DESPITE a bad stomach upset and diarrhoea, Mr Lim Nghee Huat is soldiering on, along with fellow ultramarathoner Yong Yuen Cheng, who has hurt his foot.
The duo are attempting to run 50km for 50 days in a row to celebrate Singapore's 50th birthday, and pay tribute to its founding fathers and pioneers.
They have made it halfway, but are also paying a price for the pounding to their bodies.
For Mr Lim, 61, symptoms appeared on Saturday when he vomited, and had to rush to the toilet several times along the route.
Things had improved yesterday but he still felt nauseous.
"I felt unwell, but giving up was never on my mind," he said.
At the end of their run yesterday, the two men had each clocked 1,250 km. They finished yesterday's run in 8hr 20min.
Dr Jason Lee, head physiologist at DSO National Laboratories, which studies military endurance, explained that constant strenuous exercise can cause fatigue, weakening the runners' immune systems. This makes them prone to viral or bacterial infections.
He advised Mr Lim to slow down or walk throughout the journey, if necessary. "I am worried because for many endurance athletes, their minds are stronger than their bodies," added Dr Lee.
Mr Lim's wife Deborah, 56, a businesswoman, is supportive.
"We've seen similar conditions in previous ultramarathons, yet he completed them," she said.
"Once, he pulled out of an ultramarathon because of a leg injury, so that tells me he knows how to listen to his body."
Ultramarathons cover distances longer than the traditional marathon's 42.2km.
Both men are feeling the strain.
Mr Lim, an editor, struggled through his 50km on Saturday in almost 10 hours - the duo's longest run time in 25 days - and Mr Yong is nursing a strained Achilles' tendon.
Dr Lim Baoying of Changi Sports Medicine Centre warned that constant running could lead to a higher risk of strains and fractures.
Concurring, Dr Wang Mingchang of National University Hospital Sports Centre said: "Running 50km for 50 days may result in the body not getting enough rest to repair impact to the bones, tendons and muscles. This increases one's risk of over-use injuries, such as stress fractures."
Mr Yong, who is consulting senior sports physiotherapist Yeo Hwee Koon of Singapore Sports Institute, said he is coping well with special tape to support his injury.
"We are only halfway there, slow and steady wins the race." he said.
Click here for more stories and videos on the Go50 ultramarathon.