madman
Super Moderator
Figure 2. Body composition by DXA and skeletal muscle mass by D3-creatine dilution in 1,376 older men from the MrOS cohort, as a percentage of body weight. The men are ranked from the highest to the lowest D3-creatine muscle mass (left to right). DXA bone, fat and lean mass sum to 100%; muscle mass by D3-creatine dilution is overlaid on the lean mass of which it is part, and the points are DXA appendicular lean mass. Redrawn from Figure 1 of Orwoll et al. [11] as reproduced in Evans and Cawthon [12], an open-access article distributed under the Creative Commons Attribution 4.0 International License (creativecommons.org/licenses/by/4.0/).
ABSTRACT
Two critiques dispute the claim that 1.6 g/kg/day of protein is a ceiling for people who perform resistance training. This commentary replies to both and uses the disagreement to examine what the evidence can and cannot show. Nearly every trial reports fat-free or lean mass, which is not muscle: in one comparison, lean mass by dual-energy X-ray absorptiometry exceeded muscle mass by D3-creatine dilution by about 20 kg (roughly half), and changes in lean mass track changes in muscle but not well. At face value, about three months of training adds 1.1 kg of fat-free mass and supplemental protein adds a further 0.3 kg. Head-to-head trials of 1.6 against up to 3.2 g/kg/day, two of them in an energy deficit, found no clear advantage for lean mass. A worked example shows that even 20 kg of new muscle in a year deposits a small amount of muscle protein, and even generous estimates show substrate supply as unlikely to be limiting. A position stand that steers female athletes toward the upper end of 1.4 to 2.2 g/kg/day is also examined: its sources do not support that advice, and direct comparisons of women and men show no difference in whole-body estimates or in relative gains in muscle size. Small, short trials; self-reported diets; between-study comparisons; and whole-body tracer estimates limit inference on all sides. An intake of 1.6 g/kg/day appears sufficient, and the trial needed to show otherwise is described.
Conclusion
Based on the evidence reviewed here, a protein intake of 1.6 g/kg/day is sufficient for people who resistance train, whether the aim is fat loss, muscle gain, or both. The evidence provides no reason to set the figure higher for women, in any phase of the menstrual cycle or after menopause. Older adults of either sex do have reason to eat more than the RDA, and 1.2 to 1.6 g/kg/day covers what the trials support. Nothing in this commentary argues against eating more. The objection is to the claim that more is needed. Both critiques of the author’s work make fair points: the breakpoint was imprecise, and the original statement’s absolute wording invited objection. It is also true that the earlier recommendation of up to 2.2 g/kg/day was a hedge, and that small trials with null results cannot rule out a small benefit if it exists. None of those points supplies what is missing: a demonstration, in a study that measures muscle, that more protein produces more of it. The author has used a colloquial metaphor and said that: “Lifting bakes the cake. Protein adds a very thin layer of icing.” On present evidence, that metaphorical description holds. Resistance training accounts for most of the gain, and protein adds a small increment. A second layer, from intakes above 1.6 g/kg/day, has yet to be shown with a method that measures muscle.
