Body Composition
This content is for informational purposes only and does not constitute medical or nutritional advice. Speak with your health professional before starting this protocol.
Measurement disclaimer: Body fat percentage formulas and circumference-based methods are population estimates. Accuracy varies by individual. If your measurement result places you near a threshold (eligibility minimum or sprint level boundary), seek a more precise method — DEXA scan or professional skinfold assessment — before proceeding.
Why the Scale Isn't the Point
Scale weight is the most common measure of progress. It's also the least informative. Two people at 90 kg can have completely different body compositions — one at 15% body fat with significant muscle mass, the other at 35% body fat with much less. Their health risks, caloric needs, protein targets, and sprint parameters are entirely different.
Your Fat Loss Sprint is built on two numbers: body fat percentage and lean body mass (LBM). These determine everything. Scale weight is a secondary signal.
The Two Numbers That Drive Your Sprint
Body Fat Percentage is fat mass expressed as a share of total body weight:
Body Fat % = (Fat Mass ÷ Total Body Weight) × 100
Lean Body Mass is everything that isn't fat: skeletal muscle, bone, organs, connective tissue, and water. You calculate it like this:
LBM = Total Body Weight × (1 − Body Fat % ÷ 100)
Example: 90 kg bodyweight at 30% body fat → LBM = 90 × 0.70 = 63 kg
Your protein targets are calculated on LBM. Your sprint duration, refeed timing, and refeed carbohydrates are set by your body fat percentage. Without an accurate body fat measurement, you cannot correctly set up your sprint.
Your FLS Sprint Level
Every protocol parameter is determined by your FLS Sprint Level — a 3-tier classification based on your measured body fat percentage at the start of your sprint. It does not change during the sprint.
| FLS Sprint Level | Male Body Fat % | Female Body Fat % |
|---|---|---|
| 1 — Lean | < 16% | < 25% |
| 2 — Moderate | 16–25% | 25–35% |
| 3 — Overweight | > 25% | > 35% |
What Your Sprint Level Sets
| Protocol Parameter | Sprint Level 1 (Lean) | Sprint Level 2 (Moderate) | Sprint Level 3 (Overweight) |
|---|---|---|---|
| Protein base | 3.0 g/kg LBM | 2.7 g/kg LBM | 2.2 g/kg LBM |
| Dietary fat | 30 g/day | 27 g/day | 25 g/day |
| Sprint duration | 14 days | 21 days | 28 days |
| Mid-program refeed | Day 7 | Day 10 | Day 14 |
| Final day refeed | Day 14 | Day 21 | Day 28 |
| Refeed carbs | 7 g/kg LBM | 5 g/kg LBM | 3 g/kg LBM |
Why Leaner People Need More Protein Per Kg
This is counterintuitive but physiologically sound. The leaner you are, the less absolute fat you carry as available fuel. That means your muscle protein becomes a larger proportion of available energy substrate during restriction. Sprint Level 1 participants need more protein per kg of LBM to protect against lean mass loss. Sprint Level 3 participants have greater fat reserves to draw on, making extreme protein density less critical — though targets still sit well above conventional dieting recommendations.
Minimum Eligibility
Not everyone should start a Fat Loss Sprint. These minimums apply:
| Sex | Minimum Body Fat % |
|---|---|
| Male | ≥ 12% |
| Female | ≥ 18% |
Below these thresholds, the risks of lean mass catabolism, hormonal disruption, and metabolic complications are too high. See the eligibility guidelines for the full criteria.
Body Fat Reference Ranges
Males
| Category | Body Fat % |
|---|---|
| Essential fat | 2–5% |
| Athletic | 6–13% |
| Fitness | 14–17% |
| Average | 18–24% |
| Obese | >25% |
Females
| Category | Body Fat % |
|---|---|
| Essential fat | 10–13% |
| Athletic | 14–20% |
| Fitness | 21–24% |
| Average | 25–31% |
| Obese | >32% |
Based on American Council on Exercise and ACSM guidelines (ACSM, 2021). Women carry approximately 8–12% more essential fat than men for reproductive and hormonal function. The Sprint Level thresholds reflect this difference.
How to Measure Your Body Fat
The right method depends on your body type.
If You're Athletic or Have Above-Average Muscle Mass
Standard equations were developed on typical body compositions. They systematically over- or underestimate body fat in muscular individuals. Use one of these instead:
1. DEXA Scan (most accurate) Dual-energy X-ray absorptiometry uses low-dose X-ray beams to differentiate bone, lean tissue, and fat. Clinical gold standard (Shepherd et al., 2017). Accuracy: ±1–2% body fat. Cost: $50–200 per scan. Provides regional data including visceral fat and limb-specific lean mass.
2. Skinfold Calipers Measures subcutaneous fat directly at specific body sites. When performed by an experienced technician: ±2–3% accuracy in athletic populations (Jackson & Pollock, 1978). Inexpensive. Highly dependent on technician skill.
3. Medical-Grade BIA (multi-frequency) Devices like the InBody 770 or Tanita MC-980 use multiple frequencies and electrode contacts. More accurate than consumer scales. Still affected by hydration — test under consistent conditions. Less accurate than DEXA or skilled skinfolds.
