A 40-year Follow-up of the Dallas Bed Rest Study

Abstract

BACKGROUND: In 1966, five 20-year old men underwent a comprehensive physiologic evaluation of the capacity for adaptation of the cardiovascular system in response to 3 weeks of bed rest and 8 weeks of heavy aerobic training; these same subjects were studied again at the age of 50. The aim of the present study was to re-examine the cardiovascular response to maximal exercise in these men after a 40-year interval from their initial assessments. METHODS: In all three study years, minute ventilation and expired gases were determined with Douglas bags and mass spectrometry respectively. Cardiac output was determined using the C2H2 re-breathe technique in 1996 and 2006. RESULTS: On average, compared with 1966, body weight and percent fat mass increased 19 and 73% respectively, while lean body mass declined 10%. Additionally, absolute VO2max decreased 28% (3.3 vs 2.4 L/min), attributable to a 22% decline in maximal arterio-venous oxygen difference (16.2 vs 12.7 vol%). Age-related changes in cardiovascular capacity were significantly greater in the interval between 1996 and 2006 compared with the interval between 1966 and 1996 (Table 1).Table 1: Relative changes in select variables between the two follow-up intervals.CONCLUSIONS: In the original 5 participants of the Dallas Bed Rest Study, maximal oxygen uptake declined over the 40-year study interval, initially because of a reduction in maximal peripheral oxygen extraction, and subsequently due to a balanced decrease in delivery and extraction. Also, the rate of decline in VO2max and its determinants accelerated after the age of 50 years. Impressively, the decline in VO2max after 40 years of life was comparable (27% vs 26%) to that experienced after three weeks of strict bed rest at 20 years of age.