Citrulline = The Dieter's Amino Acid? Citrulline Maintains Muscle Protein Synthesis & Strength Endurance During Caloric Deficits Better Than Leucine!?

Can citrulline supplementation prevent you from hitting a catabolic wall, when you are dieting? And is it more potent than leucine?

You have been told "leucine is the most anabolic amino acid known to man", by the guy at your local GNC, the bros in the gym and the "experts" on the board.

And yeah, in a way, they all are "right", but the surprising negative effects of HMB supplementation on the muscle catabolism during overtraining (read more) should have reminded you that this does not imply that it will also protect your muscles against muscle breakdown and/or have similar "anabolic" effects on a diet.

Dieting is a major change in the metabolic stage and another stage means a different cast, among whom citrulline could turn out to be the new star... at least if we trust the results of a recent rodent study.

Different metabolic stage - new stars on the scene

In their most recent paper Ventura et al. describe the results of a rodents experiment in the course of which they  evaluated the effect of sequential administration of leucine (LEU) and citrulline (CIT) to preserve lean body mass during food restriction. In a 2009 study, Moinard et al. had already observed that the provision of 1.0 g/kg/day of CIT (HED ~10-15g) to exert beneficial effects on body composition in aged rats (Moinard.2009) and if you go by the abstract of the study at hand, it would sound as if citrulline was not simply "lean mass protective", but also much more potent than leucine:

Only CIT administration (1 g/kg) was able to restore MPS [muscular protein synthesis] (CIT1: 3.4±0.3 vs.R: 2.5 ±0.2 %/day,p=0.05) and increase muscle maximum tetanic force (CIT1: 441 ±15 vs.R: 392 ±22 g,p=0.05) and muscle strength (CIT1: 4,259±478 vs. R: 3,045 ±663 A.U., p=0.05). LEU had no effect and CIT+LEU supplementation had few effects, limited to adipose mass and fatigue force. The results of this study highlight the ability of CIT alone to preserve muscle function during dietary restriction. Surprisingly, LEU antagonized some effects of CIT." (Ventura. 2013)

This observations have been made after the rats dietary provisions had been cut by 60% for 2 weeks while the amino acid composition of their diet had been increased by the provision of additional amino acids: 

• R-CIT 0.2 - low dose citrulline: 0.2g/kg
• R-CIT 1- high does citrulline: 1.0g/kg
• R-LEU - leucine: 1.0g/kg
• R-LEU-CIT - leucine + citrulline: 1.0g/kg + 1.0g/kg

By addding valine (130 mg/kg/day) and isoleucine (220 mg/kg/day) to the diet, the researchers had also ensured that the natural BCAA balance would be maintained and ....

Figure 1: Changes in body composition during the 2 weeks on 60% of the regular energy intake with different amino acid supplements in the diet (Ventura. 2013)

... well if you look at the "net result" in terms of weight loss, it would in fact seem that citrulline is the way to go... if you do yet take a look at the lean mass measurements, it becomes plain obvious that there was no difference to the starved control group in any of the AA supplemented rodents.

Figure 2: Muscle contractile properties (fatigue AUC), myofibrillar and sarcoplasmic protein synthesis (PS) after 2 weeks on the different 40% dietary restricted diets (Ventura. 2013)

This is interesting, as it stands in contrast with the directly measured influx of protein into the myofibrillar part of the skeletal muscle of the rodents, and does not mirror the pronounced benefits on muscular fatigue the researchers observed and is not appropriately discussed in the study, the authors of which were so fascinated by the miniscule increase in protein synthesis that they did not even notice that they effectively produced a null-result.

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The hormonal response (esp. testosterone & GH) to workouts is another of those things that don't predict real world results (learn more)

Real results count: So does it really matter that the protein synthesis increased? No, just as it does not matter in the countless post-exercise protein synthesis studies. If you want to inflate a tire, you are not interested in how much air you can pump into it, but rather how much of the air will stay inside and the results of the study at hand only confirm that the former cannot predict the latter.

And let's face it: None of the treatments actually had to prevent lean mass loss, because much contrary to the bro-scientific believe that you would lose tons of muscle mass within a day, if you don't get all your shakes and pills in just in time. The rodents lost no lean mass at all.

So if you want take home messages, don't rely on protein synthesis rates alone and don't freak out about muscle loss too much.