Abstract
Background: The 776C→G polymorphism of the vitamin B-12 transport protein transcobalamin gene (TCN2) (rs1801198; Pro259Arg) is associated with a lower holotranscobalamin concentration in plasma. This effect may reduce the availability of vitamin B-12 to tissues even when vitamin B-12 intake is adequate. Clinical outcomes associated with vitamin B-12 insufficiency could potentially be worsened by high folate intake.
Objective: We determined the association of the TCN2 776C→G polymorphism and folate intake with peripheral neuropathy in elders with normal plasma concentrations of vitamin B-12.
Design: Participants in this cross-sectional study (n = 171) were from a cohort of community-based, home-bound elderly individuals aged ≥60 y who underwent an evaluation by physicians including an assessment for peripheral neuropathy. Participants were administered food-frequency and general health status questionnaires, anthropometric measurements were taken, and a fasting blood sample from each subject was collected.
Results: Odds of neuropathy were 3-fold higher for GG genotypes than for CC genotypes (OR: 3.33; 95% CI: 1.15, 9.64). When folate intake was >2 times the Recommended Dietary Allowance (800 μg), GG genotypes had 6.9-fold higher odds of neuropathy than CC genotypes (OR: 6.9; 95% CI: 1.31, 36.36). There was no difference between the genotypes in the odds of peripheral neuropathy when folate intake was ≤800 μg (OR: 1.5; 95% CI: 0.18, 12.33).
Conclusion: The TCN2 776C→G polymorphism is associated with increased odds of peripheral neuropathy in the elderly, even with a normal vitamin B-12 status, especially if their folate intake is >2 times the Recommended Dietary Allowance.
Keywords: folate, folic acid, peripheral neuropathy, TCN2 776C>G polymorphism, transcobalamin, vitamin B-12
See corresponding editorial on page 1495.
INTRODUCTION
Transcobalamin is a vitamin B-12–binding protein that transports the vitamin from the ileum to the tissues (1). The 776C→G single nucleotide polymorphism in the gene for transcobalamin (TCN2) (rs1801198) results in a Pro259Arg substitution in the transcobalamin protein (2). This polymorphism affects the binding of vitamin B-12 by transcobalamin and is associated with a lower concentration of holotranscobalamin (vitamin B-12–bound transcobalamin) in plasma, which, in turn, may affect the availability of vitamin B-12 to the tissues (3–5). There are 2 vitamin B-12–dependent reactions in mammalian cells as follows: the synthesis of methionine from homocysteine and methyltetrahydrofolate catalyzed by the enzyme methionine synthase and the conversion of methylmalonyl CoA to succinyl CoA by methylmalonyl-CoA mutase. Methionine is the precursor of S-adenosyl methionine, which is the biological methyl donor for methylation reactions in the cell. S-adenosyl homocysteine, which is generated during every methylation reaction, is hydrolyzed to homocysteine. When vitamin B-12 availability is limited, homocysteine and methylmalonic acid concentrations increase in plasma. Thus, total homocysteine and methylmalonic acid in plasma serve as functional biomarkers for vitamin B-12 status in tissues. The GG genotype of the 776C→G polymorphism of TCN2 is associated with a higher concentration of homocysteine in individuals with lower plasma vitamin B-12 (6). However, the G allele has also been reported to be associated with lower homocysteine under low vitamin B-12 status (7). In a population with normal plasma values for vitamin B-12, the concentration of methylmalonic acid was higher in GG genotypes (4). These studies indicate that the TCN2 polymorphism is functional and affects the availability of vitamin B-12 in tissues for metabolic reactions.
