Cutting carbohydrates is often promoted as a strategy that can deliver benefits well beyond weight loss.
Supporters of low-carb eating plans say reducing foods such as bread, rice and pasta may help put type 2 diabetes into remission, cut liver fat by as much as 70 per cent and even reduce epileptic seizures.
But if the potential benefits are so wide-ranging, why do many scientists remain cautious about recommending low-carb diets?
Professor Tom Sanders, one of the UK’s leading nutrition science experts, argues that these diets often increase the amount of fat people consume. In his view, that could damage blood vessels and eventually raise the risk of heart disease.
Adding to the debate, an alarming study published earlier this year suggested that eliminating carbohydrates might promote the growth of cancerous tumours.
Researchers at the Massachusetts Institute of Technology found that mice fed a low-carbohydrate diet were more likely than control animals to develop cancer in the small bowel.
So where does the evidence point? Leading nutritionists examine the often-confusing science surrounding low-carb and keto diets.
Can cutting carbs really shift stubborn weight when other approaches have failed? How much bread can you eat? And who should steer clear of this diet altogether?
Keto, also known as a low-carb diet, forces the body to switch its primary fuel source from carbohydrates to fat, including fat found in foods such as meat and eggs
How does it work?
Most low-carb plans, from the Atkins diet that became popular in the 1990s to meat-focused carnivore regimes, fall broadly within the wider category of ketogenic diets.
They restrict carbohydrates sharply, while most calories come from fat and protein.
As a result, foods such as bread, rice, pasta, potatoes and cereals are generally excluded. Sugar-rich fruits, including bananas, and root vegetables may also be limited or removed.
Meat and fish provide protein, while foods such as cheese, butter, eggs and olive oil supply fat. Leafy green vegetables are also commonly included.
The precise balance remains contested, but a standard ketogenic diet usually consists of roughly 70 to 75 per cent fat, 20 to 25 per cent protein and only 5 to 10 per cent carbohydrates.
Put simply, carbohydrates become a small accompaniment rather than the foundation of a meal. Instead of centring dinner on pasta or a jacket potato, a typical keto plate might feature a protein such as salmon, additional fat from avocado or oil, and green vegetables.
Many studies, however, use an intake of about 30 grams of carbohydrates per day. That is approximately the amount found in two slices of bread or one large banana.
When carbohydrate intake falls, the body has less of the nutrient it normally uses to produce or store energy. Insulin levels then begin to decline.
Insulin is a hormone made by the pancreas. It helps transport sugar from the bloodstream into the body’s cells, where it can be used as energy or stored for future use.
After several days of severe carbohydrate restriction, the body can enter a metabolic state called ketosis. At this point, it moves away from burning sugar, or glucose, and begins relying more heavily on fat stored in the liver.
The liver turns that fat into substances known as ketones. The brain and other organs can then use these compounds as an alternative source of energy.
Studies have found this process can help people lose weight and improve blood sugar control.
But whether keto is superior to other healthy diets in the long term remains fiercely contested.
Will going low-carb help me shed a load of weight – and keep it off?
One of keto’s biggest selling points is the speed at which weight loss can occur.
Within the first week of adopting a keto-style diet, slimmers can lose up to six pounds, studies suggest.
Other research has found that going low-carb can reduce levels of hunger hormones like ghrelin, with some papers suggesting the appetite-suppressant effect is comparable to blockbuster jabs such as Mounjaro and Wegovy.
But this doesn’t mean carbohydrates themselves are responsible for weight gain – or that removing them entirely is an effective approach, explains nutritionist and author Rhiannon Lambert.
‘One of the reasons people often notice a relatively rapid drop in weight when starting a low-carb or keto diet is because of changes in the body’s stores of glucose,’ she says.
‘This means that much of the initial weight loss can be attributed to water rather than body fat – and this is an important distinction given how often rapid weight loss is promoted as evidence that keto diets are somehow superior.’
What’s more, the speedy drop in weight isn’t the case for everyone – it can take two to four days on a low-carb diet to enter ketosis, Ms Lambert adds.
The most reliable evidence shows a weight loss of roughly 10 per cent of body weight within a year. For the average person weighing 13st this is roughly a stone.
Several reviews have concluded that, ultimately, low-carb results in roughly the same weight loss as any lower calorie diet. However, experts say it may appear easier to shift pounds on low-carb because it is a simple plan, making it easy to follow.
Dr Federica Amati, head nutritionist at health company ZOE, does not recommend staying on a low-carb diet for ever, but says it can be a helpful place to start for those trying to lose weight
But even when weight is lost, in many cases, at least some of it is regained once the diet is stopped.
A recent study involving 294 adults on a low-carbohydrate diet found that after an average follow-up of about three years, half had regained at least a quarter of the weight they had initially lost.
