What does weight loss consist of when losing weight through dieting?
Guest writer: Rob van Berkel, Research dietitian and writer on nutrition and health
What does weight loss consist of when losing weight through dieting?
When an individual is overweight and aims to lose weight, the goal is to reduce fat mass rather than muscle mass. Some loss of muscle mass is often unavoidable. It is important to monitor this and understand which factors influence it.
Body composition
Body weight composition can be expressed in different ways, for example using compartment models. These include the 2-compartment, 3-compartment, and 4-compartment models (see table).
| 2-compartment model | 3-compartment model | 4-compartment model |
|---|---|---|
| Fat mass | Fat mass | Fat mass |
| Fat-free mass | Body water | Body water |
| Proteins and minerals | ||
| Proteins | Minerals |
Table: Differences between the 2-, 3-, and 4-compartment models.
Fat mass is a clearly defined component of the body, but fat-free mass may be less well understood. Fat-free mass consists of muscle mass (skeletal muscle, smooth muscle, and cardiac muscle), bone mass (skeleton), organs, water, blood, and other bodily fluids. When an individual is overweight and seeks to lose weight, the objective is to reduce fat mass rather than fat-free mass.
The gold standards for measuring fat mass, fat-free mass, bone mass, and muscle mass are Dual-Energy X-ray Absorptiometry (DXA) and Magnetic Resonance Imaging (MRI). However, these methods are costly and not universally available. A practical alternative is Bioelectrical Impedance Analysis (BIA). This method involves passing a weak electrical current through the body. Because fat-free mass contains a high proportion of water, which conducts electricity well, it produces low electrical resistance. In contrast, fat mass contains relatively little water, resulting in poorer conductivity and higher resistance. Using this information in combination with specialised algorithms, the proportions of fat mass, fat-free mass, and muscle mass can be estimated. BIA is also relatively affordable, widely available, user-friendly, and quick to perform.
Composition of weight loss
In energy-restricted diets, approximately 10–30% of total weight loss consists of fat-free mass (Chaston et al., 2007; Hall, 2007). This proportion remains relatively consistent regardless of the magnitude of weight loss (5–35%) (Magkos et al., 2016). As a general rule, it is therefore often stated that approximately 25% of weight loss consists of fat-free mass, with the remaining portion consisting of fat mass. Although this serves as a useful guideline, several factors influence this proportion, including (Chaston et al., 2007; Heymsfield et al., 2014; Cava et al., 2017; McCarthy & Berg, 2021; Binmahfoz et al., 2025):
-
Initial body weight and composition. A higher body fat percentage is generally associated with a greater proportion of fat mass loss.
-
Dietary strategy. Greater energy restriction and insufficient protein intake are associated with a higher proportion of fat-free mass loss as a percentage of total weight loss.
-
Physical activity. Physical activity, particularly resistance training, reduces the loss of fat-free mass.
-
Rate of weight loss. Faster weight loss is generally associated with a greater proportion of fat-free mass loss. This was also observed in a meta-analysis of seven studies, although the observed difference of 1 kg was not statistically significant (Ashtary-Larky et al., 2020). These studies had limitations, and further research is warranted.
-
Individual factors, including sex, age, and genetic predisposition.
Conclusion
When weight loss occurs through an energy-restricted diet, an average of approximately 25% of the total weight loss consists of fat-free mass. However, several modifiable factors influence this proportion, resulting in substantial variability between individuals. Monitoring the composition of weight loss provides valuable insight and allows appropriate lifestyle recommendations to be tailored accordingly.
References
Ashtary-Larky D, Bagheri R, Abbasnezhad A, Tinsley GM, Alipour M, Wong A. Effects of gradual weight loss v. rapid weight loss on body composition and RMR: a systematic review and meta-analysis. Br J Nutr. 2020;124(11):1121-1132.
Binmahfoz A, Dighriri A, Gray C, Gray SR. Effect of resistance exercise on body composition, muscle strength and cardiometabolic health during dietary weight loss in people living with overweight or obesity: a systematic review and meta-analysis. BMJ Open Sport Exerc Med. 2025;11(3):e002363. Published 2025 Sep 2.
Cava E, Yeat NC, Mittendorfer B. Preserving Healthy Muscle during Weight Loss. Adv Nutr. 2017;8(3):511-519. Published 2017 May 15.
Chaston TB, Dixon JB, O'Brien PE. Changes in fat-free mass during significant weight loss: a systematic review. Int J Obes (Lond) 2007; 31(5): 743-50.
Hall KD. Body fat and fat-free mass inter-relationships: Forbes’s theory revisited. Br J Nutr. 2007;97(6):1059-1063.
Heymsfield SB, Gonzalez MC, Shen W, Redman L, Thomas D. Weight loss composition is one-fourth fat-free mass: a critical review and critique of this widely cited rule. Obes Rev. 2014 Apr;15(4):310-21.
Magkos F, Fraterrigo G, Yoshino J, et al. Effects of moderate and subsequent progressive weight loss on metabolic function and adipose tissue biology in humans with obesity. Cell Metab. 2016;23 (4):591-601.
McCarthy D, Berg A. Weight Loss Strategies and the Risk of Skeletal Muscle Mass Loss. Nutrients. 2021;13(7):2473. Published 2021 Jul 20.
