Do any diet plans actually work?

Diet planning notebook

The media is awash with claims about the best ways to lose weight, but do these diets risk your health? We give the lowdown on which diets work and which to avoid.

Battling the bulge is big business, and there’s no shortage of celebrity, fad and commercial diets promising an easy way to weight loss. Not all of these diet plans are healthy, particularly if you’re living with a long-term condition.

Maintaining a healthy weight and body shape is important for good health, so how can you do this with a healthy balance?

We explore 12 popular diet plans and debunk some of the claims surrounding them. Scroll down the page or click to read about:

1. Intermittent fasting (the 5:2 diet)

Salmon with broccoli

Dmitry Zuev / Via Flickr

What is intermittent fasting? 

You restrict your calorie intake for part of each day or week (two days per week in the case of the 5:2 diet). The aim is to reduce overall energy intake and therefore lose weight.

What does a typical week look like if you're doing intermittent fasting or the 5:2 diet?

Fast days (usually twice a week):

  • Breakfast – one slice of toast with yeast extract spread (no butter or margarine)
  • Lunch – low-fat vegetable soup (no bread)
  • Dinner – salmon and broccoli; no snacking.

Other days – eat as normal.

Does the 5:2 diet work? 

There is some limited evidence that intermittent fasting leads to weight loss, but not much is known about its long-term effects. Some people do find this approach to dieting easier to stick to, but it won’t work if you overindulge on your non-fasting days. Usually, your guideline daily energy intake is 2,000kcal for a woman and 2,500kcal for a man, although requirements vary depending on your age, physical activity levels and metabolism. To lose weight at a gradual, safe rate without intermittent fasting, you'd usually be advised to stick to a calorie limit of around 1,900kcal a day for men, and 1,400kcal for most women.

There are different versions of intermittent fasting. The 5:2 diet is based on having a normal diet five days a week, and then reducing your calorie intake to a quarter of your normal needs (about 500 calories for women, or 600 for men) two days a week. That works out roughly the same as reducing your intake to 1,600kcal per day, every day. So most intermittent fasting plans are essentially calorie-controlled diets.

Fasting may make you feel lethargic and less active, and you may be tempted to overcompensate on non-fasting days. For some people, fasting may create health risks, such as increasing the risk of low blood sugar in people with diabetes who are using certain types of medication, so take advice from your GP or diabetes specialist if you are thinking of trying this diet.

A healthy, sustained approach to weight loss requires control of your portion sizes and engaging in regular physical activity

How much does the 5:2 diet cost?

There doesn't need to be any additional cost.

2. The paleo diet

Roast beef salad 

What is the paleo diet? 

Also known as "eating like our ancestors", this means you only eat foods available to our prehistoric ancestors, such as vegetables, fruits, nuts and meat, and exclude dairy products, grains, salt, pulses and processed foods.

What does a typical day on a paleo diet look like?

  • Breakfast – steak and eggs cooked with coconut oil
  • Lunch – roasted butternut squash soup or a chicken salad
  • Dinner – beef stew cooked with blueberries, carrots and onion
  • Snack – boiled egg, prawns or nuts

Does the paleo diet work?

By excluding whole food groups, you risk deficiency in certain nutrients. For example, excluding dairy could lead to low calcium levels. Claiming these diets are prehistoric may also be misleading. Evidence suggests cave people ate starchy carbohydrates and other foods ‘forbidden’ on the modern diet, depending on the season and where they lived. Some foods permitted on this diet wouldn’t have been available, including butter and coconut oil, which are high in saturated fat. Too much saturated fat may increase your risk of coronary heart disease and high cholesterol.

Why not try the Mediterranean diet instead? It’s also an ancient diet but there’s sound evidence it promotes heart health and weight loss.

How much does the paleo diet cost?

It can be signficantly more expensive depending on your choice of meats, fish, fruit and nuts.

3. ‘Sugar-free’ diets

Chicken thighs with roasted vegetables  

What is a 'sugar-free' diet?

These cut out added white sugars and often ‘natural sugars’ (eg maple syrup or honey). Some versions cut starchy carbohydrates (eg pasta and rice), which the body converts to sugar, and fruits, which contain sugars.

What does a day look like on a 'sugar-free' diet?

  • Breakfast – poached eggs with avocado
  • Lunch – tuna quinoa salad
  • Dinner – chicken thighs with roasted vegetables
  • Snack – carrot sticks with cashew nut butter

Does a sugar-free diet work? 

We’re all consuming too much sugarfree sugars include added sugars in food and drink, and the sugar in fruit juices. These account for around 12 per cent of UK adults’ daily energy intake, more than double the recommended maximum of five per cent.

