"Your metabolism – how much energy you burn at rest – will go down. Your appetite hormones will change, telling you to eat more. And all of these systems work against you to take you back to your starting weight, and beyond."
We have evolved these responses over centuries as a way of surviving food shortages – now most of us are surrounded by fatty processed foods, all it means is that many of us are waddling towards obesity. In short, he says, diets are making us fat. Studies pitch the number of dieters who end up gaining more weight than they lost in the initial stages at around 90 per cent.
"We have to tell people to stop what they're doing because they're ending up in a worse-off position, not only physically because of the weight regain, but the psychological ramifications that come with dieting and food restriction are enormous," he says.
"It's particularly relevant in the time of Covid. Various studies from the western world have reported that anywhere between 22 and 49 per cent have experienced weight gain during lockdown and that weight gain was more prevalent among people who were already overweight or obese," he says.
Fuller developed the plan over 15 years of working in an obesity clinic. He noticed that most of the patients sitting sadly in front of him were telling him exactly the same story: that they'd tried every diet in the book and their situation had only worsened.
"With time, they were finding it harder to lose weight and they'd say, 'I can't even seem to shift a couple of kilos now, I don't know what to do…'''
What the £2bn UK dieting industry doesn't want you to know, says Fuller, is that "they're not failing due to a lack of willpower – they're failing due to their biology".
46-year-old Laura, a mother-of-two from Durham, knows this feeling all too well. Having tried everything from Weight Watchers to the Hairy Bikers and weight-loss app Noom, she estimates that she has spent hundreds of pounds on subscriptions and books over the years. Despite success in the first few weeks and months, this has always tailed off and leave her back where she started – or worse.
"The restrictive diets made me eat more. If I was only allowed to eat two squares of chocolate, I'd get fixated on that. And then when I put weight back on I felt like it was my fault, that I love cake, chocolate and biscuits too much and I have no self control," she says.
After a year of lockdown, Laura bought Fuller's book on the recommendation of her mother, who had heard him talk to Dr Rupy Aujla on the Doctor's Kitchen podcast.
"I had read the theories around set point weight, but needed more specifics about how to shift it. This programme pulls a lot of principles together and has helped me get my head around it, as well as offering lots of real things I can do," she says. She describes it as a "relief" to now be following a healthy eating plan more easily integrated into life with a demanding job and two young boys.
"I feel less pressure and I'm not counting calories or having to weigh food. I'm also back to eating foods that I instinctively thought were healthy such as avocados and nuts, which are often restricted on other diets," she says.
Aujla, a London based GP who is currently creating the UK's first culinary medicine course, accredited by the Royal College of General Practice, believes the method is "very very credible" in enacting long-term change. "Even though it does take time… that's generally how things work: it's an investment of years rather than months," he says.
This time investment means it's hard to measure how well Fuller's plan holds up: his first book was released in 2017, with the online community only launched 18 months ago. While he's satisfied with the efforts of his nearly 10,000 members, he believes success can only really be counted when weight loss has been maintained for more than five years.
Aisling Fleury, a 39-year-old geriatrician from Ireland, is not far off – three and a half years into the programme, she has maintained a weight loss of 15kg. The process has been "very, very slow," she admits, and "I would be lying if I said I didn't reconsider it more than once … but I thought well, if this actually works … then let's just do it."
For Laura, too, this slow burn approach takes some getting used to "I'm 46 now so I can wait, but," she concedes, "if I was under 30 I know I'd choose the lose-weight-quick option and not think too much beyond that."
The Interval Weight Loss principles:
During the "weight loss" months
• Five meals per day – biggest meal when you get up and smallest meal at the end of the day.
• Home-cooked meals on six days per week – use left-overs each night for lunch the next day.
• One treat food per week (for example, an ice cream) and one meal out.
• 30 minutes of exercise six days per week, of varying intensity and different types of activity.
• Sleep 6–8 hours per night.
• TV-free days on three days per week.
• No more than two hours TV per day on the other four days.
During "weight-maintenance" months:
• Five meals per day – the biggest meal at the start and the smallest meal at the end of the day.
• Home-cooked meals on five days per week.
• Two treat foods and two dining-out meals per week.
• 30 minutes of exercise on five days per week of low to moderate intensity without a need to vary the type of activity each day.
• Sleep 6–8 hours per night.
• TV-free days on two days per week.
• No more than two hours of TV per day on the other five days.
