As a mother of four I am not sure how I am supposed to feel about sugar. If I believe the anti-sugar lobby, it's "the new tobacco". Sugar rather than fat, the argument goes, is responsible for ever-rising levels of obesity. "Sugar is not addictive like tobacco," explains Professor Graham MacGregor, chairman of the campaign group Action on Sugar, "but it causes just as much harm in other ways. It is an unnecessary source of calories and a major cause of obesity, thereby causing many deaths and diabetes."

The more sugar you eat or drink, the more the body stores it as fat. Hence the links to obesity. But what is emerging is just how much of what we eat is stuffed with "hidden" sugar, not just in fizzy drinks and doughnuts, but sauces, cereals, fruit juices, even fruit itself. This month Britain's chief medical officer Dame Sally Davies said "we may need to introduce a sugar tax" to help reduce the amount manufacturers put in their products.

This can be confusing for the average person who is just trying to feed their kids. And that's before addressing the counter-argument that sugar is taking too much of the blame and that overall nutrition, as well as exercise, are important too.

To try to make sense of it all, I go cold turkey for 30 days, dragging my family along for the ride. If I cut out sugar, would I feel better? Being the kind of person who steams my children's vegetables while allowing them a sticky bun at the weekend, I am a pretty good guinea pig. How much hidden sugar is really in my children's diet? And how much of life without sugar can I tolerate?

Week One

The mood of this first week free from sugar is, to quote Thomas the Tank's Fat Controller, "confusion and delay".

I've done a lot of reading: Michael Moss's bestseller Salt Sugar Fat; the blog Kate Quit Sugar; the NHS Choices website; endless press coverage; James Duigan's Clean & Lean Diet. I've watched Dr Robert Lustig's convincing lecture, Sugar: the Bitter Truth, on YouTube.

Here's what goes in the bin: Cheerios, Fruit and Fibre, Petits Filous yoghurts (my son has been known to eat three on the trot), baked beans, tomato sauce, tomato and mascarpone pasta sauce and the children's Saturday evening "treat": pizzas. Also on the way out are jam, honey and anything, frankly, that kids find tastes nice.

Where I am confused is on the issue of fruit and moderation. Fruit is laden with sugar (fructose). If, as Lustig says, fructose is "poisonous", what is moderation? Smoothies and fruit juices, if you believe some research, are as bad as Coke (35g of sugars – nearly 9 teaspoons per can). Get rid of fruit in my children's lunchboxes? They have two pieces every day and often fruit for pudding. But a banana can have 7 tsp, grapes 1½, and a melon 12 tsp – all their favourites.

The NHS is less hardline, more sane, advising a "balanced diet" and so too is Kate Quit Sugar: "I eat fruit because it is delicious. The whole fruit includes the fibre of the flesh and also the naturally occurring fruit sugar. People have a million opinions on fruit … make up your own mind!"

But when it comes to mass-market fruit juices, even some of those sold as having relatively lower sugar content, everybody is pretty much united. They are bad. Spawns of the devil.

I decide that the fruit stays for the kids but not bananas, and no fruit for me. I supplement bananas with kiwis and lower fructose fruit such as berries, and switch to raw vegetables such as peppers and sugar snaps, which the children welcome (thank God). Brown pasta and granary bread also stay. The children are small, and I'm not prepared to experiment with them in the way I can with myself.

There's some grumbling about the cereals (20.9g of sugars – 5 tsp – per 100g ); "why do they tell us on the box they are healthy?" asks my eight-year-old. A quick life lesson there. But when I hide the chocolate biscuits (two each after school, normally: 1 tsp of sugar each) there is a riot. "You're lying! You're lying!" It's a routine for them. I relent.

As for me, the diet is a drastic change, not from cutting out sweet stuff (I'm not big on biscuits and chocolate) but from eating no carbohydrates at all (all sugar in the end). By day two, I have no energy. I have to go to bed straight after the children at 8.30. The running I started a few months ago – which brought me such mental relaxation and quick weight loss – is off the cards. I feel cross and resentful.

