One day I'm going to give up this talk of moderation and balance and a wide variety of whole foods and write a diet book.

It won't just be a book, there'll be a website, of course, with a 10-week plan you can follow (for a small fee of $99). There will be a Facebook page where people can share their amazing success stories. And a range of ancillary products to help you along will be on offer.

I've been inspired by Dr Kellyann's Bone Broth Diet, which recently popped into my inbox.

"Can't imagine losing up to 15 pounds in 21 days? Or finishing a cleanse without a cheat day? This broth may make you a believer!" says the blurb.

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The diet won't only help you lose weight, they say. It will heal your gut, make your wrinkles disappear and improve your joints, too. Amazing!

Dr Kellyann describes herself as a "weight-loss and natural anti-ageing transformation expert" and she's the author of Living Paleo for Dummies.

She's also very pretty, and her website features lovely photos of her looking youthful and promising that you too can "reverse the ageing process, regain your energy, lose weight, stop your hair from thinning, erase your wrinkles, and feel strong and attractive again".

So I'll need to become a doctor, probably, to really make my own diet fly.

That doesn't seem to be as important, anyway, as The Promise.

The Promise can't just be weight loss. The Promise has to be that by doing this diet, you'll become a better person. That's what all diets really promise, one way or another. It's why the diet industry still exists.

Dr Kellyann promises nothing less than total transformation, describing how, after hitting 40, "people who used to look at me when I walked into a room were looking right past me".

Her diet will change that.

"As you regain your glow on the inside, you'll also transform yourself on the outside. Your hair will get fuller. Your skin will get softer and smoother. Your body will become more sculpted. Within weeks, you'll look 10 years younger. As a result, you're not going to feel invisible any more."

And that, right there, is the huge advantage the diet industry has over anyone with a message of moderation.

Do I think that eating well can help you look and feel better? Absolutely.

Will it help you lose weight? Potentially, yes.

But I can't tell you eating lots of veges and not too much processed food will sculpt your body and erase your wrinkles.

Ten years younger? Probably not without Photoshop. These are the promises of plastic surgery. None of us, in our hearts, really believes them.

But it doesn't stop us being vulnerable to The Promise, and trying these diets again and again and again.

• Niki Bezzant is editor in chief of Healthy Food Guide