Body composition assessment (revision 15)
Old revision·02:07, 22 May 2025·FormularyFrida
| Body composition assessmentMeasurement | |
|---|---|
Fat and lean compartments change in different proportions during weight loss. | |
| Reference methods | DXA, four-compartment models |
| Field methods | Bioimpedance, skinfolds, circumference |
| Limitation of weight alone | Cannot distinguish the compartments |
| Analytical method infobox · conventions | |
Body composition assessment measures the fat and lean compartments of the body separately. Body weight cannot distinguish them, and a given weight change may comprise very different proportions of each.[1]
The reference methods are dual-energy X-ray absorptiometry and multi-compartment models combining several measurements. Field methods — bioimpedance, skinfolds, circumference-based equations — are more accessible and considerably less precise.[1]
The question this measurement is usually asked in this field is how much of the weight lost under incretin therapy is lean mass, and whether the proportion differs from that seen with other forms of caloric restriction. See Sarcopenia and Lean mass preservation.[2]
Methods and their precision
[edit]| Method | Basis | Precision for change |
|---|---|---|
| DXA | Differential X-ray attenuation | Good; the practical reference |
| Four-compartment model | Combines density, water, mineral | Best available; research only |
| Bioimpedance | Electrical conductance | Sensitive to hydration state |
| Skinfolds | Subcutaneous thickness | Operator-dependent |
| Circumference equations | Anthropometric proxies | Population-derived; poor for individuals |
Bioimpedance is the method most often used outside research and the one whose limitations are least appreciated. It infers composition from conductance, which depends on total body water; changes in hydration produce apparent changes in lean mass that did not occur.[1]
For tracking change in one person, the same method on the same device under the same conditions is worth more than a nominally better method used inconsistently — the same principle that governs a laboratory baseline.[3] Composition measured across a withdrawal period behaves differently again, since the compartments do not regain in the proportions they were lost.[4]
Lean mass loss during weight reduction
[edit]Loss of lean mass accompanies loss of fat mass in essentially every form of substantial weight reduction. Reported proportions vary with method, population and rate of loss, and figures in the region of a fifth to a quarter of total loss are commonly cited for caloric restriction.[1]
Whether incretin therapy produces a different proportion is disputed. The trials were not designed to answer it, body composition was a secondary outcome measured in subsets, and the comparison group in most analyses is a placebo arm losing very little weight — which makes the proportional comparison awkward.[2]
Lean mass is not synonymous with muscle. The compartment includes organs, connective tissue and body water, and some reduction is expected simply because a smaller body requires less of all of them.[1]
See also
References
- ^ a b c d e 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." Obesity Reviews 15(4):310–321 (2014). PMID 24447775.
- ^ a b Wilding JPH, Batterham RL, Calanna S, et al. "Once-weekly semaglutide in adults with overweight or obesity." New England Journal of Medicine 384(11):989–1002 (2021). PMID 33567185.
- ^ American Diabetes Association. "Standards of Care in Diabetes." Diabetes Care 47(Suppl 1) (2024).
- ^ Rubino D, Abrahamsson N, Davies M, et al. "Effect of continued weekly subcutaneous semaglutide vs placebo on weight loss maintenance (STEP 4)." JAMA 325(14):1414–1425 (2021). PMID 33755728.