I seem to live on boiled eggs, almonds, coconut flakes, protein in various forms, avocado and kale. I go to the health food shop in my country town and spend more than £40 on chia seeds, quinoa, flaxseed, more coconut flakes, hazelnuts, coconut water (hideously expensive) and more kale. I'm like a crazed celebrity. By Sherborne standards, I've spent so much in one hit the shopkeeper throws in protein shake samples for free, for my planned smoothies. In Sainsbury's I buy expensive, unsweetened almond milk and a small jar of coconut oil that costs £6. Six pounds! That's the price of a chicken for the kids.

Breakfast is my main problem. The low glycemic index granola from the health shop, bought at vast expense, disappears in one sitting. It's all very well for Hollywood stars to whip up smoothies of avocado, kale, blueberries and chia seeds, but you try doing that without a housekeeper or a nanny when you've got four kids, a job and lunchboxes to pack.

So I'll leave you with an image at the end of this first week: determined to make said smoothie but not owning a smoothie maker, following a recipe, I throw into the food processor the frozen berries (low sugar), kale, chia seeds, coconut water. It all explodes over the top and onto the floor. The dog laps it up and is sick. The little one is banging his spoon on the table singing at the top of his voice "No no no sugar, never never never!" I am in so much pain with my back – which I later find out is severe constipation – that I can hardly move. I cry out in such anguish that my husband looks worried rather than bemused.

But by day seven, everybody is eating full fat Greek yoghurt sweetened with berries and topped with protein-packed nuts. Perhaps this is a new definition of good mothering? Never mind if the mother can't move.

Week Two

The backache has eased. I make a string of rather joyless suppers with quinoa and various leaves (no balsamic in the dressing) and spiced-up protein or smoked salmon. The husband confesses that one day he is so starving he eats two hot lunches at work.

The NHS recommends that we try to limit ourselves to 10tsp (40g) of added sugars a day, but some have said this should be 6 tsp for women and 8tsp for men. The World Health Organisation now recommends just 6tsp (25g) for adults. Roughly, you divide the grams by four to get the teaspoons.

I am a sad woman in the supermarket, squinting at the "carbohydrates (of which sugars)" labels. ( A free smartphone app, FoodSwitch, scans labels – measuring total fat, saturated fat, sugars and salt – and offers healthier alternatives.) Anyway, according to the NHS high sugar is more than 22.5g of total sugars per 100g and low is 5g or less per 100g. A lot of innocent-looking stuff – like a boeuf bourguignon or coq au vin packet flavouring – is out for being way too high.

I must be on about 1 tsp a day now, allowing for the odd oat cake and glass of red wine (half a teaspoon), which, paradoxically, the stress of the whole project makes necessary. I realise that a significant amount of my previous sugar intake came from bread and rice, and white wine. When I reintroduce a fruit yoghurt to my son as an experiment, he goes nuts with the sugar rush.

This week's progress is shaped by a visit to London to see Dr John Briffa, author of the weight loss guide Escape the Diet Trap, who makes sense of these subtle changes in my life. The low energy is my body recalibrating its metabolism, switching its system of fuel from carbs to fats and proteins. "Hang in there," he advises. "You are going to start feeling a lot better very soon."

Eat fats to fill up, he says: "Historically, we've had this focus on fat but it appears that fat is not inherently fattening. Insulin plays a key role in fat storage and the more insulin you secrete, the more you are likely to become insulin-resistant." Basically, you eat a lot of sugar, you store a lot of weight.

Briffa is hardline on carbs, even porridge: "just a big bowl of starch". My constipation is due to a lack of water and vegetables. "I've seen hundreds of clients on this kind of diet and none of them suffer from constipation … People weren't eating granary bread and porridge two million years ago." (When I quote this back to my husband, he retorts "People didn't live long two million years ago".)

I sense my need for clear answers is irritating Briffa. I basically want him to tell me what to feed my kids. Is it not absurd for a middle-class mother, committed to fresh food, to be stressing about a chocolate biscuit and worse, fruit, in itself full of soluble fibre and goodness?

"Look," he says, "if you have normal kids who exercise, with no weight problems and no history of diabetes, a rule of thumb would be that natural sugars from fruits are OK. But if you brought an obese kid in here, I would certainly be telling you to take the fruit out of the lunchbox. And the granary roll is OK, but only as a vehicle for getting a healthier filling inside them. I can't tell you that a biscuit as a snack is good. It has no nutritional value at all.'

