Sticking to a regimen for 2 days a week can be more achievable than 7 days, so you may be more likely to persevere with this way of eating and successfully lose weight.
Two days a week on a restricted diet can lead to greater reductions in body fat, insulin resistance and other chronic diseases.
The non-restricted days don't mean unlimited feasting. While you don't need to be as strict about your calorie consumption, you still need to make healthy choices and be physically active.
There's a risk that your restricted eating days may not be nutritionally balanced.
Skipping meals could make you feel dizzy, irritable, give you headaches, and make it hard to concentrate, which can affect work and other daily tasks.
Other reported side effects are difficulties sleeping and daytime sleepiness, bad breath and dehydration.
The 5:2 is a simple way to reduce calorie intake. There are lots of versions of this diet, with some being less safe than others.
Many studies on IF are short term, involve small numbers of subjects or are animal-based.
If you choose to follow this diet, choose an evidence-backed plan based on healthy, balanced eating and written by a dietitian, such as the "2-Day Diet".
It's vital for your health to avoid nutritional deficiencies, dehydration and overeating on non-fasting days.
Never attempt to delay or skip meals if you're pregnant, or have had or are prone to eating disorders or diabetes.
The Dukan diet is a low-carb, high-protein diet. There's no limit to how much you can eat during the plan's 4 phases, provided you stick to the rules of the plan.
During phase 1, you're on a strict lean protein diet. This is based on a list of 72 reasonably low-fat, protein-rich foods such as chicken, turkey, eggs, fish and fat-free dairy. This is for an average of 5 days to achieve quick weight loss. Carbs are off limits, except for a small amount of oat bran.
Unlike the Atkins diet, Dukan's phase 1 bans vegetables and seriously restricts fat. The next 3 phases of the plan see the gradual introduction of some fruit, veg and carbs, and eventually all foods.
The aim is gradual weight loss of up to 2lb a week and to promote long-term weight management. There's no time limit to the final phase, which involves having a protein-only day once a week and taking regular exercise.
You can lose weight very quickly, which can be motivating.
It's a very strict and prescriptive diet, which some people like. It's easy to follow, and you don't need to weigh food or count calories.
Apart from keeping to low-fat, low-salt and high-protein foods, there's no restriction on how much you can eat during your first 2 weeks.
At the start of the diet, you may experience side effects such as bad breath, a dry mouth, tiredness, dizziness, insomnia and nausea from cutting out carbs.
The lack of wholegrains, fruit and veg in the early stages of the diet could cause problems such as constipation.
Rapid weight loss can be motivating, but it's unsustainable and unhealthy. The Dukan diet isn't nutritionally balanced, which is acknowledged by the fact you need a vitamin supplement and a fibre top-up in the form of oat bran.
There's a danger this type of diet could increase your risk of long-term health problems if you don't stick to the rules. The diet lacks variety in the initial phases, so there's a risk you'll get bored quickly and give up.
The paleo diet, also known as the caveman diet, consists of foods that can be hunted and fished – such as meat and seafood – or gathered – such as eggs, nuts, seeds, fruits, vegetables, herbs and spices.
It's a regime based on the supposed eating habits of our hunter-gatherer ancestors during the Paleolithic era, before the development of agriculture, around 10,000 years ago. That means cereal grains including wheat, dairy, refined sugar, potatoes – as well as anything processed or with added salt – are strictly off the menu.
There's no official "paleo diet", but it's generally seen as a low-carb, high-protein diet, with some variations on carbohydrate and meat intake.
Advocates say the paleo diet is a long-term healthy eating plan that can help you lose weight and reduce your risk of diabetes, heart disease, cancer and other health problems.
Most studies on the paleo-type diet are small, and more long-term research is needed to show conclusively whether or not it's as effective as some people claim.
A 2015 review of current studies found some moderate evidence for short-term health improvements and weight loss. It concluded that while the modest carbohydrate, healthier fats and lower salt were beneficial, it was less clear whether the restriction on wholegrain foods and dairy was beneficial.
The paleo diet encourages you to eat less processed food, less high-fat and high-sugar foods (such as cakes, biscuits, crisps), and more fruit and vegetables. Reducing your consumption of high-calorie foods will reduce your calorie intake and help you lose weight.
The diet is simple and doesn't involve calorie counting. Some plans are more flexible, which can make the diet easier to stick to and increase your chances of success.
There are no accurate records of the diet of our Stone Age ancestors, so the paleo diet is largely based on educated guesses, and its health claims lack any scientific evidence.
