‘Don’t go on a diet.” That’s one tip from personal trainer Graham Waugh, 50, and may be surprising given that he once weighed three times as much as he does now. A few years ago, he was 330kg (52st) and stuck at home, feeling as if he was waiting to die. Exercise had never been a part of his life, but after bariatric surgery, he began visiting the gym. When his weight reached 111kg (17.5st), the gym owner suggested he become a personal trainer. “I said: ‘You’re joking, aren’t you? I’m too fat and old.’” But then Waugh realised he could specialise in training people who had had similar problems with their weight and health. “I’d seen a lady in the waiting room at the obesity clinic and she looked plump, at worst. I thought to myself: ‘If she had a gym she felt comfortable in, maybe she wouldn’t need surgery.’”

Waugh isn’t the only personal trainer with a powerful life story. For many trainers, it is the enormous change to their health and happiness that they experience at the gym that leads them into their chosen field – whether they have lost weight, gained strength or tackled physical or mental health issues. A prominent recent example is the former deputy Labour leader Tom Watson, 52. When he announced he was stepping down from politics in November, he also revealed he was retraining as a gym instructor. Watson has been on what he describes as a “health journey”, losing 51kg (8st), and reversing his type 2 diabetes diagnosis through diet and exercise.

What lessons can we take from the trainers who have transformed their lives? “Don’t look at how much you’ve got to lose,” says Waugh. “If someone had told me I had to lose 35 stone, what do you think that would have done for my morale? What I had to do was make each day the best I could, try to make the best choices.” A balanced diet is important, of course, but so is learning to forgive yourself. If you slip up in the morning with biscuits or cake, he says, don’t write off the rest of the day.

For Alma White, it was learning consistency and self-respect, and moving past the allure of fad diets, that changed everything. In the space of one year, she transformed the way she ate, discovered exercise, lost 70kg (11st) and left a “toxic” relationship. “My whole life did a huge about-turn,” she says. Then she changed careers. She had been a busy, successful makeup artist but decided to become a personal trainer instead. “I was so passionate about makeup, I never saw myself doing anything else. But my life had changed so much, I thought: ‘I can’t do this career because it’s not fulfilling me in the way I know training and coaching would.’ I decided that this is what I wanted to do for the rest of my life – I wanted to be able to give women the same feeling and confidence that I gained from my journey.”

Graham Waugh, who became a personal trainer after losing 210kg (33st). Photograph: Image supplied by Graham Waugh

White, from London, studied for a year before becoming a full-time trainer. “Yes, it’s about the weight loss and there is a huge aesthetic difference, but the mental difference is something you can’t see in a picture and that’s what I get the most fulfilment from.” She has seen clients come off antidepressants, improve their relationships and change their lives in all sorts of ways.

At one point, she weighed 159kg (25st). She is now 80kg (12st 6lb). “I had a massive issue with emotional eating – that was how the weight piled on. I was in a really toxic relationship at the time, so that further fuelled my eating issues. I was a little bit overweight throughout my childhood but I put on a massive amount in that year, during that relationship.” She thinks she gained about 40kg (6st). “I had major confidence issues and anxiety. I suffered with depression.”

She moved in with her partner soon after they met, but the relationship turned sour. “It made a massive dent in my confidence. I was cut off from friends and family and because I’d put on so much weight I didn’t want to go out anyway. I felt trapped.”

She had seen some posts by a nutritionist on social media and decided to start tackling her diet. “There is so much misinformation and so many fads and you can get caught in a vicious circle of dieting. I dieted for as long as I can remember. I tried anything and everything.” The nutritionist encouraged her to take a “before” photograph of herself, wearing only her underwear. “I remember looking at it and thinking: ‘My God, how have I got here?’ I don’t think I was aware of how overweight I was. That was a big wake-up call and I decided enough was enough and to seek help because I wasn’t getting anywhere on my own.” With the nutritionist’s help, White lost quite a bit of weight over several months. “Then I got myself a personal trainer and did some sessions with them and that’s when I fell in love with weight training. It was the first time I felt a sense of achievement – that I could do this and that I really enjoy it.”

It was hard work, she says, but what surprised her was how simple it was. “All I needed to do was to be consistent in my habits – continually train and eat well – and the weight would fall off. I loved weight training because when you lift heavy weights, you put yourself in an uncomfortable position, but being able to push past it made such a big difference in my life outside of the gym.” She soon had the confidence to leave her relationship.

White advises working out why you want to become fitter, rather than having it as a vague goal. Then accountability can help you stick with the exercise – through either a personal trainer, or a friend you go for a walk or run with. You don’t need to go to the gym “but I think it can be easier because naturally our lifestyles don’t give us that activity, so we need to plan it. Once you’ve done it for a while, it becomes a habit, and the great thing with training is you’ll see immediate benefits to your life. And you’ll see there are no negatives.

