How Quickly Will You Lose Weight?

This content is for informational purposes only and does not constitute medical or nutritional advice. Speak with your health professional before starting this protocol.

Results disclaimer: Weight loss projections in this article are estimates based on calculated caloric deficits. Actual results vary based on adherence, water retention, activity level, hormonal factors, and individual metabolic variation. Do not use projected figures as a measure of success or failure on any given day or week.

What the Scale Is Actually Telling You

Weight loss on the scale is not a single thing. It is composed of multiple components that shift at different rates. Understanding what you are losing — and when — prevents frustration and helps you interpret your results accurately.

The Five Components of Scale Weight Change

Fat mass. This is the target. Fat tissue contains approximately 7,700 kcal per kilogram. The energy deficit you create determines how much fat you lose per day. A typical sprint deficit of 1,500–2,500 kcal/day produces approximately 0.2–0.3 kg of fat loss per day, or 1.4–2.3 kg per week.

Water. Water loss can be dramatic in the first 1–2 weeks. Glycogen depletion, reduced sodium intake, and the diuretic effect of ketosis can produce 2–5 kg of water loss in the first week alone. This is not fat, but it produces real changes in scale weight, appearance, and how clothes fit.

Glycogen. Your body stores roughly 400–600 grams of glycogen in muscles and liver. This depletes within the first 2–3 days of the sprint, accounting for 0.4–0.6 kg of weight loss plus an additional 1.2–2.4 kg of associated water.

Gut contents. Eating significantly less food means less food in your digestive tract at any time. This can account for 0.5–1.5 kg of apparent weight loss in the first few days.

Lean body mass. Some lean mass loss is physiologically unavoidable during severe restriction. The sprint protocol is specifically designed to minimise this through high protein intake and resistance training. Under optimal conditions, lean mass should represent less than 25% of total weight loss.

How Weight Loss Unfolds Over Time

Multi-Sprint Cycle Projections. The FLS has three sprint durations: 14 days (Level 1), 21 days (Level 2), and 28 days (Level 3). The projections below for weeks 5 and beyond represent results from consecutive sprint cycles with mandatory diet breaks — not a single continuous sprint.

Expected Rates by Starting Profile

The size of your daily energy deficit is the single strongest predictor of how fast you lose fat. This is shaped by your total daily energy expenditure, your starting weight and body fat, your sex, your activity level, and how much metabolic adaptation has occurred.

Note on the tables below: "Weeks 3–8" and "Weeks 9–16" columns represent results across consecutive sprint cycles with mandatory diet breaks between each — not a single continuous sprint. A single FLS lasts 14 days (Level 1), 21 days (Level 2), or 28 days (Level 3) at maximum.

Males with Obesity (BMI 35 or above, body fat above 30%)

Expected across multiple sprint cycles (12 weeks total, with diet breaks): 20–30 kg total scale weight, 15–22 kg fat mass.

Females with Obesity (BMI 35 or above, body fat above 40%)

Expected across multiple sprint cycles (12 weeks total, with diet breaks): 15–24 kg total scale weight, 11–17 kg fat mass.

Males with Moderate Overweight (BMI 27–35, body fat 20–30%)

Expected across multiple sprint cycles (12 weeks total, with diet breaks): 14–20 kg total scale weight, 10–16 kg fat mass.

Females with Moderate Overweight (BMI 27–35, body fat 30–40%)

Expected across multiple sprint cycles (12 weeks total, with diet breaks): 10–16 kg total scale weight, 8–13 kg fat mass.

What You Lose Matters as Much as How Much You Lose

The ratio of fat to lean mass lost is the critical metric — not total weight on the scale.

The Forbes Curve: Starting Body Fat Determines Lean Mass Risk

Gilbert Forbes' foundational research established that the ratio of fat to lean mass lost is primarily determined by starting body fat percentage. The more fat you have, the more your body preferentially mobilises fat stores.

