Results:
In total 383 obese adults (123M, 260F) aged 36.0 to 55.0 years old (Table 3) assigned to eight hypocaloric diets (five high-protein, and three non-high-protein diets) were included across the studies. The initial mean BMI and primary outcomes measured are listed in table 2, they include changes in pre- and post- diet mean BMI, LDL-C, HDL-C, TAG, and TC levels. Studies were 4 weeks to 9 months in length. Retention rates ranged from 88.9%-100%. All studies reported statistical powers as compared to baseline values as well as between diet groups, all p-values are stated in table 2. Mean BMI change was significant from -21.3% to -60.9% of a greater decrease in the high-protein diet group as compared to the control diet groups in the two out of three studies which had both a control and high protein diet group (Abete et al. , 2009; D.A. et al. , 2015). In one study both diet groups were high in protein and significant mean BMI changes were present in both diet groups as compared to baseline (Johnstone et al. , 2011). Significant mean BMI changes were present therefore in three studies (Abete et al. , 2009; D.A. et al. , 2015; Johnstoneet al. , 2011), the high-protein group and control diets had a mean BMI change of -2.28 kg/m2 vs -1.85 kg/m2 respectively. There were no significant changes in mean LDL-C, HDL-C, TAG, and TC levels among diet groups in the three studies which had both a control and high protein diet group (Abeteet al. , 2009; D.A. et al. , 2015; Petrisko et al. , 2020). The one study where participants withdrew was before randomization, and therefore whether or not an intention-to-treat analysis would have been applied to that or any of the other studies with 100% retention rates was unknown. The risk of bias assessment (Figure 2) noted a high risk of bias in three out of four studies (Abeteet al. , 2009; D.A. et al. , 2015; Petrisko et al. , 2020) The high risks of bias in the “deviations from the intended interventions” domain in these three studies was due to a likely lack of compliance to diets as participants may have eaten more than the prescribed diet as subjects were not resident at a dietary unit. The study by Johnstone et al. had complete control over dietary input as participants were resident at a Human Nutrition Unit over the course of the study, nonetheless some concerns arose in this study as participants left the unit to attend their workplace, again indicating possible calorie intake outside of the prescribed diet. However ethical concerns arise in the nature of such dietary studies when complete surveillance of participants is present, and therefore a dietary control design as presented in the study by Johnstone et al . has attempted to decrease bias in terms of dietary control to it’s best abilities whilst remaining within ethical guidelines of such studies, as such the relevance of the bias score in this domain is lessened in this study. All the studies had not stated available pre-specified planned outcome analyses (as seen in registered studies on ISRCTN or an equivalent register of randomized control trials) and therefore leading all of the studies to having some concerns in the “selection of the reported result” domain. Finally the study by D.A. et al. had stated that subjects were allocated “alternatively” to diets, which arises concern in whether or not the subjects assigned to the diets were randomly allocated.