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It's pretty common to hear that a new dieting trend is apparently backed by a favourite celebrity - and as people prepare for their post-Christmas healthy-eating plans, they're even more common.

But which diets actually work? The British Dietetic Association, which is made up of food and nutrition experts, has taken a look at some of the most well-known and given its verdict.

1. Clean eating

Those who adhere to this diet, who reportedly include the actresses Miranda Kerr and Jessica Alba, eat "clean" foods and avoid all processed foods, which means cutting out refined sugar, making everything from scratch and eating foods in their "natural state".

Some versions of the diet even exclude gluten and grains and dairy.

The BDA says: The idea that foods are "clean" or "dirty" is concerning, and could potentially lead to "orthorexia nervosa" - where someone becomes obsessed with sticking to foods deemed to be healthy and eliminating unhealthy ones.

Even foods that are actually nutritionally beneficial are deemed to be unhealthy, such as those containing whole grains, fruit and dairy, with no basis in scientific evidence.

There is no need to avoid foods unless you have an allergy or intolerance that has been medically diagnosed.

And sometimes, suggested substitutions - such as coconut oil, and syrups to sweeten foods - are as high in calories, no better nutritionally and more expensive.

2. Diet pills

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These claim to prevent your body absorbing fat, to "melt" fat away, or to reduce appetite. Kim Kardashian and Sam Faier from The Only Way is Essex are reported to have used them.

The BDA says: No-one should take diet pills without talking to a doctor, pharmacist or dietician.

The pills can cause side-effects such as diarrhoea - and if bought from unregulated websites, there is no way of knowing what is in them.

Some have been found to contain elements such as pesticides that are unfit for consumption and have proven fatal.

3. Tea-toxes

Nicki Minaj and Kylie Jenner's Instagram accounts have featured these products.

The term is short for "tea detoxing", and products claim to improve skin, reduce bloating and help you to lose weight.

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The BDA says: These teas often contain extra caffeine, diuretic ingredients (that make the body get rid of water) or laxatives such as senna, which is not safe to take for longer than a week without medical supervision.

Any inches lost are probably to do with water-weight loss.

Replacing foods or higher calorie drinks with these teas might lead to extra weight loss, but also risks side-effects such as diarrhoea, dehydration and even gut damage.

4. The 6:1 diet

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We've all heard of the 5:2 - this goes just a little further. Reportedly followed by Coldplay singer Chris Martin, it involves eating your usual diet for six days - and then not eating at all on the seventh.

The BDA says: If people fast without managing it properly, they will often struggle to concentrate, feel very tired and experience low mood.

It can be dangerous too, depending on your age, health and lifestyle.

If fasting sounds like something you could manage, speak to a medical professional to make sure it's healthy and safe for you.

5. Green juices

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Green means healthy, right? These juices or smoothies are made up of various fruits, veg and powders. Rosie Huntington-Whiteley, Blake Lively and Gwyneth Paltrow are said to be fans, Benefits are said to range from detoxing to rejuvenation and weight loss.

The BDA says: These juices are unnecessary. They will certainly not make an otherwise unhealthy diet OK, and the body is "perfectly able" to detox itself.

Plus, people are often tempted to add extras such as nuts, coconut oil and avocadoes to their morning juice - meaning they can come in at up to 400kcal per glass.

Eating a normal breakfast as well is likely to mean you will gain weight, not lose it.

The BDA advice is: "Keep your veg and fruit whole, and limit juice/smoothies to 150ml [half a cup] per day."

So what is the advice?

These points all feature in the Eatwell Guide - backed by both the BDA and NHS Choices:

Aim for those five portions of fruit and veg each day

Choose a variety of types and colours in order to get a range of vitamins, minerals and fibres

Include one starchy food - ie bread, chapattis, cereals, potatoes, rice, oats and pasta (preferably wholegrain)

Opt for lean meats - and eat fish a couple of times a week, one oily

Three portions of dairy foods offer the right level of calcium - such as a small pot of yoghurt or a matchbox sized piece of cheese - reduced fat versions are best

Low fat oils and spreads are better

Try to cut down on foods and drinks with high levels of sugar or fat

Make small sustainable changes you need to make forever

Enjoy a rich variety of foods in appropriate portion sizes

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