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Background: The evident weight loss observed during the first six months after bariatric surgery is due to the reduction in body fat and muscular mass. There is, however, the need to keep track the body composition of obese patients during this period of time in order to monitor the different body composition fractions such as water, fat-free mass, and fat. Bioimpedancy is a useful tool to assess body composition and a portable and easy-to-use alternative to determine variations of these fractions in obese patients during their weight loss period.

Methods: Thirty-six morbid obese patients were subjected to gastroplasty with silastic ring and Roux-en-Y distal gastric bypass, and their percentages of fat, water, and fat-free mass were measured the day before the surgery, as well as 2, 4, and 6 months after the surgery. A four-channel Bioelectrical Body Composition Analyzer, which measures the difference between upper right limb and lower right limb was used.

Results: The results indicated a pattern in the measurements obtained in all cases. A linear reduction of the average fat content was observed, ranging from 50.9% at the pre-operatory to 40.8% 6 months after the surgery. The results also showed a linear reduction of average BMI, which varied from 55.1 kg/m2 to 37.7 kg/m2 in 6 months, and a linear increase of water content varying from 35.9% to 43.4% during the same period. Although there was a decrease in the total fat-free mass (76.9 kg to 62.7 kg), an increase in its percentage relative to the total weight (49.1% to 58.9%) was observed during the study.

Conclusions: Bioimpedancy measurements carried out up to 6 months after the surgery did not reveal significant loss in muscular mass, indicating only a minor effect of malnutrition associated to weight loss, and an increase in the percentage of fat-free mass relative to the total weight.