When protein is consumed as part of a high carbohydrate meal, amino acid oxidation increases,resulting in nitrogen losses. Less protein is stored and more is burned (1).

The higher a diet is in carbohydrates, the greater the amino acid oxidation and nitrogen losses.

Conversely, amino acid oxidation may be spared on a high fat diet as more fatty acids are used for energy. For example, science has demonstrated in man and animals that when high fat meals are consumed with protein, digestion and use of protein is enhanced and nitrogen retnetion is higher. (1-2).

The high fat diet leads to protein sparring, which is reduction in excretion of nitrogen compounds in the urine. THus consuming fat in conjunction with protein would be useful in promoting protein synthesis neccessary for muscle growth.

Summary: If you want to build lean muscle mass more effectively, follow a higher fat diet consisting of the right types of fats.

References

1. McCargar LJ,Clandinin MT,Belcastro AN and Walker K Dietary carbohydrate-to-fat ratio:influence on whole body nitrogen retention,substrate utilization, and hormone response in healthy male subjects.Am J Clin Nutr, 1989;49:1169-1178

2. Hartsook WE, Hershberge TV, and Nee JCM. Effects of dietary protein content and ratio of fat to carbohydrate calories on energy metabolism and body composition of growing rats. J Nutr, 1973;103:167-178