Ketogenic Diet Simply Explained

1. What It Is About

The ketogenic diet is a special form of nutrition. It is used as a treatment method and not as ordinary food. The proportion of carbohydrates is very small, the largest part of the energy comes from fat and a medium part from protein. Carbohydrates are found in bread, pasta, rice, potatoes and sugar.

When the body receives hardly any carbohydrates, it switches its operation. It then forms ketone bodies. These are fuels that it produces itself, and it uses them as the main energy source. This state is called ketosis.

The method was developed in the 1920s for the treatment of epilepsy. Today there are several variants, for example the classic ketogenic diet, the modified Atkins diet or very low-carbohydrate diets with a ketogenic focus. How strict the rules are depends on the goal and the group of people. The transition is often accompanied by measuring ketone bodies in the blood, urine or breath.

2. What Studies Show

The effects on metabolism, that is on blood sugar, blood lipids and weight, are the best studied. There are controlled studies in which one group received the ketogenic diet and another group received a comparison diet.

An analysis of nine controlled studies with 734 people with type 2 diabetes found: with a low-carbohydrate diet the long-term blood sugar value fell more than with the comparison diets 5. This value shows how high the blood sugar was on average over the past weeks. The “good” HDL cholesterol also rose, and the blood lipids fell. The advantage in weight was only visible in the short term. A second analysis of 14 studies with likewise 734 participants found the same pattern for ketogenic diet versus low-fat diet: better long-term blood sugar, less weight and more favourable blood lipids 6. Most of these studies lasted twelve weeks on average.

A two-year study followed 262 adults with type 2 diabetes who followed a ketogenic diet and received remote supervision 2. The wall thickness of the carotid artery remained unchanged. Limitation: the participants were not randomly assigned to the two groups. In people with type 2 diabetes the blood lipid particles changed favourably: the small ones decreased, the larger ones increased 4.

In a controlled feeding study 164 adults ate different amounts of carbohydrates over 20 weeks 7. The group with the fewest carbohydrates showed the clearest improvement in a value that indicates how well the body responds to insulin.

In the DIETFITS study, 609 overweight adults compared a low-carbohydrate diet with a fat-reduced diet for twelve months 1. Whether someone carried a certain genetic pattern or how strongly their body responded to sugar with insulin made no difference to the result.

In a small pilot study with 20 patients with bipolar disorder or schizophrenia, abdominal fat decreased after four months of ketogenic diet, and the psychiatric assessments improved. There was no comparison group.

3. What Remains Unclear

Many questions are open. Almost all studies last less than two years. Therefore it is not clear whether the improvements persist and how the diet affects the heart, vessels and bones over a long period.

It is also unclear where the observed effects come from: from the few carbohydrates, from the lower calorie intake or from the type of fat. The studies are too differently designed for that.

For neurological, psychiatric and oncological questions the statements are based on reports of individual patients, on studies without a comparison group or on experiments with animals and cells. Confirmatory studies in humans with sufficient statistical power are still outstanding here.

Constipation, fatigue and elevated blood lipids have been reported as side effects. In type 2 diabetes hypoglycaemia can occur if the medication is not adjusted. How frequently these side effects occur with longer use is not precisely quantified.

4. What This Means in Everyday Life

For metabolism the situation is clear: in type 2 diabetes and overweight several controlled studies show short-term better blood sugar values, more favourable blood lipids and greater weight loss than with low-fat comparison diets. This is considered well proven.

For other areas of application, for example in diseases of the nervous system or in cancer therapy, the confirmatory studies are still outstanding. In the studies the diet was carried out as treatment and medically supervised.

It remains to be clarified whether the advantage over other forms of diet persists over years, what role the ketone bodies themselves play and whether certain groups benefit more than others. These questions can be answered with longer controlled studies.

All details with study numbers and sources: detailed page on the ketogenic diet. For children: ketogenic diet for children.