Fad diets explained

Niamh Hennessy
Lead Dietitian, Cromwell Hospital
13 January 2021
Next review due January 2024

If you want to lose weight or eat more healthily you might be tempted to try a new diet. Perhaps you’ve seen some diet plans online, read about them in the news, or been recommended one by friends and family. Here I talk about some well-known fad diets and whether they can help you eat more healthily.

What is a fad diet?

There isn’t an official definition of a fad diet, but they tend to be diets that you follow for a short amount of time. They usually claim to help you lose weight. Fad diets tend to go in and out of fashion, and there have been many different ones over the years. They’re often endorsed by celebrities or promoted on social media.

Low carb diets

You might have heard of people reducing the amount of carbohydrates they eat in order to try and lose weight. This is often called a ‘low carb’ diet. Famous low carb diets include:

  • the Atkins diet
  • the Dukan diet
  • the ketogenic diet
  • the fast 800

The idea behind low-carb diets is that when you eat fewer carbohydrate foods your body starts burning fat for energy, causing you to lose weight. There is some evidence to suggest that low carb diets can help you lose slightly more weight than low fat diets. If you’re somebody who wants to cut down the amount of less nutritious carbohydrate foods you eat, this diet may help you to do that.

But, if you follow a low carb diet you might also find you eat fewer fruits, vegetables, and wholegrains. This will reduce the amounts of vitamins, minerals and fibre in your diet. Some people also experience unpleasant side-effects from low carb diets, including:

There isn’t a lot of evidence about how safe it is to eat a low carb diet long term. It’s important to bear in mind that some people have become severely unwell from following low carb plans. You might also find it harder to stick to a restrictive diet like this for a long time.

Cutting out specific foods

There are also diets which restrict specific lists of foods. These lists are usually based on claims with little or no scientific evidence behind them. Well-known examples include:

  • the paleo diet, also referred to as the caveman diet
  • the alkaline diet
  • blood type diets

Some of these diet plans might help you to form healthier habits. This is because they often encourage you to eat fewer processed foods, less salt and sugar, and more fruits and vegetables. This can help you to feel better and lose weight.

But these diets can also be very restrictive and difficult to follow. For example, they might require you to:

  • give up healthy foods like grains, pulses and dairy products
  • spend more money than usual to buy specialist foods
  • spend a lot of time planning and preparing your meals

You might also struggle to get all of the nutrients you need from a restricted list of foods. More research is needed about the long-term safety of this kind of diet.


These diets involve eating a normal, healthy diet between periods of fasting. ‘Fasting’ can mean eating no food, or only eating a small amount. You might have heard of:

There’s some evidence to show that periods of fasting could have some benefits. For example, studies have found that you can lose similar amounts of weight in the short-term as you can from a calorie-controlled diet.

These plans can be appealing for if you don’t want to count calories. But keep in mind that there isn’t any long-term data on how well they work. Many people also find it hard to stick to them. There are also side-effects that can happen when you fast, such as:

  • a lack of energy
  • trouble concentrating
  • feeling light-headed
  • constipation
  • headaches

We also don’t know how safe or helpful it is to follow a diet which includes fasting over a long period of time.

How to lose weight safely

This list by no means includes all the diets out there. There are many others we haven’t covered. If you’re thinking about changing your diet, it’s important to be aware of which ones are based on unscientific advice or have misleading claims.

Some of the main things to watch out for are diets that:

  • involve avoiding or severely restricting entire food groups
  • involve eating only one type of food
  • promise rapid weight loss
  • are trying to sell products such as bars or shakes
  • aren’t backed up by any good evidence

Bear in mind that many diets are popular because they’re seen as a ‘quick fix’, but there’s really no such thing. Although you may lose weight at first, you often won’t sustain the weight loss in the long run. As soon as you stop restricting what you eat, you will probably put the weight back on. And, strict restrictions can be dangerous, and put you at risk of long-term health problems.

It’s far better to think about your overall diet and lifestyle. Aim to eat a healthy, balanced diet and build some activity into your daily life. Eat regular meals with sensible portion sizes.

If you’re considering any fad diet or eating plan which involves restricting certain foods or fasting, you should speak to your GP or a dietitian first. They can help you to make sure you aren’t putting your health at risk.

Bupa offers digital GP services through different routes to suit you. If you have Bupa health insurance you have unlimited access to Digital GP appointments through the Digital GP app (in partnership with Babylon) and if you don't have health insurance, our remote private GP service is available to anyone who wishes to book a self-pay video appointment with a private GP via Bupa Health Clinics.

Niamh Hennessy
Niamh Hennessy
Lead Dietitian, Cromwell Hospital

    • Fad Diets. British Dietetic Association., published October 2017
    • Obesity. Oxford Handbook of Nutrition and Dietetics (3rd edition). Oxford Medicine., April 2020
    • Low Carb. Diabetes UK., 15th January 2019
    • What to Eat on The Paleo Diet., accessed 07 January 2021
    • Popular diets. Oxford Handbook of Nutrition and Dietetics (3rd edition). Oxford Medicine., April 2020
    • Preventing excess weight gain. NICE., published 13 March 2015
    • The Fast 800., accessed 12 January 2021

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