I wanted to circle back around to discuss
positive nitrogen balance before leaving this thread alone for a while.
As discussed in
Post #52, the amount of nitrogen in urine can be used to assess protein breakdown. Researchers refer to these markers as
surrogate endpoints, which are "physiological or biochemical markers that can be quickly and easily measured, and that are thought to be predictive of a true clinical end point." Such markers are convenient, but are not necessarily indicative of the "true clinical end point" in question.
In this case, the clinical end point we are after is muscle mass gain or loss over time. Skeletal muscle turns over at a very slow rate (most sources I've seen say 1-2% per day). This would be very difficult to measure on a day-to-day basis, so researchers use nitrogen balance as a surrogate, which can be measured daily through urine samples and calculated based on dietary intake. The problem with this is, acute daily measurements of nitrogen balance aren't necessarily predictive of long-term loss or gain of muscle mass.
Certainly, in the case of the starvation study posted by Peg, the marker seems to have been predictive, which is to say their calculations of nitrogen excreted in the urine can be correlated to lean mass loss over time. But keep in mind, these participants were starved for 21 days and the only exercise they got was to walk around the hospital hallways. The very purpose of the study was to understand why obese people tend to die of starvation while they still have fat to metabolize. What they concluded was that protein was necessary for survival, and oxidizing fat was insufficient, so when you run out of muscle to catabolize, you die.
As a counterpoint to the use of this surrogate endpoint, however, have a look at this study -
Protein requirements and muscle mass/strength changes during intensive training in novice bodybuilders. It took 12 men in their early twenties and randomized them into a high protein diet group and a low protein diet group. They followed the same beginner bodybuilding resistance training program for 1 month. Their body composition was measured before and after with hydrostatic weighing, along with strength indicators like 1 Rep Max for Squat and Bench Press, and of course, urine samples were collected to assess nitrogen balance.
Taken from the Results section of the study:
Although the PRO supplement (high protein diet) produced greater gains in some measures (body density, midthigh muscle area, and leg strength), these differences were small and not statistically greater than those observed with the CHO supplement (low protein diet). These data indicate that during the 1st mo of intensive bodybuilding training, if dietary Protein = 1.35 g/kg/day (157% of current recommendations), additional protein supplementation leads to similar gains in muscle mass/strength. This is surprising, given the NBAL (nitrogen balance) results, because over a 1-mo period the observed positive NBAL (+8.9 g N/day) should have resulted in a net muscle accretion of ~7.8 kg (assuming wet muscle tissue is 20% protein and protein is 16% N-by weight). This increase would have been detectable using the body density method employed (hydrostatic weighing) and definitely did not occur.
So, what they're saying is, using the same type of calculation the starvation study used, the nitrogen excretions in the high protein diet group predicted over 17 pounds of muscle mass gain. Instead, both groups showed gains on the order of 1-2 pounds during the month. This is further complicated by the fact that the low protein group was indeed in a NEGATIVE nitrogen balance overall, which predicts a net loss of lean body mass, and that also did not occur. It's worth noting both groups experienced increases in muscle strength during the trial.
This research seems to confirm my suspicion that nitrogen balance day over day is less important to long term muscle tissue loss or gain than other factors, such as training methodology and perhaps long-term nitrogen balance. Also from the study linked to above:
... despite the negative NBAL in the CHO treatment, the anabolic stimulus of the exercise program may have made adequate N available for skeletal muscle from endogenous N stores, e.g., gastrointestinal tract, liver, and kidneys (24). If so, short-term muscle gains during both treatments could be similar. However, such N mobilization (-95 g) could not continue indefinitely, and eventually one would expect to observe reduced gains in muscle mass/strength with the CHO treatment. To confirm this possibility, exercise studies with longer training programs and measures of labile protein mobilization are needed.
Fair enough, but IF protocols shouldn't have this problem when implemented intelligently, because any negative nitrogen balance would be temporary. The implication here is that for an IF protocol to be successful at retaining or gaining muscle mass over time, it would need to be combined with a good resistance training program, and the overall diet would need to include the equivalent of ~1.3g/kg/d of protein over time (probably a little more since the low protein group was in a slight negative NBAL). More generally, this study seems to show, at least for this small cohort over this short time span, that significantly more dietary protein than that does not lead to significantly more muscle gain.
