In men with overweight or obesity, eating a 5-week diet with less protein and more carbohydrates, while keeping total calories the same, leads to a loss of about 1.1 kg of fat mass, with little to no change in muscle mass.
See the scientific wording
In men with overweight or obesity, a 5-week isocaloric diet with reduced protein (0.9 g/kg/day) and increased carbohydrates results in a significant reduction in fat mass by approximately 1.1 kg and a non-significant trend toward reduction in lean body mass.
Very strong evidence
Randomized trialsOne good-quality study supports this claim.
What the research says
1 study reviewedSupporting (1)
Randomized Controlled TrialHuman2026
In a study, men who ate less protein and more carbs — but the same total calories — lost about 1.1 kg of fat over five weeks, and their muscle barely changed. So yes, this diet helped them lose fat without losing much muscle.
Contradicting (0)
No contradicting studies found yet
That doesn't mean it's settled — it just means no study has tested the opposite.
Quality-weighted scoring: we follow the GRADE framework — each study is rated High, Moderate, Low, or Very Low based on study design, methodology rigor, and risk of bias. A single high-quality RCT can outweigh several weaker observational studies.
Scores reflect study quality, not just count.
When you eat less protein, your liver releases a hormone called FGF21. This hormone signals your body to burn more energy, especially from fat, leading to fat loss. At the same time, your body slows down the breakdown of muscle to save the amino acids from the protein you eat. This means you lose fat while preserving most of your muscle.
Score breakdown, mechanism chain, raw evidence, ideal studies needed & 1 supporting study
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In men with overweight or obesity, eating a 5-week diet with less protein and more carbohydrates, while keeping total calories the same, leads to a loss of about 1.1 kg of fat mass, with little to no change in muscle mass.
Mechanism
1 studyEating less protein makes your liver release a hormone that helps you burn more fat. Your body also slows down muscle breakdown to save protein, so you lose fat without losing much muscle. This is how the diet works.
When you eat less protein, your liver releases a hormone called FGF21. This hormone signals your body to burn more energy, especially from fat, leading to fat loss. At the same time, your body slows down the breakdown of muscle to save the amino acids from the protein you eat. This means you lose fat while preserving most of your muscle.
Reduced dietary protein intake leads to lower amino acid availability and reduced amino acid oxidation.
The body decreases muscle protein breakdown to conserve amino acids.
Total and individual plasma amino acid concentrations are maintained, indicating amino acid conservation.
This adaptation limits lean body mass loss during weight loss.
Simultaneously, reduced dietary protein intake triggers increased FGF21 production and secretion by the liver into circulation.
FGF21 binds to the FGFR1c/β-klotho receptor complex on target tissues.
Activation of intracellular signaling (e.g., ERK1/2 phosphorylation) in target tissues.
Increased energy expenditure (thermogenesis) leading to body weight and fat mass loss.
Evidence from Studies
Supporting (1)
Community contributions welcome
Dietary Protein Reduction During Isocaloric Conditions Reduces Body Weight in Men With Overweight or Obesity.
In a study, men who ate less protein and more carbs — but the same total calories — lost about 1.1 kg of fat over five weeks, and their muscle barely changed. So yes, this diet helped them lose fat without losing much muscle.
Contradicting (0)
Community contributions welcome
Score Breakdown
No multi-axis breakdown available yet. The overall Pro / Against score above is the best signal.
- No clinical evidence is available; the score reflects mechanistic plausibility only.
What Would Prove This
Per GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this claim, ordered from strongest to weakest.
Systematic Review of Isocaloric Low-Protein Diets on Body Composition in Overweight and Obese Men
Population: Men with overweight or obesity; Intervention: 5-week isocaloric diet with 0.9 g/kg/day protein and increased carbohydrates; Comparator: Isocaloric diet with standard protein intake (e.g., 1.6 g/kg/day); Outcome: Change in fat mass and lean body mass measured by DEXA or MRI; Duration: 5 weeks; Number of studies: ≥10 RCTs.
Double-Blind Randomized Trial of Low-Protein vs Standard-Protein Isocaloric Diets on Body Composition in Overweight Men
Population: 100 men with BMI 25–35; Intervention: 5-week isocaloric diet with 0.9 g/kg/day protein; Comparator: 5-week isocaloric diet with 1.6 g/kg/day protein; Outcome: Primary = change in fat mass (DEXA); Secondary = change in lean body mass; Duration: 5 weeks; Blinding: Dietitian and outcome assessors blinded; Randomization: Stratified by baseline BMI.
Prospective Cohort Study of Dietary Protein Intake and Body Composition Changes in Overweight Men Over 5 Weeks
Population: 200 overweight men; Intervention: Self-selected 5-week diet with protein intake ≤0.9 g/kg/day and increased carbohydrates; Comparator: Men maintaining usual protein intake; Outcome: Change in fat mass and lean body mass measured by bioimpedance; Duration: 5 weeks; Follow-up: Weekly dietary logs and body composition scans.
Cross-Sectional Analysis of Protein Intake and Body Composition in a Sample of Overweight Men
Population: 500 overweight men; Intervention: None; Comparator: Grouped by self-reported protein intake (≤0.9 g/kg/day vs >1.2 g/kg/day); Outcome: Single measurement of fat mass and lean body mass via bioimpedance; Duration: Single time point.
Case Report of Fat Mass Reduction and Lean Mass Changes in a Single Overweight Man Following a 5-Week Low-Protein Isocaloric Diet
Population: One man with BMI 30; Intervention: 5-week isocaloric diet with 0.9 g/kg/day protein and increased carbohydrates; Outcome: Daily body weight, fat mass (DEXA), and lean mass measurements; Duration: 5 weeks; Monitoring: Daily food logs and weekly scans.