According to Dr Federica Amati, one of the UK’s leading nutritional scientists, low-carb is not supposed to be a ‘forever diet’ – but rather a way to help people start to lose weight before transitioning to a more flexible Mediterranean-style diet.
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This approach prioritises gut-healthy fibre, lean proteins and healthy fats, while keeping some low-carb principles in place. The key mistake made by most people, Dr Amati says, is that they forget the transition and instead stick to low-carb which, in most cases, is doomed to eventually fail.
‘When we think about long-term health, legumes, healthy fats, wholegrains and fibre have been shown to be some of the most important food groups when it comes to longevity and healthy ageing,’ Dr Amati says.
‘Low carb diets are not sustainable long-term interventions, but they can help people achieve the initial weight loss they need to see to drive long-term habits.’
Could going low-carb help cure diabetes?
In 2021, a team of international experts published a major analysis that would go on to influence diabetes treatment across the globe.
Writing in the British Medical Journal (BMJ), the scientists reported results that showed low-carb diets were more likely to achieve diabetes remission than any other eating plan.
The findings were so influential that even the NHS recommends the diet for type 2 diabetics.
‘As carbohydrates are broken down into glucose during digestion, reducing carbohydrate intake can help limit rises in blood sugar after eating,’ Ms Lambert explains.
However, more recent evidence seems to suggest that much of the beneficial impact for type 2 diabetics is a result of weight loss – rather than the reduction of carbs specifically.
In the BMJ study, the researchers found that many of the patients saw their diabetes return after a year because they had regained weight.
Harvard researchers found that those on a vegetable-based low-carb diet were far more likely to see weight loss than those eating animal-based or junk-food heavy diets
‘Sustained weight loss can play a role in achieving remission – and a low carb diet is one way to do this,’ says Ms Lambert.
Today, the NHS recommends a very low calorie diet for type 2 diabetes, which involves eating just 800 or 900 calories a day for 12 weeks, before calories are increased gradually over the course of a year.
Studies show that around a third of diabetics who follow the plan can put their condition into remission and lose around two and a half stone in a year.
The diet CAN reverse liver damage
Contrary to popular belief, eating more fat and fewer carbohydrates could help dramatically reduce dangerous fat in the liver.
In a study – published in the journal Cell Metabolism – researchers split 55 volunteers into three groups, each instructed to follow a popular diet programme.
All were obese and had prediabetes as well as fatty liver disease, where fat builds up in the organ and can eventually cause serious liver damage.
Together, the conditions are known to increase the risk of heart attack and stroke.
But researchers found those who ate a keto diet – restricting carbs in favour of high-fat foods such as meat, eggs and cheese – not only lost around 10 per cent of their starting weight, but nearly 70 per cent of fat in their liver.
And, there were no significant differences between the diets when harmful LDL cholesterol was tested for – despite the diet’s high fat content.
Experts say this could be due at least in part to the fact that overeating carbs and carrying excess weight can trigger inflammation, increasing the risk of chronic disease.
The dark side of low-carb…there’s a serious cancer risk
One of the major drawbacks of low-carb diets is that many people end up eating less fibre than they already do.
In the UK, adults are advised to eat around 30g per day of the nutrient that’s found in wholegrains like brown rice and wholemeal bread – but most of us fall short, consuming just 17g a day.
It’s thought that around a third of bowel cancers in the UK could be prevented by eating more fibre.
As the body cannot break down this type of carbohydrate, it passes to the large intestine where it plays a vital role in digestion – helping to move waste products out of the digestive tract.
The downside of cutting out foods like wholegrain bread through a keto diet is that it limits our fibre intake
‘Eliminating carbohydrate-rich plant foods also means missing out on valuable sources of fibre, vitamins, minerals and beneficial plant compounds,’ Ms Lambert said.
‘Fibre plays an important role in digestive health, cholesterol regulation and supporting our gut microbiome, so severely restricting these foods may have wider implications for long-term health.’
A recent study from the University of Bath found that after 12 weeks, people following a keto diet had a lower fibre intake, higher bad cholesterol and reduced beneficial gut bacteria.
There is also some evidence that following this restricted eating plan can cause a number of nutrient deficiencies including magnesium, zinc and B vitamins which are typically found in wholegrains.
Over time, this can cause brain fog, persistent headaches, hair loss and fatigue – as resources are directed towards more important organs and processes.
Experts are also concerned that keto diets high in processed meat, butter and cheese could increase levels of LDL cholesterol and potentially raise the risk of bowel cancer, although evidence remains mixed.
But slimmers can mitigate the risk by upping their intake of leafy green vegetables and prioritising unsaturated fats, found in oily fish, nuts and seeds.
An analysis published earlier this year found intriguing links between people who ate a low-carb diet and a reduced risk of dying young.
However, the association only applied to those who ate a ‘healthy’ low-carb diet – consisting of leaner protein and plenty of fibrous fruit and vegetables.
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