The ketogenic diet has long been popular in Hollywood and in 2020, “keto” was the most Googled food-related topic in the world with 25.4 million searches. The value of the global keto diet market is predicted to hit £12.5 billion by 2027.
The principles were originally devised for medical reasons in 1911, after doctors identified that epilepsy patients showed an improvement after fasting. Then it was found that the effects of fasting could be replicated through a low-carbohydrate, high-fat eating plan, and so the keto diet was born in the 1920s as an epilepsy treatment.
That is was embraced by the weight-loss industry isn’t surprising. Cheese, steak and butter on the menu? It feels like cheating the system. The “caveman” narrative also appeals to much of the recent wellness culture: eating like our ancestors might seem preferable to modern food processes. After all, meat, fish, eggs, nuts and leafy greens are commonly regarded as “wholefoods”.
But is the keto diet safe? What are the side effects? And can it help you lose weight? Over to the experts...
What is a ketogenic diet?
Simply, it’s a very low-carbohydrate and high-fat food plan. How much of each you can eat per day is the subject of some debate online, but the standard ketogenic diet typically comprises a daily intake of 70-75 per cent fats, 20 per cent protein and 5-10 per cent carbs.
The word “ketogenic” is derived from “ketosis”, the metabolic state the diet aims to send our bodies into, so that we burn fat for energy instead of glucose and hence lose weight.
So what is ketosis?
When we eat carbohydrates, our bodies use the glucose we get from them to make energy, or we store it as glycogen for future use. When we don’t eat carbs, we go into a state of ketosis – where our liver breaks down fat stores to make “ketones” that we use for fuel instead. It usually takes between two and seven days to enter ketosis, depending on your body type.
“It can [help with quick weight loss] for some people, but that weight loss is not maintained for the vast majority,” says Tim Spector, a professor of genetic epidemiology at King’s College London and the co-founder of the Zoe personalised nutrition programme.
This is indeed what Fiona*, 43, discovered. While she “loves” keto it proved tricky to keep up. “When you first start, it’s a huge shock to the system, and I definitely experienced 48 hours of feeling headachy and lethargic. Then suddenly I felt incredible – really energetic and I found exercise was easy,” she says. “The problem came when life took over.”
The problem for Fiona was remaining in ketosis. “I couldn’t control every meal and there would be the odd glass of wine – and while an occasional cheat won’t hurt, taking your eye off the ball every weekend will. Then you find yourself in trouble: I’d got used to snacking on cheese and, fundamentally, if you aren’t in full ketosis (that is, where your liver breaks down fat stores to use for fuel) it’s not the healthiest diet.
Many people choose to track this using a blood ketone meter, where you prick your finger to collect a drop of blood on a ketone test strip, then insert it into the meter.
How do ketogenic diets help you to lose weight?
When we eat carbohydrates like white bread, white rice and sugary cereals, our insulin levels rise then rapidly fall, so our bodies tell us we’re hungry. On the keto diet, the lack of carbs can keep our insulin levels steadier, and fats take longer to digest, meaning we can feel fuller for longer and have fewer cravings. It means that you might notice fast weight loss in the first week or so, potentially up to 4.5kg a week, but the studies into this tend to be small and often focus specifically on obese patients or those with diabetes. A 2013 review of academic research found that in studies that had followed dieters for at least 12 months, those on a keto diet lost about one kilogram more than those on low-fat diets.
Experts refer to the initial weight loss as “water weight”, since lowering your carb intake means your body releases the water that becomes bound to glycogen when it is stored in the muscles.
After that phase, your body will switch to burning fat, meaning weight loss will slow to under a kilo per week. Academics are sceptical that a keto diet can keep the pounds off long-term – partly because it’s so hard to stick to, not to mention that many of the foods required to remain in ketosis (such as meat, fish, avocados and high-fat dairy products) are expensive.
A review published in the Journal of Clinical Lipidology in 2019 found that while the keto diet is increasing in popularity, after six months it doesn’t lead to significantly higher weight loss than other approaches.
“Likely the reason why people initially feel better is that the keto diet mostly excludes UPFs [ultra-processed foods] and highly refined carbohydrates, which we all eat too much of,” says Spector, who calls UPFs “nutritionally empty mush” that lack essential vitamins, minerals and fibre. Data suggests that eating too much of them is linked to higher levels of ill health, including cancer, cardiovascular disease, high blood pressure or diabetes. “Having said that, you can now buy keto cereal, keto bars and chocolate”, says Spector, “which are all UPFs designed to entice this market”.
