Can a high protein intake help resistance-trained individuals retain muscle mass while dieting? A systematic review and meta-regression provides updated recommendations.
Overview
- What did they test? The study examined the dose-response relationship between daily dietary protein intake and fat-free mass (FFM) change in energy-restricted, non-obese, resistance-trained individuals.
- What did they find? The authors found a linear dose-response relationship between dietary protein intake and FFM changes and the relationship was stronger when protein intake was scaled to FFM rather than body mass. Furthermore, longer intervention durations (>4 weeks), lower BF%, and male participants exhibited stronger associations with FFM retention.
- What does it mean for you? Protein intake up to 1.9 g/kg of body mass per day or 2.5 g/kg of FFM per day is associated with less FFM loss during energy restriction. Intakes above these values, up to 3.2 g/kg of body mass or 4.2 g/kg of FFM are linearly associated with larger FFM gain.
What’s the problem?
Protein intake plays an important role in supporting fat-free mass (FFM), particularly in individuals who resistance train (RT). Previous meta-analytic data suggest that an average protein intake of ~1.6 g/kg/day is optimal for maximizing FFM gains with resistance training. Notably, the upper confidence interval in these findings reached 2.2 g/kg/day 1 , aligning with the upper confidence interval estimates (~2.2 g/kg/day) observed in male bodybuilders 2. However, recent research suggests the acute upper limit to protein intake is higher than we previously thought, so these recommendations may change in the future 3.
In 2011, Phillips and Van Loon recommended even higher protein intakes (1.8–2.7g/kg of body mass) during periods of negative energy balance and weight loss compared to those for maximizing FFM growth 4. A few years later in 2014, a systematic review suggested protein needs for energy-restricted resistance-trained athletes should be scaled to FFM, recommending protein intakes of 2.3–3.1 g/kg of FFM 5. The authors also recommended that intake be increased with severity of caloric restriction and leanness 5. However, this review only included six studies and no study tested protein intake higher than 2.7g/kg of FFM. In 2020, our review for physique athletes recommended 1.8-2.7 g/kg/day (of body mass), or up to 3.5 g/kg/day for those specifically attempting to mitigate hunger 6. You can see the underlying questions here: should we scale protein intake to body mass or FFM? And how much protein do we need to maintain FFM when dieting?
Energy deficit magnitude and duration play important roles in determining FFM retention. Pasiakos et al. (2013) found that severe caloric restriction (1560 kcal deficit per day) suppresses muscle protein synthesis, even when protein intake is 2.4g/kg of body mass, but that’s a fairly extreme scenario designed to help military operators 7. A recent meta-regression found that larger deficits lead to greater FFM losses and people hoping to gain FFM should avoid energy deficits of >500 kcal/day 8. This somewhat aligns with our recommendation that rates of weight loss (≤0.5% of body mass per week) is preferable for reducing the loss of FFM while dieting 6.
Another important factor is starting body fat. It would be fair to speculate that those with a higher body fat percentage may be less prone to losing FFM while dieting. They have more energy stored to use. According to one review, that’s true, but only in extreme cases like the Minnesota Experiment where participants were below ~7% body fat. In all other studies in the review there were no associations between baseline body fat and loss of FFM 9. It could be because as people get very lean (i.e., physique athletes) they have hormonal changes that make FFM retention more difficult (i.e., low testosterone), but that’s not necessarily translatable to the average person trying to lose weight. There’s also Forbes rule 10, which states that a quarter of weight loss will be FFM, but this has been challenged and doesn’t include people who RT while dieting 11.
With all that in mind, the current systematic review and meta-regression sought to fill these gaps by synthesizing findings across multiple studies, providing a better estimate of how protein intake influences FFM retention in trained populations.
Purpose
The purpose of this article was to examine (a) the pattern and strength of the dose-response relationship between daily protein intake (g/kg of body mass and g/kg of FFM) and FFM (b) the impact of key variables—such as intervention duration, energy deficit magnitude, baseline body fat percentage, and participant sex—on this relationship in energy-restricted, resistance-trained individuals.
Hypothesis
The authors did not state a hypothesis, but did use a Bayesian analysis with noninformative priors, meaning they did not start with any strong expectations about how protein intake would affect fat-free mass.
What Did They Test and How?
Participants
Studies were included if they were longitudinal trials involving healthy, non-obese adults (18–50 years old) with at least three months of resistance training experience. Participants followed a standardized or habitual resistance training protocol at least once per week, and the intervention lasted a minimum of seven days, incorporating an energy restriction that led to fat mass loss.
Studies were excluded if participants were using hormone replacement therapy, anabolic agents, or muscle-enhancing supplements (except protein or amino acids). Only peer-reviewed, English-language original research articles were considered, with case studies excluded.
