Soldiers undertaking the arduous tactical phase of the Section Commanders Battle Course (TAC SCBC) are shown to be in energy deficit (Richmond et al., 2010, Med Sci Sports Exerc, 42, S287). An energy deficit of >1000 kcal·d-1 blunts the positive anabolic adaptations to heavy training by decreasing circulating insulin-like growth factor (IGF) -1 and IGF binding protein (IGF BP) -3 (Nindl et al., 2003, J Appl Physiol, 95, 1083-89). PURPOSE: To investigate the effect of a mixed nutritional supplement on the IGF axis in soldiers undergoing TAC SCBC. METHODS: Thirty males (age 25 ± 3 yr; body mass 80.9 ± 7.7 kg; body fat 15.4 ± 3.3 %) undertaking the 8-week TAC SCBC consumed a habitual diet alone (CON, n = 15) or received an additional food supplement (SUP, n = 15). All soldiers were given ∼3,550 kcal·d-1 during in-barrack feeding between wk 0 and 6, and 4,242 kcal·d-1 during a final 2-wk field exercise (Field EX). SUP were provided an additional 1,350 kcal·d-1 during wk 0-6 and 850 kcal·d-1 during Field EX. The macronutrient breakdown of the supplement was ∼45% carbohydrate, ∼40% fat and ∼15% protein. Measures of body composition (DEXA) and resting blood samples were collected at 0530h at baseline (wk 0) and at the end of TAC SCBC (wk 8). Physical activity was monitored by accelerometry (3DNX, Biotel) during Field EX. RESULTS: There were no between group differences in baseline demographics or physical activity during Field EX. The loss in body mass (CON 5.0 ± 2.3; SUP 1.6 ± 1.5 kg: P < 0.001), lean mass (CON 2.1 ± 1.5; SUP 0.7 ± 1.5 kg: P < 0.05) and fat mass (CON 3.0 ± 1.6; SUP 0.9 ± 1.8 kg: P < 0.01) over the 8 wk period was greater in CON. Training reduced circulating IGF-1 (28%), testosterone (19%) the testosterone: cortisol ratio (25%) and increased sex hormone binding globulin (38%) (P < 0.01) with no effect of SUP. However, there was a significant decrease in IGF BP-3 in CON (wk 0, 2.3 ± 0.3; wk 8, 1.8 ± 0.4 μg·ml-1; P < 0.05) that was abolished in SUP (wk 0, 2.2 ± 0.5; wk 8, 2.1 ± 0.4 μg·ml-1). There were no changes in circulating insulin, cortisol, IGF BP-1, triiodothyronine or thyroxine. CONCLUSION: A mixed nutritional supplement did not blunt the unfavourable decreases in IGF-1 or testosterone concentration during 8 wks of arduous military training, but did prevent the decrease in IGF BP-3 concentration. Sponsored by UK Ministry of Defence (Army).