The ketogenic diet – or keto for short – is the most radical and scientifically extensive form of carbohydrate-reduced diet. While moderate low-carb approaches merely reduce carbohydrate intake, keto aims to fundamentally reprogram the metabolism: The body is to be freed from its primary dependence on glucose and learn to use fat as its main energy source – a metabolic state known as ketosis.

The term 'ketogenic' derives from the ketone bodies the body produces in this state. When carbohydrate intake is drastically reduced—typically to below 20 to 50 grams per day—glycogen stores are depleted, and the liver begins converting fatty acids into ketone bodies. These ketone bodies—beta-hydroxybutyrate, acetoacetate, and acetone—can be used as an energy source by virtually all organs, including the brain, which normally relies almost exclusively on glucose.

The history of the ketogenic diet stretches surprisingly far back. It was developed as a therapeutic approach to treating epilepsy as early as the 1920s – and remains a recognized medical therapy for patients who do not respond adequately to medication. In the last two decades, keto has gained popularity far beyond this medical context: as a tool for weight loss, improving metabolic health, increasing mental clarity, and even as a potential adjunctive therapy for various neurological and metabolic disorders.

What distinguishes keto from other diets is the depth and consistency of the metabolic changes. While a moderate reduction in carbohydrates lowers insulin levels and promotes fat burning, keto goes a crucial step further: The body is put into a state that was normal for our ancestors during periods of fasting, but which most modern people never experience – a state of efficient fat burning with consistently low insulin levels and increased ketone production.

This guide leads you through the science of ketosis, shows you how to practically implement a ketogenic diet, and explains how you can monitor your metabolic health using lab results. Because keto is more than a diet—it's a fundamental reorientation of your metabolism.

To truly understand the ketogenic diet, we need to delve deep into the biochemistry of energy metabolism. The human body is a remarkable system capable of switching between different energy sources—a capability known as metabolic flexibility, which was essential for the survival of our ancestors.

Normal glucose metabolism

In a typical high-carbohydrate diet, glucose is the preferred energy source. Carbohydrates are broken down into glucose, blood sugar levels rise, and insulin is released. Glucose enters the cells, where it is converted in the mitochondria—the powerhouses of the cells—into ATP (adenosine triphosphate), the body's universal energy currency. Excess glucose is stored as glycogen in the liver and muscles; anything beyond that is converted into fat.

The transition to ketosis

When carbohydrate intake is drastically reduced, a fascinating metabolic transformation unfolds. In the first 24 to 48 hours, the body uses up its glycogen reserves – roughly 300 to 500 grams, which, together with the bound water, can weigh several kilograms. Once these reserves are depleted, the body receives the signal: 'External glucose is not available – activate Plan B.'

Plan B is fat burning. Fatty acids are released from fat stores and transported to the liver. There, a process called ketogenesis begins: The liver converts fatty acids into ketone bodies. This process always occurs at a low level, but it is massively increased when carbohydrates are restricted. The ketone bodies are released into the bloodstream and transported to the cells, which use them as an alternative energy source.

Ketone bodies as a superfuel

The three ketone bodies – beta-hydroxybutyrate (BHB), acetoacetate, and acetone – are more than just emergency energy sources. BHB, the main ketone body in the blood, has remarkable properties: it provides more ATP per unit of oxygen than glucose, it easily crosses the blood-brain barrier and supplies the brain with energy, and it has anti-inflammatory and neuroprotective effects that extend beyond its pure energy function.

The brain is particularly interesting during ketosis. Normally, it relies almost entirely on glucose – about 120 grams per day. In ketosis, the brain can meet up to 75 percent of its energy needs from ketone bodies. The rest is supplied by glucose, which the body produces itself from protein and glycerol via gluconeogenesis. Many people report improved mental clarity and concentration in ketosis – an effect attributed to the efficient and stable energy supply to the brain provided by ketone bodies.

The hormonal changes

Ketosis is accompanied by profound hormonal changes. Insulin levels drop to a minimum – a state that maximizes fat release and burning. Glucagon, the antagonist of insulin, rises, signaling the liver to release stored energy. Human growth hormone (HGH) levels increase, promoting muscle maintenance and fat loss. This hormonal state is the opposite of what occurs with a high-carbohydrate diet and frequent meals.