A lack of sufficient vitamin B-12 is associated with peripheral neuropathy, anemia, depression, and cognitive impairment (8–10). Peripheral neuropathy with characteristic features is a frequent clinical outcome in vitamin B-12 deficiency, which has been observed even in the absence of anemia or macrocytosis (11). It has also been reported that older adults may present with clinical conditions and elevated concentrations of plasma biomarkers that are associated with vitamin B-12 insufficiency when their plasma concentrations of vitamin B-12 are low but in the normal range (8, 9, 12, 13). The objective of this study was to investigate the association between the TCN2 polymorphism and peripheral neuropathy in individuals aged ≥60 y with normal plasma concentrations of vitamin B-12. Early reports on folic acid supplementation of vitamin B-12–deficient patients have shown that folic acid ameliorates the megaloblastic anemia and hematologic abnormalities seen in severe vitamin B-12 deficiency but has no effect on the neurologic conditions that are associated with the deficiency (14). Current evidence has suggested that excess folate intake worsens the clinical and metabolic manifestations of vitamin B-12 insufficiency as indicated by increases in the prevalences of anemia and cognitive dysfunction and the concentrations of homocysteine and methylmalonic acid in plasma (15–19). Hence, we hypothesized that high folate intake can modify the association between the TCN2 polymorphism and peripheral neuropathy.
METHODS
Study population
This cross-sectional study consisted of participants from the Nutrition, Aging, and Memory in Elders study, which is a cohort of community-based, home-bound elderly participants aged ≥60 y who were recruited during 2003–2007 from the Boston area as described in detail previously (20). Participants were recruited from Boston’s 3 Aging Services Access Points, which are home care agencies that provide services to the low-income elderly to facilitate independent living. The exclusion criteria were a lack of English fluency, severe auditory or visual impairment, brain tumor, severe mental retardation, schizophrenia, bipolar disorder, HIV or AIDS, or a diagnosis of epilepsy as documented in the case records of Aging Services Access Points. Informed consent for all tests including genotyping was obtained. Individuals with severe cognitive impairment or dementia [a score ≤10 on Mini-Mental State Examination (score range: 0–30)] or who had an estimated intelligence quotient <75 (as measured with the use of the North American Adult Reading Test) were excluded from the study. The participants were administered food-frequency and general health status questionnaires, anthropometric measurements were taken, and a fasting blood sample was collected from each subject. From the total 1248 participants in the cohort, a planned subset of 366 participants who consented to additional tests and were able to visit the hospital by taxi or wheelchair van underwent an evaluation by physicians that included an assessment of peripheral neuropathy and gait and balance at Tufts Medical Center (20). Participants were excluded from the current study if data were missing for the TCN2 genotype (n = 53) or any of the covariates (n = 86), if they had a stroke (n = 43), or if they had a plasma vitamin B-12 concentration <148 pmol/L (n = 2), which was indicative of deficiency, or >701 pmol/L (n = 11), which may have been indicative of malignancies or abnormalities (21–23). A total of 171 participants who underwent an evaluation by the physicians met the criteria for the current study (Supplemental Figure 1). The study was approved by the Institutional Review Board at Tufts University and Tufts Medical Center.
Blood analyses
Blood samples were collected after ≥7 h of fasting. Concentrations of vitamin B-12 and folate in plasma were measured with the use of Quantaphase II vitamin B-12/folate radioassay (BioRad Laboratories) (interassay CVs of 4.3% for vitamin B-12 and 7.1% for folate), plasma total homocysteine was determined with the use of an HPLC method (24) (CV: 6%), and serum creatinine was determined with the use of a modified Jaffe reaction (25) (CV: 4%). The plasma concentration of pyridoxal 5′-phosphate, which is the functional form of vitamin B-6, was determined with the use of a tyrosine-decarboxylase-apoenzyme method (CV: 8%) (26). Blood glucose was measured with the use of a hexokinase method with a Beckman Coulter AU400e instrument (Beckman Coulter Inc.) (CV: 3.4%).
DNA isolation and genotyping
DNA was isolated with the use of QIAmp DNA Blood Mini Kit (Qiagen). The TCN2 776C→G polymorphism was determined with the use of a Taqman allelic discrimination assay, with primers and probes that were specific for the polymorphism, and Taqman Universal Master Mix (Applied Biosystems). No-template controls and duplicate DNA samples were included in the assays for quality control.
