Long-term effects of calorie or protein restriction on serum IGF-1 and IGFBP-3 concentration in humans.
Level 3 - non-randomized controlled study
Non-randomized comparative cohort studies and an uncontrolled before-and-after dietary intervention.
PubMed 18843793 · doi:10.1111/j.1474-9726.2008.00417.x
What was done
Evaluated serum IGF-1 concentrations and IGF-1:IGFBP-3 ratios across two long-term calorie restriction (CR) studies (1 and 6 years) in humans practicing severe CR without malnutrition. In addition, total and free IGF-1 were examined in moderately protein-restricted individuals, and a 3-week intervention lowering protein intake from 1.67 g/kg/day to 0.95 g/kg/day was conducted in 6 volunteers already practicing CR.
What was found
Severe long-term CR for 1 and 6 years did not change serum IGF-1 levels or the IGF-1:IGFBP-3 ratio (no numerical values reported in the abstract). Moderately protein-restricted individuals had significantly lower total and free IGF-1 concentrations. In the 6 CR volunteers, reducing protein intake for 3 weeks lowered mean serum IGF-1 from 194 ng/mL to 152 ng/mL.
Why it matters
Unlike rodent models where calorie restriction robustly lowers IGF-1, circulating IGF-1 in humans appears largely unresponsive to calorie restriction alone unless protein intake is concurrently reduced.
Limits
The abstract does not report sample sizes, baseline demographics, or quantitative data for the 1- and 6-year CR cohorts. The prospective protein restriction trial was very small (n=6), short (3 weeks), and uncontrolled. Clinical endpoints such as disease incidence or lifespan were not measured.
Cited by
- supports In human caloric restriction studies by Fontana and colleagues, calorie restriction alone did not reduce IGF-1 levels until dietary protein was also restricted.