Serum pyridoxal concentrations in patients with diabetic neuropathy

Vitamin B6 and diabetes and its role in counteracting advanced glycation end products

Naturally occurring forms of vitamin B6 include six interconvertible water-soluble compounds: pyridoxine (PN), pyridoxal (PL), pyridoxamine (PM), and their respective monophosphorylated derivatives (PNP, PLP, and PMP). PLP is the catalytically active form which works as a cofactor in approximately 200 reactions that regulate the metabolism of glucose, lipids, amino acids, DNA, and neurotransmitters. Most of vitamers can counteract the formation of reactive oxygen species and the advanced glycation end-products (AGEs) which are toxic compounds that accumulate in diabetic patients due to prolonged hyperglycemia. Vitamin B6 levels have been inversely associate with diabetes, while vitamin B6 supplementation reduces diabetes onset and its vascular complications. The mechanisms at the basis of the relation between vitamin B6 and diabetes onset are still not completely clarified. In contrast more evidence indicates that vitamin B6 can protect from diabetes complications through its role as scavenger of AGEs. It has been demonstrated that in diabetes AGEs can destroy the functionality of macromolecules such as protein, lipids, and DNA, thus producing tissue damage that result in vascular diseases. AGEs can be in part also responsible for the increased cancer risk associated with diabetes. In this chapter the relationship between vitamin B6, diabetes and AGEs will be discussed by showing the acquired knowledge and questions that are still open.

The Role of Vitamin B6 in Peripheral Neuropathy: A Systematic Review

INTRODUCTION

Vitamin B6 is a water-soluble vitamin that is naturally present in many foods and is accessible in many dietary supplements. The three natural forms are pyridoxine, pyridoxal, and pyridoxamine. Both vitamin B6 deficiency and high B6 intake have been described as risk factors for developing peripheral neuropathy (PN). The aim of this systematic review is to characterize and comprehensively describe B6-related PN.

METHOD

A systematic, computer-based search was conducted using the PubMed database. Twenty articles were included in this review.

RESULTS

Higher vitamin B6 levels, which usually occur following the taking of nutritional supplements, may lead to the development of a predominantly, if not exclusively, sensory neuropathy of the axonal type. After pyridoxine discontinuation, such patients subjectively report improved symptoms. However, although low vitamin B6 levels can be seen in patients suffering from peripheral neuropathy of various etiologies, there is no firm evidence that low B6 levels have a direct causal relationship with PN. Many studies suggest subjective improvement of neuropathy symptoms in patients suffering from PN of various etiologies after receiving B6 supplementation; however, no data about B6 administration as a monotherapy exist, only as part of a combination treatment, usually with other vitamins. Therefore, the potential therapeutic role of B6 cannot be confirmed to date. Supplementation with vitamin B6, even as part of a nutritional multivitamin supplement, has not been proven harmful at permitted daily doses in patients who already suffer from PN.

CONCLUSION

Current scientific evidence supports a neurotoxic role of B6 at high levels. Although some studies suggest that low B6 is also a potential risk factor, further studies in this area are needed.

B vitamins as a treatment for diabetic pain and neuropathy

WHAT IS KNOWN AND OBJECTIVE

B vitamin therapy is a common treatment for diabetic pain and neuropathy, yet its use remains controversial in patients lacking B vitamin deficiencies. The aim of this review was to summarize the current evidence for the efficacy of B vitamin therapy in diabetic patients with neuropathy.

COMMENT

We screened the English literature for clinical studies evaluating B vitamins as a therapy for pain and neuropathy in diabetic patients. We selected 43 relevant studies for qualitative analysis based on our selection criteria. Our survey of the literature revealed substantive heterogeneity with respect to efficacies of reported outcomes, as well as study design. Most beneficial outcomes were reported against baseline measures, with few positive comparisons against placebo. This highlights the need for larger, placebo-controlled studies.

