Lisa has kept six stone off for 12 years, since she was a size 18, left, and has now worked on developing her six-pack, right
Diet tips: 'I completely cut out alcohol and began to eat more protein - I really like chickpeas, fat-free yoghurt and Quorn - and within six weeks I saw a big difference'
A mother who shrunk from a size 18 to a size six in just six months by dramatically changing her lifestyle has developed such a passion for keep-fit that she has become a fitness instructor.
Lisa Rennison, 40, from Peterborough, Cambridgeshire, has kept six stone off for 12 years by dedicating herself to a strict diet and gruelling exercise regime, and over the past four months has succeeded in her next challenge: to develop an impressive six-pack.
Speaking to MailOnline, Lisa explained how she decided at Christmas that she would like to turn her flat stomach into a super-toned six-pack and embark on her new career.
She said: 'Basically I just love exercising so much ever since I've lost the weight, and I wanted a new career, something I was passionate about.
'I worked for the council as a customer service rep, but always hated working in an office. Friends kept telling me I should do something to do with keeping fit but I didn't have the confidence. It was last September, when I was 39, that I was really unhappy in my job and decided to do something about it.'
Lisa now teaches ab-toning, spin, body combat, circuit training and body conditioning classes at three different gyms near her home.
She cycles to and from work (a total of one hour every day, 70 miles per week) and runs two back-to-back hour-long classes at the gym, five or six days a week.
'When I was big I didn't do any exercise at all,' she said. 'And eight years ago I ran and cycled but wasn't keeping fit to anything like the extent I do now. Then in 2010 I joined a gym and didn't look back.'
It was also towards the end of 2012 that she also decided she wanted to work on toning her already flat stomach into something more defined: 'There was a definition there because of all my cardio work, but just before Christmas I told my instructor i wanted six-pack, so I decided to look at my diet and do more weights.
'I completely cut out alcohol and began to eat more protein - I really like chickpeas, fat-free yoghurt and Quorn - and within six weeks I saw a big difference.
'I started doing body-pump workouts - sort of weight training to music - and really working my upper body. Other people started noticing a big difference in my body, too.
'The great thing is that the exercises you do don't have to be done for a massive length of time - just a few minutes a day. And they really work. It took about six weeks to notice a difference and three months for my six-pack to arrive.'
'My body combat class is sort of weight training to music - it's all areas of the body using weights, a bit like boxercise and martial arts together,' she said.
'I do spin classes three times a week and I'm also a big advocate of my circuit training classes beacuase they work all the muscle groups - it's a big cardio workout that has lots to do with toning.'
Lisa is also a big believer in the Tabata work-out: short, sharp bursts of high-intensity sprints and training that keep your heat rate high and are said to improve fitness and help you burn more calories.
Alongside her fitness routine, Lisa's super-healthy diet is also strictly regimented.
Her daily menu consists of oats and water for breakfast, fruit snacks, wholewheat grains and plenty of salad for lunch and Quorn and vegetables for dinner. She also drinks lots of water and says apples, carrots and celery are the best foods to make yourself feel full.
'I never get bored of it and I do feel slightly panicky if I have to go out to a restaurant and always like to look at the menu first to see if there's something I can eat.
'I have the occasional treat but usually just cocoa powder in yoghurt - junk food doesn't interest me and i don't really get cravings for rubbish anymore.'
Overweight as a child and called 'fat a***' by bullies at school, a lifetime of unhealthy eating had seen Lisa's weight soar to 12st, which at 5ft 2in put her in the dangerously overweight category.
But it was after the birth of her son Ben, now 15, that her weight really spiralled out of control. She eventually decided to do something about it, taking a photo of herself at her heaviest and carrying it around with her for 'thinspiration'.
She dieted by cutting out bread and chocolate and lost a stone in a month and followed it by stopping greasy takeaways and fast food.
The mother-of-one added: 'I didn't use Slimming World, or Weight Watchers or muscle in on any fad diets. I literally stopped eating the rubbish.
'I stopped eating the chocolate and the cakes. I looked at my portion sizes and got rid of the takeaway meals.'
In just six months Lisa managed to drop to a healthy 7st, dropping from a size 18 to a size six.
She says her next goal is to work on her body strength: 'I'm happy with weight, I don't want to lose any more. Now it's more about developing strength and muscle which I'll do by lifting heavy weights. I don't want to become a body builder, I just want to be as fit as I can be.
'I'm 40 now and not as young as everyone else in the industry - so I need to work twice as hard!'
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