Not medical advice. I am a software engineer, not a physician. This page documents my own protocol and the research I read while building it. Talk to a qualified clinician before changing your supplementation, diet or any treatment. Lab reference ranges quoted here vary by laboratory and by individual.
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Body fat percentage (BF%) represents the proportion of your total body weight that is composed of fat tissue. Unlike body weight or BMI alone, BF% distinguishes between fat mass and lean mass (muscle, bone, organs, water), making it one of the most meaningful biomarkers for health assessment.
Your body needs essential fat to function. Essential fat supports hormone production, vitamin absorption, temperature regulation, and organ protection. For men, essential fat is approximately 3-5% of body weight; for women, it is 10-13% due to reproductive and hormonal needs.
Body fat ranges vary by sex, age, and fitness level. The American Council on Exercise (ACE) and the American College of Sports Medicine (ACSM) provide the following general classifications:
| Category | Men | Women |
|---|---|---|
| Essential Fat | 3-5% | 10-13% |
| Athletes | 6-13% | 14-20% |
| Fitness | 14-17% | 21-24% |
| Acceptable | 18-24% | 25-31% |
| Obese | >25% | >32% |
Optimal target for health and aesthetics: Most research suggests 10-15% for men and 18-25% for women provides the best balance of metabolic health, hormonal function, and physical performance.
Age matters: body fat naturally increases with age due to hormonal changes, decreased muscle mass (sarcopenia), and reduced physical activity. A 45-year-old man at 18% body fat may be in excellent health, while the same percentage in a 25-year-old competitive athlete might indicate room for improvement.
BIA is the technology used in most consumer scales (including the Xiaomi Body Composition Scale). It sends a small electrical current through your body and measures resistance. Since fat tissue has higher impedance than lean tissue (which contains more water), the device estimates body composition.
Accuracy: BIA scales typically have a margin of error of 3-5% compared to DEXA. However, they are excellent for tracking trends over time when used consistently (same time of day, same hydration state). A 2015 meta-analysis by Earthman et al. reviewing 50 studies found that BIA can systematically underestimate body fat in obese individuals and overestimate it in lean individuals.
Earthman, C. P. (2015). Body Composition Tools for Assessment of Adult Malnutrition at the Bedside. Journal of Parenteral and Enteral Nutrition, 39(7), 787-822. PubMed
DEXA is considered the clinical gold standard for body composition. It uses two X-ray beams at different energy levels to differentiate between bone mineral, lean tissue, and fat tissue. A DEXA scan takes about 10 minutes, involves minimal radiation (less than a chest X-ray), and provides regional body composition data (arms, legs, trunk).
Accuracy: DEXA has approximately 1-2% error margin. A study by Shepherd et al. (2017) comparing DEXA with the four-compartment model (the absolute gold standard) across 1,026 adults found DEXA had a mean bias of only 0.7% body fat.
Shepherd, J. A., et al. (2017). Body composition by DXA. Bone, 104, 101-105. PubMed
Practical tip: For home monitoring, 4-electrode BIA scales (feet only) are less accurate than 8-electrode devices that also measure through the hands. Regardless of device, always measure at the same time (ideally morning, fasted, after using the bathroom) for consistent tracking.
While lower body fat is generally associated with better metabolic health, going too low introduces serious risks:
Rossow, L. M., et al. (2013). Natural bodybuilding competition preparation and recovery: a 12-month case study. International Journal of Sports Physiology and Performance, 8(5), 582-592. PubMed
Excess body fat accumulation is fundamentally caused by a sustained caloric surplus (consuming more energy than your body expends). However, several factors modulate how easily fat accumulates and where it is stored:
Spaeth, A. M., et al. (2013). Effects of experimental sleep restriction on weight gain, caloric intake, and meal timing in healthy adults. Sleep, 36(7), 981-990. PubMed
A caloric deficit is the only way to lose body fat. No supplement, exercise routine, or meal timing strategy works without it. A moderate deficit of 500 calories per day produces approximately 0.5 kg (1 lb) of fat loss per week. Research shows the maximum rate of fat oxidation is approximately 69 calories per kg of body fat per day (Alpert, 2005), meaning leaner individuals must use smaller deficits to avoid muscle loss.
Learn more: The Science of Caloric Deficit and Fat Loss
Resistance training preserves muscle mass during fat loss, keeping metabolic rate higher. A meta-analysis by Clark (2015) across 18 studies found that resistance training during caloric restriction preserved an average of 93% of lean mass compared to diet alone (which preserved only 75%). Train 3-4 times per week with compound movements (squats, deadlifts, bench press, rows).
Learn more: Muscle Mass and Strength: A Complete Guide
During fat loss, protein needs increase. The landmark meta-analysis by Morton et al. (2018) — analyzing 49 studies with 1,863 participants — found that protein intake of 1.6 g/kg/day maximized muscle protein synthesis. However, during a caloric deficit, the optimal range increases to 2.2 g/kg/day or even up to 2.7 g/kg for very lean individuals (Helms et al., 2014). This higher intake helps preserve lean mass and increases satiety.
Morton, R. W., et al. (2018). A systematic review, meta-analysis and meta-regression of the effect of protein supplementation on resistance training-induced gains in muscle mass and strength in healthy adults. British Journal of Sports Medicine, 52(6), 376-384. PubMed
Cardio increases caloric expenditure. Both steady-state cardio and HIIT (High-Intensity Interval Training) are effective, but HIIT may have a slight edge for fat loss relative to time invested. A 2019 meta-analysis by Viana et al. (36 studies, 1,012 participants) found HIIT reduced total body fat by 28.5% more than moderate-intensity continuous training. However, the best cardio is whichever type you will consistently perform. Walking 8,000-10,000 steps daily is an often-underestimated fat loss tool.
Sleep deprivation is a potent fat-loss saboteur. The Nedeltcheva et al. (2010) study at the University of Chicago took 10 overweight adults and placed them on the same caloric deficit for 14 days, randomized to either 5.5 hours or 8.5 hours of sleep. Both groups lost similar total weight, but the sleep-restricted group lost 55% more lean mass and 60% less fat. Poor sleep also increases cortisol, which promotes visceral fat storage.
Nedeltcheva, A. V., et al. (2010). Insufficient sleep undermines dietary efforts to reduce adiposity. Annals of Internal Medicine, 153(7), 435-441. PubMed
A 2025 study in Military Medicine compared multi-frequency BIA devices (at 500 kHz and 1000 kHz) against DEXA in 62 military personnel and found strong correlations (r = 0.84-0.93, ICC = 0.89-0.94), significantly better than older single-frequency devices. This confirms that modern consumer scales using multi-frequency bioimpedance are closing the accuracy gap with clinical DEXA scans, particularly for trend-tracking in healthy adults. Notably, octopolar multi-frequency BIA devices now achieve ICC values of 0.87 compared to just 0.39 for older single-frequency models, making device generation a critical factor when interpreting BIA readings.
Additionally, a 2022 retrospective analysis of 3,655 body composition measurements confirmed that BIA systematically underestimates fat percentage in individuals with BMI above 30 and overestimates in lean individuals (BMI below 20). For the most accurate home tracking, researchers now recommend using the same device at the same time daily (ideally morning, fasted, after voiding) and focusing on 4-week moving averages rather than day-to-day fluctuations.
Military Medicine (2025). Comparison of Four Body Composition Methods: Circumference Measurements, Eight-Point BIA up to 500 and 1000 kHz to DEXA. Military Medicine, 190(3-4), e642. Oxford Academic