Peripheral neurologic examination
A diagnosis of peripheral neuropathy was made by a board-certified neurologist from the neurologic history and a complete elementary neurologic examination was performed that included both lower limbs. The sensory examination included a timed vibration with a 128-Hz tuning fork that was measured bilaterally at the distal phalanges of great toe. The distal vibration test was scored on the basis of the report of a loss of vibration by the participant <10 s before the examiner perceived the loss of vibration (normal to mild), ≥10 s (mild to moderate), or the inability of the patient to feel the maximum vibration of the tuning fork (severe to total). The predictive value and reproducibility of the tuning fork test for peripheral neuropathy has been shown to be better than that of a monofilament test (27). In addition, subjects were evaluated for slow and fast proprioceptive sensations at the distal phalange of the great toe. Proprioceptive loss in either of these modalities was considered to be evidence of large-fiber neuropathy. Sensitivity to a pinprick, temperature, and a light touch were also determined and mapped to define a level of involvement that was typically in a distal to proximal pattern. Deep-tendon reflexes at the Achilles, patella, biceps, and triceps were tested. Peripheral neuropathy was diagnosed as the presence of a consistent pattern of large-fiber polyneuropathy that was characterized by a loss of either vibratory sense or proprioception. Typically, these subjects had decreased or absent Achilles deep-tendon reflexes. Neuropathy was also diagnosed if signs of small-fiber loss that were characterized by a loss of pain or temperature sensation, numbness, or dysesthesia were shown on the examination, which was typically in a distal-to-proximal pattern of loss. This pattern of sensory loss along with a loss of deep-tendon reflexes in the Achilles as well as the patella was considered evidence of small-fiber polyneuropathy. All subjects were examined by the same neurologist.
Other covariates
Folate and vitamin B-12 intakes were determined with the use of a validated semiquantitative food-frequency questionnaire (28). Information on the other covariates that were used in this study were collected as follows: age, sex, race or ethnicity, education, and smoking status (on the basis of a participant self-report); diabetes (on the basis of a blood glucose concentration >7 mmol/L and reported use of diabetes medication); hypertension (on the basis of systolic pressure >140 mm Hg or diastolic pressure >90 mm Hg and reported use of medications for hypertension); and BMI (in kg/m2), which was calculated as measured weight divided by the square of measured height).
Statistical analysis
ANOVA was used to describe participant characteristics by genotype categories for continuous outcomes, and Pearson’s chi-square test was used for categorical variables. For diabetes and hypertension, adjustments were made for age, sex, and race or ethnicity. The association of the TCN2 776C→G polymorphism with plasma concentrations of vitamin B-12 and total homocysteine was determined with the use of ANCOVA. Logistic regression was used to determine the OR with 95% CIs of having peripheral neuropathy in the GG and CG genotypes compared with in the reference genotype CC. In a secondary analysis to determine whether folate intake modified the association of peripheral neuropathy with the TCN2 polymorphism, we stratified the cohort on the basis of folate intake >2 times the Recommended Dietary Allowance (>800 compared with ≤800 μg dietary folate equivalent/d) or on the basis of the median plasma folate concentrations (>27.9 compared with ≤27.9 nmol/L) before calculating the association or OR as previously described. The interaction between the TCN2 genotype and folate intake in relation to risk of peripheral neuropathy was determined with the use of the interaction term genotype × folate intake in a logistic regression analysis. For all analyses, adjustments were made for the following covariates: age, sex, BMI, race or ethnicity, education (as an indicator of socioeconomic status), serum creatinine concentration (as an indicator of kidney function), smoking status, alcohol consumption, diabetes, hypertension, and vitamin B-12 intake. We adjusted for vitamin B-12 intake in the analyses to distinguish the effect of the TCN2 polymorphism from that of vitamin B-12 intake on the outcomes. In addition, plasma concentrations of folate and pyridoxal 5′-phosphate were used as covariates for the plasma total homocysteine concentration to identify the effect of the TCN2 polymorphism separate from that of these vitamins. Plasma concentrations of homocysteine were log transformed before analyses because of a skewed distribution. Adjustments were made for multiple comparisons (Tukey’s test). There were 10 participants whose vitamin B-12 intakes were less than the recommended 2.4 μg/d. All the analyses were repeated after the exclusion of these participants. All analyses were performed with SAS 9.3 software (SAS Institute Inc.), and a 2-sided P < 0.05 was considered significant.
