|Image 1: While these images are from a study on premature aging in mice, they go to show that the little critters are by no means better of than human, when their time is near. First they get fat, then they lose all their muscle and then they pass away (img from de Boer. 2002)|
Too little, too short or simply too inefficient?
Apart from the absence of the necessary complementary amino acids, which is in fact a design flaw in many of the trials, both the amount of supplemental leucine, as well as the total duration of the dietary intervention could possibly explain why "[c]hronic free leucine supplementation alone did not improve lean body or muscle mass during resistance training or in elderly" (Balage. 2010). Against this background Daiana Vianna and her colleagues from the Universities of Sao Paulo and Texas conducted a trial in which they fed 6-month old adult (and thusly not yet old!) mice diets to which either a standardized amino acid mixture or a huge bolus of leucine (cf. figure 1) for 40 weeks (Vianna. 2011):
|Figure 1: Basal diet composition (left), additional amino acids (middle), vitamin and mineral mix (right) of the diet the mice in the control and leucine groups of the study were fed (data adapted from Vianna. 2011)|
The intake of "large" amounts of leucine is totally benign
From the scientists' perspective, the first important (yet not actually surprising) result of the Vianna study was thusly that (I quote) "despite the relatively high dose of leucine in the diet", the mice in the leucine group did not exhibit a "significantly higher mortality rate". However, as the data in figure 1 shows, the expected and highly desirable effects on the retention of lean muscle mass in the aging mice were similarly non-significant (cf. figure 2):
|Figure 2: Relative change (expressed relative to adult mice) in lean body mass during aging in old mice on control and old mice on high leucine diet (data calculated based on Vianna. 2011)|
|Figure 3: Relative change (expressed relative to adult mice) in leptin, triglycerides, blood glucose, insulin, and HOMA-IR during aging in old mice on control and old mice on high leucine diet (data calculated based on Vianna. 2011)|
Leucine is no magic bullet
Overall the results of this long term, high dose (allegedly rodent) study are thusly similarly disappointing as previous human trials, after all the desired ameliorative effect on age-related declines in muscle mass did not occur and the ameliorative effect on body fat accumulation may be statistically significant, but does not detract from the fact that the animals in the leucine group...
[...] still presented a marked accumulation of body fat compared with young adult animals (78% and 57% more body fat in the control and leucine groups, respectively, compared with the adult group). Thus, the attenuation of fat gain observed with leucine supplementation was not sufficient to promote an equivalent decrease in the cardiovascular risk indicators studied.So that the main take home messages of this study probably are that
- in situations like aging, where the protein synthetic machinery is already stuttering, neither "high dose" (whether a human equivalent of ~6g is actually "high" may be debatable, though and the fact that this were no "bolus ingestions", but the 6g of leucine were distributed across one day certainly didn't help either) nor long-term leucine supplementation alone is able to reduce the age induced decline in muscle mass, and
- either leucine or the "absence" (in view of the fact the diet contained enough casein that there would be "all" of the N-EAAs present in the diet, we should probably say "lower amounts") of any of the other non-essential amino acids (N-EAAs) the control group received must have a non-negligable effect on the age-induced increase in body fat levels, we do not yet fully understand