The food is called ProLon. It comes in small, neatly organised, white boxes. They contain vegetable-based soups, tiny energy bars, kale biscuits, herbal teas and an energy drink, together with a warning that the Fasting Mimicking Diet (FMD) is “not intended to diagnose, treat, cure, or prevent any disease”.

The meals were first supplied to me in 2013, free of charge as part of the clinical trial, by researchers at the University of Southern California. They are made by L-Nutra, a Los Angeles-based company founded by Longo. ProLon is not commercially available, although L-Nutra has plans to start selling it sometime in 2016. (For the purposes of full disclosure, Longo says he has no financial interest in the product. He has started a non-profit foundation called Create Cures, which, he says, will focus on providing fast interventions, “similar to fasting”, to treat conditions like cancer, Alzheimer’s disease and multiple sclerosis, for which there are currently no cures. Longo plans to donate all of his shares in L-Nutra to the foundation.)

The clinical trial was set up by Longo’s group to see whether people could cope with such a strict dietary regime. The laboratory-formulated food is designed to deliver maximum nutrition through minimum calories. Based on two decades of research, the idea is to produce the benefits of fasting without actually fasting.

The practice of fasting is an age-old tradition in many societies and religions. It leads to a vast array of changes, at a cellular level – including a drop in levels of IGF-1. In other words, it has a similar effect on my body to that of the genetic mutation that causes Laron syndrome.

But very few people are prepared to endure an extended period of time with little food, even if it is beneficial to their health. For most people, even a five-day regime is a daunting prospect. The FMD aims to lessen the burden of a complete fast while still promoting the positive aspects of food deprivation.

Longo and his team of nutritionists believe it is necessary only to do the diet three or four times a year to reap the benefits, although there is no ‘one size fits all’ and the recommended frequency will depend on an individual’s health and general eating habits. Longo favours a low-protein (preferably plant-based) diet, in between stints of FMD, to optimise its effects. Those who default to a typical Western high-calorie diet, rich in red meats and processed foods, may, Longo says, need more frequent interventions to have a lasting impact on their health.

Longo’s team of researchers refer to it as a “periodic fasting regime”. Each cycle brings a rollercoaster of emotions and physical challenges, as I found out as one of their first human guinea pigs.

© Michael Driver

The regime requires 100 per cent adherence. My first instinct was to binge-eat in the days before it all started. This, as I soon learned, is futile and counter-productive. The best preparation is to gradually ease into the five-day plan. It is far easier to eat smaller meals in the run-up and cut out snacking and caffeinated drinks (which also come with high calories) altogether, over a period of days before the FMD, than to suddenly and extremely change your food intake.

On paper, the first day should be the easiest, with ProLon meals containing 1,090 calories (10 per cent protein, 56 per cent fat, 34 per cent carbohydrates) – more than the four remaining days, which only have 725 calories (9 per cent protein, 44 per cent fat, 47 per cent carbohydrates). Yet I found it could be the most challenging.

Some volunteers complain of pounding headaches, which can be caused by dehydration as cutting down on food also reduces your water intake (the FMD does allow for unlimited drinking water). Headaches can also occur as the brain gets used to burning fat instead of glucose for fuel. And for heavy coffee drinkers, the sudden drop in consumption can play havoc. With the researchers’ permission, though, I continued to drink one cup of black coffee per day. They allowed it for trial subjects like me who felt that their daily espresso was so important that they might not be able to complete the diet without it (a single cup, with no milk or sugar, did not significantly affect my daily intake of calories).

The headaches normally ease after a few hours and, after a number of cycles, were not a problem at all for me. It is as if the body learns what to expect and quickly adapts to the temporary state of caloric restriction. Hunger pangs come and go. After enduring the first few waves of discomfort, stomach rumbles give way to a Zen-like state of mind and body.

This makes some sense – a feeling of being cognitively sharper is a commonly reported effect of ketosis, the process by which the body burns ketone bodies, an alternative fuel produced by breaking down fat when glucose stores in the body are depleted. A majority of participants in the trial also said they felt more clear-minded but others reported that they were mentally fatigued during the five-day cycle. I experienced both. Sheer exhaustion set in some days but at other times my brain was buzzing.

© Michael Driver

What was remarkable was that 95 per cent of the participants stuck to the regime without cheating. This is a key issue: those who found the diet the most difficult tended to be people who live on convenience food – microwave meals and the like – and ate mostly meat-based meals rich in animal protein. Arguably this is also the group that would benefit the most from the diet.

“The people who came from a very poor diet, where they were eating lots of processed foods, lots of sweets and fast food and such, they found it really challenging,” says Dr Felice Gersh, an unpaid medical adviser to L-Nutra, who says she has overseen 48 patients on the FMD. For people on a “terrible diet” Gersh recommends a four-week detox from their regular eating pattern before the FMD, “otherwise it can be a little bit overwhelming”.

© Michael Driver

I talk with Sandra, another dieter, who has completed one round of FMD and plans to do more. She describes herself as a “typical American – I like to eat”. “If I could do it, anybody could do it,” she says. Her goal was weight loss. The 55-year-old lost just under 2 kg, which is on the low side of what most people achieve – with each fasting cycle I lost approximately 3–4 kg.

Sandra speaks enthusiastically of FMD. She, like many others on the trial, says she would do it again. I try to be dispassionate, but to be honest I felt the same. So after completing the 2013 trial, I decided to continue. I did it four times in 2014 and twice in 2015.

The biggest tangible benefit was weight control. Although I inevitably regained some weight once I start eating normally again, I, like Sandra, found it reduced my tendency to snack, even when not on the regime. Longo’s group have reported a significant drop in the amount of IGF-1 in the bodies of their study participants after a five-day round of FMD. Once we returned to a normal diet, IGF-1 increased, but not to its original level. Subsequent rounds of the diet followed a similar pattern.

It is possible that all this good feeling is simply selection bias – that people who volunteer for a diet trial are more inclined to react enthusiastically than those in a real-world setting. But immediately after each five-day intervention, I felt reinvigorated. I felt stronger at the gym, though whether this can be directly linked to the diet is unclear. After all, this isn’t much different from how others say they feel after following any other diet or detox fad. And I still get sick. But the potential rewards for my life down the road continue to pique my interest.