When it comes to measuring progress, many people focus on body weight. However, body composition is often a far more meaningful measure of health. Understanding how much of your body is muscle, fat, bone and water provides valuable insight into your metabolic health and helps guide more personalised nutrition and exercise recommendations.
The challenge is that not all body composition technologies measure these tissues with the same level of accuracy.
Bioelectrical Impedance Analysis (BIA)
Many commercial body composition scanners, including InBody, Evolt and Tanita devices, use Bioelectrical Impedance Analysis (BIA). This technology sends a very small electrical current through the body and estimates body composition based on how easily that current travels.
Because water conducts electricity well, lean tissue generally appears more conductive than fat tissue. The scanner then uses sophisticated algorithms to estimate muscle mass and body fat percentage.
While BIA is quick, non-invasive and useful for monitoring general trends, it is highly influenced by factors including:
- Hydration status
- Recent food or fluid intake
- Exercise performed before the scan
- Glycogen stores
- Menstrual cycle
- Skin temperature
Even drinking several glasses of water or completing a workout before a scan can alter the results.
The Hidden Challenge - Intramuscular Fat
One of the biggest limitations of BIA occurs in individuals with higher levels of body fat or insulin resistance.
As metabolic health declines, fat is no longer stored only under the skin. It also accumulates around organs (visceral fat) and within muscle tissue itself. This is known as intramuscular fat.
Because intramuscular fat exists within the muscle, BIA devices can struggle to distinguish it from healthy contractile muscle tissue. The result is an overestimation of lean mass and an underestimation of body fat percentage.
This can create the impression that someone has considerably more muscle than they actually do, even when their metabolic health suggests otherwise.
Why DEXA Is Considered More Accurate
Dual-energy X-ray Absorptiometry (DEXA) uses low-dose X-rays to directly differentiate between bone, lean tissue and fat tissue.
DEXA is far less affected by hydration and provides a much more accurate assessment of regional fat distribution, lean tissue and visceral fat. For this reason, it is widely regarded as the clinical gold standard for body composition assessment.
Why It Matters
At AstonRX, we focus on metabolic health rather than simply body weight. Our body composition algorithms are designed to estimate metabolically active tissue so we can prescribe appropriate protein, carbohydrate and fat requirements for each individual.
Accurately estimating lean mass is essential. If lean tissue is overestimated, nutritional requirements may also be overestimated, potentially slowing progress and making fat loss more difficult.
As metabolic health improves and intramuscular fat decreases, many people notice their BIA scans become progressively more accurate. Interestingly, some individuals may even see a reduction in reported "lean mass". While this can sound concerning, it often reflects the loss of fat stored within muscle rather than the loss of functional muscle itself.
The most important measure is not a single number on a scan, but improvements in metabolic health, strength, waist circumference, energy levels, blood markers and overall body composition over time.
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