The science behind six popular diet fads, from Keto and Paleo to Low GI and 5:2 – The Telegraph

The science behind six popular diet fads, from Keto and Paleo to Low GI and 5:2 – The Telegraph

Meal plans for weight loss can be a daunting prospect. However, planning ahead can help you make healthy eating choices and stick to your weight loss goals.

There are many meal planning resources available to choose from. The most efficient way to do this is to use our Meal Plan Selector tool to find the plan that is right for you.

The word fad may immediately spring to mind when you hear someone mention their latest diet regime. We all know it’s not rocket science. “Eat less and move more” is a mantra that’s hard to quibble with. 

But there’s an emerging understanding that it’s not as simple as that: the health of our gut microbiome may play a part. Research is illuminating just how individual our response is to ­different types of foods.

It is clear that genetics and personality play a role in determining our physical dimensions and wellbeing. However, when we’re feeling the need to lose weight for health reasons, it can be tempting to follow the herd and adopt an approach that is regimented and restrictive. 

To cut through the hype, we consulted experts at the British Nutrition Foundation (BNF) to explain the science behind some well-known and often well-publicised diets.

“Although some of these diets have evidence to show they can lead to weight loss and other health benefits, many of them are also restrictive diets and involve the exclusion of key food groups that, in the long term, could have health consequences,” warns BNF dietitian Dr Louise Durrant.

Before embarking on a restrictive diet, you should consult your doctor and you should also view such a diet as a temporary measure, rather than a long-term lifestyle.

A more moderate tactic, Dr Durrant advises, would be to follow the BNFs healthy-eating model, the Eatwell Guide, which describes how much of what we eat overall should come from each food group to achieve a healthy, balanced diet. “Just making small, simple changes to what and how much you are eating and drinking can really help you lose the pounds, whilst maintaining a healthy, balanced diet,” Dr Durrant says.

Calorie counting

When calorie counting, it’s important not to avoid healthier foods that are higher in calories – such as oily fish, avocados and plain nuts Credit: Getty Images

The original weight-loss approach has been dropped in preference to the view that it’s what you eat, rather than how many calories that really counts. However, it remains a popular metric and in April 2022 the Government introduced calories on restaurant menus. 

Calories are a measure of the amount of energy provided by an item of food or drink; the average woman needs roughly 2,000 calories (kcals) per day and the average man 2,500, but the exact number of calories each person needs will depend on factors including their size and how active they are. 

Regularly consuming more calories than we need can lead to weight gain over time. 

If you need to lose weight and calorie counting is a suitable option for you, the NHS recommends that the average person should reduce their daily calorie intake by about 600 kcals. That means reducing calories from the recommended daily allowance to 1,400 kcals for women and 1,900 kcals for men.

“It has been noted that regularly monitoring your own dietary intake may support weight loss, as part of multi­faceted weight-loss interventions. However, it is recognised that motivation can decrease over time,” says Dr Stacey Lockyer, senior nutrition scientist at the British Nutrition Foundation.

A systematic review and meta-analysis, which combined the results of studies into the effects of multicomponent behavioural weight-loss programmes (which provide diet and physical activity recommendations, including behavioural support sessions) in overweight and obese adults, reported that there was evidence (potentially confounded) that asking participants to count calories led to a 7lb greater weight loss than similar programmes that did not do this.


It’s important not to become obsessed with calories, as the quality of the foods that you are eating matters, too.

It is possible to consume a low number of calories simply by eating small amounts of less healthy foods and no healthy foods – this isn’t advisable and is likely to leave you hungry.

It’s important not to avoid healthier foods that are higher in calories – such as oily fish, avocados and plain nuts – completely. 

The non-calorie-counting approach

This regime focuses on cooking healthy meals from scratch Credit: Getty Images

Interestingly, a large randomised clinical trial (Dietfits) conducted in 609 overweight/obese and non-diabetic adults, who were instructed by dietitians to follow either a healthy low-fat or a healthy low-carbohydrate diet (without calorie restriction), reported that both diets resulted in significant weight loss: 5.3kg (12lb) for the low-fat diet and 6kg (13lb) for the low-carbohydrate diet, over 12 months, and both groups consumed fewer calories, even though they had not been instructed to do so.

“This resulted from diets focusing on cooking healthy meals from scratch, low intake of added sugars and high intake of fruit, vegetables and whole grain products,” says Dr Lockyer.


To eat healthily and lose weight, you will still need to think about portion sizes – you can find out more about managing portion sizes at


Keto diets have recently gained attention due to reports that they may also help with weight loss and blood-sugar control Credit: Getty Images

You’ve no doubt met a ketovangelist. The diet has become increasingly ­popular in the past few years; the chef Tom Kerridge and the politician Tom ­Watson have both followed the diet with success.