The intake of children and teenagers is even higher. This can contribute to weight gain, which can lead to type 2 diabetes – two risk factors for cardiovascular disease (CVD).

We don’t need to exclude starchy carbohydrates. These provide us with energy and maintain digestive health.

Honey and syrups may be ‘natural’, but they’re still sugars. Dairy products and fruit contain sugars, but come with nutrients too. It’s better to cut down on food and drinks with free sugars instead, and these are usually the types of sugar in our diet that we are eating the most of.

We don’t need to exclude starchy carbohydrates (pasta, bread and cereals), which should make up a third of our diets. These provide us with energy and maintain digestive health, as well fibre from wholegrain versions.

How much does a sugar-free diet cost?

It may be cheaper if you're not buying sugary foods.

4. Slimming groups

Salmon and beetroot salad

What are slimming groups?

Local groups with weekly meetings where an instructor weighs you and offers motivational advice. Some are online only.

What does a typical day look like on a slimming group diet?

  • You are given an allowance for different types of foods to guide your meal choices each day.
  • No foods are banned, but healthier choices mean your allowance goes further.
  • Most vegetables are ‘free’ so you are encouraged to eat plenty of them.

Do slimming groups work?

The NHS sometimes refers people to slimming groups. Their strong focus on calorie counting and portion control (both sound principles) can encourage sustained weight loss. The aim is to make it simpler for people to limit their calorie intake while allowing occasional treats, which is a sensible long-term approach.

The support of others at the groups may help, too. However, many also offer ‘add-ons’ such as ready meals, snacks and breakfast bars. It is healthier (and cheaper) to cook from scratch or swap a snack bar for a portion of fruit, which will also help you towards your 5-a-day. And although slimming group products are calorie-controlled, they may be high in salt.

How much do slimming groups cost?

It depends on the group - usually a joining fee (around £10) and weekly group fee (around £20-25 per month).

5. High-protein or high-fat diets (eg the Atkins diet)

Eggs with bacon 

cyclonebill / Via Flickr

What are high protein (or high fat) diets?

It means you reduce carbohydrate intake and increase intake of fat and/or protein to compensate. This reduces overall calorie intake and can induce ‘ketosis’ – a state where the body breaks down fat into ‘ketones’ to provide energy, causing rapid weight loss.

What does a typical day look like on a high-protein/ high-fat diet?

  • Breakfast – bacon and fried eggs
  • Lunch –a burger with salad instead of a bun
  • Dinner –steak with broccoli

Do high-protein diets work? 

While you may lose weight quickly at first, there’s no evidence that these diets are any more effective than others in the long-term. High-fat diets can be unhealthy, too. While fats are important, too much fat will unbalance the diet.

We need to ensure we’re including the right types of fat (unsaturated fats like olive, sunflower or rapeseed oil and nuts). Take care if you’ve got high blood pressure; processed meats like bacon and sausages can mean the diet is high in salt.

While fats are important, too much fat will unbalance the diet

High-protein diets may also increase your risk of kidney damage and osteoporosis. Due to the low carbohydrate intake, bad breath can be a problem, along with other side effects like fatigue, dizziness, insomnia and nausea.

Fruits, vegetables and wholegrain carbohydrates should make up three quarters of your diet, with 20 per cent from protein-rich foods like meat, fish and dairy products. Unsaturated oils and spreads should account for just 1%.

How much do high-protein diets cost?

They can be significantly more expensive depending on your choice of meats, fish and fruit.

6. Juicing

A glass of cucumber and apple juice 

bertholf / Via Flickr

What is a juicing diet?

A juicing diet or juice diet is a short-term diet consisting entirely of fruit and vegetable juices. Often advertised for ‘detoxing’ as well as weight loss. Juices can be made at home or bought pre-made from a commercial company.

What does a typical day of juicing look like?

  • You will consume only fruit and vegetable juices for a short period of one to five days.

Do juicing diets work?

These diets are not nutritionally balanced. You’ll miss out on important nutrients like calcium and protein. Side effects include fatigue, headaches, diarrhoea and bad breath. Drinking only juices is likely to make you feel hungrier, too.

When fruits are blended into juices, it causes the ‘natural’ sugars, which are incorporated into the cellular structure of the fruit, to become ‘free’ sugars, which are more likely to have a negative effect on dental health and weight.

Juicing, even for a short time, could be dangerous for people with certain pre-existing conditions. People taking certain medications should also be careful.

Juicing, even for a short time, could be dangerous for people with certain pre-existing conditions. People taking certain medications should also be careful. For example, if you have diabetes, too much fruit juice can raise your blood glucose levels.

