Background Acute illness during training for an endurance running event reduces the likelihood of completing the event. A greater understanding of the risk factors for acute illness during training may inform prevention strategies and advice.

Objective To describe training factors and acute medical conditions amongst participants in mass-participation community-based marathon events.

Design Observational questionnaire-based study.

Setting Two large UK city mass-participation marathon events.

Patients (or Participants) Entry to both events was open to novice runners, with no qualifying time for general entry. All registered participants were invited to complete an online questionnaire in the week preceding the event. 11809 runners completed the survey.

Interventions (or Assessment of Risk Factors) Demographics including age, gender, experience and training history.

Results The average age of respondents was 40.9y (range 18–83y) and 54.1% were male. 22.3% of respondents developed a new illness in the 4 weeks prior to the event. Upper respiratory tract infection (URTI) was most common (64.3%), followed by GI problems (15.4%) and headache/migraine (14.6%). 28.5% of respondents who had been training for <2 months developed an acute illness, compared with 19.8% of those trained for >6 months (p=0.0002). Lower average weekly training distance (22.9% of those training from <20 to 40 miles/week vs. 18.7% training from 40 to >50 miles/week; p<0.05) and shorter longest training run (24.4% whose longest run was <20 miles vs. 19.4% whose longest training run was >20 miles; p<0.05) were associated with higher incidence of acute illness. 25.0% of novice runners (running <1y) developed an acute illness compared to 20.3% of those who had been running >10y (p<0.05).

Conclusions Novice runners who train for <2 months with low average weekly training mileage were more likely to develop an acute illness during marathon training than more experienced runners. Further research is needed to establish the direction and relationship between these factors before guidance can be issued.