Objective: To examine the hypothesis that the increase in fibrinogen concentration and respiratory infections in winter is related to seasonal variations in vitamin C status (assessed with serum ascorbate concentration).
Design: Longitudinal study of individuals seen at intervals of two months over one year.
Setting: Cambridge.
Subjects: 96 men and women aged 65-74 years living in their own homes.
Main outcome measures: Haemostatic factors fibrinogen and factor VIIC; acute phase proteins; respiratory symptoms; respiratory function.
Results: Mean dietary intake of vitamin C varied from about 65 mg/24 h in winter to 90 mg/24 h in summer; mean serum ascorbate concentration ranged from 50 mumol/l in winter to 60 mumol/l in summer. Serum ascorbate concentration was strongly inversely related to haemostatic factors fibrinogen and factor VIIC as well as to acute phase proteins but not to self reported respiratory symptoms or neutrophil count. Serum ascorbate concentration was also related positively to forced expiratory volume in one second. An increase in dietary vitamin C of 60 mg daily (about one orange) was associated with a decrease in fibrinogen concentrations of 0.15 g/l, equivalent (according to prospective studies) to a decline of approximately 10% in risk of ischaemic heart disease.
Conclusion: High intake of vitamin C has been suggested as being protective both for respiratory infection and for cardiovascular disease. These findings support the hypothesis that vitamin C may protect against cardiovascular disease through an effect on haemostatic factors at least partly through the response to infection; this may have implications both for our understanding of the pathogenetic mechanisms in respiratory and cardiovascular disease and for the prevention of such conditions.