Body composition and progress

What each method really measures (scales, skinfolds, DEXA…) and how to track your progress without obsessing over a single number.

Your body fat % depends on which 'model' measures it

The simplest model (2 compartments: fat vs. the rest) assumes the density of the 'rest' of your body is always the same, which is false: it varies with hydration, training level and age. That is why comparing your body fat % measured by bioimpedance with someone else's measured by DEXA is not a fair comparison: they are different models with different assumptions.

How to apply it

• Do not compare your body fat % with someone else's if you used different methods
• To track your own progress, always use the same method under similar conditions

DEXA, hydrostatic weighing and the Bod Pod: the reference methods

DEXA (minimal radiation, equivalent to minutes of background exposure) is today's practical standard: it measures fat, muscle and bone separately with more precision than almost any other accessible method, including where the fat is distributed. Hydrostatic weighing and the Bod Pod are reasonable alternatives but less practical. No field method (scales, skinfolds) comes close to this precision.

How to apply it

• If you want a genuinely precise figure, an occasional DEXA (1–2 times a year) is the best option available
• You do not need to repeat it often: for day to day, a consistent field method is enough to see trends

Bioimpedance scales: what they measure and why they vary so much

Bioimpedance estimates your composition from how well your body conducts electricity, and water is the factor that most influences that conductivity. That is why the result shifts by several percentage points just from being dehydrated, having trained, or the time of day: it is not measuring new fat, it is measuring different water. The equations are also built for specific populations and fail more in very muscular people or those with obesity.

How to apply it

• Always weigh/measure yourself under the same conditions (same time, fasted, before training) so the changes are comparable
• Do not be alarmed by a jump of several points from one day to the next: it is almost always water, not fat

Skinfolds: the most underrated method

Done by the same trained person, with the same callipers and at the same sites, measuring skinfolds is surprisingly accurate for tracking your progress over time, even if the exact body fat % the formulas produce (Jackson-Pollock, etc.) carries a margin of error. Its weakness is that it only captures fat under the skin, not visceral fat, and the formulas are calibrated for specific populations.

How to apply it

• If someone takes your skinfolds regularly, make it always the same person and the same sites
• Give more weight to the trend (are they falling over the weeks?) than to the exact number on a single day

Circumferences and BMI: useful for what they are, not for everything

BMI (weight/height²) does not distinguish muscle from fat: a muscular athlete can come out as 'overweight' without being so. It is useful only as a population screening tool, not as an individual diagnosis. Waist circumference, on the other hand, does correlate well with visceral fat (the kind most linked to metabolic risk) and is a cheap, sensible measure to track alongside others.

How to apply it

• Do not define yourself by BMI if you train strength regularly: it probably misclassifies you
• A tape measure at the waist is a simple, useful measure genuinely worth logging

Visceral vs. subcutaneous fat: total % does not tell the whole story

Subcutaneous fat (under the skin) is relatively harmless metabolically; visceral fat (around the abdominal organs) is hormonally active and is associated with insulin resistance and inflammation. Two people with the same total body fat % can have very different metabolic risks depending on how much of that fat is visceral: there is even a 'thin outside, fat inside' (TOFI) profile in people with a normal BMI.

How to apply it

• Do not assume that being slim on the outside means being healthy on the inside: waist circumference still matters even if your weight is normal
• Abdominal fat that feels 'hard' or distended usually has more of a visceral component than fat you can easily pinch

Your body fat is an active hormonal organ

Adipose tissue is not a simple energy store: it secretes leptin (which regulates appetite and rises with more fat), adiponectin (which improves insulin sensitivity and falls with more visceral fat) and inflammatory substances when there is excess visceral fat. Also, the number of fat cells is largely fixed in childhood/adolescence; in adults, gaining or losing fat is mostly about enlarging or shrinking those cells, not creating many new ones.

How to apply it

• Understand that losing fat also changes your hormonal profile (less leptin, hence more hunger in a deficit), not just your appearance
• Do not try to 'eliminate' fat cells from an area: what changes is their size, not their number

How muscle mass (and sarcopenia) is really measured

Beyond 'looking muscular', skeletal muscle mass can be quantified (for example with the ASMI index: lean mass of your arms and legs divided by your height squared) and the diagnostic criteria for sarcopenia prioritise actual strength, not just the amount of muscle. You can have a low body fat % and still have little relative muscle mass if you have never trained for strength: that is why the goal is not just to 'lose weight' but to gain or maintain muscle while you do it.

How to apply it

• Do not chase the number on the scale alone: strength and muscle mass matter as much or more for your long-term health
• If you train strength and eat enough protein, weight loss is more likely to come from fat rather than muscle

Why counting calories is harder than it looks

Energy balance (eating less than you burn to lose fat) is a non-negotiable principle, but measuring both sides accurately in real life is very hard: food logs systematically underestimate real intake by 20–40%, and activity watches/apps have errors of 10–30% in estimated expenditure. The scale not moving 'even though you eat little' almost always reflects this margin of error, not an exception to thermodynamics.

How to apply it

• Log your food honestly and consistently, knowing the exact number will always carry a margin of error
• If your weight does not drop for several weeks in a row, suspect your logging before your metabolism

How to track your real progress without obsessing over one number

No method is perfect on its own, so the best strategy combines several signals: your weight as a trend (a weekly average, not a single day's reading), regular photos in the same light and pose, how your clothes fit, and if you can, a tape measure at the waist plus some composition measurement every few months. When several signals agree in the same direction, you can trust the trend even though each individual measure has its margin of error.

How to apply it

• Use your weekly average weight, not a single day's number, to judge your progress
• Combine at least 2–3 signals (weight, photos, measurements) before drawing conclusions about whether something is working