Background The maintenance of phosphate and calcium homeostasis alongside optimal vitamin D status is essential for bone health. Moreover, low vitamin D is associated with an increased risk of stress fracture, impaired muscular remodelling and increased risk of upper respiratory tract infections.

Objective The primary objective was to assess the efficacy of an individualised in-season vitamin D supplementation programme. The secondary aim was to observe for regression of vitamin D levels during the longer daylight months (April-October) in players where vitamin D was not routinely supplemented.

Design Serum 25 Hydroxyvitamin D (Serum 25[OH]D) levels were collected twice a year (October and April). Levels were categorised as: deficient (<30 nmol/l), insufficient (31–75), sufficient (76- 175) or overloaded (>175). Players with sufficient levels were given a maintenance protocol (20,000iu once weekly) October-April. Players who were deficient/insufficient were given a loading regimen (20,000iu twice weekly for three weeks) and the maintenance protocol thereafter.

Setting A category one Premier League Elite Player Performance Plan (EPPP) youth football academy.

Participants 13 participants from the u23s and u18s squads.

Interventions Serum 25[OH]D levels were collected (via venous blood sample) twice a year (October and April). Levels were categorised as: deficient <30nmol/l, insufficient 31–75nmol/l, sufficient 76–175nmol/l or overloaded >175nmol/l.

Players with sufficient or insufficient levels were given a maintenance protocol (20,000IU once weekly) during October-April. Players who were deficient were given a loading regimen (20,000iu twice weekly for three weeks) and the maintenance protocol for the remainder of winter (22 weeks). Oral supplementation was administered using Synergy Biologics LTD Pro D3 Sport 20k (UK).

Results There was no significant increase in serum 25[OH]D during the supplementation period (October-April). However, 20,000iu of supplemental Vitamin D over a 25-week period was sufficient to maintain vitamin D status. Baseline serum 25[OH]D was not significantly increased after a period of supplemental vitamin D during winter months. Further studies in this population group would be beneficial to assess the effects of longer term season-on-season supplementation.