It is an eye-catching promise: a simple diet that can dramatically reduce dangerous fat in the liver – without giving up indulgent foods such as bacon, sausages, eggs and cheese.
And, given recent disappointing headlines about the potential of processed meats to increase cancer risk, it sounds almost too good to be true.
But that’s exactly what a group of American scientists investigating the body-wide impact of a range of eating plans have discovered.
In a study, published recently in the journal Cell Metabolism, researchers split 55 volunteers into three groups, each instructed to follow a popular diet plan.
All were obese and had prediabetes – dangerously high blood sugar that can progress to type 2 diabetes – 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 substantially increase the risk of heart attacks and strokes.
During the study, one group was given a Mediterranean diet, rich in fruit and vegetables, whole grains, fish, nuts and olive oil.
Meals included oatmeal with almonds and blueberries, minestrone, salmon, quinoa and black beans, chicken pesto pasta and roasted sweet potatoes.
The ketogenic diet can dramatically reduce dangerous fat in the liver – and it doesn’t involve giving up indulgent foods such as bacon, sausages, eggs and cheese
The second group followed a very-low-fat, ‘plant-forward’ diet based largely around fruit, vegetable, grains, beans and other pulses, with 70 per cent of calories coming from carbohydrates and just 15 per cent from fat.
They ate meals such as oatmeal and fruit, red beans and rice, vegetable wraps, mixed grains, sweet potato and black bean soup and vegetable chilli. It was not strictly vegetarian – some meals included salmon.
The final group was put on a ketogenic diet, also known as keto, which severely restricts carbohydrates in favour of high-fat foods such as meat, eggs and cheese.
Just four per cent of their calories came from carbohydrates, while 73 per cent came from fat.
Their meals included omelettes with bacon and sausage, cheesy beef, pulled pork, salmon with cauliflower mash, pot roast and chicken with asparagus.
After five months, volunteers in all three groups lost roughly the same amount of weight – about 10 per cent of their starting weight.
However, when other health factors were looked at, the differences were striking.
Most significantly, the amount of fat in the livers of the keto diet group dropped on average by 67 per cent, compared to 45 per cent in both the Mediterranean and plant-forward diets.
Half of the keto group experienced prediabetes remission – with blood sugars returning to normal – versus 29 per cent in the Mediterranean group and just 7 per cent in the plant-forward group.
And volunteers in the keto group saw a 20 per cent drop in blood sugar, on average, compared to a fall of just 8 per cent in the other two diet groups.
And there was another surprise: despite the keto group’s high fat and saturated fat intake, there were no significant differences between the diets when harmful LDL cholesterol and ApoB were tested for.
Both of these markers, found in the blood, increase the risk of heart disease.
The researchers said the findings showed that the benefits were not simply down to losing weight – what the volunteers ate appeared to matter too.
‘Our study demonstrates the superiority of a very-low-carbohydrate ketogenic diet in improving hepatic [liver] metabolic function… and lowering other risk factors for diabetes and cardiovascular disease,’ they wrote.
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In other words, despite all three groups losing roughly the same amount of weight, the ketogenic diet appeared to deliver additional benefits – particularly for the liver and blood sugar control.
The researchers acknowledged several limitations.
The study was small, with just 42 people completing it, and it remains unclear whether people would need to stick to the restrictive ketogenic diet to maintain the benefits.
Nor could the researchers say whether a less extreme low-carbohydrate diet might have produced similar results without requiring people to enter ketosis.
They said larger and longer studies were now needed to confirm the findings.
But the research throws up an intriguing question: Is the keto diet the healthiest option, when it comes to reversing liver damage and blood sugar issues? And could more of us benefit from following a low-carb plan such as this, even if we aren’t struggling with life-threatening health problems?
First, it’s useful to look at exactly what the keto diet is – and why it remains controversial.
The plan was developed more than a century ago, not as a way to lose weight, but as a treatment for epilepsy.
Doctors had observed that fasting appeared to reduce seizures in some patients. But starving people was obviously not a practical long-term treatment.
So in 1921, Dr Russell Wilder, a physician at the Mayo Clinic in Minnesota, developed a diet designed to mimic some of the effects of fasting. He called it the ketogenic diet.
Normally, the body gets much of its energy from glucose, which comes from carbohydrates such as bread, pasta, rice and potatoes.
But when carbohydrate intake is drastically reduced, the body begins breaking down fat and producing chemicals called ketones, which it can use as an alternative source of energy. This state is known as ketosis.
The diet remains an established treatment for some people with drug-resistant epilepsy, but over recent decades it has become far better known as a weight-loss regime.
Studies have found that ketogenic and other low-carbohydrate diets can help people lose weight and may improve blood sugar control and triglyceride levels, particularly in people who are obese or have type 2 diabetes.