WHAT IS NEW AND CONCLUSION

B vitamins should be considered a plausible therapy for diabetic neuropathy, but its overall efficacy remains uncertain and requires further study.

Revisiting the evidence for neuropathy caused by pyridoxine deficiency and excess

Pyridoxine deficiency and excess have been implicated as a cause for peripheral neuropathy. As a result, unrelated neuropathies are often treated with pyridoxine based on questionable evidence. However, neurological practitioners frequently discourage patients from taking pyridoxine in excess of 50 mg/d given concerns around the development of a toxic sensory neuronopathy. There is no systematic review to support either of the 2 practices. To address this gap in knowledge, we reviewed the available literature on neuropathy attributed to pyridoxine deficiency and excess. Based on the current limited data, it can be concluded that very low doses of daily pyridoxine are required to prevent peripheral neuropathy. There is inadequate evidence to support routine pyridoxine supplementation in patients with disorders of peripheral nervous system. Supplementation with pyridoxine at doses greater than 50 mg/d for extended duration may be harmful and should be discouraged.

Metanx in type 2 diabetes with peripheral neuropathy: a randomized trial

PURPOSE

To determine whether a combination of L-methylfolate, methylcobalamin, and pyridoxal-5'-phosphate (LMF-MC-PLP [Metanx; Pamlab LLC, Covington, La]) improves sensory neuropathy.

RESEARCH DESIGN AND METHODS

This multicenter, randomized, double-blind, placebo-controlled trial involved 214 patients with type 2 diabetes and neuropathy (baseline vibration perception threshold [VPT]: 25-45 volts), who were randomly assigned to 24 weeks of treatment with either L-methylfolate calcium 3 mg, methylcobalamin 2 mg, and pyridoxal-5'-phosphate 35 mg or placebo. The primary end point was effect on VPT. Secondary end points included Neuropathy Total Symptom Score (NTSS-6) and Short Form 36 (SF-36), as well as plasma levels of folate, vitamins B(6) and B(12), methylmalonic acid (MMA), and homocysteine.

RESULTS

There was no significant effect on VPT. However, patients receiving LMF-MC-PLP consistently reported symptomatic relief, with clinically significant improvement in NTSS-6 scores at week 16 (P=.013 vs placebo) and week 24 (P=.033). Improvement in NTSS scores was related to baseline MMA and inversely related to baseline PLP and metformin use. Quality-of-life measures also improved. Homocysteine decreased by 2.7±3.0 μmol/L with LMF-MC-PLP versus an increase of 0.5±2.4 μmol/L with placebo (P=.0001). Adverse events were infrequent, with no single event occurring in ≥2% of subjects.

CONCLUSIONS

LMF-MC-PLP appears to be a safe and effective therapy for alleviation of peripheral neuropathy symptoms, at least in the short term. Additional long-term studies should be conducted, as the trial duration may have been too short to show an effect on VPT. In addition, further research on the effects in patients with cobalamin deficiency would be useful.

Improving neuropathy scores in type 2 diabetic patients using micronutrients supplementation

AIM

The aim of the present study was to determine if micronutrients supplementation can improve neuropathy indices in type 2 diabetes.

MATERIALS AND METHODS

In this randomized, double-blind, placebo-controlled clinical trial, 75 type 2 diabetes patients were assigned to three treatment groups, receiving one of the following daily supplement for 4 months: Group MV: zinc (20 mg), magnesium (250 mg), vitamin C (200 mg) and E (100 mg); Group MVB: both of the above mineral and vitamin supplements plus vitamin B1 (10 mg), B2 (10 mg), B6 (10 mg), biotin (200 μg), B12 (10 μg) and folic acid (1 mg); Group P: placebo.