Most versions of the diet encourage eating a lot of meat, which runs counter to current health advice on meat consumption. Many versions ban dairy products and wholegrains, which form part of a healthy, balanced diet. Unless it's for a medical reason, there's no need to cut out whole food groups from your diet. Cutting out food groups without careful substitution can lead to nutritional deficiencies.
The paleo diet can be expensive. For example, it advocates eating only grass-fed meat.
Most versions of the paleo diet exclude key food groups, raising the potential for nutritional deficiencies unless careful substitutions are made, and dietary supplements may be necessary.
The diet has some positive aspects, so an adapted version that doesn't ban any food groups – such as wholegrains, dairy and legumes – would be a better choice.
The diet lacks variety, so there's a risk you'll get bored quickly and give up. If you want to copy your paleolithic ancestors, you're better off mimicking their activity levels rather than their alleged diet.
New Atkins diet
The Atkins diet promises to turn your body into a fat-burning machine. The theory is that by starving yourself of carbohydrates, your body will start burning fat for energy. During the first phase of the diet, designed for rapid weight loss, you're on a protein-rich diet, with no restrictions on fat, and a daily carb allowance of 20 to 25g.
During the next 3 phases, the weight loss is likely to be more gradual, and regular exercise is encouraged. More carbs are introduced to your diet with the aim of working out what your ideal carb intake is to maintain a healthy weight for life.
Phase 1 is designed to help you lose up to 15lb in 2 weeks, reducing to 2 to 3lb during phase 2.
You can lose weight very quickly, which can be motivating.
The diet also encourages people to cut out most processed carbs and alcohol. With its diet of red meat, butter, cream, cheese and mayonnaise, it's one of the diets that appeals most to men.
Initial side effects can include bad breath, a dry mouth, tiredness, dizziness, insomnia, nausea and constipation from cutting out carbs, and potential for lower fibre intake.
The high intake of saturated fat may increase your risk of heart disease, and there are concerns about the recommendation to add salt.
The amount of processed meat, red meat and saturated fat in this type of diet is an issue, as is the advice to add salt. These all contradict current health advice.
Some could still find it complicated and time-consuming, but the promise of initial rapid weight loss may appeal to and motivate some.
The alkaline diet is based on the idea that modern diets cause our body to produce too much acid. The theory is that excess acid in the body is turned into fat, leading to weight gain.
High acidity levels have also been blamed on conditions such as arthritis, osteoporosis, tiredness, and kidney and liver disorders. There is however no scientific evidence for this.
The diet involves cutting back on acid-producing foods such as meat, wheat and other grains, refined sugar, dairy products, caffeine, alcohol and processed foods in favour of "alkaline foods", which reduce the body's acidity levels. This translates into plenty of fruit and vegetables. The idea is that an alkaline diet helps maintain the body's acidity at healthy levels.
There is no evidence that you can change your body's blood acidity (pH level) through what you eat. The weight loss observed among followers is more likely to be the result of eating plenty of fruit and vegetables, and cutting down on sugar, alcohol and processed foods, which is standard healthy weight loss advice.
The diet contains plenty of good healthy eating advice, such as cutting down on meat, avoiding sugar, alcohol and processed foods, and eating more fruit and veg, nuts, seeds and legumes. This means you'll be cutting out foods you may normally eat and replacing them with healthier choices, which will also reduce your calorie intake.
Your body regulates its acidity levels, regardless of diet. When cutting down on dairy products such as milk, cheese and yoghurt, you need to find other calcium substitutes, as cutting out an entire food group is never a good idea.
Getting to grips with what you can and can't eat on the diet can be time-consuming, particularly in the beginning.
The theory of the alkaline diet is that eating certain foods can help maintain the body's ideal acidity levels to improve overall health. But the body carefully maintains its pH balance (called homeostasis) regardless of the food we eat.
The diet is not supported by any evidence. Any weight loss is likely to be because you are being careful about what you are eating, reducing high-fat and high-sugar foods as well as overall calories.
South Beach Diet
The South Beach Diet is a low-GI (glycaemic index) diet originally developed for heart patients in the US. There's no calorie counting and no limits on portions. You're encouraged to eat 3 meals and 2 snacks a day, and follow an exercise plan. People who have more than 10lb to lose start with phase 1.
This is a 2-week rapid weight loss regime where you eat lean protein, including meat, fish and poultry, as well as some low-GI vegetables and unsaturated fats. Low-GI carbs are reintroduced during phases 2 and 3, which encourage gradual and sustainable weight loss.
If you can avoid phase 1 and start on phase 2, there are fewer dietary restrictions in the rest of the plan than some other popular diets.
