For anyone who hasn’t seen the series before, how would you describe How to Lose Weight Well?

This show looks at all the different weird and popular diets that are out there at the moment that you might be thinking about in the New Year, and all the other odd weight loss methods and we put them to the test. We’re pretty open minded about them. The core of the show is we take three pairs – friends, housemates, family members, for example – and we put them on different diets and we see how they do. We divide them into Crashers, who usually have a long term goal like a wedding or a party or a beach holiday; Shapeshifters, who have got more of a medium-term goal; and then the Life-Changers, who go on their diets for three to four months and they normally have real concerns about their weight or some health problems they are at risk of.

In between meeting those people, I go on a series of journeys trying to learn about some of the weirder, more extreme weight loss methods out there. In this series I have my stomach zapped with radio frequency, try a fat destruction gadget to see if it will get rid of my tummy, I go to a Chinese restaurant with my father who shocks me with electricity to try and stop me from eating. I wear a body suit that claims it can burn hundreds of calories in half an hour to melt my fat away and make my muscles work without much effort. There’s all kinds of mad stuff. I think it’s really weird! It’s a nice programme because we don’t want to body shame anyone or tell people they have to lose weight. We’re just interested in all the different things that are out and which things work in the real world and which ones don’t. You just see real people having a real go with the things you might be thinking of trying.

A lot of people who maybe want to lose weight can be put off by the more structured types of diet that you evaluate in the show. Why do you think that is and how do you change that perception for people?

We just want to do whatever we want to do, don’t we? All I want to do is order exactly what I want from my local takeaway and watch what I want on the telly? I think it’s that way more than ever. You could have four people over for dinner and everyone can order different food and it will all be brought quickly by a guy on a bicycle and you can all watch your separate shows on your own devices. We can all have whatever we want whenever we want, and you might think “Why should I have to go on an annoying regime”. I think the appeal of these diets is that in a very confusing world, some of them give you a structure. I think there are some people who do just crave a decision. I think the hardest thing I find about losing weight is knowing what to do when, and all the different possibilities. So sometimes it’s nice to have a book or a set of guidelines that say “Here are the rules. Stick to them. We will hold your hand and guarantee you’ll lose weight if you stick to the rules” and that works for a lot of people, and sometimes it’s not sustainable. I think people’s suspicion around diet books is that they’re not sustainable. The book I wrote (How to Lose Weight Well, 2016) had very few rules in it. My rules were like “just tidy up your life”, and that’s quite a complicated discussion to have with an editor who wants “10 clear principles of good eating from Dr Xand van Tulleken”.

You lost a lot of weight yourself. What was your motivation for losing weight?

I have an identical twin brother, so I have a thin avatar out there in the world reminding me that I had disgraced my genes! We were filming something with a genetic scientist who turned to me and looked at my tummy and said “You’re a disgrace to your DNA!”, so that was a wakeup call for me. But also I was unhealthy. I woke up in the morning and my feet ached from carrying the weight of body. I was nearly twice as heavy as I am now. I was 19 stone, so that’s like carrying another human around all day. My feet went up one shoe size. My blood sugar was disordered. My blood fats were disordered. I looked and felt terrible. I was meant to be on the telly and I didn’t feel happy about the way I looked.

What methods helped you?

Most of all, it was figuring out why I had gained weight. I had gained weight because I was miserable and because eating was one of my few sources of pleasure, and I was not addressing the other problems that I had in my life. A huge part of what I had to do was to look at the problems I had to do was ask “Well, why am I struggling with this?” and I think a lot of people find that when they get to about 30, life becomes more complicated. Be that because you have children, your relationships become more difficult, you have bigger financial responsibilities, you may have more anxiety about your career or responsibility at work. It’s not uncommon for people to have difficulties like that in their 30s and 40s and all of those things added up for me. I was very lucky in that I knew approximately how to eat well – although doctors know less than you may think – but mainly it helped having my friends and family support with getting my life back on track, and then that meant I could turn my attention to eating a bit less.

You’re probably one of the most experimented upon factual presenters on TV. Do you enjoy being a lab rat?

Erm… For this programme I’ve had enemas, I took that pill that made me faecally incontinent and I shat all over my brother’s floor. It would be hard to imagine that it would always be enjoyable! What I do love, is that the only way of getting this information is to actually do it. Even if you think you know, you don’t know. I had heard about that fat loss pill before and I knew what I thought it would be like, but my experience of it was totally different.

What were the most challenging treatments you’ve done this series?

This series I do the radio frequency fat ablation, where radio waves zap my tummy, and I thought that the negatives of that it would be that it would either leave me misshapen or burned, or that it just wouldn’t work. In fact the really awful thing about doing that is that I had to sit there and look at my stomach for FORTY FIVE MINUTES twice a week. 45 minutes in an empty room with a machine about the size of a drinks trolley with a radio blaster shooting at my abdomen, and I couldn’t use my phone because it would be bad for the machinery, so I was just sitting there feeling bored and angry. And I did think “What are you doing here, mate? Why don’t you just go for a walk?!” 45 minutes of walking will lose some fat while 45 minutes sitting under a radio frequency machine, in my opinion, won’t. That surprised me the most. That I was more psychologically vulnerable than I thought going into these ‘aesthetics laboratories’. But it is good and enjoyable getting the information about them, even if it’s not fun in the moment. In this series I get zapped! I have to wear an electric bodysuit and get shocked while I’m doing exercise.

What’s the most remarkable thing you’ve learned about your own physiology from doing the programme?

In this series I learn quite a big thing about my own body fat distribution and composition. I have an MRI scan and find there’s a problem that we may be able to fix with diet and exercise. At the moment I’m seeing a personal trainer twice a week and going to the gym five times a week and we’ll see if it works. It’s quite significant for me, but that’s all I can say for now.

Have you encountered any products or treatments that fall short of their claims?

I’m much less sceptical about the diets than I was before. I’d be much more open minded about telling people “Try a few of them. If that one doesn’t work for you, try this one”. However, I have yet to really encounter a product or a treatment that is worth the money. There’s so much money involved in exploiting people’s unhappiness and inability to take charge of their lives.

Were there any particularly extraordinary treatments you saw this series?

There’s one – and I’d say viewers will have to make their own minds up about this – in one episode we meet a guy, Jason, who wants to get rid of his double chin and his rather unique problem is that he doesn’t like his double chin but he doesn’t mind being a bigger guy. He has a surgery that involves injections of detergent into his chin to melt the fat away, and I thought this was a daft idea and so did a lot of people he encountered, and at the end of it, in my view, he looked a lot more like a superhero than he did at the beginning of the treatment.

Do you have one bitesize tip for losing weight well?

One would be to change your life; figure out why you have gained the weight, and change that thing in your life. If you’d like a simpler tip than that, it would be buy a sharp kitchen knife that you can use to cook all your own food with. That’s the thing that’s missing from most people’s kitchens – a good sharp chef’s knife.

How to Lose Weight Well Series 3 continues Mondays at 8pm on Channel 4. Catch up on All 4