Fad diets explained

Expert reviewer, Mr Paul McArdle, Registered Dietitian
Next review due January 2021

If you're looking to lose weight or improve your diet, it can be tempting to follow the latest diet you’ve seen in the media. There always seems to be a new one that claims to outshine all others and will miraculously help you to rapidly lose weight.

Here we take a look at 10 of the most popular diets and the claims behind them.

Green soup and green vegetables

Paleo diet (caveman diet)

The Paleolithic (Paleo) diet consists of foods thought to have been eaten by our ancestors thousands of years ago. It’s also referred to as the caveman diet. It’s based around foods that may have been hunted (meat and fish) or gathered (uncultivated plant foods, such as vegetables, fruits and nuts). Grains, beans, dairy products, salt, refined sugars and processed oils are all excluded. Supporters claim it has beneficial effects on diseases such as type 2 diabetes and cardiovascular disease, which are associated with a modern Western diet.

You may well lose weight when you start this diet, if you reduce your calorie intake. However, there’s no evidence that this can be sustained over the long-term. More importantly, it can be risky to your health to cut out entire food groups – grains and dairy products are an important part of our diet. More research is needed about the long-term safety of this kind of diet.

Atkins diet

The Atkins diet is a high-protein, low-carbohydrate diet. You begin by cutting your carb intake right back for the first couple of weeks, before gradually reintroducing them, but at a much lower level. Meanwhile, you can have unrestricted amounts of protein – including meat, fish, eggs and fats such as butter and cheese. The idea is that with reduced carbs, your body starts burning fat for energy – and so you lose weight.

As with many restrictive diets, you're likely to have some rapid initial weight loss on the Atkins diet. However, it's not thought to be any more effective in the long term than a standard low-fat diet. And you might find it harder to stick to a restrictive diet like this for very long.

As you eat less fruit, vegetables, and wholegrain bread and cereals on the Atkins diet, it really restricts your fibre intake. So constipation is a common side-effect. You may also suffer with bad breath, muscle cramps, diarrhoea and general weakness. There’s no good evidence to say what the long-term effects on health are of following this diet. But the high levels of saturated fat associated with the diet means it could increase your risk of heart disease.

5:2 diet

In this diet, you eat a normal, healthy diet for five days of the week, followed by two days of fasting. You shouldn’t eat to excess on the five ‘normal’ days. Although you don’t restrict your diet as much on these days, you might still need to eat less than you’re used to.

There’s some evidence to show that the 5:2 diet could have some benefits. Studies have found that you can lose similar amounts on the 5:2 diet over the short-term as you can on a standard calorie-controlled diet. People who have gone on this diet have also shown similar improvements in markers for conditions like diabetes and cardiovascular disease. For some people, it might be a worthwhile alternative to try. But keep in mind that there isn’t any long-term data on how well it works. There are also side-effects associated with fasting, such as lack of energy and concentration, feeling light-headed, bad breath, constipation and headaches. Many people find it hard to stick to for a long period of time.

For more information about this, read our blog on intermittent fasting.


This is another low-carb, high protein diet, which follows four phases. In phase one, you're restricted to certain high-protein foods, before you gradually reintroduce certain vegetables and then limited amounts of fruit, starchy and dairy foods. You’re expected to then live by three strict rules, which include eating three tablespoons of oat bran a day and having one pure protein day a week.

The Dukan diet promises rapid weight loss in the first phase of the programme. It’s hard to say how effective it is as no clinical studies have evaluated the diet. Such strict restrictions can be dangerous though, and put you at risk of long-term health problems.


The alkaline diet is based on a belief that controlling the acidity (pH) in your body through your diet will improve your health. You eat fewer foods considered to be acidic, such as meat, fish, dairy and grains, and more alkaline foods, such as fruits, nuts and vegetables. People on the diet are advised to test the pH of their urine to make sure that it is alkaline and not acidic.

In reality, the pH of your urine is a very poor reflection of the pH more generally in your body, particularly of your blood. Your body has various effective mechanisms for maintaining your blood pH within a very narrow range. The food you eat may alter the pH of your urine, but not the pH of your blood. The types of food recommended on the alkaline diet are actually quite healthy – but there’s no evidence behind the claims of how the diet works. And there are no clinical studies to support the diet as an effective means for weight loss, or for improving health.

An icon of a DNA helix Smarter living. It’s in your DNA.

