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Background: Age-related sarcopenia and central adiposity are common concerns among middle-aged adults. A high-protein diet has been proposed as a nutritional strategy to preserve lean mass and modulate fat distribution. Objective: To evaluate the effects of a 12-week high-protein dietary intervention on lean muscle mass and regional fat distribution in healthy middle-aged adults. Methods: In this controlled trial, 80 participants (aged 45–60 years) were randomly assigned to either a high-protein diet (HP; 1.6 g/kg/day, n = 40) or a control diet (C; 0.8 g/kg/day, n = 40). Lean mass (via DXA) and fat distribution (visceral and subcutaneous abdominal fat, via MRI) were assessed at baseline and post-intervention. Dietary intake was closely monitored. Data are expressed as mean ± SD. Group differences were tested using ANCOVA adjusting for baseline values. Results: At 12 weeks, the HP group exhibited a significantly greater increase in total lean mass than the control (HP: +1.8 ± 0.9 kg vs. C: +0.5 ± 0.8 kg; p < 0.001). Visceral fat decreased by –8.5 ± 3.2% in HP vs. –3.1 ± 3.5% in C (p = 0.002), while subcutaneous abdominal fat decreased by –5.2 ± 2.8% vs. –2.0 ± 3.1% (p = 0.010). Lean mass gain relative to body weight was +3.2 ± 1.4% in HP vs. +0.9 ± 1.2% in C (p < 0.001). No adverse events were reported. Conclusion: A 12-week high-protein diet significantly enhances lean mass gains and induces greater reductions in visceral and subcutaneous fat compared to a standard protein intake in middle-aged adults. These results support dietary protein optimization for healthy body composition in this population.
Age-associated changes in body composition, particularly the decline in lean muscle mass (sarcopenia) and the accumulation of central adiposity, contribute to increased cardiometabolic risk and functional decline in middle-aged populations [1]. Dietary strategies that target preservation or enhancement of lean mass, while reducing adiposity, are of high clinical interest. Protein intake plays a pivotal role in muscle protein synthesis, with higher protein consumption linked to maintenance of muscle mass in aging adults [2]. Moreover, some evidence suggests that elevated dietary protein may modulate fat distribution, particularly by reducing visceral adipose tissue [3].
Randomized controlled trials have demonstrated that high-protein diets, often defined as >1.2 g/kg/day, are more effective in promoting lean mass retention compared to lower-protein regimens in older adults [4], and may also facilitate fat loss when combined with resistance training [5]. However, studies focusing specifically on healthy middle-aged individuals without concurrent exercise interventions are limited. Current literature largely addresses older, overweight populations or combines dietary and physical activity components, leaving a gap in understanding the isolated effects of increased protein intake on body composition in mid-life [6].
Therefore, this controlled trial aimed to determine the impact of a 12-week high-protein diet (1.6 g/kg/day) versus a standard protein intake (0.8 g/kg/day) on lean mass outcomes and abdominal fat distribution—assessed through DXA and MRI—in healthy, non-exercising, middle-aged adults.
Study Design and Participants
This was a single-center, parallel-group, controlled dietary intervention conducted over 12 weeks. Eighty healthy adults aged 45–60 years were enrolled via local advertisements and screened for eligibility. Sample size (n = 40 per group) was calculated to detect a between-group difference of 1.2 kg in lean mass change with 80% power and α = 0.05, assuming an SD of 1.5 kg, allowing for 10% dropout.
Inclusion criteria included BMI 20–30 kg/m², stable weight (±2 kg in last 3 months), no structured exercise (>1 session/week), and no metabolic or chronic illness. Exclusion criteria included protein-modifying medications, renal impairment (eGFR < 60 mL/min/1.73 m²), diabetes, or MRI contraindications.
Dietary Intervention
Participants were randomized 1:1 to either:
Outcomes and Measurements
Lean mass was measured by dual-energy X-ray absorptiometry (DXA; Hologic QDR) at baseline and 12 weeks. Regional fat distribution was assessed by abdominal MRI at the level of L4-L5 to quantify visceral and subcutaneous adipose tissue areas (cm²), which were converted to percent change. Body weight was measured to nearest 0.1 kg. Safety labs (renal function) were monitored at baseline and 12 weeks.
Statistical Analysis
Analyses used intention-to-treat with multiple imputation for missing data. Primary outcomes were change in total lean mass and percent change in visceral fat. Between-group differences were assessed using ANCOVA adjusting for baseline values. Secondary outcomes included subcutaneous fat change and lean mass relative to body weight. Data are presented as mean ± SD; significance threshold at p < 0.05. Analyses performed using SPSS v27.
Participant Characteristics and Retention
Of the 80 participants randomized (HP, n = 40; C, n = 40), 2 dropped out from HP (1 for personal reasons, 1 lost to follow-up) and 3 from Control (all lost to follow-up). The final analysis included 78 participants (HP, n = 38; C, n = 37). Baseline characteristics were well matched (mean age: HP 52.1 ± 4.3 years, C 51.8 ± 4.5 years; BMI: HP 25.2 ± 2.8 kg/m², C 25.5 ± 2.6 kg/m²; all p > 0.1).