Keto diets were first developed for the treatment of seizures in patients with epilepsy, but they have more recently gained attention due to reports that they may also help with weight loss and blood-sugar control. 

“Keto” is short for “ketogenic”, which refers to the state in which your body begins to generate something called “ketone bodies” from fat, due to a lack of readily available carbohydrate. Our bodies do this to provide a metabolic fuel for our brain, heart and other organs that usually run on glucose (sugar) and can’t use fat as an energy source. 

When the supply of carbohydrate in our diet is not sufficient to meet demand, ketone bodies are produced in the liver. This is also referred to as entering into a state of “ketosis”. 

“There are several types of keto diet out there,” says Dr Simon Steenson, nutrition scientist at the BNF, “including cyclical, targeted and high-protein versions, but the most studied is the ‘standard’ keto diet, where you typically consume fewer than 50 grams of carbohydrates per day.

“A number of trials in patients with type 2 diabetes suggest that following a very low-carbohydrate ketogenic diet (VLCKD) is effective for losing weight, and may also help improve blood-­glucose control and reduce cholesterol levels.

For example, an analysis published in 2021 combined results from eight clinical trials, including almost 650 people with type 2 diabetes. The researchers found that a VLCK diet led to a greater reduction in bodyweight and HbA1c (a marker of long-term glucose control) compared to following a control diet.”

However, there was no difference between diets at 12 months. “This might be partly explained by the difficulty many people have sticking to a low-carb dietary pattern in the long-term,” Dr Steenson adds. More trials are needed to know if a keto diet is superior to other weight-loss diets in the long term.


Side effects may include headaches, lack of energy and dizziness, and many people find that this type of diet can be difficult to stick to in the long term.

Pulses such as beans, lentils and chickpeas are not allowed on a keto diet – a problem if you are trying to source your protein other than from meat. Also, eating too many fatty meats can increase your high saturated fat intake.

The 5:2

Dr Michael Mosley first popularised what is often referred to as intermittent fasting Credit: Shutterstock

It seemed like a magic solution when the idea was first popularised by Dr Michael Mosley: “fast” for two days in the week, consuming between 500 calories (women) and 600 calories (men), and then eat a healthy, balanced diet for the remaining five days.

Since then, the term intermittent fasting has been used to describe any window where you don’t eat; typically, eight hours. 

“The idea is that fasting creates a calorie deficit, which means that once the body uses up its carbohydrates stored in the liver and muscles, it relies on fat stores for energy instead, which can lead to weight loss,” says Sarah Coe, nutrition scientist at the British Nutrition Foundation. 

“There is some evidence from clinical trials to suggest that intermittent fasting can help with weight loss. But the evidence for the 5:2 diet specifically is limited compared to other weight-loss diets.”

Importantly, evidence from clinical trials also suggests that forms of intermittent fasting such as the 5:2 diet don’t appear to be any more effective than more traditional weight-loss diets with continual daily calorie restriction. 

“There is also some evidence to suggest that intermittent fasting could bring other health benefits, including improved blood pressure and blood-glucose control, and lower cholesterol levels, which could reduce the risk of disease. But compared to other types of weight-loss diets, the amount of evidence for any specific health benefits of intermittent fasting is limited,” says Coe. 

Many of the studies are short term or in small numbers of subjects, or have been done in animals, so more research is needed to explore the possible long-term health benefits in humans.


The 5:2 diet can be restrictive, and some people may find it challenging to stick to in the long term, particularly as it can result in hunger and low energy levels, disrupting work or social plans. 

For the 5:2 diet to work, it’s important to stick to the calorie limits on “fast” days and not go overboard on the “normal” days, as this can hinder any weight loss or could even lead to weight gain. 

The 5:2 diet isn’t recommended for women who are pregnant or breastfeeding, people with a history of an eating disorder, those who are prone to fainting, and children and teenagers.

Fast 800

The creation of Dr Michael Mosley, who helped to popularise the 5:2 diet, the Fast 800 diet involves three stages: first, a very low-calorie stage with about 800 calories a day for the first few weeks, which could include meal-­replacement products.

The second stage involves intermittent fasting, restricting calorie intake to 800 calories a day for two days a week, and a lower-carb, Mediterranean-style diet, rich in vegetables, fruit, protein sources and unsaturated fats, for the rest of the week.

The third stage is a Mediterranean-style diet to help maintain a healthy weight. 