What foods can I eat on the keto diet?
Meat such as chicken, turkey, pork, venison and beef are all allowed, plus cheese, butter, eggs, oils, nuts, fish, avocados and some vegetables – largely non-starchy ones like tomatoes and courgettes and low-carb leafy greens.
What do I need to avoid on a keto diet?
Out are bread, rice, pasta, potatoes, bananas, baked goods, legumes (like lentils and peas), root vegetables and most fruit. Also most types of alcohol are out, although wine is lower in carbohydrates than beer, as are pure spirits such as gin or whisky. Any alcohol you drink on the keto diet will mean the liver has to process it and can distract your body from fat burning and producing ketones.
Basically, watch the carbs – you’re limited to just 40-50g a day. For context, a 6-inch long banana contains roughly 30g of carbohydrates.
But being desperate enough to lose weight quickly that you avoid so many foodstuffs – and therefore miss out on nutrients – is bad news, say the experts. “All restrictive diets are unsustainable,” says Spector, the author of the books Food for Life and Spoon Fed. “There is no reason to try and sustain a diet that excludes the majority of foods that we know are good for our overall health, such as fruits, vegetables and wholegrains.”
“It’s ultimately unsustainable”, says registered nutritionist Laura Thomas, the bestselling author of Just Eat It, “and we don’t know all the potential consequences and outcomes and it’s quite far outside of nutritional guidelines.”
She emphasises that The NHS Eatwell Guide is an important tool to ensure we all eat in a healthy, balanced way that is least likely to lead to health problems. It recommends only eating fats in “small amounts” and specifies that starchy foods such as wholewheat pasta, skin-on potatoes and brown rice should make up just over a third of what we eat – so far more than a keto diet allows.
Are there any health benefits to a keto diet?
Apart from epilepsy, there are a number of conditions that may benefit. Studies are being done into the potential impact of a ketogenic diet on some cancers and Type 2 diabetes. A study published in the Journal of the American Medical Association found that a low-carbohydrate diet helped cut blood-sugar levels in people with pre-diabetes, though the results hung on patients self-reporting their food intake (notoriously unreliable) and could be explained by the fact that by cutting carbs, most people lose weight, which helps the body respond better to the insulin it makes.
“Yes, a keto diet might initially be marginally better for blood-sugar control than just eating healthily. But after six to 12 months, that’s not supported by the evidence,” says Thomas.
Other areas of investigation include Alzheimer’s disease. A very small pilot study by the University of Kansas suggested an improvement in the cognitive scores of mild sufferers after three months on a ketogenic diet, which declined once they had resumed their regular eating habits. Researchers considered the results significant enough to warrant further study.
Thomas agrees that caution is necessary “because of question marks over the sustainability of the diet and how it can predispose people to disordered eating. I would never start with it as a possible treatment for any condition,” she says.
What are the risks of a keto diet?
A 2023 study, presented at an American College of Cardiology conference and using data from the UK Biobank, suggested that keto diets could be linked to higher levels of LDL (low-density lipoprotein) or “bad” cholesterol and may therefore double the risk of cardiovascular events, such as angina, heart attack and stroke. As it was an observational study, the researchers concluded there was an “association” rather than a causal link and further research would be required to spell out the reason for the apparent rise in LDL cholesterol.
But several studies have pointed to the wider health impacts of a diet heavy in red meat: in 2021, University of Oxford research monitored data from almost 475,000 UK adults and found that eating higher levels of red meat was associated with an increased risk of ischaemic heart disease, pneumonia, diverticular disease, colon polyps and diabetes. Meanwhile a significant intake of fats such as cheese – known to contain saturated fats – has long been associated with an increase in LDL cholesterol.
But keto-specific research is in its infancy. “We don’t have enough data on long-term keto consequences because not many people can sustain it in the long term,” says Spector.
When the keto diet can take a dangerous turn
Thomas outlines the more immediate risks: “Whenever we enforce rules and restrictions around our diet, it can quite easily turn into a preoccupation. It can start to consume our thoughts and interfere with our enjoyment of daily life,” she says. “You might find yourself constantly thinking about food because you’re hungry and scrolling for keto-friendly food on social media – it can become very obsessive very quickly, to the detriment of your relationships and friendships. If it interferes with your social life, we would consider that to be a red flag for disordered eating.”