After phase 1, the diet broadly follows the basic principles of healthy eating. No major food groups are eliminated, and plenty of fruit, veg and low-GI carbs are recommended.
The severe dietary restrictions of phase 1 may leave you feeling weak, and you'll miss out on some vitamins, minerals and fibre. You may initially experience side effects such as bad breath, a dry mouth, tiredness, dizziness, insomnia, nausea and constipation.
The first 2 weeks are the most difficult to get through. We're concerned this diet promises such significant weight loss – up to 13lb – in the first 2 weeks. But this won't be all fat: some of the weight loss will include water and carbs, both of which will be replaced when you begin eating more normally.
Once you get past the initial phase, the diet follows the basic principles of healthy eating and should provide the nutrients you need to stay healthy.
Slimming World diet
Slimming World's weight loss plan encourages you to swap high-fat foods for naturally filling low-fat ones.
You choose your food from a list of low-fat foods they call "Free Foods" that are generally filling and low in energy, such as fruit, vegetables, pasta, potatoes, rice, lean meat, fish and eggs. These can be eaten in unlimited amounts. There are additional healthy extras, such as milk, cheese, cereals and wholemeal bread. There's no calorie counting, no foods are banned and you're still allowed the occasional treat.
You can get support from fellow slimmers at weekly group meetings and follow an exercise plan to become gradually more active. The plan is designed to help you lose 1 to 2lb a week. You can also join an online programme.
No foods are banned, so meals offer balance and variety, and are family-friendly. There is one main plan, called Extra Easy, which is flexible.
The "Body Magic" booklet provided offers ideas to help you raise your activity levels, and meeting as a group can provide valuable support.
The programme advocates eating plenty of low-energy and filling foods. However, while following the Free Foods list, you may choose to eat more lean protein foods and starchy carbohydrate foods than recommended, as these are unrestricted, and this would limit any weight loss if you went over your daily calorie requirements.
Higher-energy treat foods are still allowed but in small quantities. They are known as "syns", which is short for synergy but the similarity to the word "sin" won't be lost on anyone.
The group meetings encourage members to share successes, ideas and recipes with each other, but they may not appeal to everyone. The web-based programme may be helpful for others.
The list of low-energy, filling foods can help to promote a healthy, varied and balanced diet including plenty of fruit and vegetables.
Members gain an appreciation of which foods are higher in energy and should therefore be limited. This is helpful for long-term healthy eating.
The SlimFast diet is a low-calorie meal-replacement plan for people with a BMI of 25 and over. It uses SlimFast's range of products. The plan recommends 3 snacks a day from an extensive list (including crisps and chocolate), 2 meal-replacement shakes or bars, and 1 regular meal taken from a list of recipes on the SlimFast website.
You can stay on the diet for as long as you want, depending on your weight loss goal. Once reached, you're advised to have 1 meal-replacement shake a day, up to 2 low-fat snacks, and 2 healthy meals. The plan is designed to help you lose about 1 to 2lb a week.
Meal-replacement diets can be effective at helping some people lose weight and keep it off. The plan is convenient, as the products take the guesswork out of portion control and calorie counting.
No foods are forbidden, although you're encouraged to eat lean protein, fruit and vegetables.
On their own, meal-replacement diets do little to educate people about their eating habits and change their behaviour. There's a risk of putting the weight back on again once you stop using the products.
You may find it hard to get your 5 A Day of fruit and veg without careful planning.
If you don't like the taste of the meal-replacement products, you won't stay with the plan.
The SlimFast plan can be useful to kickstart your weight loss regime, but it's important that you make full use of the online support to learn about the principles of healthy eating and how to manage everyday food and drink.
The LighterLife weight loss plans combine a very low-calorie diet (VLCD) with weekly counselling. With LighterLife Total, for people with a BMI of 30 or more, you eat 4 meal-replacement food packs a day – consisting of shakes, soups, mousses or bars – and no conventional food. LighterLife Lite, for those with a BMI of 25 to 30, involves eating 3 food packs a day, plus 1 meal from a list of approved foods. There is a new LighterLife Fast Plan based on the 5:2 intermittent fasting plan.
The meal plans can lead to very rapid weight loss, and you're advised to see your GP before starting. How long you stay on the diet depends on how much weight you have to lose.
The counselling can help you understand your relationship with food, so hopefully you can make lasting changes to keep the weight off for good.
With the meal replacements, there's no weighing or measuring, so it's a hassle-free approach to weight loss. Having a break from real food may kick start your weight loss, and the initial rapid weight loss can be motivating.
Initial side effects of the diet can include bad breath, a dry mouth, tiredness, dizziness, insomnia, nausea and constipation from cutting down on carbs and fibre.
