Despite decades of work, the case of daily protein requirement is still heatedly debated, although we have already enough information to give at least some educated guidelines.
Truth be told, this ludicrous controversy stems from an ignorance of context, which unfortunately occurs rather frequently in our beloved field when making various recommendations.
It's important to note that your optimum daily protein requirement is not static over time, so you shouldn't diligently adhere to any predetermined norm without considering your particular situation – body composition and specific goals.
And although the current body of research has not yet investigated all possible scenarios, we can always rely on the physiological facts or available data for interpreting and making the best of the existing empirical evidence.
Brief Primer On Protein Metabolism
Your body is continuously regenerating itself, including skeletal muscle, through a continuous process of breakdown and resynthesis known as tissue turnover.
Essentially, the resulting amino acids (from tissue breakdown but also dietary protein) are released into the blood stream, and stored in a common pool that is used to support protein metabolism.
In other words, most of the incoming protein is reused to rebuild the various tissues.
It's important to mention that some of the protein will be inevitably lost in the process, and has to be supplied through nutrition. (This value represents the base used for RDI.)
Therefore, protein turnover dynamics determines if you gain or lose muscle. If synthesis is greater than breakdown, there will be an increase in the amount of that lean tissue. Conversely, if breakdown surpasses synthesis, there will be an overall loss in the volume of the lean tissue.
A key point that must be made is that protein turnover is an energetically expensive process, accounting for 10-25% of your resting energy expenditure. This is important to know because weight training has a dramatic effect on this process.
Do you remember when, a few years ago, all fitness authorities were heralding that gaining a few pounds of muscle will send your metabolism through the roof?
As it turned out, muscle tissue itself has only a modest contribution to metabolic rate, and in fact most of the difference is coming from the raise of both protein synthesis and breakdown that occur after resistance training.
(Protein synthesis increases only if you provide enough amino acids through nutrition. Duh!)
It should be noted that food intake has probably the greatest impact on protein turnover as it mediates the changes in the concentrations of various hormones; this will both inhibit protein degradation and stimulate synthesis.
It has long been believed that carbohydrates (insulin prevents protein breakdown) were better for improving nitrogen balance. (As a side note, nitrogen balance refers to net protein gain/loss.)
However, it seems that nutritional status, rather than a single macronutrient, plays a more important role in protein retention.
Hence, your main concern should be how much food you put in your mouth and not to worry about the protein sparing qualities of carbohydrates – especially when setting up calories for maintenance:
“It has long been known that dietary amino acid adequacy is markedly influenced by energy balance but in recent years the importance of this has been generally underestimated. The common assumption that the type of energy influences protein utilization is probably incorrect with fat as effective as carbohydrate in maintaining NB at energy maintenance.” (Millward, 2004)
Now you may ask yourself: If dietary amino acids are necessary for protein synthesis, why not eating 10kg of chicken breast a day? Well, very simple. The reason has to do with a psysiological mechanism which regulates protein retention, called diurnal cycling.
When you eat more protein, both synthesis and breakdown will increase, so the net result may be nil. (This equation is greatly influenced by resistance training.) Conversely, when protein intake goes down, your body will use it more efficiently.
Along these lines of thought, some researchers even argue that strength athletes lose more protein for the simple reason that their intake in too high, which may be partially correct but mostly erroneous if the resistance training is done properly.
It should be noted that daily protein requirement may diminish as the athlete approaches his/her genetic potential.
In the early stages, a more substantial amount was necessary to sustain muscle hypertrophy, but if you have a few years of productive training under your belt, you've already reached an inevitable plateau.
On the other hand, at this level you should also be more proficient in increasing training intensity (which would have more of an impact on protein turnover), so your protein intake should be adjusted accordingly.
Daily Protein Requirement: Putting It All Together
As I already mentioned, your daily protein requirement should be set according to how much body fat you carry and your fitness ambitions.
For consistency and convenience, I recommend that you base this calculation on target bodyweight rather than on your current condition. How do you know your target bodyweight? Simple...
My assumption is that you want to see your six pack abs. Right? This means single digit body fat, mind you. Let's say your goal is to get to 5-6%. At this level of leanness, the genetic limit for more than 99% of the population can be calculated easily with the following formula:
(height in centimeters - 100) = bodyweight in kilograms
In other words, if your height is 182cm or 6ft like me, it would be almost impossible to weight more than 82kg in a contest like condition.
Now you'll say that there are a few guys at the gym who doesn't fit this formula. And no, they are are not among the 1%. They are just among the brainless individuals who use anabolic steroids.
Ok, so let's get back to our topic.
In my practice, daily protein requirement usually oscillates between 2-3g/kg (0.9-1.36g/lb). If you are obese, and want to limit calories as much as possible for weight loss maximization, a good idea is to choose a high range.
Besides an increase in satiety, when your calorie deficit is substantial, you want to make sure that you consume enough protein, essential fatty acids and vitamins/minerals to avoid potential health complications.
At the other extreme are the already lean athletes who are preparing for a contest or some other event. They should also increase their protein intake (even above 3g/kg) because in their case the danger of losing lean mass increases exponentially.
On the other hand, if you are a gym veteran, and your goal is to maintain whatever you already have, this figure can be lower. However, as I mentioned, you should make adjustments whenever you train harder/longer because your daily protein requirement will be higher.
Final Note: If you are interested in nutrition and metabolism, and looking for a solid source of information, I strongly recommend Advanced Nutrition and Human Metabolism. This comprehensive book is myth/pseudoscience free, and gives you a detailed perspective on how things really work.