Dietary preparation and per cent fat measurement by hydrostatic weighing

To examine if the dietary preparation for hydrostatic weighing (HW) alters the % fat measurement, seven men and three women (age 29 +/- 6, Males 11.7 +/- 7.3% fat and Females 24.1 +/- 5.4% fat, mean +/- SD) were assessed before and after three meals. On separate days and in random order, each subject (1) ate a salad with toppings (600 g) with small beverage, (2) ate two bean burritos and one bean tostada (900 g) with small beverage, and (3) drank 800 ml of carbonated beverage. The subject was reweighted 45 min after the meals and immediately after the beverage ingestion. Functional residual capacity was assessed simultaneously with underwater weight. Dependent t-tests indicated that body weight in air increased after each of the three means (Bean 0.78, Salad 0.92, Beverage 0.90 kg, all p less than 0.05), but underwater weights were unchanged. Following the salad, body density (1064.6 vs 1061.8 kg.m-3) and % fat (15.0 vs 16.3%, p less than 0.05) were significantly different pre-meal vs post-meal. Likewise, beverage ingestion resulted in decreased body density (1064.7 vs 1061.2 kg.m-3) and increased % fat (15.0 vs 16.5%, p less than 0.05). The bean meal did not cause a change in measured body density (1062.8 vs 1062.8 kg.m-3) or % fat (15.8 vs 15.8%). These results indicate that for the most accurate analysis, preliminary dietary preparation for HW should include food and beverage restriction.