What is a ketogenic diet?

How do low-carb keto diets work, and what do ketosis and ketones mean? We look at their efficiency for weight loss and the potential side effects.

Selection of low-carb foods on a chopping board

If you’ve ever considered following a diet, make sure you have all the facts first. BBC Good Food and nutritional therapist Kerry Torrens take a closer look at ketogenic diets – what are they, what are the health claims behind the headlines, and are they healthy?

How does the ketogenic diet work?

The aim of ketogenic diets is to send the body into a state of ‘ketosis’ by using a very strict low-carb diet. This umbrella term can include diets such as the Atkins diet, Dukan diet and LCHF (low carb, high fat) diets, although the ratios of fat, protein and carbs and other specific features of each diet (e.g. ‘phases’) can vary.

What is ketosis?

Under normal circumstances our body uses glucose from carbohydrate foods for energy. In the absence of glucose a process called ketosis occurs. This is a state in which the body burns fats instead of carbohydrates as its main fuel source. When we don’t eat carbs, the liver breaks down fat stores to produce energy. This energy is in the form of (and also creates) molecules called ‘ketones’. 

How are ketogenic diets used in a medical setting?

Ketogenic diets were originally developed to treat epilepsy in children as it appears to reduce the frequency of seizures. It should be noted that using the diet in this context should not be attempted without the supervision of a specialised doctor. From these medical origins, the diet was picked up by the mainstream media and marketed as a weight loss regime – it is in this context that we will be discussing the diet for the purpose of this article.

What foods are allowed on a ketogenic diet?

Foods that are generally allowed include high-fat meats, fish, oils, nuts, high-fat dairy such as cheese, and low-carb vegetables. 

Unsurprisingly, reducing carb levels means cutting out bread, pasta, rice, and most conventional baked goods. However, achieving such low levels of carbs also means skipping legumes, root vegetables, most fruits and starchy veggies, such as potatoes. 

Fresh avocado that has been cut in half

We asked nutritional therapist Kerry Torrens for her view...

Is it safe?

This eating plan certainly contradicts most people’s understanding of a healthy, balanced diet which typically promotes the consumption of protein, fat and carbohydrates. From an evolutionary perspective, ketosis is a normal adaptive response which enabled humans to withstand periods of famine throughout history. Today, this natural physiological mechanism has been exploited by a number of low-carb diet regimes. 

Following such a diet means you will be replacing carbs with foods rich in fat and protein, and if followed over an extended period of time this may have consequences. An intake of high fat foods is likely to increase your saturated fat intake which current UK government guidelines recommend that we limit to 30g for men and 20g for women. High levels of dietary protein are thought to be an issue if you have an underlying kidney condition. However, most ketogenic diets supply moderate rather than high levels of protein. 

Is it effective for weight loss, and is it sustainable?

Ketogenic diets usually do cause weight loss and may improve insulin sensitivity in patients with diabetes. In fact when compared to a low-fat diet a ketogenic diet appears to achieve greater long term reductions in body weight. However, the success long term is dependent on your ability to adapt your dietary habits once you start to introduce a more balanced and healthy approach to eating.

How do the various versions of ketogenic diets differ?

The Standard Ketogenic Diet (SKD) appears to be the most researched particularly with regard to its ability to help people lose weight and control blood sugar. It typically comprises 70-75% fat, 20% protein and 5-10% carbs. When following a ketogenic diet be sure to include plenty of non-starchy vegetables, like kale and spinach which are very low in carbohydrate. 

Woman standing on bathroom scales
What should I do before starting a ketogenic diet?

If you are interested in adopting this sort of diet you should consult your GP to confirm it is appropriate and safe for you to do so. Before you start the diet, focus on liver-supportive foods like garlic and onions and try to reduce your intake of sugar, caffeine and alcohol.

Who should be more careful about following a ketogenic diet?

Diabetics, especially, type 1 diabetics are at risk of complications if they attempt to follow a ketogenic diet. For this reason diabetics and anyone with a blood sugar management issue should discuss the potential implications with their GP and healthcare team before embarking on such a regime. Similarly anyone with kidney disease or a family history of such should consult their GP. 

What are the long-term effects of ketogenic diets?

The symptoms associated with ketosis are often temporary and may relate to dehydration. These may include headache, dry mouth, bad breath, fatigue and nausea. However, it’s worth noting that because the diet restricts carbs it is typically low in dietary fibre which may have a negative impact on gut health including the presence of gut friendly bacteria. 

Please note: if you're considering attempting any form of diet, please consult your GP first to ensure you can do so without risk to health.

Now read...

How to eat a balanced diet
Six things you should consider before starting a diet
All our popular diet guides


This article was published on 11th May 2017.

A registered Nutritional Therapist, Kerry Torrens is a contributing author to a number of nutritional and cookery publications including BBC Good Food. Kerry is a member of the The Royal Society of Medicine, Complementary and Natural Healthcare Council (CNHC), British Association for Applied Nutrition and Nutritional Therapy (BANT).

All health content on bbcgoodfood.com is provided for general information only, and should not be treated as a substitute for the medical advice of your own doctor or any other health care professional. If you have any concerns about your general health, you should contact your local health care provider. See our website terms and conditions for more information.

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IBikeNYC's picture
IBikeNYC
25th May, 2017
I eat a diet that varies from ketogenic to simply very low carb, and I love it. After a lifetime of being overweight and unfit, I am now, in middle age, in the best shape of my life. I weigh less and wear a smaller size than I did in my teens. I am also stronger and fitter than ever. In addition, my physician was "keep[ing] an eye on" a number of health issues of the type that most people assume are the inevitable result of ageing: Elevated blood lipids; elevated blood pressure; elevated blood sugar. I attribute a great deal of my success to the fact that I was motivated this time much more by health than vanity. They wanted me to go on statins, and I just refused. I applied myself in earnest to a low-carb way of eating, and in the course of my research I learned about ketogenic diets. I normally eat about 75 grams a day of fat; when I am riding regularly, that's often 100 or more; during long-distance rides, 150+.
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