Ketogenic diet: what it is, what to eat and who it may suit

What is ketosis and how many carbohydrates does it involve?

Nutritional ketosis occurs when the liver produces more ketones in response to a low carbohydrate intake. A ketogenic diet usually keeps carbohydrates below 50 g per day [3]. Some versions reduce them further, but responses vary from person to person [3]. This figure describes what studies have examined; it is not an instruction to start on your own.

The body still needs nutrients, fibre and enough energy. Being in ketosis also does not prove that you are losing fat. The scales may go down at first because of water loss associated with glycogen, the body’s carbohydrate reserve.

Ketosis and ketoacidosis are different conditions. The latter is a medical emergency linked mainly to a lack of insulin and to certain diabetes treatments [4]. If you have diabetes, your medical team will tell you when and how to measure ketones [4].

What to eat and which foods are limited

The table shows the practical cost of keeping carbohydrate intake so low. “Almost excluded from the diet” describes a typical ketogenic diet; it is not a judgement about the food’s quality.

Fruit, pulses and whole grains provide fibre and nutrients. Leaving them almost entirely out means you need to review the overall quality of what remains on your plate. To compare the role of these foods, see the guide to foods high in carbohydrates.

Versions based on cured meats, cheese and butter are not equivalent to a healthy diet either. Choosing unsaturated fats and enough vegetables matters, even when carbohydrates are restricted.

What does the evidence say about weight loss?

A review of low-carbohydrate diets found little difference in weight compared with balanced diets at one and two years [1]. It included diets with varying levels of restriction; it did not study the ketogenic diet alone. In many trials, both groups received similar guidance on reducing energy intake [1].

A review focused on ketogenic diets with comparable prescribed energy intake found a small short-term advantage [2]. The certainty was low, and robust data on how long it lasts are lacking [2]. So we cannot promise greater fat loss from producing ketones.

The diet worth following is the one you can sustain without losing valuable foods or putting your health at risk.

We consider the long-term plan before choosing a restriction. The weight-loss guide explains how to work with a moderate deficit, preserve muscle and maintain habits.

Medical uses: epilepsy and what is still being studied

The best-studied application is drug-resistant epilepsy, especially in children [5]. Studies find fewer seizures in some people following ketogenic dietary therapy [5]. Specialist teams recommend and supervise it; they adjust the plan and monitor growth, tolerance and test results.

Other conditions are being researched, but studying a possibility is not the same as recommending it as a treatment. We would not use it to replace medical care for cancer, diabetes or a neurological condition.

What side effects can it have?

Very low-calorie versions may cause constipation, nausea, headaches, tiredness and bad breath [6]. If these persist, we review hydration, food intake, fibre and the reason the diet was started.

A very restrictive diet can also make it difficult to meet requirements for some nutrients [6]. This is assessed by looking at food intake and clinical context and, where appropriate, blood tests. We do not recommend additional amounts of minerals without assessing the individual.

Who should not start this diet?

A ketogenic diet for weight loss is not designed for pregnancy or breastfeeding, people under 18, people with chronic kidney disease or anyone who needs to gain weight. It is also not suitable for people with diabetes treated with insulin or medicines that can lower blood sugar too much, or for anyone who has or has had an eating disorder [6]. If you have a condition that requires its own dietary plan, such as coeliac disease, a diagnosed allergy, or heart or liver failure, that plan takes priority. These cautions are especially clear for very low-calorie versions [6]. In these cases, do not start without medical and dietetic assessment.

With type 1 diabetes or insulin use, abruptly changing carbohydrate intake alters treatment requirements [4]. Sodium-glucose cotransporter 2 inhibitors (SGLT2 inhibitors) increase the risk of ketoacidosis, even without markedly high blood glucose [4]. Do not adjust or stop medication on your own.

Nutritional status, medicines and the reason for the consultation also matter. Very low-calorie diets require different supervision from that needed by a healthy person who simply has questions about their meals [6].

How to stop the diet without going back to square one

When you start eating more carbohydrates again, glycogen and water are replenished; an initial increase in weight does not by itself mean that you have regained fat. What matters afterwards is the pattern you maintain. If you go back to eating as before, without a plan that feels manageable, the weight may return.

That is why we plan how to stop the diet before starting it. Reintroducing fruit, pulses and whole grains helps bring back variety. We adjust amounts according to appetite, activity, results and preferences. If you decide to weigh yourself, do so no more than once a week. Measuring your waist and hip circumference or using a familiar belt as a reference is more useful. Support during the transition helps you interpret changes without feeling that you have failed.

What myths come up in consultations?

“Ketones mean I am losing fat.” They indicate a change in fuel source; changes in body fat require consideration of energy balance and time.

“If I exercise, I need to cut out all carbohydrates.” A position statement on exercise finds that the effects on performance are generally neutral or unfavourable compared with diets containing more carbohydrates [3]. Some high-protein gym diets combine weight loss and muscle gain, two goals that are best considered separately. Our fitness nutrition guide explores the latter goal.

“Any intense restriction is more effective.” Cutting back can be difficult to maintain and can limit your social life. If strict rules appeal to you, review the warning signs of fad diets. If your question is about meal timing, intermittent fasting addresses a different question.