RESULTS
The prevalence of homozygosity for the variant allele G of the TCN2 776C→G polymorphism was 17.2% in this cohort (Table 1). Individuals with the GG and CG genotypes had significantly lower plasma vitamin B-12 than that of individuals with the CC genotype even after adjusting for vitamin B-12 intake (P < 0.02) (Table 2). The concentration of plasma total homocysteine was similar for all genotypes after adjusting for the covariates (Table 2).
TABLE 1.
Characteristics of study participants by TCN2 776C→G genotype1
| Characteristic | Total (n = 171) | CC (n = 79) | CG (n = 61) | GG (n = 31) | P |
| Age, y | 73.5 ± 8.02 | 73.2 ± 7.9 | 73.8 ± 8.3 | 73.8 ± 8.0 | 0.90 |
| Women, n (%) | 121 (70.8) | 55 (69.6) | 41 (67.2) | 25 (80.7) | 0.39 |
| BMI, kg/m2 | 31.9 ± 8.6 | 32.6 ± 9.9 | 31.7 ± 7.1 | 30.5 ± 7.7 | 0.50 |
| Education, n (% within genotype) | 0.66 | ||||
| Kindergarten through grade eight | 21 (12.3) | 10 (12.7) | 7 (11.5) | 4 (12.9) | |
| Grades 9–11 | 19 (11.1) | 11 (13.9) | 6 (9.8) | 2 (6.5) | |
| Grade 12 or high school | 47 (27.5) | 25 (31.7) | 17 (27.9) | 5 (16.1) | |
| Any undergraduate | 70 (40.9) | 28 (35.4) | 26 (42.6) | 16 (51.6) | |
| Any graduate | 14 (8.2) | 5 (6.3) | 5 (8.2) | 4 (12.9) | |
| Race or ethnicity, n (% within genotype) | <0.001 | ||||
| American Indian or Alaskan Native | 3 (1.8) | 3 (3.8) | 0 (0) | 0 (0) | |
| Asian | 3 (1.8) | 1 (1.3) | 0 (0) | 2 (6.5) | |
| Non-Hispanic black | 58 (33.9) | 39 (49.4) | 16 (26.2) | 3 (9.7) | |
| Hispanic | 2 (1.2) | 0 (0) | 1 (1.6) | 1 (3.2) | |
| Non-Hispanic white | 105 (61.4) | 36 (45.6) | 44 (72.1) | 25 (80.7) | |
| Current smoker, n (%) | 39 (22.8) | 18 (22.8) | 14 (23.0) | 7 (22.6) | 1.00 |
| Alcohol, g/d | 2.0 ± 5.4 | 1.3 ± 4.3 | 3.2 ± 7.2 | 1.4 ± 3.2 | 0.11 |
| Vitamin B-12 intake, μg/d | 16.6 ± 16.6 | 14.8 ± 16.8 | 18.2 ± 16.0 | 17.8 ± 17.5 | 0.44 |
| Folate intake, μg DFE/d | 928.3 ± 576.0 | 863.5 ± 616.0 | 985.4 ± 543.0 | 980.8 ± 531.5 | 0.40 |
| Plasma metabolites | |||||
| Folate, nmol/L | 32.0 ± 19.7 | 31.0 ± 18.8 | 31.5 ± 21.1 | 34.4 ± 20.2 | 0.71 |
| Pyridoxal 5′-phosphate, nmol/L | 79.7 ± 83.34 | 76.5 ± 89.0 | 82.9 ± 76.9 | 82.1 ± 83.8 | 0.89 |
| Creatinine, μmol/L | 78.7 ± 26.5 | 79.6 ± 26.5 | 77.8 ± 35.4 | 75.1 ± 26.5 | 0.68 |
| Diabetes, n (%)3 | 49 (28.7) | 20 (25.3) | 20 (32.8) | 9 (29.0) | 0.54 |
| Hypertension, n (%)3 | 142 (83.0) | 68 (86.1) | 53 (86.9) | 21 (67.7) | 0.21 |
Between-genotype differences were analyzed with the use of Pearson’s chi-square test for categorical variables and a 1-factor ANOVA for continuous variables. DFE, dietary folate equivalent; TCN2, transcobalamin gene.
