Even as vitamin E appears to be losing some of its memory-saving appeal, the B vitamin folate (i.e. folic acid) is becoming the nutrient du jour. Besides helping to prevent birth defects, it probably reduces the risk of heart disease, stroke, and some cancers. And now, two articles suggest that folate supplementation could help prevent or even treat Alzheimer's disease.

The value of folate-found especially in leafy greens, fruits, vegetables, and yeast-stems from the fact that it helps keep the amino acid homocysteine at low levels. But how might homocysteine contribute to Alzheimer's? In today's Journal of Neuroscience, Mark Mattson, Inna Kruman, and colleagues at the National Institute on Aging and Johns Hopkins University, both in Baltimore, Maryland, directly link folate and homocysteine to amyloid toxicity. Hypothesizing that high homocysteine levels promote the accumulation of DNA damage, they demonstrate that hippocampal neurons cultured without folate, or with added homocysteine, undergo elevated levels of apoptosis. This culture environment further rendered neurons vulnerable to added Aβ1-42. Rather than working in concert with Aβ to damage neurons, or damaging DNA directly, high homocysteine levels appear to interfere with the cell's efforts to repair Aβ-induced oxidative modification of DNA bases.

These in vitro results stood when tested in AβPP-transgenic mice, which overproduce and deposit Aβ. Relative to mutant mice fed a normal diet, those deprived of folate showed increased cellular DNA damage and hippocampal neurodegeneration. This occurred despite any difference in brain Aβ levels between the two groups.

What does the epidemiology say? Conflicting studies on the proposed link between Alzheimer's and high homocysteine levels have left the question unsettled. Enter Philip Wolf, Sudha Seshadri, and colleagues at Boston University and at the U.S. Department of Agriculture Human Nutrition Research Center on Aging at Tufts University in Boston. In the February 14 New England Journal of Medicine, these authors report that a five-micromolar increment in plasma homocysteine level increased the risk of getting Alzheimer's by 40 percent. The data come from a large prospective study, in which 1,092 dementia-free subjects from the Framingham cohort had their homocysteine levels tested eight years prior to the baseline examination for dementia, and were followed for an average of eight years. The effect was independent of age, sex, ApoE genotype, and other AD risk factors.

Where does all this leave humans, besides awaiting more studies? If you simply follow the familiar nutrition pyramid-high in green vegetables, fruits, and whole grains-you eat the foods highest in folate, but may still not be getting enough to reduce homocysteine to innocuous levels. For that reason, the FDA now mandates that some staple foods contain added folate. The question this raises for future studies is whether even higher quantities are required to protect against Alzheimer's and other diseases, and whether such quantities are safe.-Hakon Heimer.

References:
Kruman II et al. Folic acid deficiency and homocysteine impair DNA repair in hippocampal neurons and sensitize them to amyloid toxicity in experimental models of Alzheimer's disease. J Neurosci 2002 Mar 1;22(5):1752-1762. Abstract

Seshadri S et al. Plasma homocysteine as a risk factor for dementia and Alzheimer's disease. N Engl J Med 2002 Feb 14;346(7):476-83. Abstract

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  1. Data obtained in the recent prospective epidemiological studies of Wolf et
    al. and in our studies of a mouse model of Alzheimer's disease (AD) provide
    a strong case for folic acid supplementation as a preventative approach for
    AD. The study of the Framingham cohort suggests that elevated plasma homocysteine
    levels is an independent risk factor for AD, but did not allow a conclusion
    as to if and how homocysteine promotes neuronal dysfunction and death. We found
    that maintaining AβPP-mutant mice with Aβ
    deposits in their brains on a folic acid deficient diet results in elevated
    plasma homocysteine levels and degeneration of neurons in their hippocampus.
    The endangering effect of folic acid deficiency was not the result of increased
    production of Aβ peptide; instead, homocysteine rendered
    hippocampal neurons vulnerable to Aβ peptide-induced
    cell death. The mechanism whereby homocysteine endangers neurons involves an
    impairment of DNA repair, and the resulting accumulated DNA damage triggers
    apoptosis. Thus, we have established a cause-effect relationship between elevated
    homocysteine levels and neuronal degeneration.