RESULTS

67 patients completed the study. Neuropathic symptoms based on the MNSI questionnaire improved from 3.45 to 0.64 (p=0.001) in group MVB, from 3.96 to 1.0 (p=0.001) in group MV and from 2.54 to 1.95 in placebo group after 4 months. There was no significant difference between three treatment groups in MNSI examinations after 4 months supplementations. Over 4 months of treatment, patients showed no significant changes in glycemic control, capillary blood flow or electrophysiological measures in MV and MVB groups compared with placebo group.

CONCLUSIONS

These studies suggest that micronutrients supplementation might ameliorate diabetic neuropathy symptoms.

Experimental model of pyridoxine (B6) deficiency-induced neuropathy

A pyridoxine (B6) dietary deficiency was studied in female adult Sprague-Dawley rats by hind-limb walking-track analysis. Serum levels of pyridoxine and three metabolites were quantified by high-pressure liquid chromatography with fluorescence measurement. Morphometric analysis of the sciatic and posterior tibial nerves (from within the tarsal tunnel) was performed after 1 year on a diet deficient in vitamin B6. The B6-deficient rats developed abnormal walking-track patterns by 8 months, and these track parameters were different from age- and sex-matched normal diet control rats at the p < 0.05 level. Adding B6 at 10 parts per million to the diet then partially corrected these parameters, whereas the addition of 30 parts per million B6 corrected the abnormal pattern completely. Serum pyridoxal concentration correlated with the functional parameters, dropping from a mean of 115 mg per liter to 39.5 mg per liter (p < 0.05), and correcting with the B6 additive. Morphometric analysis demonstrated that the B6-deficient nerve from the tarsal tunnel had a decreased nerve fiber density (p < 0.001), with a normal total myelinated nerve fiber number, and an increased axon-to-myelin ratio (p < 0.003). It is concluded that a diet totally deficient in vitamin B6 results in a peripheral neuropathy.

Pyridoxine and pyridoxamine inhibits superoxide radicals and prevents lipid peroxidation, protein glycosylation, and (Na+ + K+)-ATPase activity reduction in high glucose-treated human erythrocytes

Vitamin B(6) (pyridoxine) supplementation has been found beneficial in preventing diabetic neuropathy and retinopathy, and the glycosylation of proteins. Oxygen radicals and oxidative damage have been implicated in the cellular dysfunction and complications of diabetes. This study was undertaken to test the hypothesis that pyridoxine (P) and pyridoxamine (PM) inhibit superoxide radical production, reduce lipid peroxidation and glycosylation, and increase the (Na+ + K+)-ATPase activity in high glucose-exposed red blood cells (RBC). Superoxide radical production was assessed by the reduction of cytochrome C by glucose in the presence and absence of P or PM in a cell-free buffered solution. To examine cellular effects, washed normal human RBC were treated with control and high glucose concentrations with and without P or PM. Both P and PM significantly lowered lipid peroxidation and glycated hemoglobin (HbA(1)) formation in high glucose-exposed RBC. P and PM significantly prevented the reduction in (Na+ + K+)-ATPase activity in high glucose-treated RBC. Thus, P or PM can inhibit oxygen radical production, which in turn prevents the lipid peroxidation, protein glycosylation, and (Na+ + K+)-ATPase activity reduction induced by the hyperglycemia. This study describes a new biochemical mechanism by which P or PM supplementation may delay or inhibit the development of complications in diabetes.

Carpal tunnel syndrome, pyridoxine, and the work place

Carpal tunnel syndrome has become a major source of work-related impairment. Recent studies have suggested pyridoxine deficiency may be a cause of carpal tunnel syndrome, but all studies on this relationship have been flawed by lack of scientific design. Given the available evidence, pyridoxine is not likely to be a significant cause of carpal tunnel syndrome in the work place. Ergonomic measures are more likely to be effective than pyridoxine in reducing the incidence and cost of carpal tunnel syndrome in the work place.