Surviving on a strict diet of shakes, soups and other meal replacements isn't much fun and can feel socially isolating.
Rapid weight loss can be motivating, but it's unsustainable. People often regain weight after the diet and, overall, research suggests there is little difference between a VLCD and conventional weight loss after 1 to 2 years.
LighterLife's VLCD and its counselling component may work for some – particularly people who have struggled to lose weight for years, have health problems as a result of their weight and are clinically obese with a BMI of more than 30.
A VLCD that involves eating 1,000 calories or fewer should not be followed for more than 12 continuous weeks. If you're eating fewer than 600 calories a day, you should have medical supervision.
The WeightWatchers Flex programme is based on the SmartPoints system, which gives a value to foods and drink based on protein, carbs, fat and fibre content. It's essentially a calorie-controlled diet where you get a personal daily SmartPoints allowance, which you can use how you like. There's no limit to the amount of fruit and most veg you can eat as part of a list of zero-points foods.
There is a focus on keeping active and choosing exercise that you enjoy as a means of earning points, and there are plenty of recipes to help with the healthy eating weight loss plan. The weekly meetings and confidential weigh-ins provide support and extra motivation to encourage long-term behaviour change. The plan is designed to help you lose up to 2lb a week.
No foods are banned, so you can eat and drink what you want provided you stick to your points allowance.
The SmartPoints system is flexible, easier to follow for some than calorie counting and less restrictive than other plans. There is also online support and mobile apps, with barcode scanners to help with shopping.
When you begin, working out the points system can be just as time-consuming as simply counting calories. Some people may feel pressured into purchasing WeightWatchers-branded foods.
Some of the zero-points foods are low in fat and good sources of protein, and can be quite filling. However, although some may be difficult to eat in large quantities (such as lean chicken or eggs), these foods will still contribute to overall calorie intake so should probably not be completely unlimited.
WeightWatchers Flex is generally well balanced and can be a foundation for long-term changes in dietary habits. The support-group approach can help keep people motivated and educate them about healthy eating.
There is a focus on including exercise as part of the plan, which can help ensure weight loss success. It's important to appreciate the connection between the points system and calories in order to aid long-term weight management.
Rosemary Conley diet
Rosemary Conley's diet and fitness plans combine a low-fat, low-GI diet with regular exercise. You can follow her recipes or her various diets and fitness programmes.
You're encouraged to eat food with 5%-or-less fat, with the exception of oily fish, porridge oats and lean meat. Her online weight loss club has a range of tools and videos covering cooking classes; medical, psychological and nutritional advice; and exercises for all fitness levels. There's also support and motivation from trained coaches.
You learn about calorie counting and portion size, which can help you sustain your weight loss beyond the programme. The diets are designed to help you lose 14lb in 7 weeks and encourage lifestyle change. How long you stay on the plan depends on your weight loss goal.
The programme is based around calories, with a focus on cutting fat. The "portion pots" – used to measure foods such as rice, cereal, pasta and baked beans – teach you about portion control.
Physical activity is an integral part of the plan, with exercise videos suitable for all ages, sizes and abilities offered online.
Some low-fat products aren't necessarily healthier, as they can still be high in sugar and calories.
It's unrealistic to expect people to go out with their portion pots, which means portion control may be tricky away from the home.
The diet and exercise plans offer a balanced approach to weight loss that teaches you about portion size, the importance of regular exercise and making healthier choices. The educational element is very useful for long-term weight management once you've left the programme.
As its name suggests, a sugar-free diet plan involves avoiding most, if not all, types of sugar.
Plans usually require you to cut out food and drink high in free sugars, such as fizzy drinks, breakfast cereals, flavoured yoghurts and biscuits. Some plans involve eliminating carbohydrate in all its forms – free sugar, starchy foods and fibre – but these play an important role in a healthy diet.
Cutting down on free sugars (the sugar added in foods) is a good idea because, as a nation, we consume too much sugar overall. Getting used to understanding the sugar in foods and checking the labels can be helpful.
Going completely sugar-free can be almost impossible, as that would also mean cutting out the sugar in milk and milk products, fruit and vegetables, which would not be a balanced approach.
Cutting down on sugar in things like sugary drinks, biscuit and cakes is a good idea, but removing all sugar, including sugar in milk, fruit and vegetables, is not a sensible approach. The sugar in these foods is slowly absorbed, and these foods contain important nutrients.
Beware of some of the alternative sugar products recommended in some sugar-free plans, such as palm sugar, coconut sugar, agave and honey: these are still all sugars.