But whether keto is superior to other healthy diets in the long term remains fiercely debated.
Research comparing popular diets has generally found that several different approaches can produce meaningful weight loss, with differences between them tending to shrink over time.
There are also concerns about what happens when carbohydrates are replaced with large quantities of fat – particularly saturated fat from foods such as butter, cheese and fatty meat.
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However, there is no single ketogenic menu. The defining feature is that carbohydrates are kept extremely low – typically below about 10 per cent of daily calories, and often considerably lower – forcing the body into ketosis.
That means the diet can be followed in very different ways.
Some versions emphasise fish, nuts, seeds, avocados, olive oil and vegetables, while others have become synonymous with bacon, butter, cheese, red meat and other foods high in saturated fat.
Indeed, some keto enthusiasts have interpreted the freedom to eat fat as a licence to consume large quantities of these foods.
This distinction matters because diets high in saturated fat can raise levels of LDL, or ‘bad’, cholesterol – a major risk factor for heart disease – and studies have found LDL can rise substantially in some people following ketogenic diets.
The diet can also be difficult to stick to because staple foods such as bread, pasta, rice and many fruits are basically banned.
It is against this backdrop that the new study is particularly intriguing: the researchers wanted to know whether keto could produce metabolic benefits above and beyond those caused simply by losing weight. And it did appear to.
The need for answers is clear.
Roughly 70 per cent of men and 62 per cent of women in England are either overweight or obese.
Meanwhile, 70 per cent of adults have a waist-to-height ratio above the recommended level of 0.5 – a warning sign that they may be carrying unhealthy amounts of visceral fat.
Unlike the fat that sits just beneath the skin, visceral fat accumulates deep inside the abdomen, surrounding vital organs such as the liver and pancreas.
It is considered particularly dangerous because it is strongly linked to insulin resistance, type 2 diabetes, heart disease and other chronic illnesses.
One consequence can be fatty liver disease – now known as metabolic dysfunction-associated steatotic liver disease, or MASLD – which occurs when excess fat accumulates in the liver itself.
For decades, serious liver disease was primarily associated in the public mind with heavy drinking. But doctors now know that obesity and its associated metabolic problems can also cause potentially devastating liver damage, even in people who drink little or no alcohol.
In its early stages, fatty liver disease may cause no symptoms or obvious harm. But in some people the liver becomes inflamed and damaged – a more serious form of the condition known as MASH.
Over time, this can lead to permanent scarring, or cirrhosis, liver failure and liver cancer.
And the dangers of carrying excess fat extend beyond the liver. Obesity and excess abdominal fat are also linked to type 2 diabetes, heart disease and a number of cancers.
All of which raises an intriguing question: given the dramatic results of the new study, should we be reconsidering the role of carbohydrates in a healthy diet?
Naveed Sattar, Professor of Metabolic Medicine at the University of Glasgow, welcomed the findings but urged caution.
Professor Naveed Sattar warns that people following low-carbohydrate diets in the real world may end up eating more saturated fat, which can raise LDL, or ‘bad’, cholesterol
‘What this study showed is that keeping refined carbohydrates down might have a preventative benefit for liver health, beyond simply losing weight by other methods and improving blood sugar,’ he said.
But he stressed that the study was not designed to establish whether eating this way is safe over many years.
‘We don’t have a long-term trial of the ketogenic diet looking at cardiovascular outcomes,’ he said. ‘That’s my main concern.’
Professor Sattar said this was particularly important because people following low-carbohydrate diets in the real world may end up eating more saturated fat, which can raise LDL, or ‘bad’, cholesterol.
He added that observational studies of people who habitually eat low-carbohydrate diets have not generally produced reassuring results when it comes to their long-term cardiovascular health.
‘I’m not fully convinced that keto is safe longer term in cardiovascular terms,’ he said.
There is another, more practical problem: sticking to it.
‘If you asked me to go without bread and carbohydrates, I would find it incredibly difficult,’ Professor Sattar said. ‘Many people can do it for a short period, but they can’t do it sustainably.’
Crucially, Professor Sattar said this contrasts with the Mediterranean diet, for which researchers already have evidence that goes beyond improvements in blood tests or weight.
The landmark PREDIMED trial followed thousands of people at high risk of cardiovascular disease for several years and found that those assigned to a Mediterranean diet supplemented with extra-virgin olive oil or nuts suffered fewer major cardiovascular events than those assigned to a lower-fat control diet.
No comparable large, long-term trial has yet shown that a ketogenic diet prevents heart attacks and strokes.
Professor Sattar said that was the evidence he would want to see before recommending keto for long-term health.
‘We need a big cardiovascular outcomes trial,’ he said. ‘That would answer everything – is the keto diet sustainable, is there a heart benefit, and are there harms?
‘Without that, I don’t recommend it.’