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Updated: 29 Nov 2021

Do popular diets really work?

We assess the evidence behind popular weight loss plans such as keto, paleo and Slimming World, plus how to lose weight sustainably
Shefalee Loth

The diet and weight loss industry in the UK is big business - each year we collectively spend around £2 billion on diet plans, books, apps, and related foods and supplements. 

Despite this, government figures show around two-thirds of adults in the UK are overweight or obese. So what's going wrong?

The problem with diets for weight loss

Dr Giles Yeo, obesity geneticist, told us: 'Our bodies have evolved to stop us losing weight. When we were hunter-gatherers our bodies stored fat to use during times of famine and equated the amount of fat we carried to our chances of survival.

When we lose weight our brain interprets this as a lower chance of survival and fights back, making it harder to lose weight, and trying to get us back to our starting weight'.

He also adds it's important we accept we're not all going to be one size and that our weight is determined by many things including our genetics. Dr Yeo adds: 'We all have a different "set" weight which is where our weight naturally sits.'

Age is also a factor - weight can creep up as we age due to our metabolism slowing, possibly moving around less and eating habits not adapting accordingly.

This is why it can be challenging to lose weight and keep it off.

Many people turn to diet plans for help. None are a magic fix, but some are more useful than others. 

Get our verdict on the pros and cons of popular diets below, plus tips for sustainable weight loss that sticks.

Does calorie counting work?

Bowl of salad and a calorie tracker app
A bowl of salad and a calorie tracker app

This is a popular approach with many people using apps such as MyFitnessPal and Nutracheck to help them stay on track.

The approach of calorie counting does sound sensible - to lose weight you need to create a calorie deficit and eat fewer calories than you consume.

But all calories aren't equal. A lollipop and an apple might contain the same number of calories but the apple fills you up in a way the lollipop doesn't. The apple also contains nutrients that the lollipop lacks.

During digestion our bodies extract calories from different foods differently - known as caloric availability.

An apple contains fibre we can't digest and some of the calories are tied up in this fibre making them unavailable. In the case of the lollipop which is sugar almost all the calories are available.

The different macronutrients, carbohydrates, protein and fat, also have different caloric availability.

Dr Yeo explains in his book, Gene Eating, that it takes our bodies more energy to digest protein compared to carbohydrates and fat. For example, for every 100 calories of lean meat you eat your body uses around 30 calories to digest it.

In the case of carbohydrates, for every 100 calories consumed, five to 10 calories are used in the digestion process. Fat is the most efficient fuel for our bodies, it only takes around three calories to digest 100 calories of fat.

VERDICT: You'll come unstuck sticking rigidly to calories, you also need to think about the foods you're eating to achieve healthy and sustainable eating habits.

Read more: Should I buy a fitness tracker or smartwatch?

Do juicing and detox diets work?

Picture of woman pouring a green smoothie
Woman pouring a green smoothie

Detox diets range from avoiding specific foods or ingredients such as sugar, gluten, dairy and caffeine, to consuming nothing but juices or teas for a period of time to 'cleanse' your system.

These diets often involve spending money on products such as special juices, teas and supplements or foods which can be delivered to your house pre-prepared.

Influencers and brands will wax lyrical about clearer skin, improved digestion and boosted immune systems. 

But there's nothing wrong with most of the foods these diets demonise when they're consumed moderately (as long as you don't have an allergy, for example in the case of gluten in coeliac disease). 

What's more our bodies are already equipped with effective detox systems in the form of our liver, kidneys, lungs and skin.

On the plus side, many detox diets encourage eating lots of fruit and vegetables but these alone aren't enough to sustain us. And when consumed in the form of juices or smoothies, these can contain lots of 'free sugar' which is damaging to teeth and much of the fibre is lost or broken down in the juicing process.

Detox diets and juice cleanses can lead to rapid weight loss as calorie intake nosedives. However, these diets aren't sustainable for long and the weight will return as soon as you return to eating normally.

VERDICT: Unnecessary and unsustainable.

Find out more: Do you need a blender, juicer or smoothie maker?

Keto, Atkins and Paleo: do low-carb diets work?

Fish, meat, eggs and vegetables
Fish, meat, eggs and vegetables

The Keto, Paleo and Atkins diets are all trendy variations on a theme - low-carb diets.

They contain very little carbohydrate, around 20-50g a day or 5% of calories (compared to the national average of around 50% of calories coming from carbohydrates) and are instead high in protein and fat.

The carbs that are allowed are generally non-starchy veg such as green leafy veg and broccoli, nuts and seeds.

Removing carbs such as bread, pasta, potatoes and rice effectively excludes a whole food group from your diet. This results in a lower calorie intake and weight loss.

People on low-carb plans usually experience significant weight loss quite quickly, which can be very motivating. However, this weight loss is mainly from losing stored carbs and fluid.

The lack of carbs can cause headaches and brain fog, caused by the lack of glucose in the diet, as well as unpleasant digestive issues due to lack of fibre.

Low-carb plans exclude or severely limit wholegrains, legumes, most fruit and starchy veg so it can also be hard to get a good balance of nutrients.

When excluded foods are re-introduced any weight lost usually returns.

VERDICT: Restricts some important foods and can have unpleasant side effects.

Does the 5:2 diet or intermittent fasting work?