“You also have to be consistent with your nutrition. For a lot of people, overeating is a way of suppressing emotions. I see it time and time again – people don’t know how to deal with their most powerful emotions so they use food as an outlet.” She still has treats: “That is how you stay sane. I’m not an advocate of cutting anything out of your diet; it’s about having everything in moderation, and as much as your life needs. If you’re sitting at a desk all day, you’re going to need less.”

Waugh’s issues with weight stretched back to his childhood. “I was always really big,” he says. “When I was 17, I was 17st; when I was 18, I was 18st and it just went on.” He lives in Luton, and when he was about 30, and weighing around 28st (178kg), he walked up a hill one day and started having heart palpitations. “I assumed I was having a heart attack and I became scared to go out. Because I was stuck indoors I was getting depressed and because you associate happy times with food I would eat. I would be happy because I was eating something I really loved, then I’d get depressed afterwards because I was eating crap.” He wouldn’t eat much during the day but a typical evening meal, often from the takeaway, might consist of a doner kebab with extra meat, a couple of portions of chips and four burgers.

Facebook Twitter Pinterest ‘I fell in love with weight training’ … White. Photograph: Linda Nylind/The Guardian

Waugh became a recluse for about 15 years. He rarely left his sofa, didn’t go into the kitchen (his wife cared for him) and slept on a special medical bed. Did he want to get fitter? “My mental health wasn’t allowing me to do that.” He says he was largely left alone by the healthcare system, couldn’t work and had no friends.

One day, he got a call from his sister, who told him that their brother, who was only 40, had died suddenly. “He had lived a healthy life and I felt cheated. I felt that death was meant for me.” Then, he says, he told himself: “You’re a selfish bastard just sitting waiting here for death to happen.”

His GP referred him to an obesity clinic and eventually he was put on a waiting list for bariatric surgery – a gastric sleeve procedure to drastically reduce the size of the stomach. “I said: ‘If you save my life, I’ll pay it back.’ I think I only had a 20% chance of survival and there was a huge team at my operation – two or three anaesthetists, four surgeons.” Another operation – a duodenal switch, which reduces absorption of food by rerouting the small intestine – was scheduled, but first Waugh had to lose more weight. “I started going to the gym and my addiction changed from eating to being at the gym. I worked my backside off, lost a lot of weight really fast, and then a year later the surgeon did the switch.”

His aim now is to train people to either prevent obesity surgery, or to lose weight in preparation for it, as well as people who have fibromyalgia or other illnesses. He says his own story gives him an understanding of their emotional challenges and physical capabilities. “That person is going to walk into an environment they’re not going to be comfortable in, they’re going to be nervous, they might have bad anxiety. What I have to do is say: ‘Are you all right walking in to the gym on your own? Do you want to come in the back way? Do you want me to meet you in the car park? Can you manage the stairs?’ These are all hurdles before they walk in the door.”

Facebook Twitter Pinterest ‘Helping people with back problems is my calling’ …Trevor Harrison-Phipps. Photograph: Image supplied by Trevor Harrison-Phipps

Trevor Harrison-Phipps, from Croydon, was in his late 40s when he discovered exercise could cure the back pain he had endured for nearly 30 years. He had been a builder when he suffered a back injury while mixing concrete. He couldn’t work for six months and was told he would need surgery, but refused even though he was in agony and his movement was restricted. Things improved to the point that he was able to work as a graphic designer but the pain remained. He remembers when his son was three, lying on the floor with him, trying to take a paracetamol and in tears.

He was on constant painkillers, plus antidepressants at one point, and he saw countless osteopaths who couldn’t help. “Having that pain eating away at you day in, day out, destroys you. You can’t see the light at the end of the tunnel.” One friend, an ex-wrestler in his 70s, advised him to do a series of stretches but Harrison-Phipps ignored the advice for years. “When I was at my lowest ebb and overweight and depressed, I thought: ‘Let’s just do these four stretches in bed.’” Within days, his back pain had improved and he says it felt miraculous. “From that I got the confidence to go to the gym. The stretches were just the start of everything.”

He had been going to the gym for a couple of years before looking around at the personal trainers there and thinking: “This is my calling – I’ve got to help people with back problems.” He became a personal trainer four years ago at the age of 50, and now trains people who also have joint issues or arthritis.

What does he tell people? “Sleep well, exercise and eat well and the rest looks after itself. Try to always exercise, even if it is just for 10 or 15 minutes. It’s not hard to make healthy food – you can have the bad stuff, but you’ve got to have a balance so the good stuff outweighs it. Sleep and rest periods, along with relaxation, put you in a better mindset.”

And his own mindset? “This is the best job in the world,” he says. “I love people and I’m with people all day long. Recently I got one lady who couldn’t walk up the stairs – she had really bad arthritis. She’s done 24 sessions with me now and she runs up and down stairs all day long.” He says he has prevented at least two clients from needing shoulder surgery. He loves, he says, “the buzz of seeing people’s lives change”.

Waugh feels the same. It’s one thing to help people lose a bit of weight to fit into a dress, he says, “but I’m talking about changing people’s lives, if not saving their lives.”