4. Visual Comparison Comparing your physique to validated body fat reference photos. Useful for rough categorization. Not sufficient as a sole method for protocol setup.
If You're from the General Population (Non-Athlete)
US Navy Circumference Method Uses neck, waist, and hip circumference plus height. Validated on general population samples (Hodgdon & Beckett, 1984). Accuracy: ±3–5%. Free, self-administered, and reproducible for trend tracking.
For Men:
Body Fat % = 86.010 × log₁₀(waist − neck) − 70.041 × log₁₀(height) + 36.76
For Women:
Body Fat % = 163.205 × log₁₀(waist + hip − neck) − 97.684 × log₁₀(height) − 78.387
All measurements in centimetres. Measure waist at the navel, neck below the larynx, hips at the widest point (women only).
If you strength train regularly and carry above-average muscle mass, do not use the Navy method. Its circumference-based formula does not account for muscle tissue and will distort your estimate. Use DEXA, skinfolds, or medical-grade BIA instead.
Worked Example: Setting Up a Sprint
Alex's measurements:
- Male, age 35
- Weight: 95 kg
- Height: 178 cm
- Waist: 97 cm, Neck: 40 cm
Step 1: Estimate body fat (Navy Method)
Body Fat % = 86.010 × log₁₀(57) − 70.041 × log₁₀(178) + 36.76 = 151.04 − 157.62 + 36.76 = 30.2%
Step 2: Assign Sprint Level Male at 30.2% body fat → exceeds 25% threshold → Sprint Level 3 — Overweight
Step 3: Calculate LBM and Fat Mass
LBM = 95 × (1 − 0.302) = 66.3 kg Fat Mass = 95 − 66.3 = 28.7 kg
Step 4: Set Protein (Sprint Level 3 base = 2.2 g/kg LBM)
2.2 × 66.3 = 146 g/day (584 kcal)
Step 5: Set Dietary Fat (Sprint Level 3 = 25 g/day)
25 g/day (225 kcal)
Step 6: Account for Trace Carbohydrates Net carbs come from non-starchy vegetables, eaten freely. Fibre is not counted. Target is 50 g net per day. Cruciferous and leafy vegetables are mostly fibre, so generous portions still land inside that range, and the app allows up to ~70 g before flagging over so veg-heavy days stay in range.
Estimate: 30–50 g net carbs (120–200 kcal)
Step 7: Total Daily Calories
584 + 225 + 120–200 (vegetables) = ~929–1,009 kcal/day 929 kcal > 800 kcal floor ✓
Step 8: Sprint Duration and Refeeds
Sprint Level 3 = 28 days Refeeds: Day 14 + Day 28 Refeed carbs: 3 g/kg LBM × 66.3 = ~199 g (both refeeds)
Step 9: Estimated Weekly Fat Loss
BMR (Mifflin-St Jeor) = 10 × 95 + 6.25 × 178 − 5 × 35 − 5 = 1,882 kcal TDEE = 1,882 × 1.55 = 2,917 kcal (FLS activity multiplier is fixed at 1.55 for all users — 2 strength sessions + 7,000–10,000 steps/day) Daily deficit: 2,917 − 929 = 1,988 kcal Weekly deficit: 13,916 kcal → ~1.8 kg fat/week
Over 28 days, Alex could expect to lose approximately 7 kg of fat, plus additional water and glycogen for a total scale reduction of roughly 10–12 kg.
Alex's Sprint Summary
| Parameter | Value |
|---|---|
| Starting weight | 95 kg |
| Body fat % | 30.2% |
| FLS Sprint Level | 3 — Overweight |
| LBM | 66.3 kg |
| Fat mass | 28.7 kg |
| Protein target | 146 g/day |
| Dietary fat | 25 g/day |
| Net carbs (vegetables) | 50 g/day target (~70 g app cap) |
| Total calories | ~929–1,009 kcal/day (varies with vegetable intake) |
| Sprint duration | 28 days |
| Refeeds | Day 14 + Day 28 |
| Refeed carbs | ~199 g (3 g/kg LBM) |
| Estimated weekly fat loss | ~1.8 kg |
Tracking Progress During Your Sprint
Body fat percentage provides more useful data than scale weight alone. Use this monitoring approach:
- Daily scale weight — fluctuations of 1–2 kg are normal day to day
- Weekly average weight — the mean of 7 daily weigh-ins reveals the real trend
- Waist circumference — measured weekly at the navel, first thing in the morning
- Optional: Progress photos — front, side, and back, weekly, consistent lighting
- Optional: Body fat reassessment — using the same method as baseline, every 2–4 weeks
Waist circumference often decreases faster than overall body weight during a sprint. This reflects preferential mobilization of visceral fat and is a reliable signal that the protocol is working.
Key References
- Romero-Corral et al. (2008): Body fat percentage predicts cardiovascular risk better than BMI. Normal-weight individuals with high body fat carry significantly elevated metabolic risk.
- Heymsfield et al. (2015): The two-compartment model (fat mass + lean mass) is sufficient for clinical body composition assessment and dietary planning.
- Shepherd et al. (2017): DEXA provides the most accurate and reproducible body composition data, with regional analysis including visceral fat and limb-specific lean mass.