Impact of a Fish-Based Restrictive Ketogenic Diet on Body Composition and Strength Capacity: A Pre-Post Study

Background: The ketogenic diet (KD) is becoming a popular nutritional model for athletes. One limitation of this diet is the use of animal-meat products, which may be unacceptable to some people. Moreover, the relationship between this diet's use, body composition, and strength capacity has not been thoroughly investigated. This study aimed to assess the effects of a two-week fish-based restrictive KD on body composition, strength capacity (isometric muscle strength), and somatic disorders in healthy adults. Methods: Participants were recruited through advertisements, posters displayed at the university, and information shared among colleagues. Ultimately, 14 individuals qualified for this study. For two weeks, they followed a designated ketogenic diet enriched with fish and omega-3 fatty acids, with a calorie reduction of -500 kcal/day. The study was designed and conducted as a single-group pre-post study. Before and after completing the diet, participants' body composition (body mass, body fat, fat-free mass, and visceral fat) and strength capacity (knee extensor peak torque [KEPT] and knee flexor peak torque [KFPT]) were measured. The survey also analyzed participants' somatic disorders such as nausea, vomiting, energy level, diarrhea, constipation, drowsiness, sleep problems, hunger, thirst, and bad breath. The average results of both trials obtained before and after the diet were compared using the paired Student's t-test or non-parametric test. The significance level was set at 0.05. Results: After two weeks of a fish-based restrictive KD, significant increases in ketone bodies were observed in both the blood (pre: 0.21 mmol/L ± 0.15 vs. post: 2.20 ± 1.19, p < 0.001) and urine (pre: 0.00 mmol/L ± 0.00 vs. post: 4.64 mmol/L ± 3.24, p < 0.001). Body composition changes included significant reductions in total body mass (pre: 76.66 kg ± 11.06 vs. post: 73.77 kg ± 10.76, p < 0.001), fat mass (pre: 21.34 kg ± 7.36 vs. post: 19.73 kg ± 7.20, p < 0.001), and visceral fat (pre: 708.50 g ± 627.67 vs. post: 603.21 g ± 567.82, p = 0.0012). Skeletal muscle mass decreased by 2% (pre: 27.75 kg ± 5.80 vs. post: 27.03 kg ± 5.47, p = 0.01), though no significant changes were seen in muscle strength when adjusting for body mass or skeletal muscle mass. No major adverse effects were noted in relation to somatic disorders, although some participants reported increased thirst and bad breath. Conclusions: After 14 days of a fish-based restrictive KD, a significant reduction in body mass and changes in body composition were observed, with no loss of muscle strength. This type of diet may serve as an effective method for rapid body mass reduction in sports with weight categories, while preserving muscle strength, which is crucial for athletes. It could also be an alternative means for individuals to eliminate animal meat from their diet.