Abstract 326: Salt Intake, Drinking Behavior, And The Body Corporate

Abstract

Clinical practice and public health advice on salt consumption relies on the generally accepted notion that accurately collected 24-hour urine collections are valid estimates for salt and fluid intake. Reduced salt intake would also reduce thirst and ingestion of sugary drinks accordingly. To test these widely accepted ideas, we performed two independent ultra-long term salt balance studies lasting 105 and 205 days in 10 men simulating a flight to Mars. We fixed dietary intake of all constituents for months at salt intakes of 6, 9, and 12 grams per day and collected all urine. We controlled all environmental factors for rigorous quantitative comparison of urine volume and salt excretion with daily fluid and salt intake. In 1646 collected 24-hour urine samples, our ten subjects recovered 92% of salt they ingested. Due to infradian rhythmical renal salt excretion, only 781 out of 1646 daily urine samples correctly classified a 3 g difference in salt intake (47%). Reducing salt intake from 12 to 6 g reduced 24-hour cortisone excretion (78.3±19.6 vs. 67.2±19.3 μg/d; P<0.001) and increased water intake by 299 ml/d due to reduced glucocorticoid-driven metabolic water production. Even accurately collected 24-hour urine collections at intakes ranging from 6-12 grams salt per day provide no reliable information on individual salt intake. Reducing salt intake leads to reduced glucocorticoid levels decreased metabolic water production and a counterintuitive increase in fluid intake. Traditional views on fluid and electrolyte homeostasis may lead to misinterpretations in clinical practice, public health research, and policy-decision making.