(Forbes, 2000, Annals of the NYAS)

As body fat decreases, the body becomes increasingly reluctant to mobilise fat and more prone to catabolising lean tissue — unless protective measures are in place.

How the Sprint Protocol Shifts the Ratio

High protein intake and resistance training significantly improve the fat-to-lean ratio beyond what unprotected caloric restriction produces:

Standard VLCD (low protein, no resistance training):

• 56–75% of loss from fat
• 25–44% from lean mass

Sprint protocol (high protein plus resistance training):

• 75–90% of loss from fat
• 10–25% from lean mass

Optimal adherence (obese individual, high protein, consistent training):

• 85–100% of loss from fat
• 0–15% from lean mass

One study found that a VLCD combined with resistance training produced zero measurable lean mass loss over 12 weeks, while a VLCD-only group lost 4.6 kg of lean mass. (Donnelly et al., 1993, Am J Clin Nutr)

The sprint is specifically designed to beat the population average. Heymsfield et al. (2014) reviewed the commonly cited "one-quarter rule" (that 25% of weight loss is lean mass) and confirmed this represents an average across all dieting conditions — and that protein intake, exercise, and starting body composition dramatically shift the actual ratio. (Heymsfield et al., 2014, Obesity Reviews)

Managing Your Expectations Through the Sprint

Week 1 Losses Are Not All Fat

If you lose 5 kg in your first week, approximately 2–3 kg is water, glycogen, and gut contents. The remaining 1.5–2.5 kg is genuine fat loss. This is still an excellent result. Expecting 5 kg of fat loss every single week will lead to frustration.

Plateaus Are Usually Water, Not Failure

Weight stalls lasting 1–3 weeks despite consistent adherence are common. They are almost always caused by water retention masking continued fat loss. Common triggers include:

• Elevated cortisol from stress or poor sleep
• Luteal phase water retention in women
• Increased sodium intake
• Inflammation from beginning a new exercise programme
• Post-refeed water retention from glycogen replenishment

If you are maintaining your targets, you are losing fat regardless of what the scale shows that day.

The "Whoosh" Pattern

Many people experience weight stalling followed by a sudden drop. The likely explanation: adipocytes (fat cells) that have released their stored triglycerides temporarily fill with water before that water is also released. The practical experience is well documented even if the exact mechanism is still debated.

Better Than the Scale Alone

Use multiple metrics. No single measurement tells the complete story.

Worked Examples

Mark: 42-year-old male, 118 kg, 175 cm, BMI 38.6, estimated 35% body fat. Three consecutive sprint cycles (28 days each, with a 14-day diet break between each), with one diet break shown at weeks 6–7.

• Weeks 1–2: 7–10 kg total (3–4 kg fat, remainder water and glycogen)
• Weeks 3–6: 2.0–2.5 kg/week (mostly fat) = 8–10 kg
• Week 6–7: Diet break (weight stabilises or rises 1–2 kg from water and glycogen)
• Weeks 8–12: 1.5–2.0 kg/week = 7.5–10 kg
• Total scale loss: approximately 22–28 kg
• Estimated fat mass lost: 16–20 kg
• Estimated lean mass lost: 1–3 kg with proper protein and training
• Ending weight: approximately 90–96 kg, BMI 29–31, body fat 25–28%

Sarah: 35-year-old female, 92 kg, 165 cm, BMI 33.8, estimated 42% body fat. Two sprint cycles with a diet break in between (diet break shown at weeks 5–6).

• Weeks 1–2: 4–6 kg total (2–3 kg fat, remainder water and glycogen)
• Weeks 3–5: 1.5–2.0 kg/week = 4.5–6 kg
• Week 5–6: Diet break (weight stabilises or rises 1–2 kg)
• Weeks 7–10: 1.0–1.5 kg/week = 4–6 kg
• Total scale loss: approximately 12–16 kg
• Estimated fat mass lost: 9–13 kg
• Estimated lean mass lost: 0.5–2 kg with proper protein and training
• Ending weight: approximately 76–80 kg, BMI 28–29, body fat 34–37%

James: 28-year-old male, 98 kg, 180 cm, BMI 30.2, estimated 25% body fat, experienced with resistance training. 8-week sprint with refeeds every 5 days.

