In this study we found clinical diabetes was associated with a marked deficiency of thiamine in blood plasma linked to increased ClThiamine and FEThiamine. The low plasma thiamine concentration in diabetes was linked inversely to plasma sVCAM-1.
The strengths of this study were the use of a validated assay for thiamine and phosphorylated metabolites, a comprehensive assessment of thiamine-related variables (including thiamine transporter content of erythrocyte membranes, urinary excretion, ClThiamine
) and investigation of the association of thiamine-related variables with markers of metabolic and vascular dysfunction. The decreased plasma thiamine concentration in clinical diabetes was probably not due to a deficiency of dietary input of thiamine. The urinary excretion of thiamine of diabetic patients and normal healthy volunteers was within the normal range (>0.20 μmol/24 h) [19
], except for one type 2 diabetic patient with a urinary thiamine excretion of 0.08 μmol/24 h. Urinary excretion of thiamine accounts for a minor part of normal thiamine turnover: pyrimidine and thiazole degradation products of thiamine are also excreted in the urine and account for the major component of thiamine turnover [20
]. Rather, low plasma thiamine concentration was linked to a profound increase in ClThiamine
. The molecular mass of thiamine is <500 Da and hence it is filtered from plasma in renal glomeruli [10
]. Increased ClThiamine
is probably due to decreased re-uptake of thiamine in renal proximal tubules. Thiamine clearance was dysfunctional in diabetic patients with normal GFR, as assessed by creatinine clearance. This study suggests renal mishandling of thiamine in diabetic patients is an early marker of renal dysfunction in diabetes and, linked to the locus of renal thiamine re-uptake, particularly relates to proximal tubule dysfunction.
Re-uptake of thiamine occurs in the proximal tubules by thiamine transporters THTR-1 and THTR-2 via a sodium-independent, proton antiport mechanism with regulation by Ca2+
]. The expression of the genes encoding THTR-1 and THTR-2 (and also RFC-1) transporters are regulated via SP1 promoter elements [22
]. SP1 signalling in the tubular epithelium is impaired in hyperglycaemia associated with diabetes by increased O
-glycosylation of the SP1 via enhanced hexosamine pathway activity [25
]. Re-uptake of thiamine by the tubular epithelium in experimental diabetes [26
] and clinical diabetes (this work) may be impaired by hexosamine pathway-linked decreased expression of the genes encoding THTR-1 and THTR-2, inhibition of these thiamine transporters by dicarbonyl glycation [27
] and acidification of the tubular lumen [28
]. This deserves further investigation.
A weakness of the study was the finding of no strong link to a recognised clinical endpoint such as incipient nephropathy judged by microalbuminuria, although there was a weak negative correlation of erythrocyte TK activity with urinary albumin excretion. It is becoming increasingly evident, however, that urinary albumin excretion may not be a reliable marker of early stage decline in renal function in diabetes [29
]. Future studies would be probably better addressed to investigation of the link between changes in thiamine homeostasis and early decline in GFR assessed by cystatin C measurement [30
Decreased plasma thiamine concentration has been reported in diabetic patients previously in studies with small patient numbers [31
] but ClThiamine
, thiamine and TMP transporter content of erythrocyte membranes and plasma sVCAM-1 were not evaluated and hence links to these variables were not disclosed. Thiamine deficiency is conventionally assessed by measuring the percentage unsaturation of erythrocyte TK with TPP cofactor [12
]. Herein we found, however, that plasma deficiency of thiamine in diabetes was masked in erythrocytes by increased levels of THTR-1 and RFC-1 transporter protein. As erythrocytes lack protein synthetic capability, increased protein levels of these transporters was produced by increased expression of the genes encoding THTR-1 and RFC-1 in the erythrocyte precursors, reticulocytes and erythroblasts. Indeed, increased levels of THTR-1 and RFC-1 transporter proteins were also found in MNLs in these diabetic patients (data not shown). Increased thiamine transporter gene expression and protein level is a response to thiamine deficiency [21
] and is consistent with the low plasma thiamine concentration found in diabetic patients herein. This explains why a low plasma thiamine concentration in clinical diabetes has hitherto gone unrecognised. This study indicates that the measurement of the ‘thiamine effect’ based on erythrocyte TK activities is an inadequate assessment of clinical thiamine status as it can be masked by changes in thiamine transporter protein levels.
Low plasma thiamine concentration in diabetes may be of limited significance if tissues can upregulate the gene expression and protein levels of thiamine transporters and maintain normal TK activity. Although this occurs in the normoglycaemic state [21
], our recent studies of experimental diabetes indicated it does not occur in renal glomeruli in the diabetic state [26
] where enhanced hexosamine signalling may block increased expression of the genes encoding THTR-1, THTR-2 and RFC-1 and their protein levels. In diabetic glomeruli, TK activity and level of TK protein was decreased 60% [26
], and similar impairment of thiamine uptake and metabolism may occur in the diabetic retina and peripheral nerve [34
]. Indeed, RFC-1 has impaired gene expression and protein levels in the diabetic retina [35
Decreased availability of thiamine in vascular cells in diabetes exacerbates metabolic dysfunction in hyperglycaemia. Increased plasma sVCAM-1 is a marker of endothelial dysfunction [36
] and increased risk of atherosclerosis [37
]. sVCAM-1 was increased in diabetic patients with normal renal function [38
] and was linked to microvascular and macrovascular complications in diabetes [39
], although the link to glycaemic control is less certain [39
]. Low plasma thiamine concentration may be a confounding factor linked to increased sVCAM-1 in diabetes.
This study indicates that type 1 and type 2 diabetic patients in the UK exhibit low plasma thiamine concentration. The conventional indicator of thiamine sufficiency, erythrocyte TK activity, is masked in clinical diabetes by increased protein levels of thiamine and TMP transporters, THTR-1 and RFC-1. The deficiency of thiamine in clinical diabetes may increase the fragility of vascular cells to the adverse effects of hyperglycaemia and thereby increase the risk of developing microvascular complications. Correction of the low plasma thiamine concentration with thiamine supplements may decrease the risk of microvascular complications in diabetes.
Important areas for future study are: (1) confirmation of low plasma thiamine concentrations in diabetic populations of other countries independent of local dietary and culinary practice; (2) the evaluation of thiamine and thiamine derivatives to correct low plasma thiamine concentration in diabetes, reverse vascular dysfunction and prevent vascular complications; and (3) investigation of the mechanism of increased ClThiamine in diabetes. These studies are either ongoing or in preparation.