Vitamin B6 Disease Prevention

Homocysteine and cardiovascular diseases

Even moderately elevated levels of homocysteine in the blood have been associated with increased risk for cardiovascular diseases, such as heart disease and stroke. When we digest protein, amino acids, including methionine, are released. Homocysteine is an intermediate in the metabolism of methionine. Healthy individuals utilize two different pathways to metabolize homocysteine. One pathway results in the conversion of homocysteine back to methionine, and is dependent on folic acid and vitamin B₁₂. The other pathway converts homocysteine to another amino acid, cysteine, and requires two vitamin B₆ (PLP)-dependent enzymes. Thus, the amount of homocysteine in the blood is regulated by at least three vitamins: folic acid, vitamin B₁₂, and vitamin B₆. Several large observational studies have demonstrated an association between low vitamin B₆intake or status with increased blood homocysteine levels and increased risk of cardiovascular diseases. A large prospective study found the risk of heart disease in women who consumed, on average, 4.6 mg of vitamin B₆ daily to be only 67% of the risk in women who consumed an average of 1.1 mg daily. Another large prospective study found higher plasma levels of PLP to be associated with decreased risk of cardiovascular disease, independent of homocysteine levels. In contrast to folic acid supplementation, studies of vitamin B₆ supplementation alone have not resulted in significant decreases of basal (fasting) levels of homocysteine. However, vitamin B₆ supplementation has been found effective in lowering blood homocysteine levels after an oral dose of methionine (methionine load test) was given, suggesting it may play a role in the metabolism of homocysteine after meals.

Immune function

Low vitamin B₆ intake and nutritional status have been associated with impaired immune function, especially in the elderly. Decreased production of immune system cells known as lymphocytes, as well as decreased production of an important immune system protein called interleukin-2, have been measured in vitamin B₆ deficient individuals. Restoration of adequate vitamin B₆ status resulted in normalization of the lymphocyte proliferation and interleukin-2 production, suggesting that adequate vitamin B₆ intake is important for optimal immune system function in older individuals. However, one study found that the amount of vitamin B₆ required to reverse these immune system impairments in the elderly was 2.9 mg/day for men and 1.9 mg/day for women, more than the current RDA.

Cognitive function

A few recent studies have demonstrated an association between declines in cognitive function or Alzheimer's disease in the elderly and inadequate nutritional status of folic acid, vitamin B₁₂, and vitamin B₆ and thus, elevated levels of homocysteine. One observational study found higher plasma vitamin B₆ levels to be associated with better performance on two measures of memory, but unrelated to performance on 18 other cognitive tests. It is presently unclear whether marginal B vitamin deficiencies, which are relatively common in the elderly, contribute to age-associated declines in cognitive function or whether both result from processes associated with aging and/or disease.

Kidney stones

A large prospective study examined the relationship between vitamin B₆ intake and the occurrence of symptomatic kidney stones in women. In a group of more than 85,000 women without a prior history of kidney stones, followed over 14 years, those who consumed 40 mg or more of vitamin B₆ daily had only two thirds the risk of developing kidney stones compared with those who consumed 3 mg or less. However, in a group of more than 45,000 men followed over 6 years, no association was found between vitamin B₆ intake and the occurrence of kidney stones. Limited data have shown that supplementation of vitamin B₆ at levels higher than the tolerable upper intake level (100 mg) decreased elevated urinary oxalate levels, an important determinant of calcium oxalate kidney stone formation, in some individuals. However, it is less clear that supplementation actually resulted in decreased formation of calcium oxalate kidney stones. Presently, the relationship between vitamin B₆ intake and the risk of developing kidney stones requires further study before any recommendation can be made.