Sarah Vine is on a mission to get her mojo back and lose weight, but it's not going to be an easy journey...

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Until relatively recently, the extra weight I've been lugging around since the birth of my son eight years ago hasn't really bothered me that much. Granted, it's not ideal. But life is full and there always seems to be something - or someone - more important requiring my attention.

Then the other day something happened that made me realise that actually, despite my defiance and defensiveness, I do care. I was invited to join a group of friends in Ibiza in May, and I refused. It wasn't the money or the timing or the effort or the company; it was the thought of having to wear a bathing costume in front of them.

I realised that I am deeply ashamed of my weight. I may not show it, but it's true. I am at a crossroads: either I give up and become the sort of woman whose upper arms never see the light of day again; or I kick my fat arse into gear and reclaim my mojo.

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Vanity is not the only factor; I feel lousy too. I'm tired, listless and things ache that should not. I'm forever panicking about what to wear, not because I have an empty wardrobe but because my clothes don't fit properly. I become grumpy at the thought of having to dress up for a special occasion, and have more than once turned down an invitation simply to avoid the issue.

In short, it's finally time to admit that being overweight makes me unhappy. I'm being bossed around by my fat, and I don't like it one little bit. Time to regain control.

Over the next few weeks I will be charting my progress, working closely with nutritionist Amelia Freer  and trainer Steve Mellor . Both are hell bent on getting me back into shape, Freer through re-educating my eating habits, Mellor through exercise. I'm not sure who has the most impossible task.

Letting go of the lard

I can tell that Steve is a bit wary of me. For a start, I'm quite a lot bigger than him. I'm also extremely grumpy. I used to be really fit, you see. Then I had kids and various health disasters, with the result that I can barely climb the stairs without gasping for air. This makes me extremely cross, not just because it's pathetic, but because once you've known the joy of having a body that hums along if not exactly like a top of the range Ferrari, then at least a mid-range Beamer, it's depressing to suddenly find oneself at the wheel of a clapped out Nissan Micra. Still, I have to start somewhere.

We begin with a brisk walk to the park, chatting all the way. I make a point of talking, just to prove to Steve that I am least fit enough to walk and talk at the same time. I am not. By the time we reach the park gates I am out of breath. Steve politely ignores my gulps for air, and unpacks his equipment.

The routine is simple: some warm-ups on the TRX (a bright yellow pulley thing that can only have been inspired by the Spanish Inquisition), then intense bursts of exercise followed by short (very short, too short) rest periods.

There is a squidgy 5k ball which I am required to slam into the ground with vigour. This, I enjoy immensely. Since my target weight loss is 20 kilos, I visualise the ball as a wobbly wodge of fat, my fat. I am slamming it, hard, into the ground. If I could jump on top of it too, I would. I suggest to Steve that he should get one made in a horrible shade of blubber. He looks at me like I'm mad.

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While I recover from the fat-slamming, Steve whips out some paddles, and I do kicking and boxing. Then I get to push Steve around the park, literally, as hard as I can. Back and forth we go while the dog (did I mention the dog? Mars, my Jack Russell) goes nuts with excitement. On a nearby bench two fine specimens of West London yoofdom slouch over their smokes, watching us intently.

After a while I begin to feel a little giddy, and so we stop and stretch. Mars, who is a loyal and loving hound, takes this as a signal to pee on Steve's rucksack. Steve doesn't notice. I don't tell him. Mars and I walk home feeling elated. My journey has begun.

Gut feeling

Amelia Freer is very good at not mentioning the elephant in the room, i.e. me. Our entire conversation about food and health is conducted without a single mention of the f-word - until right at the end she asks me, in passing, whether as well as improving my health through diet, I would also like to lose a bit of weight.

We are sitting in her elegant white kitchen, the centrepiece of which is a giant planter bursting with herbs, discussing meal plans. She is going to help me kick-start my regime with two weeks' of nutritionally optimal meals, delivered fresh daily. The aim is a dietary re-boot, eliminating as many potential sources of irritation as possible and giving my gut a chance to rest and recuperate.

In a nutshell, Amelia thinks my gut is knackered. I've had lots of things wrong with me over the past five years, ranging from Swine flu to pneumonia, and as a result have ingested more industrial strength antibiotics than a battery hen. She reckons most of my food is just sliding through me, with very little goodness being extracted on the way.

In addition, she has concerns about food intolerances. As it happens, I have just had back the results of some allergy tests my doctor sent me for, and it turns out I am lactose and yeast intolerant. Who knew? I always thought food intolerances were an invention. Anyway, my gut is in need of some serious R&R. I leave excited about the next couple of weeks, and immediately drive to Sainsbury's, where I buy a planter and some herbs. Important to look the part, I always think.