Bupa SmartDNA examines your genetic composition to help you eat, move and think smarter. You’ll get help from a health and wellbeing coach to make sense of the science, and build a plan around your body, so that you have the tools you need to live smart. Learn more about SmartDNA >

An icon of a DNA helixSmarter living. It’s in your DNA.

Zone diet

The Zone diet sets particular proportions for different nutrients that you should have. At each meal, it recommends two-thirds carbohydrate (mainly vegetables and a little fruit); one-third protein, and a little monounsaturated fat. It’s based on an idea that this mix of foods is optimal for reducing ‘diet-induced inflammation’. And this ‘inflammation’ is proposed as being a reason for weight gain and disease.

As with many of these diets, there’s some evidence for short-term weight loss, but this doesn’t seem to be sustained over the long term. There’s no evidence to back up the theory behind the programme.

Ketogenic diet

In the Ketogenic diet, you cut your carbohydrates to very low levels, with the aim of burning your fat stores for energy. In this way, it’s similar to the Atkins diet and others where you restrict your carbohydrate intake. However, with the Ketogenic diet, you maintain low carbohydrate intake over the long term, rather than just for an introductory phase. The diet has been used successfully under medical supervision to treat epilepsy, before it gained popularity as a weight-loss diet.

There’s no denying that you may be able to lose a significant amount of weight on this diet. But a diet this restrictive is hard to stick to, and you risk serious long-term damage by cutting out important food groups.

Raw food diet

In this diet, you eat only raw, unprocessed and uncooked, mainly plant-based food. It’s mainly a vegan diet, although some people do eat animal products, such as unpasteurised milk and raw fish or meat.

There may be lots of health benefits to all the fruit, vegetables, nuts and seeds that you can eat on this diet. You’re more than likely to lose weight. In fact it’s easy to lose too much weight on this diet, and become underweight. It’s a difficult diet to follow and stick to, and restricting food to this extent can be extremely damaging to your health over the long term. Eating raw fish and meat and unpasteurised milk can also be a health risk.

DASH diet

DASH stands for ‘dietary approaches to stop hypertension’. It’s an eating plan that was developed in the US with the aim of lowering high blood pressure and improving heart health. The main principles of the DASH diet are similar to those in standard healthy eating recommendations. It’s based around eating plenty of whole grains, vegetables, fruits, meat, poultry and fish, and low-fat dairy products. DASH additionally limits the amount of fatty red meat you eat, and caps your salt intake.

DASH has been shown to be effective at reducing high blood pressure. Although it wasn’t designed as a method to lose weight, it may help to do so when combined with an exercise and weight management programme.

South beach diet

This diet was originally designed to improve heart health, but is now generally used as a weight loss diet. It’s a low-carb, high protein diet that distinguishes between ‘good’ and ‘bad’ fats and carbohydrates.

If you need to lose more than 10lb, you start with a two-week quick-weight-loss regime where you reduce your carb intake and eat more protein. After this, you can start adding in more ‘good’ low-glycaemic index (GI) carbs from whole grains, fruit and vegetables. The aim is to eventually enjoy all foods in moderation, while following the basic principles of the diet to maintain your weight over the long term.

Following a low GI diet has benefits because it means choosing a diet low in refined sugars and rich in wholegrain carbohydrates. However, like many restrictive diets, there’s no evidence that any immediate weight loss you see is sustained in the long term.

How to lose weight safely

This list by no means includes all the diets out there. There are many others we haven’t covered, including meal replacement and commercial weight loss programmes, detox diets, and diets where you’re limited to one particular food. It’s important to be aware of which ones are based on unsound, unscientific advice or have misleading claims. Some of the main things to watch out for are diets that:

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

Many diets are popular because they’re seen as a quick fix, but sadly there’s no such thing. Although you may lose a few pounds at first, you often won’t sustain the weight loss in the long run. As soon as you stop restricting your calorie intake, you will likely put the weight straight back on again.

To successfully lose weight, you really need to think about your lifestyle as a whole. Aim to eat a healthy, balanced diet and build some activity into your daily life. Eat regular meals with sensible portion sizes. See related information for more tips and advice on losing weight safely.

Did our information help you?

We’d love to hear what you think. Our short survey takes just a few minutes to complete and helps us to keep improving our health information.

About our health information

At Bupa we produce a wealth of free health information for you and your family. This is because we believe that trustworthy information is essential in helping you make better decisions about your health and wellbeing.

Our information has been awarded the PIF TICK for trustworthy health information. It also complies with the HONcode standard and follows the principles of the The Information Standard.