“The first very low-calorie stage can stimulate mild ketosis, where your body switches from burning sugar to burning fat, which can result in rapid weight loss and unpleasant side effects such as headaches, lack of energy, and dizziness,” says Sarah Coe of the BNF. 

The amount of weight loss will depend on your starting weight, how much you need to lose and how well you stick to the first challenging stage. 

“Evidence from trials such as Direct and Droplet, where people living with being overweight or obese were put on an intensive weight management programme, which included eating about 800 calories a day, supports some of the health and weight-loss ­benefits claimed by the Fast 800 diet,” says Coe. There are currently no ­published trials investigating the Fast 800 diet specifically.


Very low-calorie diets are one of the most restricted forms of dieting, and these diets are generally only recommended for people who need to lose weight fast because of obesity-related complications or surgery, and dieting under medical supervision. 

Up to 12 weeks of a very low-calorie diet may not be safe for the general population. Very low-calorie diets are particularly not suitable for children and teenagers, women who are pregnant or breastfeeding, and people who have had an eating disorder.

Low GI

Low-GI foods include wholegrain cereals like oats, and wholemeal or higher-fibre breads such as multigrain or sourdough Credit: Getty Images

The “GI” here doesn’t refer to commando rolls and shinning up ropes; it refers to the glycaemic index of how different foods affect your blood sugar.

The concept of the glycaemic index was developed by Canadian Dr David Jenkins in 1981. His research showed that some starchy foods could have ­similar effects on blood sugar to simple sugars, depending on how quickly they were absorbed.

Examples of foods with a higher GI value include white bread and white rice, non-wholegrain breakfast cereals, and sugary snacks such as doughnuts and cakes. The low-GI diet approach recommends avoiding or swapping these foods for low-GI alternatives, such as wholegrain cereals like oats, and wholemeal or higher-fibre breads such as multigrain or sourdough types. 

The science behind the GI diet is that poor blood-glucose control can increase the risk of type 2 diabetes, which is linked to a higher risk of heart disease, stroke and other complications. 

“Studies in people with prediabetes and type 2 diabetes suggest that a low-GI diet may help to improve fasting blood-sugar levels, reduce body weight, and HbA1c levels, a long-term marker of glucose control,” says Dr Simon Steenson, nutrition scientist at the British Nutrition Foundation.

Following a low-GI diet may also help with weight loss for people who have normal blood-glucose levels. “An analysis published in 2018, which included more than 100 studies and over 8,500 people, concluded that a low-GI diet led to small improvements in body weight and total cholesterol levels,” says Dr Steenson. “However, the effect on body weight and cholesterol was greater when people achieved a large reduction in the overall GI score of their diet of 20 points or more.”

It has also been reported that eating a lower GI diet might lower the risk of heart disease and some cancers: “Although these findings are based on observational studies where people report what they eat, which generally provides weaker evidence than controlled clinical trials.”


GI values do not account for the amount of food you eat, so you still need to stick to sensible portion sizes. 


Paleo is the modern interpretation of the diet that humans ate during the paleolithic or Stone Age era Credit: Getty Images

Often called the “caveman diet”, the paleolithic or “paleo diet” is the modern interpretation of the diet that humans ate during the paleolithic or Stone Age era, about 2.5 million years ago, during which humans learnt to cook meat using fire.

“Several systematic reviews and meta-analyses show that there is some evidence that the paleo diet can lead to small amounts of weight loss in the short term, but larger and longer-term, well-designed studies are needed,” says Dr Lockyer.


A paleo diet typically consists of meat, fruit and vegetables, but no pulses, dairy foods or grains. 

“This means it’s very imbalanced compared to the UK’s healthy eating model, the Eatwell Guide,” says Dr Lockyer, who warns that it could lead to fibre deficiency. 

“Diets higher in fibre reduce our risk of heart disease, type 2 diabetes and colorectal cancer. Fibre is also important for digestive health and intake of calcium, which is important for bone health. It’s important to remember that we are advised that those consuming more than 90g of red and processed meat (cooked weight) per day should cut down to no more than 70g per day (or 500g per week) due to the link with increased risk of colorectal cancer.”

Frequently consuming large amounts of fatty meat can also result in high saturated fat intakes, which can raise blood cholesterol over time.

Visit British Nutrition Foundation

Have you tried any of these diets? Did they work for you? Please let us know in the comments


Kerri Waldron

My name is Kerri Waldron and I am an avid healthy lifestyle participant who lives by proper nutrition and keeping active. One of the things I love best is to get to where I am going by walking every chance I get. If you want to feel great with renewed energy, you have to practice good nutrition and stay active.

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