But this has no effect on the number of heart attacks or deaths from heart disease, according to two long-term, random- ized clinical trials—the Heart Outcomes Preven- tion Evaluation-2 (HOPE-2) study and the Women’s Antioxidant and Folic Acid Cardiovascular Study (WAFACS)—of people at high risk for or with estab- lished cardiovascular disease.
e relationship between cancer and B vitamins— folic acid, in particular—has proved complex. ere’s evidence that people with low blood levels of folate are more prone to cancer, and several large, long- term studies suggest that people who consume more folic acid are less likely to develop colon cancer. Other research suggests that greater consumption of folic acid can lower breast cancer risk, at least among women who drink alcohol and have low folic acid lev- els. Alcohol consumption is believed to increase the risk of some cancers, including breast and colon can- cers. But folic acid seems to counteract, in part, such adverse eects of alcohol.
But while adequate amounts of folic acid appear to sti e the formation and spread of early tumors, it’s possible that too much may speed up the growth of existing tumors. In fact, one randomized trial found that folic acid supplements increased the recurrence of adenomatous polyps, which can turn into colon cancer. Several studies suggest that excess folic acid may raise the risk of cancer of the colon, breast, and prostate. A study that reviewed cancer registries in the United States and Canada (which also began folic acid forti cation in 1998) revealed a slight uptick in colon cancer rates in the early forti cation years, when aver- age blood levels of folate doubled. However, the over- all steady decline in deaths from colon cancer before and a er folic acid forti cation suggests that improved screening from colonoscopies is a more likely explana- tion for the upward blip.
blood from 816 older people. A er about four years, 112 of them had developed dementia, including 70 diagnosed with Alzheimer’s disease. An analysis of the blood test results revealed that people who started with higher concentrations of folate were less likely to have su ered cognitive decline.
Some small randomized controlled trials also sug- gest that treatment with folic acid and other B vita- min supplements may slow cognitive decline in older people, perhaps through the B vitamins’ ability to lower homocysteine, a protein that in excess can lead to in ammation of blood vessels, including those that feed the brain.
at said, a randomized controlled trial of peo- ple with mild to moderate Alzheimer’s disease found that taking high-dose vitamin B supplements did not slow cognitive decline. And three studies reviewed by the Cochrane Collaboration, an international group of independent experts, did not show any ability of these B vitamins to protect thinking skills or slow age- related decline in healthy older people.
Several epidemiological studies have shown that blood concentrations of vitamins B6, B12, and folic acid are linked to people’s performance on tests of memory and abstract thinking. In one, investigators collected