Weight change underway and in port in the American Navy

The introduction of strict weight control guidelines in the American Navy has drawn attention to a theory that obese sailors lose weight more readily at sea than ashore. I tested this hypothesis retrospectively over a 13 month period on a large American naval vessel, using data on 40 obese men who were weighed weekly as part of a mandatory health and physical readiness program. Included in the study period were both a 25 week deployment and a 29 week extended in port period. Ten subjects were enrolled for the entire 13 months, and their mean in port and underway weight changes were compared using a paired t test. Regression analysis was also performed comparing time underway or in port with net weight change for all subjects. The 10 men enrolled for 13 months had a mean weight loss of 6.6 kg during the underway period and a mean weight gain of 4.3 kg during the in port period (P less than 0.005). Number of weeks underway and number of weeks in port were found to correlate with number of kilograms lost and gained, respectively (P less than 0.005). These data suggest that men who have strong incentives to lose weight do so more effectively at sea than after returning from sea.