Using a restrictive diet for weight loss, unless done properly, can result in not only the loss of fat mass, but also skeletal muscle.

Achieving as low as possible ratio of skeletal muscle to fat mass loss is called 'High Quality Weight Loss' and is important for maintaining one's engine (muscle) and improving athletic performance.

Current recommendations for protein intakes during weight loss are set at 1.6–2.4 g protein per kilogram, per day.

However, the severity of the caloric deficit and type and intensity of training performed will influence at what end of this range you choose to be.

A recent review from Stuart Philips group at McMaster University discussed the use of protein supplementation to achieve high quality weight loss. They found that whey protein either as a supplement or incorporated into a weight loss or weight maintenance diet, promotes:

• Lean body mass retention and fat mass loss
• Positively effects strength, muscle fibre cross-sectional area and;
• Bone mass

During prolonged resistance training and may have a positive effect on high-quality weight loss for athletes (and lifeletes!) during dietary energy restriction.

BCAA's have also been of interest for promoting skeletal muscle gains and high-quality weight loss.

BCAA is a general term for the essential amino acids that the body doesn't produce naturally. These are Valine, leucine, and isoleucine that are metabolized by the body and used as sources of muscle energy. They are referred to as Branched Chain Amino Acids (BCAA) because the molecular structure of these three amino acids includes branches.

A common concern with BCAA supplementation is the antagonism of amino acids  leucine, isoleucine, and valine in competing for the BCAA transporter.

Isoleucine and valine have been shown to inhibit the absorption of leucine, a potent stimulator of muscle protein synthesis.

Based on the current data, BCAA supplementation, does not appear to have strong scientific evidence for it's utility in supporting high-quality weight loss, and may even have negative side effects.

It therefore seems equivocal to consume BCAA supplements during dietary energy restriction, when one can simply consume a complete protein supplement such as whey protein, which has been shown to promote a robust increase in muscle protein synthesis during dietary energy restriction.

The review also discussed other important considerations in high-quality weight loss during energy restriction, including the quality of protein consumed, and the timing and distribution of protein intake throughout the day.

Here is what they found:

A more balanced pattern of 4 x 20g doses of protein every 3 hrs during the day has been shown in a number of studies to increase muscle protein synthesis, lean mass and strength in both trained and elderly populations during energy restriction. 

Sleep is critical for recovery and performance and represents a prolonged post-absorptive period where muscle protein balance is negative and could be improved. Indeed, pre-sleep protein supplementation combined with exercise training has been shown to improve protein balance during the prolonged post-absorptive overnight period.

Thus, pre-sleep protein ingestion may be another strategy to improve lean body mass retention during caloric restriction.

Protein quality also plays a role in maximising lean body mass during caloric restriction.

Indeed, during dietary energy restriction, whey protein (a fast-digested protein derived from milk that contains a high proportion of the branched chain amino acid leucine) ingestion resulted in a robust increase in muscle protein synthesis that was significantly higher than consumption of an isoenergetic and isonitrogenous amount of soy protein.

During dietary energy restriction in untrained overweight/obese premenopausal women, consuming a higher protein from dairy diet combined with aerobic and resistance training, resulted in superior loss of fat mass than a lower protein, lower dairy diet, as well as gains in lean body mass.

• Reference:
• https://journals.humankinetics.com/doi/full/10.1123/ijsnem.2017-0273