Type 2 diabetes is characterized by both insulin resistance and an increased cardiovascular mortality (1). Elevated plasma total homocysteine levels are an independent risk factor for cardiovascular disease (2). Also, in patients with type 2 diabetes, elevated plasma total homocysteine levels are associated with an increased risk of cardiovascular disease and an increased mortality (3,4).

It is well known that insulin resistance is aggravated by chronic hyperglycemia. This hyperglycemia-induced insulin resistance is reversed by treatment, resulting in a prolonged period of euglycemia (5).

Although in animal studies evidence was found that hyperinsulinemia and/or insulin resistance increased total homocysteine levels (6), in humans the relation between insulin resistance and total homocysteine levels is unclear, in both healthy subjects (7–12) and patients with type 2 diabetes (13–16). In patients with type 2 diabetes, a positive association between insulin resistance and total homocysteine levels was reported (13), but this finding was not reproduced (14). Furthermore, in patients with type 2 diabetes, a positive association between the degree of metabolic control and total homocysteine levels was reported (15,16).

We hypothesized that if insulin resistance is associated with elevated homocysteine levels, thereby providing another explanation for the increased cardiovascular disease in type 2 diabetes, then homocysteine levels will decrease after amelioration of hyperglycemia-induced insulin resistance and associated metabolic abnormalities.

Here, we report the results of an intervention study (17). Eight obese type 2 diabetic patients (aged 53 ± 13 years, HbA1c 12.0 ± 1.7%, BMI 38 ± 5.8 kg/m2) with severe insulin resistance (subcutaneous insulin dose, four-dose insulin regimen 1.92 ± 0.66 units · kg−1 · day−1) were studied before and after a period of 28 ± 5 days of intravenous insulin treatment. Intravenous insulin treatment resulted in euglycemia. Before and after intervention, insulin sensitivity was assessed by a hyperinsulinemic-euglycemic clamp. Fasting plasma total homocysteine, HbA1c, and lipid concentrations were measured on the days of the clamps. Also, 24-h urine creatinine excretion was investigated before and after intervention.

Plasma total homocysteine levels were measured with a high-performance liquid chromatography method according to Fiskerstrand (18,19) with some modifications. Because all variables were normally distributed, statistical analyses of differences were performed by paired Student’s t test. P < 0.05 was considered significant. Results are given as the mean ± SD.

After a 4-week period of intravenous insulin treatment resulting in an euglycemic period of 17 ± 4 days, insulin sensitivity improved since body glucose uptake, measured by clamp, increased (12.7 ± 5.7 before vs. 22.4 ± 8.8 μmol · kg−1 · min−1 after euglycemia, P < 0.0005) and the intravenous insulin dose required for achieving and maintaining euglycemia decreased (1.7 ± 0.9 before vs. 1.1 ± 0.6 units · kg−1 · day−1 after euglycemia, P < 0.005). Metabolic control improved; HbA1c decreased substantially (12.0 ± 1.7 before vs. 8.9 ± 1.2% after euglycemia, P < 0.0001); total cholesterol decreased (5.40 ± 1.09 before vs. 4.50 ± 1.10 mmol/l after euglycemia, P < 0.05); and triglycerides tended to decrease (4.34 ± 3.32 before vs. 1.91 ± 0.69 mmol/l after euglycemia, P = 0.06). These results are described in detail elsewhere (17). Plasma total homocysteine levels were similar before and after improved insulin sensitivity (total homocysteine levels 8.5 ± 2.4 before vs. 9.2 ± 3.5 μmol/l after euglycemia, P = 0.5). Urine creatinine excretion was similar before and after euglycemia (15.0 ± 4.8 before vs. 12 ± 4.3 μmol/24 h after euglycemia).

This intervention study shows that despite amelioration of hyperglycemia-induced insulin resistance and improved metabolic control, plasma total homocysteine concentrations did not decrease. Homocysteine levels, measured in these patients with type 2 diabetes, were actually within the normal range of healthy subjects (2).

Our study is the first intervention study to investigate the relation between insulin resistance and plasma total homocysteine levels. To date only cross-sectional studies have been performed (13–15), which are more susceptible for confounding by other metabolic factors. Also in contrast to previous reports (8,9,11,12,14,15), we studied insulin sensitivity by clamp, the gold standard.

We conclude that amelioration of hyperglycemia-induced insulin resistance does not change plasma total homocysteine levels. These data refute the hypothesis that homocysteine levels are influenced by insulin resistance and by the degree of metabolic control, at least in the patients we studied. Therefore, these results do not support elevated homocysteine levels as an explanation for the link between insulin resistance and the increased risk of cardiovascular disease in type 2 diabetes.

This study was supported by a grant from the Dutch Diabetes Foundation. H.J.B. is an established investigator of the Netherlands Heart Foundation (D97.021).

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Address correspondence to Marie-Jose Pouwels, MD, Division of General Internal Medicine, Medisch Spectrum Twente, P.O. Box 50000, 7500 KA Enschede, The Netherlands. E-mail: [email protected].

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2003