Responses of bone turnover markers to repeated endurance running in humansunder conditions of energy balance or energy restriction

Distance running in humans has been associated with both positive and negat ive effects on the balance of bone remodelling. There is evidence to sugges t that the negative effects may be linked to a failure to balance energy ex penditure with an adequate energy intake. Energy restriction is known to re duce the synthesis and serum concentration of insulin-like growth factor 1 (IGF-1), which plays an important role in bone Formation. The purpose of th e present study was to compare the effects of repeated periods of prolonged treadmill running, under conditions of either energy balance or energy res triction, on markers of bone turnover and serum IGF-1 concentration in trai ned distance runners. Eight male distance runners [mean age 25.1(SD 5.9) ye ars maximal oxygen uptake 61.8 (SD 4.9) ml . kg(-1) . min(-1)] undertook an exercise and diet regime on two separate occasions, 2 weeks apart. On each occasion they performed an intensive, 60 min treadmill run on 3 consecutiv e days. On one occasion their energy intake was restricted to approximately 50% of their estimated energy requirement (RES), whereas on the other occa sion they remained in energy balance (BAL). The N-terminal pro-peptide of t ype I collagen (P1NP), osteocalcin and IGF-1 were measured in serum collect ed between 0800 and 0900 hours, when fasted and rested, on the day before a nd the day after each regime. The cross-linked N-telopeptides of type 1 col lagen and deoxypyridinoline were measured from 34 h urine collections made on the day before and the final day of each regime and adjusted for creatin ine excretion. The results showed that the serum concentration of both P1NP and IGF-1 declined by 15% (P=0.008) and 17% (P = 0.007) respectively in re sponse to RES, but did not change in response to BAL (P > 0.05). A strong r elationship was observed between the magnitude of the reduction in the seru m concentration of PINP and IGF-1 after RES (r = 0.97; P < 0.001). There we re no changes in the other bone markers in response to either regime. The r esults suggested that in trained distance runners, repeated periods of prol onged running do not affect the balance of bone turnover unless energy bala nce is simultaneously altered. These findings support the link between a ne gative energy balance, a reduced synthesis or serum level of IGF-I and redu ced collagen synthesis. They may also help to explain the bone remodelling imbalance that has been observed in some male and female distance runners.