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AlzRisk Risk Factor Discussion

Risk Factor:
Risk Factor Type: Metabolic
Current Understanding:
The evidence from observational epidemiologic studies suggests that higher levels of plasma total homocysteine (tHcy) may be associated with an increased risk of incident Alzheimer's disease (AD). This body of findings is consistent with findings from related studies of cognitive decline, of brain imaging, and of genetic polymorphisms predisposing individuals to hyperhomocysteinemia. Recent trials of the tHcy-lowering vitamins—B6, B12, and folate, however, have had mixed, predominantly null results. Further research will be required to know whether supplementation with B vitamins starting in midlife, when it could prevent chronic elevation of homocysteine over a longer span, would provide greater benefit. For a review of the putative mechanisms by which homocysteine may influence AD risk and detailed commentary on interpreting the findings below in a broader context, please view the Discussion.
Literature Extraction: Search strategy  * New *
Last Search Completed: 15 November 2013 - Last content update released on 20 Nov 2013.

Risk Factor Overview

Cite as:

Li S*, Goonesekera S*, Weuve J, Jackson JW, Blacker D. "Homocysteine." The AlzRisk Database. Alzheimer Research Forum. Available at: Accessed [date of access]. * contributed equally.

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The tables in the Risk Factor Overview summarize several observational epidemiologic studies whose results suggest that higher levels of plasma total homocysteine (tHcy) may be associated with an increased risk of incident Alzheimer's disease (AD).

Following the guidelines of AlzRisk for meta-analysis, we did not conduct a meta-analysis of the data in the Risk Factor Overview due to the sparse number of populations represented (<4) and, in the case of the studies reporting on categories of tHcy, variability in cut-off points. However, a meta-analysis by van Dam et al. of results from three prospective cohort studies found that participants with hyperhomocysteinemia (with definitions varying across the studies) had over twice the risk of developing AD of participants with lower tHcy levels (pooled RR: 2.50; 95 percent CI: 1.38-4.56) (van Dam and van Gool, 2009).

Potential Mechanisms of Action

Homocysteine is a sulfur-containing amino acid formed from methionine as a product of S-adenosylmethonine-dependent transmethylation (Refsum et al., 2004). Homocysteine can be reconverted into methionine by methionine synthase via a process that requires vitamin B12 and folic acid as cofactors, and can be transformed to cystathionine by cystathionine β-synthetase, a vitamin B6-dependent enzyme (Refsum et al., 2004). Thus, significant deficiencies in vitamins B6, B12, or folate can lead to elevated levels of plasma homocysteine.

Homocysteine may be related to the pathogenesis and progression of AD through both direct and indirect effects on the brain. Homocysteine appears to promote the generation of β amyloid plaques, a pathological hallmark of AD (Kruman II et al., 2002; Pacheco-Quinto et al., 2006). In addition, homocysteine may impair nitric oxide activity and increase oxidative stress in the brain (Selley, 2004), which may further promote the pathogenesis and progression of AD (Christen, 2000). Insufficient conversion of homocysteine to methionine may interfere with methylation of proteins, myelin, and neurotransmitters that are essential for brain function (Fuso and Scarpa, 2011). Additional evidence suggests that homocysteine may increase the permeability of the blood-brain barrier (Beard and Bearden, 2011). Potentially through these or other mechanisms, elevated tHcy is associated with hippocampal and medial temporal lobe atrophy in older adults (Williams et al., 2002; Tangney et al., 2011).

In addition to these effects, hyperhomocysteinemia is an independent risk factor for atherosclerosis and adverse cardio- and cerebrovascular events, such as stroke. Cerebrovascular pathology contributes to the risk of dementia for a given level of AD pathology (Schneider et al., 2004), and most cerebrovascular risk factors are also associated to some degree with risk of AD (Breteler, 2000; Stampfer, 2006; AlzRisk diabetes overview, AlzRisk physical activity overview); homocysteine may also influence AD risk through this mechanism (El Oudi et al., 2011; Selhub, 2006). Interestingly, data are now emerging that ischemia per se can impact Aβ generation and deposition (Tesco et al., 2007; Garcia-Alloza et al., 2007), suggesting that these mechanisms may be interrelated.

The C677T polymorphism in the methylenetetrahydrofolate reductase (MTHFR) gene elevates levels of tHcy; specifically, individuals with the T/T genotype have elevated levels of tHcy in the presence of low folate levels (Religa et al., 2003). In a meta-analysis, this allele has also been associated with increased AD risk (OR: 1.13; 95 percent CI: 1.04-1.23) (See AlzGene, Accessed Oct 16, 2011.)

Methodological Issues


The tables report on total plasma homocysteine concentrations (tHcy). Because homocysteine easily crosses the blood-brain barrier, tHcy in peripheral blood likely reflects brain levels of homocysteine. However, diurnal variation in tHcy levels and differences in instrumentation may introduce error to the measurement of tHcy and differences across studies. The timing of tHcy during the lifespan warrants consideration as another source of variability.

Diurnal variation in the exposure. There is evidence that homocysteine levels vary substantially over the course of the day (with morning levels typically lowest) (Bönsch et al., 2007). Moreover, non-fasting levels can be up to 20 percent higher (Perry, 1999). Thus, failure to implement a standard plasma collection protocol could introduce measurement error. This error, however, is unlikely to be related to AD and would generally result in attenuated estimates of homocysteine’s contribution to risk.

