Ketosis explained

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Diets may have been around since the 1830s, but people are still turning to everything from juice cleanses to protein shakes and detox teas to shed pounds. In the UK 62% of people have recently changed their diets to improve their health according to a survey by the British Nutrition Foundation. But the word on everyone’s lips right now is ketosis. 

‘Ketosis is known as a quick way to lose weight and has recently gained renewed attention,’ says Aliza Marogy, a registered nutritionist and founder of Inessa. ‘This is due to the popularity of biohacking and ‘clean living’ on social media and a more widespread interest in metabolic health. After a period where we saw a movement around body positivity it is disappointing to see that skinny is once more appearing to be the ideal’. 

Ketosis is a metabolic state brought about by a low carb diet (and what the infamous keto diet was named after) and has helped treat obesity, type-2 diabetes and fatty liver disease. 

The ketogenic diet itself was originally developed in the 1920s as a medical treatment to reduce seizures in children with epilepsy. Interest in it has waxed and waned over the years. For some it’s the holy grail of healthy living. Other experts say approach with caution as a potential trigger for eating disorders and damaging to digestive and heart health. 

‘Any significant dietary changes that involve the elimination of food groups should always be discussed with your GP or healthcare provider first,’ says Aliza. With the help of experts we investigate what ketosis is, the pros and cons and everything else you need to consider before making the switch to a ketogenic diet. 

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What is ketosis? 

To understand ketosis we first must understand what fuels the body, explains Dr Beth Godwin, a clinical psychologist and founder of Life For Every Body. She explains: ‘In basic terms, ketosis is a metabolic state where the body, deprived of carbohydrates, starts utilising fat to use as fuel. Normally, our brain and body rely on glucose (from carbs) for energy. In ketosis, because glucose is scarce, the body adapts to use ketones – an alternative energy source broken down by the liver from stored fat.

‘To achieve ketosis, someone typically cuts out foods such as bread, pasta, rice, most fruit, legumes and starchy vegetables and embraces a strict high fat, moderate protein, low carb diet.’ Scientists say we’ve evolved as humans with the ability to make this metabolic switch since it was used as a survival method by our hunter-gatherer ancestors during periods of food scarcity. 

What is the keto diet? 

‘The keto diet is what gets our bodies into this state of ketosis, says Dr Jonathan Korgaonkar, a sport medicine consultant. He says: ‘The key aim with the keto diet is to restrict carbohydrates to 5-10% of daily calorific intake [the equivalent to one medium sized banana]. In contrast, it requires a high fat consumption of roughly 70-80% to provide an alternative fuel source.

‘Protein should comprise no more than 20-25% of daily calories. This combination triggers the breakdown of stored fat into ketones – a “superfuel” which replaces glucose in providing energy for the body. A gradual dietary transition when embarking on keto can be helpful. Protein is cut back to ensure the fat is burned.’ 

Dr Beth adds: ‘It’s important to know that this diet isn’t based on casually cutting back on carbs, which has been referred to as "keto lite", "lazy keto" or "dirty keto". True ketosis, where fat is used as a fuel source, involves significant sustained restriction of carbohydrates, often rigid food tracking and a shift in how the body naturally wants to function and process food.’ 

How long does it take to be in ketosis? 

Technically if your last meal of the day was at 4pm by the time you wake up around 6-7am the next morning, your body will already be in ketosis. But Dr Jonathan says it very much depends on the individual. ‘It generally takes two to four days to enter ketosis if carbohydrate intake is kept below 20–50 grams per day,’ he says.

‘However, for some it can take a week. It all depends on prior diet, activity levels and metabolic health. Sleep quality and stress levels can also influence the onset of ketosis as can genetics and individual body composition.

‘An individual approach is increasingly relevant and one size definitely doesn’t fit all. It’s also worth noting that ketosis should be avoided by individuals who are pregnant. A significantly low carbohydrate intake can impact a baby’s growth.’ 

How do you know if you're in ketosis?

The infamous fruity smelling "keto breath" is a giveaway. Other signs it’s kicking in include fatigue, irritability and a suppressed appetite. The early days of ketosis are often marked by "keto flu" with symptoms such as headaches, dizziness and sickness. Higher ketone bodies are a sign of ketosis and these can be tracked using a urine test strip or a finger prick blood test, which you can buy online.

Dr Rochan Agha-Jaffar is a Consultant Endocrinologist at Opus says: ‘When these ketone bodies are generated, people can feel unwell. Salt imbalances can lead to cramps and because fibre is cut down drastically, people may also become constipated.’ This transition period can be very challenging, but after this people report enhanced mental clarity and sustained energy without afternoon crashes. 

Can I exercise when my body is in ketosis?

The simple answer is yes and it can even accelerate the process, but Dr Jonathan recommends being mindful of the type of exercise you do and how often you work out. He explains: ‘Exercise is beneficial to all as part of a healthy lifestyle and it can aid fat loss, muscle maintenance and overall fitness while in ketosis. Being active depletes glycogen stores faster, encouraging the body to shift to ketones for fuel more quickly.

‘Over-exercising is to be avoided especially in the early stages of ketosis, as your body will take time to adapt and regulate energy levels.’ Zone 2 exercises are recommended: mild activity when your heart rate is maintained at a level that you can still hold a conversation. 

Exercises to do in ketosis  

• Low to moderate-intensity cardio: walking, cycling, swimming.
• Light-moderate weights, bodyweight exercises or resistance bands.
• Flexibility, core strength, mobility work such as yoga and Pilates. 

‘These exercises help maintain muscle mass on a low-carb diet. Be careful to build up activity slowly and according to your own ability,’ Dr Jonathan adds. ‘If you overload, or overexert too quickly, this can lead to injury. Early on in the process avoid high-intensity interval training (HIIT) and heavy lifting. It can place undue strain on muscles and the cardiovascular system.’  