Lean Mas
At 12 weeks, total lean mass increased markedly in the HP group compared to Control (HP: +1.8 ± 0.9 kg; C: +0.5 ± 0.8 kg), with a significant between-group difference (p < 0.001). Lean mass as a percentage of body weight also rose more in HP (+3.2 ± 1.4%) versus C (+0.9 ± 1.2%; p < 0.001). (See Table 1.)
Visceral and Subcutaneous Fat
Visceral fat decreased by –8.5 ± 3.2% in HP versus –3.1 ± 3.5% in C (p = 0.002). Subcutaneous abdominal fat declined by –5.2 ± 2.8% in HP compared to –2.0 ± 3.1% in C (p = 0.010). Absolute MRI-derived adipose areas mirrored the percent changes. (Figure 1 illustrates percent changes in fat compartments.)
Weight Maintenance and Safety
Body weight remained stable in both groups (HP: –0.2 ± 0.7 kg; C: –0.1 ± 0.8 kg; p = 0.56), confirming iso-energetic intake. Renal function markers (serum creatinine, eGFR) showed no clinically relevant changes, and no adverse events occurred.
Table 1. Changes in Lean Mass and Fat Distribution After 12 Weeks
|
Parameter |
High-Protein Group (n = 38) |
Control Group (n = 37) |
Between-Group p-value |
|
Total lean mass change (kg) |
+1.8 ± 0.9 |
+0.5 ± 0.8 |
<0.001 |
|
Lean mass change (% body weight) |
+3.2 ± 1.4% |
+0.9 ± 1.2% |
<0.001 |
|
Visceral fat change (%) |
–8.5 ± 3.2% |
–3.1 ± 3.5% |
0.002 |
|
Subcutaneous abdominal fat change (%) |
–5.2 ± 2.8% |
–2.0 ± 3.1% |
0.010 |
|
Body weight change (kg) |
–0.2 ± 0.7 |
–0.1 ± 0.8 |
0.56 |
Table 2. Baseline Characteristics of Study Participants
|
Variable |
High-Protein Group (n = 38) |
Control Group (n = 37) |
p-value |
|
Age (years) |
52.1 ± 4.3 |
51.8 ± 4.5 |
0.78 |
|
Sex (M/F) |
20/18 |
19/18 |
0.92 |
|
BMI (kg/m²) |
25.2 ± 2.8 |
25.5 ± 2.6 |
0.65 |
|
Total lean mass (kg) |
49.3 ± 6.2 |
49.1 ± 6.5 |
0.88 |
|
Visceral fat area (cm²) |
118.4 ± 25.7 |
119.8 ± 26.1 |
0.81 |
|
Subcutaneous abdominal fat area (cm²) |
230.7 ± 42.5 |
233.5 ± 40.9 |
0.73 |
In this 12-week controlled trial among healthy middle-aged adults, a high-protein diet (1.6 g/kg/day) significantly enhanced lean mass gain and promoted greater reductions in both visceral and subcutaneous abdominal fat compared to a standard protein intake (0.8 g/kg/day). These findings highlight the utility of dietary protein elevation for favorable body composition shifts even in the absence of structured exercise.
Our lean mass results align with previous findings in older adults, where a higher protein intake (≥1.2 g/kg/day) enhanced muscle preservation and synthesis [7]. The gain of +1.8 kg observed here is comparable to those reported in anabolic dietary studies and underscores that middle-aged individuals can benefit similarly. Additionally, the reduction in visceral fat by ~8.5% parallels interventions combining high protein with physical activity [8], although our participants did not engage in exercise—suggesting protein itself may exert lipolytic or satiety-mediated effects conducive to central fat loss.
These outcomes contrast with studies prescribing standard protein levels (~0.8 g/kg/day), which often report minimal lean mass retention and limited fat loss—particularly in visceral deposits [9-11]. The magnitude of effect on subcutaneous fat (–5.2%) further supports the role of dietary protein in remodeling adipose tissue distribution in midlife.
Proposed mechanisms include enhanced muscle protein synthesis due to leucine-rich amino acids, elevated thermic effect of feeding, and increased satiety reducing non-lean mass accrual. Prior metabolic trials in animal models have demonstrated that high protein intake preserves lean tissue during energy balance and preferentially mobilizes visceral fat [12-15]. Although our study did not assess mechanistic biomarkers (e.g., mTOR signaling, lipolysis), the findings are biologically plausible and consistent with mechanistic hypotheses.
Strengths of the study include rigorous control of protein intake, objective measurement of lean mass and fat distribution via DXA and MRI, and maintenance of energy balance. Limitations include relatively short duration (12 weeks), lack of mechanistic endpoints, and absence of diverse populations beyond healthy, non-exercising middle-aged adults. Further research over longer durations, and among individuals with overweight or metabolic comorbidities, is warranted to generalize these results and explore durability.
A 12-week high-protein dietary regimen (1.6 g/kg/day) significantly enhances lean muscle mass gains and induces greater reductions in both visceral and subcutaneous abdominal fat compared to a lower-protein intake in healthy middle-aged adults, independent of exercise. These findings support optimizing dietary protein as a strategic intervention for maintaining favorable body composition during midlife.
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