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 Mass Index (BMI) is calculated as weight (kg) divided by height (m) squared: BMI = weight / height². Developed by Belgian mathematician Adolphe Quetelet in the 1830s, it was designed as a population-level statistical tool, not an individual health assessment metric.
| BMI Range | Classification |
|---|---|
| <18.5 | Underweight |
| 18.5-24.9 | Normal weight |
| 25.0-29.9 | Overweight |
| 30.0-34.9 | Obese (Class I) |
| 35.0-39.9 | Obese (Class II) |
| ≥40.0 | Obese (Class III) |
BMI remains valuable as a population-level screening tool. Large epidemiological studies consistently show that at the population level, higher BMI correlates with increased risk of cardiovascular disease, type 2 diabetes, and all-cause mortality. The Global BMI Mortality Collaboration (2016) analyzed 239 prospective studies with 10.6 million participants across 4 continents and confirmed a J-shaped relationship between BMI and mortality, with lowest risk at BMI 22.5-25.0.
Global BMI Mortality Collaboration (2016). Body-mass index and all-cause mortality: individual-participant-data meta-analysis of 239 prospective studies in four continents. Lancet, 388(10046), 776-786. PubMed
BMI's critical flaw is that it cannot distinguish between muscle and fat. This leads to systematic misclassification in several populations:
Romero-Corral, A., et al. (2008). Accuracy of body mass index in diagnosing obesity in the adult general population. Int J Obes, 32(6), 959-966. PubMed
Fat-Free Mass Index (FFMI) is a body composition metric that adjusts for body fat, providing a measure of muscularity independent of fat mass. The formula is:
FFMI = (lean mass in kg) / (height in m)²
Normalized FFMI = FFMI + 6.1 x (1.8 - height in m)
Where lean mass = total weight x (1 - body fat fraction). The normalized version adjusts for height, making it more comparable across different statures.
FFMI is superior to BMI because it specifically measures muscularity rather than total mass. A person with a BMI of 28 could be either a muscular athlete with FFMI of 23 or a sedentary person with excessive fat and FFMI of 18. BMI treats them identically; FFMI reveals the fundamental difference.
The landmark study defining FFMI reference values was published by Kouri, Pope, Katz, and Oliva in 1995. This study examined 157 male athletes, including:
Key finding: Among the natural athletes, no individual exceeded an FFMI of 25.0. The mean FFMI for natural athletes was 22.3 (SD: 1.9). Among steroid users, the mean was 24.8, with many exceeding 25. The study established FFMI of 25 as the approximate upper limit for natural (non-enhanced) male muscularity.
The researchers also analyzed pre-steroid era bodybuilders (1939-1959) using historical photographs and measurements, and found that even the most muscular natural bodybuilders of that era had FFMIs around 24-25, consistent with the modern data.
Kouri, E. M., et al. (1995). Fat-free mass index in users and nonusers of anabolic-androgenic steroids. Clin J Sport Med, 5(4), 223-228. PubMed
| FFMI (Men) | Classification | Interpretation |
|---|---|---|
| 16-17 | Below average | Low muscle mass, typical of sedentary lifestyle |
| 18-19 | Average | Typical for moderately active men |
| 20-21 | Above average | Regular resistance training |
| 22-23 | Excellent | Dedicated, consistent training over years |
| 24-25 | Superior | Near genetic limit for natural athletes |
| >25 | Suspicious | Extremely rare naturally; suggests AAS use |
For women, FFMI values are naturally lower due to hormonal differences. Typical ranges are approximately 3-5 points lower than men: average 14-16, excellent 17-19, and values above 21 are extremely rare without pharmacological assistance.
Important caveat: The FFMI 25 limit is not absolute. Some researchers argue it may be slightly higher (up to 26-27) for genetically gifted individuals or those at higher body fat percentages. Nonetheless, it remains the best available benchmark for natural muscular development.
The Body Roundness Index (BRI) has emerged as the leading candidate to supplement or replace BMI. Developed by mathematician Diana Thomas at the US Military Academy, BRI uses waist circumference and height (not body weight) to estimate visceral adipose tissue, scoring from 1 to 20. A 2026 national cohort study published in the Journal of Obesity using NHANES data found that BRI values outside the 4.4-5.4 range correlated with increased all-cause mortality, and BRI demonstrated narrower confidence intervals than BMI, suggesting more precise mortality risk estimates. The AMA formally recognized BMI's limitations in 2023, noting its development was based primarily on data from white men, and now recommends BMI be used alongside waist circumference, body composition, and relative fat mass for a more comprehensive assessment.
The Body Shape Index (ABSI) is also gaining research attention as a complementary metric. Both BRI and ABSI are easily calculated in clinical settings using only a tape measure and height rod, making them practical alternatives that capture central adiposity, the fat distribution pattern most strongly linked to cardiometabolic disease.
Kimura et al. (2026). Body Roundness Index and Body Shape Index as Predictors for All-Cause Mortality Beyond BMI. Journal of Obesity. Wiley