People taking certain medications should also be careful: grapefruit juice can interact with statins and calcium channel blockers, and cranberry juice can interact with warfarin, as can a dramatic change in the amount of fruit and vegetables that you consume.

If you want to include fruit or vegetable juice as part of a healthy, balanced diet, limit it to one small glass a day (150ml), which will count as one of your 5-a-day.

How much do juicing diets cost?

Companies charge around £140-£200 per week. If making your own juices, allow £30 - £100 for juicer, and £30-£70 a week for fruits and vegetables.

7. Low GI diet

Porridge with berries on top 

What is a low GI diet?

‘Glycaemic index’ (GI) is a scale that assigns a value to carbohydrate-rich food based on how much it affects blood sugar levels. Foods with a lower GI (like vegetables and pulses), are broken down more slowly, providing a more gradual rise in blood glucose.

What does a typical day look like on a low GI diet?

  • Breakfast – porridge with berries
  • Lunch – mackerel and chickpea salad
  • Dinner – chicken and white bean stew, with baked apple for dessert

Does a low GI diet work?

Milk chocolate and ice cream have a low GI, but this doesn’t make them healthy

Following a low GI diet that includes plenty of fruits and vegetables and lower GI versions of carbohydrates (eg, swapping high GI breakfast cornflakes for low GI porridge), and that limits intake of foods and drinks high in sugar and fat, can be a healthy, sensible and balanced way to lose weight.

Most GI diets also focus on reducing fat intake (especially saturated fats). However, don‘t get too caught up in the numbers – milk chocolate and ice cream have a low GI, but this doesn’t make them healthy.

Similarly, rice cakes have a higher GI than chocolate but are much lower in calories. Combinations of foods affect the GI of a meal. Some low GI plans encourage ‘phases’ of dietary restriction, which could cause you to miss out on key nutrients.

How much does a low GI diet cost?

It should be fairly similar to your normal diet.

8. Meal replacements

Meal replacement shake 

danlev / Wikimedia

What are meal replacement diets?

Manufactured products replace regular meals. Companies selling the plans promise quick weight loss. The diet works by limiting calories. Some are termed ‘very low calorie diets’, providing less than 800kcal per day. These should be followed for no more than 12 consecutive weeks and under medical supervision.

What does a typical day look like on a meal replacement diet?

  • Specially manufactured products, like shakes, soups and bars, replace most of your meals.

Do meal replacement diets work? 

The rapid weight loss can be motivating, but many people revert to old eating patterns and put weight back on afterwards. While meal replacements often contain added vitamins and minerals, some are high in salt and saturated fats.

These programmes come at a cost of £120 to £200 a month. They are restrictive when it comes to family meals and eating out, and reported side effects include fatigue, diarrhoea, constipation and nausea.

How much do meal replacements cost?

Typically around £120 - £150 per month, plus the cost of your other "normal" meals.

9. Wheat-free/gluten-free diet

Lentils salad 

What is a wheat-free or gluten-free diet?

Whilst some people need to follow a special diet for medical reasons, there is a growing trend for people to go ‘gluten-free’ or ‘wheat-free’ to lose weight, or because they perceive that they have an intolerance.

What does a typical day look like on a wheat-free or gluten-free diet?

These can vary as they are about what is out rather than in. Gluten-free would mean:

  • Eating meat, fish, dairy, fruit and vegetables, rice, potatoes and lentils.
  • Cutting out bread, pasta, pizza, cakes, biscuits and cereals (though gluten-free versions are available), as well as most beers and some ready-made foods.

Do gluten-free diets work? 

Wheat-free and gluten-free alternatives include rice, buckwheat, quinoa and oats

Any weight lost using this approach is probably due to consuming less food and therefore fewer calories, or because you’re cutting out foods like cakes and pastries (which come with lots of fat and often sugar), rather than any negative aspects of wheat.

You may find yourself eating more fruit and vegetables, whilst avoiding processed foods and decreasing your sugar and fat intake - all of these are good things. Remember to include wholegrain and high fibre carbohydrates, as they help to keep your digestive system healthy. Wheat-free and gluten-free alternatives include rice, buckwheat, quinoa and oats (gluten-free oats are available in some shops). 

Most importantly, you should speak to a GP about your symptoms before excluding food from your diet, and never self diagnose an allergy or intolerance.

How much do wheat or gluten-free diets cost? 

Wheat-free version of products like bread and pasta can be more than double the cost of regular products. But if you buy unprocessed wholegrain alternatives, like oats and brown rice, you shouldn't see much increase in cost.

10. Dairy-free diet

Lentils salad 

What is a dairy-free diet?