The macronutrient distribution in the ketogenic diet differs fundamentally from what most people know as a 'normal' diet. While an average Western diet derives about 50 percent of its calories from carbohydrates, in keto only about 5 to 10 percent come from carbohydrates – the rest comes from fat and protein.

The classic keto distribution

The typical macronutrient distribution in keto looks like this: 70 to 80 percent of calories from fat, 15 to 25 percent from protein, and 5 to 10 percent from carbohydrates. In absolute numbers, this means, for a daily requirement of approximately 2000 calories: 155 to 178 grams of fat, 75 to 125 grams of protein, and 20 to 50 grams of carbohydrates.

These numbers might seem alarming at first – especially the high amount of fat for people who have spent decades learning to avoid it. But this high fat intake is essential: on the keto diet, fat is your primary energy source and provides the fuel your body needs. If you restrict fat while also restricting carbohydrates, your body will lack energy.

Protein – moderate, not excessive

A common mistake on the keto diet is consuming too much protein. While protein is essential for maintaining muscle mass and numerous bodily functions, excessive protein intake can be problematic on keto. The reason: Excess amino acids can be converted into glucose in a process called gluconeogenesis. With very high protein intake, this can raise blood sugar and therefore insulin levels, disrupting ketosis.

The recommendation: moderate protein amounts that meet your needs but don't massively exceed them. Around 1.2 to 1.7 grams of protein per kilogram of body weight is appropriate for most people on the keto diet. For a person weighing 70 kilograms, that means about 85 to 120 grams of protein per day – certainly substantial portions of meat, fish, or eggs, but not extreme amounts.

Carbohydrates – the 20-gram rule

The carbohydrate limit is the critical factor in keto that determines whether or not ketosis occurs. As a rule of thumb, 20 grams of net carbs per day will put practically everyone into ketosis. Some people can tolerate up to 50 grams and still remain in ketosis; others need to be stricter. Individual carbohydrate tolerance depends on factors such as insulin sensitivity, muscle mass, and physical activity.

20 grams of carbohydrates isn't much – about two slices of bread or a medium-sized potato would already exceed the budget. On the keto diet, the few allowed carbohydrates come primarily from non-starchy vegetables, some dairy products, and the occasional berry. Sugar, grains, legumes, starchy vegetables, and sweet fruits are off-limits.

The ketogenic diet has received intense scientific attention in recent years, and the research findings are impressive across many applications. From weight loss and metabolic health to neurological benefits, the list of potential advantages is long.

Effective fat loss

Keto is one of the most effective dietary strategies for fat loss. The mechanism is multifaceted: consistently low insulin levels allow for maximum fat mobilization. The body becomes an efficient fat-burning machine. The satiating effect of fat and protein often leads to spontaneous calorie reduction without feelings of hunger. Many people report losing weight effortlessly on keto without counting calories – appetite regulates itself.

Stable energy and mental clarity

One of the most consistent feedback reports from keto followers is improved energy and mental performance. The explanation lies in biochemistry: ketone bodies provide a steady, stable energy supply without the fluctuations that occur with a glucose-based diet. The brain receives a constant energy flow, resulting in improved concentration, clarity, and reduced brain fog.

Improved blood sugar control

For people with insulin resistance, prediabetes, or type 2 diabetes, keto can be transformative. With minimal carbohydrate intake, blood sugar levels barely rise, less insulin is needed, and insulin sensitivity can recover. Studies show that keto can significantly lower HbA1c levels—the long-term marker for blood sugar control. Some people with type 2 diabetes can reduce or even discontinue their medication under medical supervision.

Improved lipid profile

Contrary to intuition, the high-fat keto diet typically improves blood lipid levels. Triglycerides often drop dramatically—a major independent risk factor for heart disease. HDL cholesterol increases. The triglyceride-to-HDL ratio, one of the most significant markers of cardiovascular risk, improves considerably. The situation with LDL is more complex: some people see an increase, but often in the direction of larger, less atherogenic particles.