Mean ± SD (all such values).
ANCOVA adjusted for age, sex, and race or ethnicity.
TABLE 2.
Concentrations of plasma metabolites by TCN2 776C→G genotype1
| Metabolite | Total (n = 171) | CC (n = 79) | CG (n = 61) | GG (n = 31) | P |
| Vitamin B-12,2 pmol/L | 366.0 ± 9.8 | 398.0 ± 30.5 | 336.5 ± 31.7* | 317.0 ± 33.9** | <0.01 |
| Homocysteine,3 μmol/L | 10.7 ± 1.0 | 9.9 ± 1.1 | 10.1 ± 1.1 | 10.7 ± 1.1 | 0.61 |
TCN2, transcobalamin gene.
All values are means ± SEs. ANCOVA was used to determine the association of the TCN2 polymorphism with the plasma concentration of vitamin B-12. Adjustments were made for sex, age, BMI, education, alcohol intake, race or ethnicity, smoking status, vitamin B-12 intake, and serum creatinine concentration as well as for multiple comparisons. *,**Compared with the CC genotype, *P < 0.02; **P = 0.01.
All values are geometric means ± SEs. ANCOVA was used to determine the association of the TCN2 polymorphism with the plasma concentration of homocysteine. Adjustments were made for sex, age, BMI, education, alcohol intake, race or ethnicity, smoking status, vitamin B-12 intake, serum concentrations of creatinine, and plasma concentrations of folate and pyridoxal 5′-phosphate as well as for multiple comparisons.
Of the 171 participants who were assessed for peripheral neuropathy, 75 individuals were diagnosed with peripheral neuropathy (Table 3). The odds of having peripheral neuropathy were 3.3-fold higher in individuals with the GG genotype than in individuals with the CC genotype (OR: 3.33; 95% CI: 1.15, 9.64) (Table 3). There was no difference between the CG genotype and the CC genotype in odds of peripheral neuropathy (OR: 1.26; 95% CI: 0.56, 2.81) (Table 3).
TABLE 3.
Association between peripheral neuropathy and TCN2 776C→G genotype1
| CC genotype (reference group) |
CG genotype |
GG genotype (variant) |
||||
| Cases/total, n | OR | Cases/total, n | OR (95% CI) | Cases/total, n | OR (95% CI) | |
| Overall | 33/79 | 1 | 25/61 | 1.26 (0.56, 2.81) | 17/31 | 3.33 (1.15, 9.64)* |
| Folate intake,2 μg DFE/d | ||||||
| ≤800 | 24/47 | 1 | 11/26 | 1.11 (0.31, 3.93) | 7/12 | 1.49 (0.18, 12.33) |
| >800 | 9/32 | 1 | 14/35 | 1.30 (0.38, 4.51) | 10/19 | 6.90 (1.31, 36.36)** |
Logistic regression analyses were adjusted for sex, age, BMI, education, alcohol intake, serum creatinine concentration, race or ethnicity, smoking status, vitamin B-12 intake, diagnosis of diabetes, and diagnosis of hypertension. *P = 0.03, **P = 0.02. DFE, dietary folate equivalent; TCN2, transcobalamin gene.
TCN2 genotype × folate intake, P-interaction = 0.8.