Person with a bottle of water
Person with a bottle of water

There are several versions of intermittent fasting but the most popular is the 5:2 which involves eating 'normally' for five days of the week and 'fasting' on two. On the fasting days women are restricted to 500 calories and men to 600 calories.

The 5:2 can work for people who only want to control their intake on a couple of days and carry on as normal the rest of the time. But fasting days are tough and people say they feel tired and lack concentration.

Another version of intermittent fasting is the 16:8 where followers only eat within an eight-hour window each day (noon to 8pm for example) and fast for 16 hours so effectively cut out one meal each day, usually breakfast. Some people find this easier.

Intermittent fasting works by resulting in an overall calorie deficit over the course of the week but obviously won't work if you over-eat on your non-fasting days or within your eight-hour eating window. It can result in a slight binge mentality.

VERDICT: Can work for people who want more flexibility and not to constantly watch what they're eating, but has drawbacks.

What is the blood-type diet and does it work?

Blood-type diet app on phone
Blood-type diet app on phone

The blood-type diet is one of the more wacky ones - based on the theory that your blood type affects what foods you should and shouldn't eat and the way your body digests food.

These do usually include healthy suggestions such as fruit and veg over processed foods but different blood types end up with much more restricted diets. Type A is essentially vegan while O is low-carb. 

Followers are likely to lose weight initially because the restrictive nature of the diet reduces calorie intake, but cutting out whole food groups can increase the risk of nutrient deficiencies. This diet is not supported by scientific evidence.

VERDICT: Not supported by evidence and unnecessarily restrictive

Do slimming clubs work?

Meeting of a slimming group
Meeting of a slimming group

Slimming clubs include WW (formerly Weight Watchers) and Slimming World, and cost up to £6 a week. 

Traditionally these groups involve weekly meeting where members meet to discuss experiences and share ideas. However many also now offer a digital option where this community experience is online.

Members are taught the principles of healthy eating and provided with recipes and support from coaches. 

These plans allow for occasional treats so aren't as restrictive as some diets.

The sense of community works well for those that struggle to diet alone and provides them with support.

VERDICT: Generally based on good principles and the support aspect is helpful but there is a cost attached.

Does the Alkaline diet work?

Table full of fruit and vegetables
Table full of fruit and vegetables

This diet is based on the unproven theory that acidity in the body causes disease. Because our blood is slightly alkaline, the belief is that eating alkaline foods will maintain the pH of our blood and reduce the risk of disease.

However the major fault with this plan is that the pH of food doesn't affect the pH of blood.

Counterintuitively, the diet includes acidic foods, such as lemons, limes and grapefruits, as permitted 'alkaline' foods.

On the plus side, this diet encourages followers to eat lots of fruit, vegetables, nuts and seeds. It discourages alcohol, caffeine and sugar (including cakes and biscuits) so can help overall to promote healthy eating habits. But it also discourages a range of food groups including meat, dairy, eggs and lentils leaving not very much to eat at all.

VERDICT: Based on a ropey principle and very restrictive.

Do diet pills work?

Diet pills, scales and tape measure
Diet pills, scales and tape measure
We know diets are tough and it can be tempting to turn to weight-loss pills and aids that are promoted in chemists, pharmacies and online. 

These include detox teas, diet pills, appetite suppressants in the form of lollipops and gummies and herbal remedies.

In most instances the evidence to support their use doesn't stand up to scrutiny, and we'd advise particular caution buying diet pills online and from unknown sources. These products may be ineffective and a waste of money, and some can also be dangerous for your health and contain laxatives.

Over-the-counter pills such as Alli work by binding the fat in your food and stop it from being absorbed. Instead it passes through your digestive system and many users report unpleasant side effects including oily stools. 

Some prescribed medicines such as Semaglutide, an appetite-suppressant, have shown promising results in clinical trials with patients losing significantly more weight than those on a placebo. 

However for long-lasting results these drugs have to be taken long-term. As soon as they are stopped appetite returns to normal.

Read more on How to set up a home gym

How to make weight loss stick long-term

Fad diets are not new. While we may be encouraged to eat according to our blood type now, there's always something questionable doing the rounds. 

At the beginning of the last century people were swallowing tapeworms to lose weight. In the 1950s the grapefruit, lemonade and cabbage soup diets were popular - as was the use of amphetamines to aid weight loss.

While some of the diets we've looked at above are ultimately faddy, others can be useful to kickstart weight loss journeys. Extreme diets are hard to sustain for any length of time and their restrictiveness means you could end up with nutritional deficiencies.

For sustained weight loss you need to commit to changes to your diet and activity levels that are manageable for you long-term.

Tips for sustainable weight loss:

  • Build a support network: Sustained weight loss is often easier if you have support. Whether that's the support of friends and family or an organised group it helps to have some encouragement to help keep you on track.
  • Think food not calories: Think about the foods you eat and not just the number of calories in them. Foods high in fibre and protein will slow digestion and help keep you feeling fuller for longer.
  • Don't restrict foods too heavily: Restricting foods or whole food groups is hard to sustain in the long-term. Most foods are fine in moderation and we all know if something is forbidden we crave it even more. 
  • One size doesn't fit all: Dr Yeo says: 'Our biology makes weight loss difficult, and what worked for your friend or a celebrity won't necessarily work for you. Everyone needs to find what works for them and their biology. At the end of the day the right diet for you is one you can stick to'.