Reduced inflammation

Chronic low-grade inflammation is a driver of many modern diseases. Keto, and in particular the ketone body BHB, has proven anti-inflammatory properties. BHB inhibits specific inflammatory pathways at the cellular level. Many people report improvements in inflammation-related conditions such as joint pain or skin problems.

Neurological benefits

The original medical application of keto—the treatment of epilepsy—suggests profound neurological effects. Current research is investigating keto as a potential therapeutic strategy for Alzheimer's, Parkinson's, and other neurodegenerative diseases. The neuroprotective properties of ketone bodies and the reduction of oxidative stress in the brain are promising areas of research.

The practical implementation of the ketogenic diet requires a rethinking of food choices. Some foods that are avoided in conventional diets become central – while supposedly 'healthy' options suddenly become problematic.

The basis: fats and oils

On a keto diet, high-quality fats are the foundation of every meal. Extra virgin olive oil is excellent for salads and cold dishes. Coconut oil provides medium-chain triglycerides (MCTs), which are particularly easy to convert into ketones. Butter and ghee (clarified butter) are ideal for frying and enhance the flavor. Avocado oil has a high smoke point, making it suitable for high-heat frying. Lard and tallow from grass-fed animals are also good options.

Protein sources

All unprocessed meats are allowed on the keto diet – and fattier varieties are not only acceptable, but preferred. Ground beef, pork belly, skin-on chicken thighs, lamb chops – all keto-friendly. Fish, especially fatty varieties like salmon, mackerel, and sardines, provides not only protein but also valuable omega-3 fatty acids. Eggs are a keto superfood: nutrient-rich, versatile, and perfectly balanced. Seafood such as shrimp, mussels, and crabmeat also fits well.

Vegetables – green and non-starchy

Vegetables remain important on the keto diet – but only certain types. Leafy greens like spinach, kale, arugula, and lettuce are practically carbohydrate-free and provide valuable nutrients. Broccoli, cauliflower, zucchini, asparagus, mushrooms, bell peppers, and green beans are low in carbohydrates enough for moderate portions. Cruciferous vegetables like cauliflower are particularly versatile – as a rice substitute, pureed as a potato substitute, or as a pizza base.

Dairy products – choose high-fat options

Full-fat dairy products are welcome on a keto diet. Cheese in all its varieties (hard, soft, and fresh) contains minimal carbohydrates and plenty of fat. Cream, sour cream, and crème fraîche enrich many dishes. Butter is a staple food on keto anyway. Caution is advised with milk, however, as it contains lactose (milk sugar). Reduced-fat dairy products should also be avoided – less fat usually means more carbohydrates.

What you should avoid

The list of forbidden foods in keto is extensive: all grain products (bread, pasta, rice, oatmeal), sugar in any form, potatoes and other starchy vegetables, legumes (beans, lentils, chickpeas), most fruits (except for small amounts of berries), processed foods with hidden carbohydrates, and practically all sugary drinks, including fruit juices. Even supposedly 'healthy' foods like quinoa, sweet potatoes, or whole-wheat bread are off-limits in keto.

Unlike other diets, with keto you can objectively measure whether you're 'doing it right'. The state of ketosis can be determined and quantified using various methods. Understanding these measurement methods will help you optimize your diet and track your progress.

Measurement methods for ketosis

Urine ketone test strips are the most cost-effective method. They measure acetoacetate, a ketone body excreted by the kidneys. In the first few weeks of keto-adaptation, they reliably indicate ketosis. One drawback: After complete adaptation, the body produces fewer excess ketone bodies, and the test strips may show negative results even though you are in ketosis.

Blood ketone meters measure beta-hydroxybutyrate (BHB), the main ketone body in the blood. This method is more precise and informative than urine test strips. Dietary ketosis typically shows values ​​between 0.5 and 3.0 mmol/L. The meters are affordable, but the test strips are more expensive than urine test strips.

Breath ketone meters measure acetone in exhaled air. They are convenient because they don't use test strips, but less precise than blood tests. Nevertheless, they can provide a good overview of ketosis status.

