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To examine effects of dietary fish oil supplementation with sodium restriction on blood pressure in the elderly.In a double-blind dietary intervention lasting 4 weeks, parallel comparisons of blood pressure were made in volunteers assigned to one of four treatment groups: fish oil and low sodium; fish oil and normal sodium; sunflower oil and low sodium; or sunflower oil and normal sodium.Subjects lived at home and attended our nutrition research clinic at fortnightly intervals for dietary counselling and blood pressure measurement.Health volunteers aged 60-80 years were sought by advertisement. A total of 114 men and women were enrolled in two cohorts; 106, with an initial mean blood pressure of 132/77 mmHg, satisfactorily completed the study.All subjects adopted a low-sodium diet and dietary changes were effected by double-blind administration of slow-release sodium chloride or placebo tablets, along with capsules containing either fish or sunflower oil.The primary measure was the within-subject change in blood pressure after 4 weeks of intervention in each dietary treatment group.Urinary sodium excretion in subjects on low-sodium diets decreased whilst potassium excretion was unaffected. Systolic blood pressure (SBP) fell in the group taking sunflower oil with low sodium, but there was only a transient fall in diastolic blood pressure (DBP). In those taking fish oil with normal sodium, the change in blood pressure was not significant, except after adjustment for initial blood pressure and weight changes. When fish oil was combined with low sodium, however, both SBP and DBP were substantially reduced; the reduction in DBP was significantly greater than in the other treatment groups.Dietary fish oil and sodium restriction can interact to lower DBP in the elderly.