• Weeks 1–2: 5–7 kg total (2–3 kg fat, remainder water and glycogen)
• Weeks 3–8: 1.2–1.8 kg/week = 7–11 kg
• Total scale loss: approximately 12–18 kg
• Estimated fat mass lost: 8–12 kg
• Estimated lean mass lost: 1–2 kg with high protein and experienced training
• Ending weight: approximately 80–86 kg, BMI 25–27, body fat 17–20%

When Results Are Below Expectations

Calories are higher than calculated. The most common cause. Cooking oils, condiments, beverages, and underestimated portions add up. Weigh and track everything meticulously for one week.

Water retention is masking fat loss. If you are adherent but the scale is stalled, fat loss is likely continuing underneath fluid retention. Check waist circumference. Keep going. The drop usually comes.

Metabolic adaptation. After 6–8 weeks, adaptive thermogenesis can reduce your effective deficit by 100–300 kcal/day. A structured diet break partially reverses this, then you resume the sprint.

Reduced NEAT. Unconscious reduction in daily movement can meaningfully cut your total deficit. Track daily steps and maintain a minimum of 8,000–10,000.

Medical factors. Thyroid dysfunction, cortisol disorders, or medication effects (corticosteroids, certain antidepressants, insulin) can slow results. If loss is consistently below expectation despite verified adherence, seek medical evaluation.

References

1. Forbes, G. B. (2000). Body fat content influences the body composition response to nutrition and exercise. Annals of the New York Academy of Sciences, 904(1), 359–365. DOI: 10.1111/j.1749-6632.2000.tb06482.x

2. Heymsfield, S. B., Thomas, D., Nguyen, A. M., et al. (2011). Voluntary weight loss: systematic review of early phase body composition changes. Obesity Reviews, 12(5), e348–e361. DOI: 10.1111/j.1467-789X.2010.00767.x

3. Heymsfield, S. B., Gonzalez, M. C., Shen, W., et al. (2014). Weight loss composition is one-fourth fat-free mass: a critical review and critique of this widely cited rule. Obesity Reviews, 15(4), 310–321. DOI: 10.1111/obr.12143

4. Donnelly, J. E., Sharp, T., Houmard, J., et al. (1993). Muscle hypertrophy with large-scale weight loss and resistance training. American Journal of Clinical Nutrition, 58(4), 561–565. DOI: 10.1093/ajcn/58.4.561

5. Chaston, T. B., Dixon, J. B., & O'Brien, P. E. (2007). Changes in fat-free mass during significant weight loss: a systematic review. International Journal of Obesity, 31(5), 743–750. DOI: 10.1038/sj.ijo.0803483

6. Hall, K. D. (2008). What is the required energy deficit per unit weight loss? International Journal of Obesity, 32(3), 573–576. DOI: 10.1038/sj.ijo.0803720

7. Kreitzman, S. N., Coxon, A. Y., & Szaz, K. F. (1992). Glycogen storage: illusions of easy weight loss, excessive weight regain, and distortions in estimates of body composition. American Journal of Clinical Nutrition, 56(1 Suppl), 292S–293S. DOI: 10.1093/ajcn/56.1.292S

8. Tsai, A. G., & Wadden, T. A. (2006). The evolution of very-low-calorie diets: an update and meta-analysis. Obesity, 14(8), 1283–1293. DOI: 10.1038/oby.2006.146

9. Willoughby, D., Hewlings, S., & Kalman, D. (2018). Body composition changes in weight loss: strategies and supplementation for maintaining lean body mass, a brief review. Nutrients, 10(12), 1876. DOI: 10.3390/nu10121876