The low-carb, high-fat ketogenic diet is promoted online and in the media as a weight-loss tool and seemingly all-round magical healer. Yet it was first developed in the 1920s for children with epilepsy.

There’s no shortage of celebrity and influencer fans. Jersey Shore star Vinny Guadagnino (or as he calls himself on Instagram ‘Keto Guido’), Kourtney Kardashian and Halle Berry all advocate the keto diet. But even the health and wellness influencers may not be qualified nutritionists or dieticians. “Nutritional advice is highly unregulated, particularly online”, says dietitian Sophie Medlin.

As of now, #keto has around 14 million tags on Instagram.

When you’re following a keto diet you aim to be in a state of ‘ketosis’. We ask the experts what ketosis is, if it’s worth the effort and if it’s safe.

What do you eat on a keto diet?

Your body uses whichever energy source is most readily available. This is normally glucose (sugar) converted from carbohydrates. If you drastically limit your carbohydrate consumption and replace it with fat, your body is eventually forced to use fat from food or your stores as energy instead. This process is called ‘ketosis’. “It is essentially the Atkins diet repackaged and renamed”, says Medlin.

“Most people calling their diet ‘keto’ are simply following a low or very low-carbohydrate diet”, she continues. The degree to which you need to restrict carbohydrates to be in ketosis “varies from person to person”, she adds. In a clinical or therapeutic setting, such as for children with epilepsy, the diet is designed specifically for the individual and support and monitoring are provided.

Low-carb, high-fat ingredients include avocados, meat, fish, eggs, cheese, cream, oil, butter, nuts and seeds. But not all ingredients need to be high in fat – leafy greens and berries are often included, for example. You avoid, or at least heavily restrict, wheat and other grains, potatoes, corn, pulses, beans, milk, most fruit and sugar.

To induce ketosis, a maximum of 20–50g carbohydrates can be consumed daily for someone eating 2,000 calories a day, according to a 2019 paper on the ketogenic diet. To put this into perspective, a banana contains about 20g and a plain bagel 44g. Protein is also usually kept to a moderate amount, as it can encourage the production of glucose and interrupt ketosis.

The keto diet has few rules on which high-fat foods to eat, and ‘dirty keto’ can consist of bacon and sausages fried in butter, as this technically meets the criteria. Medlin says the healthy eating guidelines should always roughly apply, even if you limit carbohydrates and regardless of what diet you're following.

Will a keto diet help you lose weight?

Research has shown a ketogenic diet can induce rapid weight loss, partly from water loss but also some fat loss. However, the “weight loss effect becomes similar to other dietary approaches after one year”, according to a 2019 review of low-carb diets.

Taking in less energy than you burn will lead to weight loss. The National Diet and Nutrition Survey reports that on average we get almost 50 percent of our energy from carbs. “Cutting this by at least half is likely to reduce total calorie intake, even if it’s replaced with fats”, says Medlin. On the other hand, if you eat more calories from fat than your body needs it will still be stored as fat.

The ‘eat like me to look like me’ ideology is flawed. “We all have different genetics and lifestyles, which means we can’t all look the same”, says Medlin. Research shows the best weight-loss diet is the one you can stick to for long enough to reduce excess body fat.

Long-term compliance with the keto diet is low. Medlin cites restricting a whole food group, symptoms known as ‘keto flu’ (including headaches, nausea, constipation, tiredness and difficulty sleeping) which can kick in after two to seven days, and the higher-than-average cost of the diet as potential reasons why people might struggle to stick to it.

“The keto diet is used for certain therapeutic situations and isn’t meant for the mainstream weight-loss market”, says dietitian Priya Tew. “Carbohydrates have an important role in our diets and whilst reducing them may aid weight loss it is not sustainable to reduce them to a very low level.”

Is a keto diet safe?

“Many of the trials investigating ketogenic diets only follow participants for around six months up to one year at the most”, says nutritionist Emma Thornton, “therefore, the longer term effects... aren’t well understood.”

Medlin says the health risks also depend on the types of food eaten. Eating an unhealthy diet containing lots of saturated fat may increase health risks such as heart disease and stroke.

Foods containing carbohydrates are often also high in fibre, which is important for a healthy gut microbiome and keeping you fuller for longer. Medlin advises anyone on a low-carb diet to ensure they eat enough fibre-rich foods from leafy greens, broccoli, cauliflower, flax seeds, nuts, coconut and avocado.

The keto diet limits starchy vegetables such as carrots, sweet potatoes, parsnips, pumpkin and squash and eliminates many fruits. This might restrict the amount and variety of nutrients, vitamins and phytochemicals (plant power) you get from your food, especially if you remove them rather than replace them.

People who restrict their carbohydrates and increase fatty foods may develop ‘keto flu’. There is very little medical research on this, but there are thousands of personal testimonies. Though it sounds unpleasant, it isn’t necessarily unsafe and many reports indicate the discomfort subsides. It is also not clear whether it only occurs on the keto diet or can be caused by other restrictive diets.

The keto diet might not be for everyone

If you take any medication or have any medical issues, consult a doctor or dietitian before starting a keto diet. For some people, a keto diet may cause harm, even with careful monitoring.

Alternatives ways to lose weight

“I recommend checking your portions sizes are sensible, with potentially a small decrease in these and an increase in your vegetable intake”, says Priya Tew. She does not recommend reducing your carbohydrate intake to less than a quarter of your diet.

“Small steps make a big difference”, says Emma Thornton. She suggests cooking from scratch at home and expanding your knowledge of fresh ingredients is a good starting point. She also says other higher-fat and lower-carb diets, such as the popular Mediterranean-style diet, can be healthy as long as plenty of fish, nuts, seeds, other foods rich in healthy fats and vegetables are on the menu.

A key way to address weight-loss is to look at “why you eat rather than what you eat”, according to Medlin. Stress, poor sleep and choosing unrealistic diets have been identified as some of the main reasons people discontinue their weight-loss diet. An indepth review suggests the most effective strategy for long-term weight-loss and heart health is a healthy diet pattern that's compatible with your food preferences and lifestyle. “It’s really important not to beat yourself up over having some birthday cake or foods you love once in a while”, concludes Medlin.