Because high folate intake can potentially worsen conditions that are associated with vitamin B-12 insufficiency (15), we determined if the association between the TCN2 polymorphism and peripheral neuropathy was modified by folate intake in a secondary analysis. There was no significant interaction between the TCN2 genotype and folate intake on risk of peripheral neuropathy (P = 0.8). However, because we had an a priori hypothesis that high folate intake may modify the effect of the TCN2 polymorphism, we stratified the samples on the basis of total folate intake. When folate intake was >2 times the Recommended Dietary Allowance (800 μg/d), GG genotypes had 7-fold higher odds of neuropathy than CC genotypes (OR: 6.9; 95% CI: 1.31, 36.36) (Table 3).There was no significant difference between genotypes in odds of peripheral neuropathy when folate intake was ≤800 μg/d (OR: 1.49; 95% CI: 0.18, 12.33) (Table 3). The mean natural folate and folic acid intake of subjects who consumed >800 μg total folate/d was higher than that of those who consumed ≤800 μg total folate/d [in >800- and ≤800-μg folate intake/d categories, means ± SDs of natural folate intakes of 312.6 ± 124 compared with 245 ± 87 μg (P < 0.001), respectively, and of folic acid intakes of 572 ± 204 compared with 149.5 ± 53 μg (P < 0.0001), respectively]. In individuals in the high-folate group, 93% of subjects were multivitamin users. The median plasma folate concentration in individuals with total folate intake ≤800 μg/d was 19.5 nmol/L, and for subjects whose intake was >800 μg/d, the median plasma folate concentration was 40 nmol/L. However, the plasma folate concentration did not modify the odds of peripheral neuropathy in GG genotypes when the analyses were conducted after stratifying by median plasma folate. When the analyses were limited only to subjects whose dietary intakes of vitamin B-12 met the Recommended Dietary Allowance, the results for all of the outcomes studied were not changed.
DISCUSSION
In this study, we report the association of the 776C→G polymorphism in the transcobalamin gene TCN2 with peripheral neuropathy in older adults with normal plasma concentrations of vitamin B-12. Peripheral neuropathy associated with vitamin B-12 insufficiency is usually reversible with vitamin B-12 treatment (11). In our cohort, despite having a normal plasma concentration of vitamin B-12, the odds of development of peripheral neuropathy was 3.3-fold higher in GG genotypes than in CC genotypes of the TCN2 polymorphism. The association of the TCN2 polymorphism with peripheral neuropathy was independent of vitamin B-12 intake.
Evidence suggests that high folate intake may worsen the clinical conditions associated with vitamin B-12 insufficiency in the elderly as indicated by the increased prevalence of anemia and cognitive impairment (15, 16, 19). The mechanism behind this association is still under investigation. The Recommended Dietary Allowance for folate is 400 μg dietary folate equivalent/d. The odds of neuropathy were significantly higher for GG genotypes than for CC genotypes when folate intakes were >2 times the Recommended Dietary Allowance (>800 μg/d) but not when the folate intakes were ≤800 μg/d. These results have been supported by a recent study on vitamin B-12 supplementation of elderly individuals who were deficient in the vitamin, in which the improvement of vitamin B-12 status after supplementation was impaired by high serum folate status (29). Both natural folate and synthetic folic acid from fortified foods and supplements contributed to total folate intake in our population. In this cohort, folic acid contributed to a large proportion of total folate intake in subjects who consumed >800 μg/d. Because 93% of the participants in the group with high folate intake also used folic acid–containing supplements, it appears that high total folate intake was mainly due to high folic acid intake. A previous study on folate intake and immune function suggested that folic acid supplementation may have a positive effect when food-folate intake is low, but when food-folate intake is adequate, supplementation may have a negative effect (30). The mean total folate intake of our cohort was 928.3 ± 576 μg dietary folate equivalent/d, which was similar to mean folate intake of the general US population aged ≥51 y as reported for the participants of the NHANES 2003–2006 (31), which indicated that excess folate intake is widely prevalent in older adults. The frequency of homozygosity for the G allele of the TCN2 polymorphism in our cohort was similar to what has been previously reported in the United States (4). Thus, the results of our study are relevant to a large population of older adults who are at increased risk of neurological impairment because of the TCN2 polymorphism.
When analyses were done after stratifying on the basis of the median plasma concentration of folate, higher plasma folate was not associated with increased odds of peripheral neuropathy in GG genotypes. This outcome might have been due to the fact that plasma folate does not always proportionately reflect total folate intake (32). In our cohort, 25.6% of subjects who consumed >800 μg folate/d had less than or equal to the median folate concentration, and 24.7% of subjects who consumed ≤800 μg/d had greater than the median folate concentration.
