How fast can you safely lose fat? Here's what the research says.

The conventional answer — "no more than 0.5–1 kg per week" — is based on the assumption that faster loss necessarily causes muscle loss, metabolic damage, and rapid rebound. The research does not support this assumption when the protocol is properly designed.

1. Protein adequacy — if protein is above the lean mass preservation threshold, speed of loss has minimal effect on muscle retention (Longland et al., 2016; Mettler et al., 2010) 2. Resistance training — provides the mechanical stimulus to maintain muscle independent of deficit depth 3. Electrolyte supplementation — addresses the primary safety concern with very low calorie intake (the 1970s cardiac deaths were caused by potassium depletion, not caloric restriction per se) 4. Defined duration — limits the period of severe restriction, preventing the complications of indefinite VLCDs 5. Structured recovery — mandatory maintenance phase prevents the rapid rebound that characterises unstructured crash dieting

The Fat Loss Sprint includes all five. This is what separates it from a crash diet at the same calorie level.

2–5 kg (water, glycogen, gut contents — expected and real, not pure fat) 1.5–2.5 kg/week (predominantly fat, at Sprint Level 2–3) 75–90% of weight lost comes from fat

Palgi et al. (1985): In 668 patients on a supervised PSMF, mean weight loss was 18.6 kg over 17 weeks — approximately 1.1 kg per week of sustained loss, consistent with the FLS trajectory.

Modern PSMF/VLCD protocols, when properly designed, have a documented safety profile:

• Zero deaths documented in supervised protocols using complete protein and electrolyte supplementation (Seim & Flanagan, 1984; Wadden et al., 1990)
• Common side effects (first week): mild headache, fatigue, constipation — transient, electrolyte-managed
• Endorsed by: NICE (UK), EASO, AHA/ACC
• Contraindicated in pregnancy, breastfeeding, type 1 diabetes, active eating disorders, severe organ disease

The 1970s liquid protein deaths (Isner et al., 1979) were caused by collagen hydrolysate (incomplete protein) plus zero electrolyte supplementation — not by caloric restriction itself. The FLS bears no resemblance to those products.