To determine the effects on physical performance of two different very low calorie diets (VLCD), 10 moderately obese women (BMI 32.7) were tested before and after an outpatient weight loss of 17 kg. Diet 1 (D1, n = 5) subjects lost 17.2 kg on 450-550 kcal/d from common food regimen adjusted for stature providing protein at 1.5 g/kg IBW and less than 10 g carbohydrate (CHO). Diet 2 (D2, n = 5) subjects lost 18.1 kg on a fixed composition 420 kcal/d formula providing 70 g (1.2 g/kg IBW) protein and 30 g CHO. Aerobic performance (VO2 max) was determined on a cycle ergometer. VO2 max did not change with D1 (2.16 to 2.01 l/min), while it decreased on D2 (2.44 to 2.06 l/min, P less than 0.05). Anaerobic function was assessed as peak quadricep strength on an isokinetic dynamometer, with anaerobic endurance taken as the time and number of repetitions to fatigue (less than 50 percent peak strength). Peak strength did not change on D1 (95.8 to 86.2 ft-lb), but it declined with D2 (102.0 to 89.4 ft-lb, P less than 0.05). Anaerobic time to fatigue did not change with either diet group. In conclusion, it is possible that the higher (and individualized) protein intake of D1 allowed the better preservation of VO2 max, while the higher CHO of D2 did not benefit anaerobic function. Our data suggest that physical performance tests could be used to assess for functional tissue preservation when diets are used for major weight loss.