There’s also, she adds, a potential physiological response. “When there isn’t enough available energy for the body, it starts to shut down what are considered to be non-essential functions. So the first thing that kind of goes offline might be reproduction – women can become amenorrheic and their periods stop. There is some evidence that there might be an increased risk of kidney stones and osteoporosis, and an increased level of uric acid, which is a risk factor for gout. It can have consequences for the whole body.”
Because the keto diet cuts out food groups containing vitamins, minerals and fibre, it is not recommended for pregnant and breastfeeding women, or those with health conditions such as IBS because the high fat content can worsen symptoms. The diet is also not good for anyone with osteoporosis, as it decreases the body’s ability to build new bone and could increase bone breakdown. And it’s a no for anyone with kidney problems, which may be exacerbated by the high levels of protein.
What about side effects?
Your gut may not thank you. “A keto diet often results in people eating even less fibre than we already do,” says Spector. “The risks are constipation, heartburn, low mood, unhealthy changes in blood fats and a lack of sustainability, which inevitably leads to yo-yo dieting. It’s an unpleasant short-term fix that negatively impacts our fibre intake and likely our gut microbiome health.”
There is also, says Thomas, some evidence that suggests restriction and dieting might reduce serotonin levels – and could therefore impact our mood. Some experts also believe that keto can lead to irritability.
“Sometimes strict regimens like the keto diet can help people feel more in control of their lives. But what I have seen in my practice is that this effect is usually short-lived and only happens when life is otherwise smooth sailing,” Thomas says. “When life throws a curveball, that seems to be where we see big mental health impacts: the stress and anxiety of trying to maintain the diet while seeing friends and family, the anxiety of not eating the ‘right’ things or being ‘tempted’. Then, if you do ‘slip up’, there’s a lot of shame and guilt. Clients often tell me that when they stop dieting they have far more headspace.”
There are also some potential nutrient deficiencies, she explains, such as B vitamins and minerals like iron, magnesium and zinc which are typically found in wholegrains. These deficiencies can have unfortunate consequences.
“On the keto diet, hair can fall out or become weaker, nails may not grow as fast and could break more easily, and skin can look dull and take longer to heal. This happens because you may not be getting all the nutrients your body needs and, what’s more, these are fairly typical responses we see when people are in an energy deficit – ‘non-essential’ functions tend to be shut down.”
Other side effects may include headaches and brain fog. Many keto dieters also report a metallic or “nail polish” taste in the mouth, leading to bad breath, in the initial stages of the diet as the body adjusts to fewer carbs. This is because your body is breaking down fat for energy through ketosis, and ketones are being expelled by your body through urinating and exhalation.
“The preferred energy source for the brain and central nervous system is glucose – it has enormously high requirements for it,” says Thomas. “When we’re in ketosis, the body runs on ketones alone, which may make people feel woolly-headed, irritable and less able to concentrate until they’ve had a sandwich. That seems like a pretty miserable existence to me.” It’s thought this could be linked to the body losing water and electrolytes, which can make us feel dehydrated.
Is the keto diet the same as the Atkins diet?
Several popular diets claim to be “keto”, when really they’re just low carbohydrate and don’t necessarily send your body into ketosis – so it can be tough to know what you’re getting into. The key difference lies in the carbs: on the keto diet, they are – as we have seen – highly restricted, whereas the Atkins allows for their slow reintroduction.
“The keto diet is a modern version of an old trend for high-protein diets,” says Spector. “The Atkins and Dukan diets were fairly similar, and both are associated with higher risks of heart disease and colon cancer.”
So what is the alternative to a keto diet?
Eat more fibre if you want to lose weight, says Spector. “More whole plants, more legumes, more nuts, more wholegrains and as minimally processed foods as possible.” Crucial for all weight loss, he adds, is not restricting entire food groups.
Thomas points to the keto diet’s ultimate lack of sticking power. “There might be some weight loss associated with it, and people tend to feel better because there’s a sense of control. Then, six or 12 months down the line, it becomes unsustainable – whether that’s cost, whether that is just because of life, or whether that’s because it’s our body fighting back against the restrictive nature of these diets. Probably a combination of all three,” she says.
“We know from Zoe research and a lot of other epidemiological data that eating in this way [more fibre, whole plants and fewer processed foods] is more likely to help achieve a healthy weight, and it also supports a healthy gut microbiome and reduces the risk of chronic diseases,” adds Spector. “Focus on positive nutrition. Add more plants to your plate and the rest will follow.”
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