BACKGROUND/OBJECTIVES: Total homocysteine (tHcy) has been associated with increased risk of several diseases in the general population. It is not clear whether these associations are causal. A less healthy lifestyle as well as a less favorable biological risk factor profile have been related to increased tHcy in cross-sectional studies. In addition, the methylenetetrahydrofolate reductase (MTHFR) C677T gene variant is an important determinant of elevated tHcy. The main objective of the study was to examine the effect of changes in biological risk factors and lifestyle on tHcy in relation to MTHFR C677T genotype. SUBJECTS/METHODS: The study is a population-based study including 1805 men and women aged 30-60 years participating in a health examination at baseline (1999-2001) and at a 5-year follow-up examination. RESULTS: Changes in tHcy were not associated with changes in dietary habits, physical activity, smoking status, coffee, tea, total alcohol or wine consumption. An inverse relationship was observed between changes in tHcy and changes in the intake of beer in TT individuals but not in CC/CT individuals (P (interaction)=0.01). In addition, changes in tHcy were positively associated with changes in several biological risk factors, such as waist circumference, diastolic blood pressure, total cholesterol and LDL cholesterol (P<0.01). The association between waist circumference and MTHFR genotype seemed stronger in TT individuals than in CC/CT individuals (P (interaction)=0.03). CONCLUSIONS: tHcy was not influenced by lifestyle changes except for the inverse association between beer consumption and tHcy observed in TT individuals. This suggests that tHcy is not a mediator in the causal pathway between lifestyle and lifestyle-related diseases such as cardiovascular disease in accordance with results of recent randomized trials. However, tHcy was related to changes in several biological risk factors, suggesting that tHcy may be a good predictor or marker of disease.