Objective. To evaluate the efficacy and metabolic impact of a high-protein, low-carbohydrate, low-fat ketogenic diet (K diet) in the treatment of morbidly obese adolescents with initial weights of >200% of ideal body weight.

Methods. Six adolescents, aged 12 to 15 years, weighing an average of 147.8 kg (range, 120.6–198.6 kg) and having an average body mass index of 50.9 kg/m2 (39.8–63.0 kg/m2), consumed the K diet for 8 weeks. Daily intake consisted of 650 to 725 calories, which was substantively in the form of protein (80–100 g). The diet was very low in carbohydrates (25 g) and fat (25 g). This was followed by 12 weeks of the K diet plus two carbohydrates (30 g) per meal (K+2 diet).

Main Outcome Measures. Anthropometric data and blood and urine were collected at enrollment, during week 1, and at 4-week intervals throughout the course of the study. Resting energy expenditure was measured by indirect calorimetry. Body composition was estimated using dual-energy x-ray absorptiometry, bioelectrical impedance analysis, and urinary creatinine excretion at enrollment and on completion of each phase of the diet. Nocturnal polysomnography and multiple sleep latency testing were conducted at baseline and repeated after an average weight loss of 18.7 kg to determine sleep architecture, frequency and duration of apneas, and daytime sleepiness.

Results. Subjects lost 15.4 ± 1.4 kg (mean ± SEM) during the K diet and an additional 2.3 ± 2.9 kg during the K+2 diet. Body mass index decreased 5.6 ± 0.6 kg/m2during the K diet and an additional 1.1 ± 1.1 kg/m2during the K+2 diet. Body composition studies indicated that weight was lost equally from all areas of the body and was predominantly fat. Dual-energy x-ray absorptiometry showed a decrease from 51.1% ± 2.1% body fat to 44.2% ± 2.9% during the K diet and then to 41.6% ± 4.5% during the K+2 diet. Lean body mass was not significantly affected. Weight loss was accompanied by a reduction in resting energy expenditure of 5.2 ± 1.8 kcal/kg of fat-free mass per day. Blood chemistries remained normal throughout the study and included a decrease in serum cholesterol from 162 ± 12 to 121 ± 8 mg/dL in the initial 4 weeks of the K diet. An increase in calcium excretion was accompanied by a decrease in total-body bone mineral content. A paucity of rapid eye movement sleep and excessive slow-wave sleep were seen in all subjects at enrollment. Weight loss led to an increase in rapid eye movement sleep (P < .02) and a decrease in slow-wave sleep (P < .01) to near normal levels.

Conclusions. The K diet can be used effectively for rapid weight loss in adolescents with morbid obesity. Loss in lean body mass is blunted, blood chemistries remain normal, and sleep abnormalities significantly decrease with weight loss.

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