Treatment of peripheral neuropathies

There are three general approaches to treatment of peripheral neuropathy. First, an attempt should be made to reverse the pathophysiological process if its nature can be elucidated. Second, nerve metabolism can be stimulated and regeneration encouraged. Third, even if the neuropathy itself cannot be improved, symptomatic therapy can be employed. This review outlines the options available for each approach.

Rationales for micronutrient supplementation in diabetes

Available evidence--some well-documented, some only preliminary--suggests that properly-designed nutritional insurance supplementation may have particular value in diabetes. Comprehensive micronutrient supplementation providing ample doses of antioxidants, yeast-chromium, magnesium, zinc, pyridoxine, gamma-linolenic acid, and carnitine, may aid glucose tolerance, stimulate immune defenses, and promote wound healing, while reducing the risk and severity of some of the secondary complications of diabetes.

An expanded concept of "insurance" supplementation--broad-spectrum protection from cardiovascular disease

The preventive merits of "nutritional insurance" supplementation can be considerably broadened if meaningful doses of nutrients such as mitochondrial "metavitamins" (coenzyme Q, lipoic acid, carnitine), lipotropes, and key essential fatty acids, are included in insurance supplements. From the standpoint of cardiovascular protection, these nutrients, as well as magnesium, selenium, and GTF-chromium, appear to have particular value. Sophisticated insurance supplementation would likely have a favorable impact on many parameters which govern cardiovascular risk--serum lipid profiles, blood pressure, platelet stability, glucose tolerance, bioenergetics, action potential regulation--and as a life-long preventive health strategy might confer substantial benefit.

Maturity-onset diabetes mellitus--toward a physiological appropriate management

"Eumetabolic" therapies of diabetes are defined as those which promote and potentiate a normal physiological pattern of insulin activity--in contrast to the less subtle effects of exogenous insulin or sulfonylureas. Effective eumetabolic therapy would provide better metabolic control and reduce the risk of secondary complications, would be more convenient to administer, and would be especially appropriate for preventive use in the early stages of diabetes. Possible components of a eumetabolic therapy include: aspirin, as a potentiator of glucose-stimulated insulin secretion; GTF, to directly enhance the efficacy of insulin; weight loss, exercise, and fasting, to help reduce tissue resistance to insulin; mitochondrial "metavitamins", to optimize the oxidative disposal of excess substrate; a high-fiber, low-fat diet, which appears superior to traditional diabetic diets as a promoter of glucose tolerance. Following a prolonged fast, obese diabetics show substantial improvement in most parameters of insulin function--an effect which is to some degree independent of weight loss; long-term remission of diabetes may be possible if the benefits of therapeutic fasting are conserved by appropriate eumetabolic measures.

The association of bacteriuria and reduced serum pyridoxal concentrations in patients with diabetes mellitus

Of 1017 patients admitted to the Royal Perth Hospital Diabetic Survey 142 were found to have significant bacteriuria. In these bacteriuric patients serum pyridoxal concentrations were significantly reduced (P = less than 0.001) when compared with 142 diabetic patients matched for age (+/- 5 years) and sex but without infection of the urinary tract. Measurements were repeated up to 6 mth after antibacterial treatment and serum pyridoxal concentrations were still low. Pyridoxal has a role in immunological competence, and it is possible that the increased incidence of urinary tract infection in patients with diabetes reflects impaired immunological competence due to pyridoxal deficiency.

Carpal tunnel syndrome, diabetes and pyridoxal

Serum pyridoxal concentrations were estimated in 13 patients who had both diabetes and carpal tunnel syndrome. The clinical features and response to treatment in these patients did not differ from that seen in non-diabetic patients with this syndrome. No significant difference in mean serum pyridoxal concentrations was noted between diabetics with carpal tunnel syndrome, diabetics without neuropathy, and normal subjects. These results indicate that carpal tunnel syndrome in diabetics is distinct from polyneuropathy in that it is not associated with a reduction in serum pyridoxal levels which is a feature of diabetic polyneuropathy.