Despite the prevalence of peripheral neuropathy and its association with the TCN2 polymorphism in the study population, macrocytic anemia, which is another condition that is frequently observed in vitamin B-12 insufficiency, was observed in only one participant. This finding was not unusual because previous studies have reported neurologic disorders that were associated with low vitamin B-12 in the absence of macrocytosis or anemia in the elderly (11).
Individuals with GG and CG genotypes had lower plasma concentrations of vitamin B-12 than did those with the CC genotype, although the values were in the normal range in our cohort. These results could be explained by the lower transcobalamin protein concentration, as well as its impaired binding of vitamin B-12 by the variant enzyme (3, 5), which may have resulted in the renal excretion of vitamin B-12. These results were different from those of previous studies that reported no differences between genotypes or that GG genotypes had higher vitamin B-12 concentrations in plasma (4, 6). We did not observe a relation between the TCN2 polymorphism and plasma concentration of total homocysteine. There have been contradictory reports of higher homocysteine in individuals with GG genotypes (6, 33) as well as a lack of a genotype effect (4, 34).
A limitation of our study is the cross-sectional design of the cohort, which did not allow us to determine a cause-effect relation. Another limitation is the small sample size, as shown by the wide CI of the OR. In a larger sample size with a smaller CI, the odds of peripheral neuropathy due to the TCN2 polymorphism might have been lower. The lack of an interaction between the TCN2 genotype and folate intake in relation to risk of peripheral neuropathy could also have been due to the small sample size. The prevalence of neuropathy was 43.9% in the study participants who were assessed for neuropathy. This value was higher than the 23.8% that was reported for individuals aged ≥60 y in the general US population in the NHANES (1999–2000) (35). One possible reason for this discrepancy could have been the fact that the NHANES population was healthier than our cohort, which was comprised of individuals who needed assistance for independent living. There were more women than men in the subset who were assessed for neuropathy, but women were at lower risk of peripheral neuropathy in our population as well as in other populations (35); consequently, a study with more men would only have strengthened the association between the polymorphism and neuropathy. Our study population was comprised of individuals from different racial backgrounds, but they were predominantly non-Hispanic whites (61.4%) and non-Hispanic African Americans (33.9%). Although lower-extremity diseases such as peripheral neuropathy are more prevalent in non-Hispanic African Americans (35), the TCN2 polymorphism is less prevalent in this group than in non-Hispanic whites (Table 1). Hence, the association between the TCN2 polymorphism and peripheral neuropathy that we observed in this study was not driven by race. Diabetes increases risk of peripheral neuropathy, but we did not make a distinction between diabetic and nondiabetic patients in our analysis because of the small sample size. The prevalence of diabetes was similar in all genotypes (Table 1), and thus the association of the TCN2 polymorphism and peripheral neuropathy was not significantly affected by the presence of diabetes.
In conclusion, our study shows that the TCN2 776C→G polymorphism is associated with increased risk of peripheral neuropathy in older adults despite having adequate vitamin B-12 status. The data also suggest that the odds of peripheral neuropathy are higher for GG genotypes if they consume excess folate. The prevalence of peripheral neuropathy in the US population in individuals aged ≥60 y is 23.8% and is higher at 34.7% in individuals aged ≥80 y (35). Intake of excess folate is also more prevalent in older adults in the United States (36). Our data suggest that increased risk of neuropathy in older adults because of the TCN2 polymorphism may be avoided by limiting folate intake to be close to the Recommended Dietary Allowance and exercising caution with regards to the consumption of folic acid supplements.
Acknowledgments
The authors’ responsibilities were as follows—HS, PRB, WQQ, TMS, and LP: conducted the research; HS, PRB, and LP: wrote the manuscript; HS and PFJ: performed statistical analysis; JS and LP: designed the research; LP: had primary responsibility for final content of the manuscript; and all authors: contributed to the critical revision of the manuscript and read and approved the final manuscript. None of the authors reported a conflict of interest related to the study.
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