Low diagnostic value of fasting and post-methionine load homocysteine tests. A study in Dutch subjects with homocysteine test indications

Variability of plasma and urine betaine in diabetes mellitus and its relationship to methionine load test responses: an observational study

Background

Since betaine is an osmolyte and methyl donor, and abnormal betaine loss is common in diabetes mellitus (>20% patients), we investigated the relationship between betaine and the post-methionine load rise in homocysteine, in diabetes and control subjects. The post-methionine load test is reported to be both an independent vascular risk factor and a measure of betaine sufficiency.

Methods

Patients with type 2 diabetes (n = 34) and control subjects (n = 17) were recruited. We measured baseline fasting plasma and 4-hour post-methionine load (L-methionine, 0.1 mg/kg body weight) concentrations of homocysteine, betaine, and the betaine metabolite N,N-dimethylglycine. Baseline urine excretions of betaine, dimethylglycine and glucose were measured on morning urine samples as the ratio to urine creatinine. Statistical determinants of the post-methionine load increase in homocysteine were identified in multiple linear regression models.

Results

Plasma betaine concentrations and urinary betaine excretions were significantly (p < 0.001) more variable in the subjects with diabetes compared with the controls. Dimethylglycine excretion (p = 0.00014) and plasma dimethylglycine concentrations (p = 0.039) were also more variable. In diabetes, plasma betaine was a significant negative determinant (p < 0.001) of the post-methionine load increase in homocysteine. However, it was not conclusive that this was different from the relationship in the controls. In the patients with diabetes, a strong relationship was found between urinary betaine excretion and urinary glucose excretion (but not with plasma glucose).

Conclusions

Both high and low plasma betaine concentrations, and high and low urinary betaine excretions, are more prevalent in diabetes. The availability of betaine affects the response in the methionine load test. The benefits of increasing betaine intake should be investigated.

Homocysteine metabolism, hyperhomocysteinaemia and vascular disease: an overview

Hyperhomocysteinaemia has been regarded as a new modifiable risk factor for atherosclerosis and vascular disease. Homocysteine is a branch-point intermediate of methionine metabolism, which can be further metabolised via two alternative pathways: degraded irreversibly through the transsulphuration pathway or remethylated to methionine by the remethylation pathway. Both pathways are B-vitamin-dependent. Plasma homocysteine concentrations are determined by nongenetic and genetic factors. The metabolism of homocysteine, the role of B vitamins and the contribution of nongenetic and genetic determinants of homocysteine concentrations are reviewed. The mechanisms whereby homocysteine causes endothelial damage and vascular disease are not fully understood. Recently, a link has been postulated between homocysteine, or its intermediates, and an alterated DNA methylation pattern. The involvement of epigenetic mechanisms in the context of homocysteine and atherosclerosis, due to inhibition of transmethylation reactions, is briefly overviewed.

Betaine and Folate Status as Cooperative Determinants of Plasma Homocysteine in Humans

OBJECTIVE

Two published studies have demonstrated that betaine in the circulation is a determinant of plasma total homocysteine, but none had sufficient power to investigate the possible effect modification by folate status.

METHODS AND RESULTS

We measured homocysteine, betaine, folate, vitamin B(6), and related compounds in serum/plasma from 500 healthy men and women aged 34 to 69 years before (fasting levels) and 6 hours after a standard methionine loading test. Choline, dimethylglycine, and folate were determinants of plasma betaine in a multiple regression model adjusting for age and sex. The increase in homocysteine after loading showed a strong inverse association with plasma betaine and a weaker inverse association with folate and vitamin B(6). Fasting homocysteine showed a strong inverse relation to folate, a weak relation to plasma betaine, and no relation to vitamin B(6). Notably, adjusted (for age and sex) dose-response curves for the postmethionine increase in homocysteine or fasting homocysteine versus betaine showed that the inverse associations were most pronounced at low serum folate, an observation that was confirmed by analyses of interaction.

CONCLUSIONS

Collectively, these results show that plasma betaine is a strong determinant of increase in homocysteine after methionine loading, particularly in subjects with low folate status. In 500 healthy subjects, postmethionine load increase in tHcy showed a stronger inverse relation to betaine than to folate and vitamin B6, whereas for fasting tHcy, betaine was a weaker determinant than folate. For both tHcy modalities, the association with betaine was most pronounced in subjects with low folate status.