Circulating IGF-I is associated with fitness and health outcomes in a population of 846 young healthy men.

CONTEXT: Insulin-like growth factor-I (IGF-I) is thought to mediate many of the beneficial outcomes of physical activity. While IGF-I has previously been shown to be positively related with aerobic fitness, few studies have examined IGF-I relationships with other fitness and health parameters. The robustness of IGF-I as a biomarker of fitness and health has yet to be fully determined.

OBJECTIVE: To determine the association of circulating IGF-I with fitness, body composition and health parameters in young, healthy men.

DESIGN AND SUBJECTS: A cross-section of 846 young, healthy Finnish men (25±5 yr, 180±6 cm, 81±13 kg). Subjects were divided into quintiles of IGF-I concentrations (Q1: lowest; Q5: highest) for statistical evaluation.

MAIN OUTCOME MEASURES: Circulating IGF-I, physical fitness: peak aerobic capacity (VO(2) peak), maximal strength of leg and arm extensors, muscle endurance (sit-ups, push-ups, and repetitive squats) and health outcome parameters (total blood cholesterol, triglyceride, high-density lipoproteins (HDL), low-density lipoproteins (LDL), systolic and diastolic blood pressure, waist circumference, % body fat, and drinking, smoking and physical activity behavior).

RESULTS: Higher IGF-I was associated with higher VO(2) peak (Q1: 39±7 vs. Q5: 44±9 mL/kg/min), sit-ups (Q1: 35±10 vs. Q5: 41±10 repetitions), push-ups (Q1: 27±13 vs. Q5: 31±14 repetitions), repetitive squats (Q2: 42±10 vs. Q5: 45±8 repetitions), HDL (Q1: 1.5±0.4 vs. Q5: 1.53±0.3 mmol/L), and lower age (Q1: 28±6 vs. Q5: 23±2 yr), %BF (Q1: 20±7 vs. Q5: 16±6%BF), waist circumference (Q1: 89±11 vs. Q5: 84±9 cm), BMI (Q1: 25.6±4 vs. Q5: 24.3m(2)/kg), diastolic blood pressure (Q1: 78.5±9 vs. Q5: 75.4±8 mm Hg), cholesterol (Q1: 4.72±0.9 vs. Q5: 4.44±0.8 mmol/L) and smoking (Q1: 44% vs. Q5: 32%). No association was observed for IGF-I and maximal leg extension (Q1: 2982±927 vs. Q5: 2932±853 N) and bench press (Q1: 895±197 vs. Q5: 919±203 N) strength, fat-free mass (Q1: 64.6±8 vs. Q5: 66.6±7 KG), LDL (2.54±0.7 vs. Q5: 2.35±0.6 mmol/L), or triglycerides (Q1: 1.05±0.6 vs. Q5: 0.99±0.5 mmol/L).

CONCLUSION: IGF-I is positively associated with aerobic fitness and muscular endurance, but not with measures of muscle strength or FFM. IGF-I is positively associated with improved health and fitness outcomes in young, healthy men.