Macro partitioning
During the early phase (Phase I) of voluntary weight loss (first 4-6 weeks), the majority of weight loss consists of fat-free mass (protein, glycogen, and associated water/electrolytes) rather than fat mass, with the fraction of fat-free mass loss being highest in the first week and decreasing over time.
Expect the first 4-6 weeks of dieting to show rapid weight loss that is mostly water and lean tissue, not just fat. This is normal physiology, not a sign that your diet is 'too harsh' or that you are losing muscle dangerously (unless protein intake is extremely low). Focus on preserving muscle through adequate protein and resistance training, and do not be discouraged when the scale slows down after the first month; this indicates you are entering the more fat-dominant Phase II.
The fraction of weight loss as fat-free mass was high at 1 week of VLCD treatment, about 0.4–0.6 across all three diet groups. The fraction of weight loss as fat-free mass subsequently decreased at weeks 2 and 4... The fraction of weight loss as fat... began low at week 4 and gradually increased up to week 12.
Why this rating
Based on systematic review of multiple studies (Calerie, Kiel, Krotkiewski) using DXA and balance methods.
Source
Voluntary weight loss: systematic review of early phase body composition changes
S. B. Heymsfield et al. · Obesity Reviews · 2010
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