The effect of a higher-protein low energy diet on weight loss in obese women

Abstract

Background. Obesity has become a global epidemic with New Zealand having the third highest adult obesity rate among all OECD countries. The optimal dietary macronutrient composition to achieve weight loss has been long debated. Whilst higher protein diets have been shown to promote weight loss, many of these diets are also low carbohydrate (CHO). It is not established whether the higher protein or the lower CHO content may be the most important factor for successful weight loss. Manipulating the macronutrient content of a low energy diet (LED) provide the opportunity to investigate these effects. LED is one of the most effective dietary treatments for obesity, it provides only 4MJ of energy while still manages to deliver all the essential micronutrients. For that reason, LED is widely used in studies to induce short term weight loss. Aim. The aim of this project was to investigate the effect of an 8-week LED which varied in protein and CHO content on (i) body weight loss; (ii) fat mass loss; (iii) lean mass loss; and (iv) satiety in a group of obese women. Methodology. Obese but otherwise healthy female adults (BMI 30kg/m²-45kg/m²) completed an 8-week LED, where they were randomized to one of four diet interventions, in a 2×2 design. Throughout the day, they were asked to consume 3 fixed meals (an oatmeal breakfast and 2 LED meal replacement products) and a variable meal where the intake was ad libitum. Two high (50en%) and two normal (35en%) protein diets, each with either low (28en%) or normal (40en%) CHO, were tested as follows: High protein, normal CHO diet (HPNC; % en from protein/CHO/fat: 50/40/10); high protein, low CHO diet (HPLC; % en from protein/CHO/fat: 50/28/22); normal protein, normal CHO diet (NPNC; % en from protein/CHO/fat: 35/40/25); and normal protein, low CHO diet (NPLC; % en from protein/CHO/fat: 35/28/38). Food records were used to assess energy and macronutrient intake at baseline, 4 weeks (mid LED), and 8 weeks (post LED). Body weight was assessed at baseline, 2 weeks (mid LED), and 8 weeks (post LED); body composition was not measured by dual x-ray (DXA) body scans at baseline and 8 weeks (post LED). Results. 46 females completed all anthropometric measurements and dietary records at the end of 8 weeks. A good level of dietary compliance to the LED treatment was observed in all diet groups of >85%. The high protein diet groups significantly increased their protein intake from baseline and achieved a significantly greater protein intake than the normal protein diet groups. Body weight and body fat mass decreased significantly in all diet groups over 8 weeks (Body weight: P<0.0001 for NPLC, NPNC, HPLC groups, P=0.0017 for HPNC group; Body fat mass: P<0.0001 for NPLC and HPLC groups, P=0.0003 for NPNC group, P=0.0006 for HPNC group). Mean weight loss ranged from -3.5 to -2.6 kg and mean % fat loss was between -4.0 to -2.9%. There was however no significant difference detected between the 4 diet groups over the 8-week LED (weight loss: treatmenttime interaction, P=0.9997; fat loss: treatmenttime interaction, P=0.9781). Lean mass decreased significantly in NPLC, NPNC, and HPLC groups, but not in HPNC group, yet the changes were also not significantly different between diet groups (treatmenttime interaction, P=0.9974). There is also no significant difference in energy intake (EI) between four diet groups mid-LED (week 4) and post-LED (week 8) (treatmenttime interaction, P=0.6473). Conclusions. In this present sub-study, a good level of dietary compliance for the LED treatment was achieved by most participants, and those who were compliant lost a significant amount of body weight. No diet group achieved a greater weight loss outcome or changes in body composition. A higher-protein LED was equally beneficial in promoting weight loss and fat loss while preserving lean mass when compared with a lower-protein LED with the same CHO content. A higher-protein LED also did not promote satiety and limit EI any better than a lower-protein LED with the same CHO content. When protein was kept constant, a lower-CHO higher-fat LED was not better at the loss of body weight and body fat loss along with the preservation of lean mass when compared with a higher-CHO lower-fat LED. However, a greater sample size might show a more significant effect, which might be more informative for the designing of future weight loss diets.