Background: Higher dietary energy as protein during weight loss results in a greater loss of fat mass and retention of muscle mass; however, the impact of protein quality on the rates of myofibrillar protein synthesis (MPS) and lipolysis, processes that are important in the maintenance of muscle and loss of fat, respectively, are unknown.
Objective: We aimed to determine how the consumption of different sources of proteins (soy or whey) during a controlled short-term (14-d) hypoenergetic diet affected MPS and lipolysis.
Methods: Men (n = 19) and women (n = 21) (age 35–65 y; body mass index 28–50 kg/m2) completed a 14-d controlled hypoenergetic diet (−750 kcal/d). Participants were randomly assigned, double blind, to receive twice-daily supplements of isolated whey (27 g/supplement) or soy (26g/supplement), providing a total protein intake of 1.3 ± 0.1 g/(kg ⋅ d), or isoenergetic carbohydrate (25 g maltodextrin/supplement) resulting in a total protein intake of 0.7 ± 0.1 g/(kg ⋅ d). Before and after the dietary intervention, primed continuous infusions of L-[ring-13C6] phenylalanine and [2H5]-glycerol were used to measure postabsorptive and postprandial rates of MPS and lipolysis.
Results: Preintervention, MPS was stimulated more (P < 0.05) with ingestion of whey than with soy or carbohydrate. Postintervention, postabsorptive MPS decreased similarly in all groups (all P < 0.05). Postprandial MPS was reduced by 9 ± 1% in the whey group, which was less (P < 0.05) than the reduction in soy and carbohydrate groups (28 ± 5% and 31 ± 5%, respectively; both P < 0.05) after the intervention. Lipolysis was suppressed during the postprandial period (P < 0.05), but more so with ingestion of carbohydrate (P < 0.05) than soy or whey.
Conclusion: We conclude that whey protein supplementation attenuated the decline in postprandial rates of MPS after weight loss, which may be of importance in the preservation of lean mass during longer-term weight loss interventions.
Alex’s Notes: When dieting for weight loss, people almost always mean fat-loss, and there are few reasons that weight loss plans shouldn’t aim to maximize fat loss and retention of lean body mass. On easy method to do so is consuming more protein. The type of protein is another consideration. We previously saw that whey protein preloads produced more pronounced reductions in appetite, caloric intake, waist circumference, body fat percentage, and weight than soy during a 12-week free living intervention in obese men. Moreover, the whey group also saw increases in lean body mass.
The above is important because muscle protein synthesis (MPS) is suppressed during fasting and postprandial times under hypocaloric conditions. Thus, the aim of the current study was to examine the efficacy of supplementation with whey protein, soy protein, or an isoenergetic carbohydrate control in affecting protein turnover and lipolysis.
Forty obese, middle-aged men and women who were otherwise healthy were recruited for the study. They were asked to maintain their physical activity levels, and were not allowed to consume any vitamin or mineral supplements or alcohol for the duration of the intervention. In a double-blind investigation, they were randomized to one of three groups: whey, soy, or carbohydrate control.
Energy requirements were estimated with the Mifflin-St Jeor equation using an appropriate activity factor, and all subjects started the intervention with a 3-day weight maintenance diet. At the end of the three days and again at the end of the study, infusion trials, DXA scans, and muscle biopsies were taken to assess the various outcomes of the study. After this first data collection period, the subjects began their 2-week hypocaloric (-750 kcal) diets with their respective whey, soy, or carbohydrate drinks included in the energy allowance and consumed twice daily (between breakfast and lunch, and between lunch and dinner). All meals were provided for the subjects, with the whey and soy providing 25g of protein per serving and the carbohydrate drink providing 25g of maltodextrin per serving.
As per study design, the whey and soy groups consumed significantly more protein per day than the control group (1.3g/kg vs 0.7g/kg) and significantly less carbohydrate. Even so, the carbohydrate intake was over 200 grams in all groups. Fat intake was not different between the groups at about 25% of total energy intake.
Whole body lipolysis (fat breakdown) was reduced in all groups with consumption of the shakes, but the effect was significantly more pronounced in the control group, with no differences between whey and soy. Conversely, the total amino acid, leucine, and sum of essential amino acids were greatest with whey, then soy, then carbohydrate. Although MPS increased from pre-meal to post-meal in the whey and soy groups only, after the weight loss diet there was a significant decrease in the fasting and postprandial MPS response. However, this reduction was significantly less in the whey group (-9%) compared to the soy (-28%) and carbohydrate (-31%) groups. Fractional synthetic rate also increased significantly more after whey ingestion than soy or carbohydrate ingestion before and after the intervention.
Despite all this, there were no significant differences between the groups in terms of body composition changes, with all groups showing similar weight, fat, and lean body loss from the intervention.
What does this mean?!
The big takeaway here is that consuming 50g of whey protein daily (as opposed to soy or carbohydrate) in a split dose resulted in an attenuation of the natural reduction in MPS that occurs during energy-restriction. Thus, it appears that not all proteins are equal and whey protein may be more effective at preserving MPS and lean body mass under longer-term diets. The results could be due to the leucine content. The data supports this via the greater peak and net exposure to leucine and EAAs with whey than soy or carbohydrate. As for the body composition, it seems unlikely that 2-weeks are a long enough duration for changes to become statistically significant. A longer duration trial would be needed.