The Patient Information Forum tick  This website is certified by Health On the Net Foundation. Click to verify.

Learn more about our editorial team and principles >

Related information

Tools and calculators

    • Klonoff DC. The beneficial effects of a Paleolithic diet on type 2 diabetes and other risk factors for cardiovascular disease. J Diabetes Sci Technol 2009; 3:1229–32
    • Anton SD, Hida A, Heekin K. Effects of popular diets without specific calorie targets on weight loss outcomes: systematic review of findings from clinical trials. Nutrients 2017; 9:822. doi:10.3390/nu9080822
    • Fad diets. British Dietetic Association., last reviewed October 2014
    • The 4 phases of Atkins. Atkins Nutritionals., accessed 21 November 2017
    • Obesity. Oxford handbook of nutrition and dietetics. Oxford Medicine Online., published December 2015
    • Fields H, Ruddy B, Wallace MR, et al. Are low-carbohydrate diets safe and effective? J Am Osteopath Assoc 2016; 116:788–93. doi:10.7556/jaoa.2016.154
    • Nordmann AJ, Nordmann A, Briel M, et al. Effects of low-carbohydrate vs low-fat diets on weight loss and cardio-vascular risk factors. Arch Intern Med 2006; 166:285–93. doi:10.1001/archinte.166.3.285
    • Astrup A, Laresen TM, Harper A. Atkins and other low-carbohydrate diets: hoax or an effective tool for weight loss? Lancet 2004; 364:897–99. doi:10.1016/S0140-6736(04)16986-9
    • Cholesterol. British Dietetic Association., last reviewed October 2017
    • Harvie M, Wright C, Pegington M, at al. The effect of intermittent energy and carbohydrate restriction v. daily energy restriction on weight loss and metabolic disease risk markers in overweight women. Br J Nutr 2013; 110:1534–47. doi:10.1017/S0007114513000792
    • Harvie MN, Pegington M, Mattson MP, et al. The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers: a randomised trial in young overweight women. Int J Obes 2011; 35:714-27
    • Dukan diet phases., accessed 23 November 2017
    • The alkaline diet: an evidence-based review. Healthline., published 9 June 2017
    • Schwalfenberg GK. The Alkaline diet: is there evidence that an alkaline pH diet benefits health? J Environ Public Health 2012; 2012:727630. doi:10.1155/2012/727630
    • The Zone diet. Dr Sears Zone., accessed 23 November 2017
    • Atallah R, Filion KB, Wakil SM, et al. Long-term effects of 4 popular diets on weight loss and cardiovascular risk factors: a systematic review of randomized controlled trials. Circ Cardiovasc Qual Outcomes 2014; 7:815–27. doi:10.1161/CIRCOUTCOMES.113.000723
    • Bueno NB1, de Melo IS, de Oliveira SL. Very-low-carbohydrate ketogenic diet v low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. Br J Nutr. 2013; 110:1178–87. doi:10.1017/S0007114513000548
    • Koebnick C, Garcia AL, Dagnelie PC, et al. Long-term consumption of a raw food diet is associated with favorable serum LDL cholesterol and triglycerides but also with elevated plasma homocysteine and low serum HDL cholesterol in humans. J Nutr 2005; 135:2372–78
    • Koebnick C1, Strassner C, Hoffmann I, et al. Consequences of a long-term raw food diet on body weight and menstruation: results of a questionnaire survey. Ann Nutr Metab. 1999; 43:69–79
    • Description of the DASH eating plan. National Heart, Lung and Blood Institute., updated 16 September 2015
    • Blumenthal JA, Babyak MA, Hinderliter A. Effects of the DASH diet alone and in combination with exercise and weight loss on blood pressure and cardiovascular biomarkers in men and women with high blood pressure: the ENCORE study. Arch Intern Med 2010; 170
    • Popular diets. Oxford handbook of nutrition and dietetics. Oxford Medicine Online., published December 2015
    • How it works. South Beach Diet., accessed 22 November 2017
    • Martin K, Jackson CF, Levy RG, Cooper PN. Ketogenic diet and other dietary treatments for epilepsy. Cochrane Database of Systematic Reviews 2016, Issue 2. Art. No.: CD001903. DOI: 10.1002/14651858.CD001903.pub3
  • Reviewed by Pippa Coulter, Freelance Health Editor, January 2018
    Expert reviewer, Mr Paul McArdle, Registered Dietitian
    Next review due January 2021