On Valentine's Day, my son gives me a shortbread heart biscuit covered in pink icing that he has made at playgroup. I eat it in an instant because it was made and given with such love. It's delicious. What could matter more than this?

Week Three

Two people tell me I look "fresh". I haven't been "fresh" for years. I'm waking up refreshed, which I've been craving for the last 10 years. Briffa had predicted this: my blood sugars have stabilised.

The children seem to have forgotten about cereal and fruit juice. We make our own pizza on Saturday night. Working mostly from home means I'm around at teatime, making it easier to control and plan the menu.

I've abandoned kale smoothies, which even Briffa said were hardcore, and we've settled into a scrambled egg/granola/yoghurt breakfast routine, with a bit of Weetabix for them too (shoot me). I'm working out that I resent hidden sugars more than the obvious sugars. In other words, yoghurts, sauces and cereals are worse than biscuits for me because I consciously choose to allow the biscuits in moderation.

Being the mother of three girls is a factor. Perhaps an important change in women of my generation is that none of us wants to create in our daughters food/body issues from things having been "forbidden", hence the biscuits in moderation. But my school-age girls surprise me: they have already learnt about traffic light food labelling in class. I never got any of this as a child.

With a history of mild dieting (and teenage years of pretty extreme dieting), I am resisting the urge to get on the scales. This is not about being on a diet. I have to keep reminding myself of James Duigan, personal trainer to Elle Macpherson, who in his own books agrees with Briffa: "Sugar is a nuclear fat bomb exploding all over your body," he says. The magic formula is to fill up on good fats and proteins and stabilise your blood sugar. So I continue to eat and snack on (mostly) good fats: nuts, avocado and a bit of cheese. I relax about bacon. After three decades of anti-fat programming, this feels like I'm breaking some kind of diet law.

The week closes with a spectacular display of bad behaviour. During a dinner party, I knock back a lot of prosecco (the worst), red wine and potato gratin. At the end of the evening I throw up in our bathroom. Classy. This has to be the sugar since I've drunk more before and not been such a wreck. I feel poisoned.

Week Four

I go running four times! A miracle given how I felt in week one.

The children and I agree to photographs in our home with all the food and drink we have cut out. Piles of it are laid on the table. My kids fall upon the sugar stuffs like locusts, clinging the packets to their chests and shouting requests for smoothies, chocolate bars and jelly babies – stuff they had supposedly forgotten about. They are slightly out of control and it panics me. This, is what happens when food is forbidden. I resolve to undo this psychology of the forbidden food by calibrating them ever so slightly in the opposite direction next week, when it's over.

As the week draws to a close, I feel relief like a convict waiting by the prison gates. On day 31, I wake up and the girls present me with chocolate cup cakes they have made in secret to celebrate. I eat one because my eldest daughter wants me to. I go downstairs and find a box of Belgian chocolates. I taste one. You know what? I don't even like it. Only Gwyneth Paltrow could be more annoying than that.

Postscript

Two weeks after my 30-day diet ends, it pains me to say that I continue to eat in the same way. I thought I'd be liberated – free from the tyranny – but my palate has been retrained. I find bread heavy now. I don't want rice or chocolate biscuits or pasta. Protein fills me up and keeps me going. I don't buy juice, smoothies, yoghurt or Cheerios for the children and I carry on putting raw vegetables and nuts alongside fruit in their lunchboxes. I'm certainly not going to be a militant anti-sugar mother, but I cannot find it in my heart to allow them a can of Coke, however much it's "a treat".

They continue to have their two chocolate biscuits after school, though, along with their treats on Saturday, whether it's popcorn, pizza or an iced bun, but I am now more conscious of their daily tally. Two biscuits, with juice, combined with a banana and grapes and perhaps a pasta sauce and a bowl of porridge with honey? That's too much sugar for my children in one day.

I don't believe that the levels of sugar my children eat compromise their health. They are fit and slim. But equally, I'm not prepared to be hoodwinked by products stuffed with hidden sugars. Thirty days of being on this diet has, ultimately, made me sugar-aware rather than permanently sugar-free.

And only now can I say it: if I carry on, I'll drop a jean size too, although for the first time in my life, that's really not the point.