This means you cut out all dairy products, including milk, butter and cheese.

What does a typical day look like on a dairy-free diet?

These can vary a lot, but it might mean:

  • Breakfast - fruit with coconut yogurt, or cereal with a dairy-free milk alternative.
  • Lunch - Jacket potato with baked beans and dairy-free spread, or a sandwich made with dairy-free bread and a dairy-free spread.
  • Dinner - Anything that doesn't contain dairy products. 

Do dairy-free diets work?

You may not lose any weight if you replace dairy products with higher-calorie alternatives

Any weight lost using this approach is probably due to consuming less food and therefore fewer calories, or because you’re cutting out foods like milk chocolate, cakes and pastries (which come with lots of fat and often sugar), rather than any negative aspects of dairy products as such. You may not lose any weight if you replace dairy products with higher-calorie alternatives, especially those based on coconut. For example, coconut yoghurt can contain significantly more calories than a low-fat dairy yoghurt. 

If you have lactose intolerance, this diet should improve symptoms. However, you should speak to a GP about this, and never self-diagnose an allergy or intolerance. 

If you do cut out dairy products from your diet, make sure you choose alternatives that are fortified with calcium and have no added sugar, so that you don't miss out on this nutrient or increase your sugar intake.

How much does a dairy-free diet cost? 

Lactose-free or dairy-free milks and dairy alternatives are usually more expensive than standard version. A 500g tub of dairy-free yoghurt, for example, costs between £1.50 and £3.50, compared with 85p to 1.50 for standard low-fat yoghurt. A litre of dairy-free milk costs around £1-£1.50, compared with around 75p for a litre of standard milk. 

11. Food combining diets

Chili con carne

What are food combining diets?

These diets are based on the principle that different foods require different acidity to be digested properly, and all have different transit times through the gut.

Combining certain foods (eg. carbohydrate and protein) together in one meal is thought to make it harder for the body to digest them. This supposedly reduces absorption of nutrients, and slows down the time food spends in the gut, promoting the build-up of gas and toxins. Sometimes foods are classified into acidic, alkaline and neutral.

What does a typical day look like on a food combining diet?

  • Breakfast – spinach with eggs
  • Lunch – chicken and salad
  • Dinner –fish with baked Mediterranean vegetables (no rice, potatoes or pasta)
  • Snack – fruit only on its own

Do food combining diets work?

These diets are based entirely on pseudoscience. It is nonsense to suggest that carbohydrates can’t be digested in an acidic environment; after all the stomach is acidic itself! We digest different nutrients at different rates and the body has adapted to an omnivorous diet over the years.

How much does a food combining diet cost? 

No significant extra costs.

12. The "mono diet" (single food diets) 

Grapefruit in a bowl 

Rebecca Sims / Via Flickr

What is the "mono diet" or single food diet?

These are diets which restrict you to one food group only, or one type of food only. Classic examples include the cabbage soup diet and grapefruit diet, but there are more recent versions with almost any fruit and vegetable you can think of, including bananas and potatoes. 

What does a day look like on a "mono diet"?

  • The same thing for every meal, every day.

Do single food diets work?

Some people claim that only eating one type of food is easier on your digestion, but there is no evidence this is the case. These diets are very strict, boring and repetitive, and may leave you craving other foods. There are many reported side effects, including bad breath and diarrhoea. These diets require lots of planning and cooking separate meals from the rest of the family.

These diets are very strict, boring and repetitive, and may leave you craving other foods

If you are on certain medications for diabetes, you may be at risk of hypos, depending on what is allowed on the diet and how much you eat. If you are on statins or calcium channel blockers, beware of the grapefruit diet as this may interact with medication.

In the short term, these diets can cause quick weight loss, simply due to the severe calorie restriction. However, because these plans are so short term (usually seven days), this weight loss will mainly consist of water and muscle loss, rather than fat loss.

How much does a single food diet cost?

It very much depends on which food you choose.

But what is a balanced diet? 

The Eatwell plate is a visual representation of a healthy, balanced diet. It shows that your diet should be mainly fruits and vegetables and wholegrain or high fibre starchy carbohydrates, with some protein foods including beans, pulses, fish, eggs, meat and dairy and alternatives.

Small amounts of unsaturated fats should also be included. But cut down on foods high in fat and sugar and limit them to small amounts.

If you need to lose weight you can maintain the proportions of the food groups in your diet, but reduce your portion sizes. This way, you eat fewer calories while still getting the nutrients you need.

  • A traditional Mediterranean diet includes all of the elements of a balanced diet - watch our animation to learn about the Mediterranean diet.

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