Eating less protein at unchanged calories lowers the growth signal; short feeding studies show fat loss and better blood sugar and blood pressure in people with metabolic problems. The only hard outcome it changes is advanced kidney disease. Muscle loss, frailty and higher death rates at low intakes narrow the window to middle age. (Full Review)
| Marker | Target | Why |
|---|---|---|
| Serum albumin | 4.0–5.0 g/dL | Detects drift from signalling into malnutrition |
| Prealbumin (transthyretin) | 20–40 mg/dL | Earliest protein-status marker |
| IGF-1 | Lower-to-middle third of age-adjusted range | The growth signal the intervention targets |
| Blood urea nitrogen | 10–16 mg/dL | Objective check that protein intake actually fell |
| Estimated glomerular filtration rate | >90 mL/min/1.73 m² | Identifies who has a kidney indication and who does not |
| Fasting insulin | 2–5 µIU/mL | Tracks the insulin sensitivity gain seen in trials |
| Glycated haemoglobin | 4.8–5.4% | Confirms metabolic benefit is durable, not noise |
| Appendicular skeletal muscle index | >7.0 kg/m² men, >5.5 kg/m² women | The primary safety endpoint of this intervention |
| Grip strength | >35 kg men, >20 kg women | Cheap functional cross-check on muscle quality |
| Vitamin B12 | 500–1000 pg/mL | Falls when animal protein is removed |
| Ferritin | 50–150 ng/mL | Iron stores fall when heme sources are cut |
| Homocysteine | <8 µmol/L | Reflects methionine cycle handling at low sulfur amino acid intake |
Cadence: Albumin, prealbumin, blood urea nitrogen at 4 weeks, then quarterly; IGF-1 and metabolic markers at 12 weeks; body composition and grip strength every 6 months