Protein density was consistently associated with higher Fat-Free Mass Index in both men and women independent of adiposity, while broad dietary patterns were confounded by body size, suggesting protein density is a more robust independent factor associated with muscle mass.
Key Findings
Results
Protein density was positively and independently associated with higher FFMI in both men and women after adjusting for adiposity.
In men, protein density was associated with FFMI (β = 0.01, P = 0.008)
In women, protein density was associated with FFMI (β = 0.01, P = 0.047)
These associations remained significant after adjustment for Fat Mass Index (FMI) in partitioning models
This indicates the relationship between protein density and muscle mass is independent of overall body adiposity
Results
The Healthy dietary pattern was positively associated with FFMI in men but lost significance after adjusting for fat mass index.
Before FMI adjustment, the Healthy pattern was positively associated with FFMI in men (β = 0.17, P = 0.003)
After FMI adjustment, the association was no longer significant (β = 0.09, P = 0.077)
No significant association was found in women for the Healthy dietary pattern
This suggests the initial association was confounded by body size/adiposity
Results
The Unhealthy dietary pattern showed a positive association with FFMI in men only after adjusting for fat mass index.
Before FMI adjustment, the Unhealthy pattern was not significantly associated with FFMI in men
After FMI adjustment, a positive association emerged in men (β = 0.10, P = 0.050)
No significant association was found in women for the Unhealthy dietary pattern
This partitioning model result suggests the relationship between the Unhealthy pattern and muscle mass was masked by co-occurring adiposity
Results
Physical activity significantly mediated the relationship between dietary patterns/protein density and fat-free mass.
Physical activity was identified as a significant mediator in the association between protein density and FFMI
This finding held for both men and women
The mediation analysis suggests the diet-muscle mass relationship is partly explained through physical activity pathways
The authors note this underscores the importance of combining dietary and physical activity interventions to optimize body composition
Methods
The study sample comprised 2,299 rural Iranian adults with overweight or obesity from the Fasa Adult Cohort Study.
Sample included 799 men and 1,500 women
All participants had a body mass index ≥ 25 kg/m²
Participants were from a rural Iranian population
Dietary intake was assessed using a validated food frequency questionnaire
Body composition was measured via bioelectrical impedance analysis
Methods
Two major dietary patterns were identified: a Healthy pattern and an Unhealthy pattern.
Dietary patterns were derived from a validated food frequency questionnaire
Pattern identification yielded two major patterns labeled 'Healthy' and 'Unhealthy'
Multivariable linear regression was used to assess associations with Fat-Free Mass Index (FFMI)
Partitioning models adjusted for Fat Mass Index (FMI) to isolate relationships independent of adiposity
Discussion
Broad dietary patterns were found to be confounded by body size, whereas protein density was identified as a more robust independent predictor of muscle mass.
The Healthy dietary pattern lost its significant association with FFMI once adiposity (FMI) was controlled for
Protein density maintained significant associations with FFMI in both sexes after FMI adjustment
The authors conclude that 'while broad dietary patterns may be confounded by body size, protein density is a robust, independent factor associated with muscle mass'
The findings suggest interventions should prioritize protein density and physical activity to optimize body composition
What This Means
This research suggests that among rural Iranian adults who are overweight or obese, the amount of protein relative to total food intake (called 'protein density') is consistently linked to having more muscle mass, regardless of how much body fat a person carries. The study followed 2,299 adults and found that while eating a generally 'healthy' diet appeared to be associated with more muscle initially, this connection disappeared once the researchers accounted for differences in body fat levels. In contrast, protein density remained a significant predictor of muscle mass for both men and women even after accounting for adiposity.
The study also found that physical activity played an important role in connecting diet to muscle mass, acting as a mediator in the relationship between protein density and fat-free mass. Interestingly, in men, an 'unhealthy' dietary pattern appeared to show a positive relationship with muscle mass only after statistically removing the influence of body fat — suggesting that some components of less healthy diets (possibly higher calorie or protein content) may contribute to muscle mass but are typically obscured by the accompanying body fat.
This research suggests that broad categorizations of diets as 'healthy' or 'unhealthy' may not fully capture what drives muscle mass differences in people who are overweight or obese. Instead, focusing specifically on the proportion of protein in the diet, alongside encouraging physical activity, may be more effective strategies for improving body composition in this population. These findings could be relevant for designing dietary interventions aimed at preserving or building muscle mass in adults with excess weight.
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M. Ariya, Paul P. Fahey, Reza Homayounfar, Evan Atlantis. (2026). Dietary patterns and protein density associations with fat-free mass in a cross-sectional analysis of rural Iranian adults with overweight and obesity. Scientific Reports. https://doi.org/10.1038/s41598-026-49776-6