Theodosis champions a type of intermittent fasting, known as the “5:2” diet. Twice a week, he eats 600 calories, only a quarter of his daily requirements. On other days, he eats normally.

“One of the reasons the 5:2 diet seems to work is you don’t need to live a miserable existence eating nothing but rabbit food every day,” explains the Australian, who moved to Hong Kong seven years ago. “Yes, you need to be restrictive on ‘fasting days’, but can sleep knowing a latte and cream cheese bagel is waiting for you in the morning.”

Research also suggests period-fasting has metabolic benefits, he says.

Theodosis wishes he’d known the impact of trimming down when he started trail running four years ago. “Running when you are heavy not only requires more effort and increases your risk of injury, it’s simply not as much fun.”

His running improvement has even sling-shotted him, unknowingly, into competitive road running after a personal best in the 2016 Tokyo Marathon of 2:44:04. While he will head back to the Tokyo marathon next year to “see what I can do”, he feels most at home on the city’s trails.

“I have a huge amount of respect for road running, especially the marathon, but it just can’t compete with the stimulation that comes from running trails,” he says.

“Even trails I’ve run dozens of times continue to surprise me in ways road running can’t – stumbling across a wild boar or aggressive snake when you least expect it.”

What do you love most about trail running?

Four things: the people, the mountains, the races, and the analytics. I spent the first half of my career as a mechanical engineer responsible for statistically driven problem solving. When you run and race regularly, you are always analysing data and solving problems, like your nutrition and hydration, stomach issues, racing tactics, gear, injuries and so on. For me, optimising this giant puzzle is fascinating, particularly during a tough race when you struggle to think straight.

Why the 5:2 diet?

In March 2015, despite taking part in numerous trail races, I was far from competitive. I realised losing weight would be the best way to improve my running economy. As best I can tell, there genuinely is no “miracle” diet; it comes down to calories consumed versus calories burned. After researching I concluded the intermittent fasting in the 5:2 would enable me to restrict my calorie intake without feeling hungry all the time.

Was it easy to lose weight?

It was relatively easy, although initially it meant missing out on the occasional social dinner. That said, I strongly believe good food is one of life’s pleasures; fasting actually allows you to enjoy good food and manage your weight. The 5:2 diet also made me very conscious of how calorie dense different foods are.

Does your diet impact your training?

At first I didn’t have much energy on fasting days, but after about three weeks I adjusted. I suspect I have become more efficient at metabolising fat for energy as I was eventually able to do tempo runs [a type of fast-paced workout] on fasting days.

You run during lunch and often run home from work. What does it bring to your working life?

By breaking up my day, it gives me time to reflect on my approach to important projects or retrospectively analyse meetings. It also helps smooth out the mid-afternoon energy slump I used to treat with coffee.

Any other secret weapon in your training arsenal, like supplements?

I used to take a daily multivitamin, and still take an omega-3 supplement to compensate for my seafood allergy, but I’ve cut out everything else. Scientific consensus seems clear that, assuming you eat a balanced diet, and do not have a particular dietary deficiency, supplements only make for expensive urine. I find being at my body’s optimal weight and getting adequate sleep are the most effective steps I can take for adequate recovery and to avoid injury.

Apart from being slimmer, what else have you learnt over the years that you wish you’d known when you started running?

The importance of recovery – getting enough sleep and taking days off, or just slowing down, when I wasn’t feeling up to a hard run. I think the most significant difference between a competitive amateur runner versus a professional is not their mileage or intensity, but their focus on genuine recovery and reducing external stressors. When you are recovered from your last training session, you are better prepared to get the most out of your next session instead of always being compromised.