What impact will ketosis have on my sleep? 

People do report improvement in their sleep quality once the process of Ketosis has kicked in. Ian Gilham, physiotherapist at Opus explains: ‘Studies suggest that ketogenic diets can increase the amount of REM sleep, which is integral to cognitive function and memory consolidation.

‘But while ketosis can improve and deepen sleep, early on people complain of restlessness at night, difficulty falling asleep and wild dreams. This is because the shift to a low carbohydrate diet can restrict production of our sleep regulating chemicals.’ 

Does ketosis help prevent Type 2 diabetes? 

Studies show that reducing carbohydrate intake can significantly improve insulin sensitivity and glucose control, which help people on the brink of type-2 diabetes. Aliza explains: ‘While most research focuses on managing existing diabetes rather than prevention, the metabolic benefits of ketosis suggest potential protective effects.

‘But you don’t need to follow an extreme ketogenic diet to prevent the onset of type 2 diabetes. There are healthier diets that work by reducing carbohydrates, but not eliminating them completely.’ 

Dr Elena Seranova, author and founder of longevity supplement brand NMN Bio, believes a keto diet can act like a ‘system reset’. ‘Many people on the verge of diabetes (pre-diabetic or insulin resistant) may benefit greatly by a ketogenic diet. High blood sugar acts like pressure in a pipe system,’ she says. ‘Ketosis reduces this pressure allowing the system time to repair and stabilise.’ 

How does ketosis compare to weight loss injections? 

Weight loss injections or ‘skinny jabs’ have hit the mainstream having been made accessible to all. Lauded as life changing for some and a concerning ‘quick fix’ for others, how do they compare to ketosis in the realm of weight management? 

‘I do believe that the mainstream use of GLP-1 medications (skinny jabs) crossing over from diabetes management to a mainstream weight-loss tool is a significant driver of this interest in ketosis,’ says Aliza.

‘Both ketosis and weight loss injections address appetite regulation and weight management - GLP-1 medications work by regulating blood sugar and mimicking satiety hormones, while ketosis creates a metabolic state that naturally reduces hunger and burns fat.

‘Rather than viewing either as "good" or "bad," I believe both can be valuable tools when used appropriately and under professional guidance. The reality is that people who fall within a healthy weight range are now using these extreme methods to achieve a “skinny” aesthetic without considering the health implications.’ 

Will the weight stay off long term? 

‘Research shows that long-term weight loss is rarely sustained,’ says Dr Beth. ‘Even when initial changes look promising. In fact, most people regain weight and sometimes even more on top (Mann et al., 2007). A 2021 systematic review found that while ketogenic diets can improve short-term metabolic markers, they are not superior to other diets in the long term and are often unsustainable with high dropout rates (Churuangsuk et al., 2021).’

‘There are compassionate, evidence-based alternatives such as Intuitive Eating,’ she adds. ‘This helps people reconnect with their bodies rather than fight against them. Carbohydrates are a macronutrient and required in large amounts in the diet for the body to function well. Carbs are also the brain’s preferred fuel source.

‘All three macronutrients, carbohydrates, fat and protein, play different and vital roles in our mental and physical health. When any one macronutrient is severely restricted, like carbs in the keto diet, it can disrupt mood and thinking, impair hormone and immune function and lead to nutrient deficiencies.’ 

What are the potential impacts on mental health? 

Psychotherapist and coach Ioana Rotaru says: ‘Following a strict diet can force us into black and white thinking whereby we see carbs as bad, sugar bad and fat good. These sorts of associations can reinforce shame and perfectionistic behaviour which can have detrimental impacts on our mental health.’ 

Dr Beth says many of her clients report on becoming obsessed with food (sometimes called "food noise") and suffer from social isolation – avoiding going out to eat or spending less time with family and friends due to their rigid diet rules or shame when the diet becomes unsustainable. ‘Even those who initially felt good on keto, might describe how the strict approach to food eventually wore them down; emotionally, socially, and physically,’ she says. 

What are the medical benefits of ketosis? 

There are medical conditions where ketosis has proved beneficial. ‘It has been a treatment for drug-resistant epilepsy in children and there is exploration into conditions like Alzheimer’s and Parkinson’s,’ says Dr Beth. ‘There is also research into whether ketogenic diets could be helpful to psychiatric conditions’

The London Psychiatry Clinic states there is a lack of high-quality clinical studies with long follow-up periods into the keto diet. ‘If you are pursuing keto for a medical reason, the British Dietetic Association strongly urges individuals to remain under close supervision of an experienced medical professional to assess risk,’ says Dr Beth.

‘To conclude, yes, ketosis can be medically useful. But for most people the benefits are often short-lived, and the psychological and physiological toll can outweigh any temporary wins.’

Is it true ketosis can boost longevity? 

A recent study, in Scientific Reports, found a keto diet lowered risk of death from any cause, by up to 24 per cent, as reported by the Telegraph newspaper. Meanwhile new research highlights the ketogenic diet could lower overall mortality by 24% — ushering in fresh perspectives on its long-term safety and effectiveness. 

Dr Elena makes a case for keto in the move towards longevity: ‘Ketosis leads to lower insulin levels and reduces the generation of inflammatory molecules,’ she says. ‘Put simply, fewer insulin spikes, less inflammation. Recently, the build up of inflammation in the body has been classified as one of the hallmarks of ageing, as it accumulates as we get older (often called inflammaging).

‘If inflammation doesn’t build up the body needs less energy and effort [resources] go towards maintaining healthy tissues. Switching to a ketogenic diet reduces inflammation and so it’s easier to preserve healthy function over time. Ketosis also tends to have a long-term neuroprotective effect – meaning better memory and brain performance.’ 

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