    Importantly, we have also shown that dietary supplementation with folic acid
    can protect neurons against amyloid toxicity, suggesting that folic acid may
    not only prevent AD, but may also slow the neurodegenerative process in symptomatic
    AD patients who already have extensive amyloid deposition in their brain. It
    should also be noted that recent studies suggest that folic acid deficiency
    and elevated homocysteine levels can render dopaminergic neurons vulnerable
    to dysfunction and death in experimental models of Parkinson disease (Duan
    et al., 2002
    ). In light of the growing epidemics of Alzheimer's and Parkinson's
    in this country, I would strongly encourage the FDA to move quickly to establish
    recommendations to the public that ensure all Americans receive a neuroprotective
    amount of folic acid, and that clinical trials move forward as rapidly as possible
    to establish efficacy of folic acid supplementation in patients with neurodegenerative
    disorders."—Mark Mattson, NIA Gerontology Research Center, Baltimore, Maryland.

    "This interesting paper by Kruman et al. demonstrates that folic acid deficiency
    and increased homocysteine levels are toxic to neuronal cells. Using cultured
    neurons and transgenic mice, these authors demonstrate that folic acid deprivation
    and increased levels of homocysteine render neuronal cells vulnerable to excitotoxic
    insults (Kruman
    et al. 2000
    ) and Aβ toxicity by increasing DNA
    damage. Furthermore, Kruman and co-workers established that folic acid deficiency
    induced DNA damage by impairing DNA repair mechanisms in neurons exposed to
    Aβ. One important implication of this study is that,
    in the Alzheimer brain, folic acid deficiency and increased homocysteine levels
    may accelerate the accumulation of DNA damage that is promoted by Aβ.

    The importance of these results is underscored by a recent prospective study
    by Seshadri et al. that established a strong correlation between increased homocysteine
    plasma levels and the development of Alzheimer's. Homocysteine plasma level
    has been shown to be a major vascular risk factor and it is generally recognized
    that cardiovascular risk factors and stroke increase the risk of vascular dementia
    and Alzheimer's disease. Future studies should be directed to determine whether
    reduction of homocysteine plasma levels could effectively reduce the risk of
    Alzheimer's disease. Taken together, these studies indicate that dietary supplementation
    of folic acid and vitamins B6 and B12 may decrease the risk of Alzheimer's disease.

  2. In this prospective study, Seshadri et al. measured plasma homocysteine
    levels in normal elderly individuals and then followed the same individuals
    for eight years and reassessed their clinical status as well as homocysteine
    levels. They found that plasma homocysteine was a risk factor for the
    development of dementia in general as well as dementia felt to be secondary
    to Alzheimer's disease. Homocysteine levels are a known risk vactor for
    vascular disease. Whether homocysteine itself is directly related to the
    risk for the dementia or is a surrogate marker for something else is not clear.

    The study is important as it suggests that further understanding of
    why homocysteine is in some way related to dementia is warranted.
    Homocysteine can be lowered by folic acid leading some to speculate that
    prospective trials of folic acid are indicated. One potential problem with
    the study is that for subjects to be called demented, they had to have a
    clinical dementia rating score of 1 (mildly demented). dementia due to
    Alzheimer's disease is often present clinically from four to eight years prior to
    someone reaching this stage. Thus, some of the individuals who were felt
    to be "normal" at baseline in this study were probably in the earliest
    clinical stages of dementia. Thus, homocysteine elevation may not necessarily
    have preceded clinical disease. Nonetheless, this is an important study
    that should lead to further work.

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References

Paper Citations

  1. . Folic acid and homocysteine in age-related disease. Ageing Res Rev. 2002 Feb;1(1):95-111. PubMed.
  2. . Plasma homocysteine as a risk factor for dementia and Alzheimer's disease. N Engl J Med. 2002 Feb 14;346(7):476-83. PubMed.

Further Reading

Papers

  1. . Folic acid and homocysteine in age-related disease. Ageing Res Rev. 2002 Feb;1(1):95-111. PubMed.
  2. . Plasma homocysteine as a risk factor for dementia and Alzheimer's disease. N Engl J Med. 2002 Feb 14;346(7):476-83. PubMed.

Primary Papers

  1. . Folic acid and homocysteine in age-related disease. Ageing Res Rev. 2002 Feb;1(1):95-111. PubMed.
  2. . Plasma homocysteine as a risk factor for dementia and Alzheimer's disease. N Engl J Med. 2002 Feb 14;346(7):476-83. PubMed.