Meta-analysis — 2022 — Energy balance · Body composition · Resistance training
Energy deficiency impairs resistance training gains in lean mass but not strength: A meta-analysis and meta-regression
What did the researchers investigate?
A systematic review of randomised trials (at least 3 weeks) of resistance training in an energy deficit. A meta-analysis compared gains in lean mass and strength with and without a deficit, and a meta-regression related the size of the deficit to lean-mass change.
What did they find?
Lean-mass gains were smaller with an energy deficit (effect size -0.57, p = 0.02), while strength gains were comparable (-0.31, p = 0.28). The meta-regression showed that a deficit of about 500 kcal a day prevented gains in lean mass. The authors advise that people lifting to build lean mass avoid prolonged deficits, and that people preserving lean mass while losing weight avoid deficits above 500 kcal a day.
What does this mean for training?
A large calorie deficit can cost lean-mass gains even while strength holds up; a moderate deficit is more compatible with keeping muscle.
How does FitX AI use this evidence?
Relevant to the fat-loss calorie target, which is 500 kcal below estimated maintenance (never below BMR). That value sits at the threshold this meta-regression discusses and is applied to every adult regardless of body size. The paper does not validate FitX AI's rule, and nothing in it supports the +150 kcal surplus used for muscle gain.
Where FitX AI's own choices go beyond this paper
Goal adjustment to the calorie target. -500 and +150 kcal are product choices. It informs the size of the fat-loss deficit only. No reviewed paper supports the +150 kcal surplus, and the fixed 500 kcal deficit is applied to everyone regardless of body size.
Limitations
Pools trials that differed in population, deficit size and duration. The 500 kcal figure is a regression estimate, not a threshold for any one person, and the abstract says nothing about fat loss or how well people stick to a deficit.