Fibrinogen is a positive acute-phase protein and its hepatic synthesis is enhanced following inflammation and injury. However, it is not clear whether fibrinogen synthesis is also responsive to oral nutrients and whether the response to a meal may be affected by age. Our aim in this study was to investigate the acute effect of oral feeding on fibrinogen synthesis in both young and elderly men and women. Fibrinogen synthesis was determined in 3 separate occasions from the incorporation of l[(2)H(5)]phenylalanine (43 mg/kg body weight) in 8 young (21-35 y) and 8 elderly (>60 y) participants following the ingestion of water (control), a complete liquid meal (15% protein, 30% fat, and 55% carbohydrate), or only the protein component of the meal. The ingestion of the complete meal enhanced fibrinogen fractional synthesis rates (FSR) by 17 +/- 6% in the young and by 38 +/- 10% in the elderly participants compared with the water meal (P < 0.02). A comparable stimulation of FSR occurred with only the protein component of the meal in both young (29 +/- 7%) and elderly participants (41 +/- 9%) compared with the water meal (P < 0.005). Similar results were obtained when fibrinogen synthesis was expressed as absolute synthesis rates (i.e. mg.kg(-1).d(-1)). The results demonstrate that fibrinogen synthesis is acutely stimulated after ingestion of a meal and that this effect can be reproduced by the protein component of the meal alone, both in young and elderly adults.