Timing of exposure measurement in the lifespan. It is unknown whether chronic elevations in homocysteine levels or acute recent elevations are most relevant to AD risk. If chronic elevations in tHcy play a stronger role in AD, the lack of repeated measurement over an extended period in most studies could result in exposure misclassification, as plasma homocysteine levels fluctuate with intercurrent illness and medication use, in addition to the diurnal variation noted above (Refsum et al., 2004). In addition, measured homocysteine levels may be inaccurate reflections of the relevant exposure level if there is a critical age window during which the risk of AD is most sensitive to plasma homocysteine, and the age at which tHcy levels were measured varied across studies. Beyond this, given the extended period over which AD develops, tHcy levels measured late in life, closer to the onset of dementia, could also reflect the consequence of disease (see Reverse Causation below).

Assay performance issues. Different methods used to measure plasma homocysteine may also result in non-comparability among study findings. Some studies reported in Table 1 used the method of high-performance liquid chromatography with fluorescence detection (e.g., Seshadri et al., 2002; Luchsinger et al., 2004), while others used automated IMx assays (e.g., Ravaglia, et al., 2005; Kivipelto et al., 2009). Methodological issues related to the time between blood collection and processing may also affect the accuracy of tHcy measurements (Ravaglia et al., 2005). However, measurement error is likely to be non-differential with respect to AD status, which would typically bias the risk estimates toward the null.

Design and Analysis

Confounding and intermediate variables. The results reported in the tables are from observational studies. Thus, failure to adequately adjust for potential sources of confounding can bias the effect estimates in either direction.

Plasma homocysteine levels typically increase with age, smoking, alcohol consumption, and dietary intake of methionine; they decrease with physical activity and intake of folate and vitamins B6 and B12; and they are generally lower among individuals with more education—all of these factors may influence AD risk. All results shown were adjusted for age. However, the studies varied in the degree to which they accounted for these other potential sources of confounding. In addition, residual or unknown confounding cannot be excluded as a potential explanation for the observed findings.

Several results were adjusted for intake of folate and vitamin B6 and B12, important direct determinants of tHcy and also potentially associated with reduced AD risk. However, adjustment for dietary influences on tHcy may be inappropriate if elevations in tissue levels of homocysteine are the only mechanism linking these dietary components to AD (Schisterman and Platt, 2009). This possibility has received some support from some studies that found no association (or a substantially attenuated one) of folate and B vitamin intake on AD risk in analyses that were also adjusted for plasma tHcy levels (Seshadri et al., 2002; Kivipelto et al., 2009; Hooshmand et al., 2010; Luchsinger et al., 2004). In any event, it appears that differences in adjustment for B vitamin intake or plasma levels do not contribute substantially to differences across studies, as the reported results with and without adjustment were very similar (Kivipelto et al., 2009; Hooshmand et al., 2010; Seshadri et al., 2002; Luchsinger et al., 2004).

The issue with potential confounding by cardiovascular disease (CVD) risk factors is more complicated because these variables may act as intermediates between homocysteine and AD, so adjusting for them probably constitutes over-adjustment and would reduce the strength of any association. When a study reported results adjusted for more than one set of covariates, the result displayed in AlzRisk is the one adjusted for the largest set of covariates that did not include cardiovascular variables. If results adjusted for fewer variables omitted too many potentially important non-intermediate confounders, then we displayed the more adjusted findings, even if they were adjusted for cardiovascular variables.

Reverse Causation. Even though studies included in this review measured tHcy in participants free of AD, these measurements often occurred only a few years prior to the onset of clinical dementia, a period during which the underlying pathological process is well underway. Therefore, associations suggesting increased AD risk with higher levels of tHcy could in theory be due to reverse causation, although no specific mechanism for an increase in tHcy during the preclinical phase of AD has been proposed. Of note, the handful of studies that have had longer intervals between tHcy measurement and AD diagnosis show equally strong associations as those with shorter intervals, making reverse causation a less plausible explanation.

Studies of Other Outcomes and Clinical Trials

Cognitive decline

Significant inverse associations between plasma Hcy and cognitive decline over time among older individuals were reported in several epidemiological studies (Kalmijn et al., 1999; Elias et al., 2005; Selhub, 2006; Haan et al., 2007). One such study found that the association was restricted to older adults, and did not change appreciably after adjustment for B vitamin intake (Elias et al., 2005). Neuro-imaging studies extend these findings in that high levels of tHcy are also associated with silent infarcts, white matter changes, and hippocampal atrophy (Vermeer et al., 2002; Polyak et al., 2003; Tangney et al., 2011).

Clinical trials

While vitamin B6, B12 and folate supplementation lowers tHcy (Dangour et al., 2010), results from clinical trials of vitamin B supplementation have had generally null results. A Cochrane review that summarized evidence from four placebo-controlled clinical trials conducted on cognitively-impaired participants reported no overall benefit on any measure of cognition or mood from folic acid supplementation with or without vitamin B12. (Malouf et al., 2003). Since then, one additional study has been negative (Ford et al., 2010) and three more have been positive, two in older adults with mild cognitive impairment, (Smith et al., 2010; de Jager et al., 2011) and one in those with normal function at baseline (Durga et al., 2007).

Discussion and Recommendations

The existing evidence suggests an association between higher plasma homocysteine and incident AD. The results from recent trials of B vitamins, which are known to lower homocysteine, however, have been mixed but predominantly null. It is possible that if such supplements were begun in midlife, when chronic elevations in tHcy could be prevented, they would be more beneficial. Further observational studies with more careful separation of midlife and late-life exposures, as well as clinical trials conducted at younger ages, will be required to answer these questions.

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