Important lab values ​​during keto

Beyond simply measuring ketosis, certain blood values ​​are important for monitoring the effects of the ketogenic diet on your health. The lipid profile – triglycerides, HDL cholesterol, LDL cholesterol, and total cholesterol – shows how your fat metabolism responds to the increased fat intake. In most people, triglycerides and HDL improve significantly on keto.

Blood glucose and HbA1c levels are relevant for documenting the effects on glucose regulation – especially important for people who have started keto due to insulin resistance or diabetes. Thyroid levels (TSH, fT3, fT4) should be monitored during long-term ketogenic eating, as some people experience changes.

The DoctorBox lipid metabolism test allows you to conveniently monitor your lipid profile from home. Especially with a high-fat diet like keto, it's helpful to see how your body reacts to the increased fat intake. Most people experience positive changes – but individual reactions vary, and objective data provides reassurance.

The transition to a ketogenic diet is a significant change for your body, and this process isn't always symptom-free. Many people experience discomfort in the first few days or weeks, often referred to as the 'keto flu' or 'carb flu'. Understanding these symptoms and their causes will help you manage them and avoid giving up prematurely.

What is the keto flu?

The keto flu isn't a true flu, but rather a collection of symptoms that can occur when transitioning into ketosis. Typical complaints include: headaches, fatigue and exhaustion, dizziness or lightheadedness, irritability and brain fog, muscle cramps or weakness, nausea, sleep disturbances, and digestive issues (constipation or diarrhea). Not everyone experiences all symptoms, and the intensity varies considerably.

Understanding the causes

The main cause of the keto flu is electrolyte loss. When carbohydrates are drastically reduced, insulin levels drop. Lower insulin signals the kidneys to excrete more sodium—and with the sodium comes water. In the first few days, you might lose several liters of water and, with it, significant amounts of sodium, potassium, and magnesium. This electrolyte imbalance causes many of the keto flu symptoms.

Additionally, your body is in a transition phase: it hasn't yet learned to efficiently use fat and ketone bodies for energy, and the usual glucose is no longer available. This metabolic 'gap' can manifest as fatigue and mental sluggishness.

Strategies for relief

The most important step: replenish electrolytes. Use salt generously—yes, more than you might consider healthy. 5 to 7 grams of sodium per day is often appropriate on keto, significantly more than the usual recommendations. Potassium-rich foods like avocado, spinach, and mushrooms should be included in your diet. Magnesium supplementation can be beneficial—magnesium citrate or glycinate are well-absorbed forms.

Staying hydrated is important, but water alone can worsen the problem by further diluting electrolytes. Bone broth is a traditional remedy for keto flu—it provides sodium, potassium, and other minerals in an easily absorbable form. Specialized, sugar-free electrolyte supplements can also be helpful.

How long does it take?

The keto flu typically lasts three to seven days, sometimes up to two weeks. The good news is that it's temporary and a sign that your metabolism is adjusting. If you manage your electrolytes and persevere, the discomfort often gives way to a new clarity and energy that many keto enthusiasts describe as transformative.

The ketogenic diet is not a monolithic concept, but encompasses various approaches with different levels of strictness and objectives. Understanding these variations will help you find the approach that suits you best.

Standard Keto (SKD)

The standard ketogenic diet (SKD) is the classic approach: a consistently low carbohydrate intake (typically under 20-50g), high fat intake, and moderate protein. This version is followed daily without planned exceptions. It is the best-researched approach and the basis on which most keto recommendations are founded. For beginners, SKD is the recommended starting point to fully adapt the body to ketosis.

Cyclical Keto (CKD)

The cyclical ketogenic diet (CKD) alternates between periods of strict ketosis and periods of increased carbohydrate intake. A typical pattern is five to six days of strict ketosis, followed by one to two days of higher carbohydrate intake (a "refeed" or "carb-up"). This approach is often used by athletes who want to combine the metabolic benefits of ketosis with occasional glycogen replenishment. CKD requires good self-awareness and discipline to prevent the carbohydrate days from becoming excessive.

Targeted Keto (TKD)

The targeted ketogenic diet allows carbohydrates only around workouts. Typically, 25 to 50 grams of rapidly digestible carbohydrates are consumed 30 to 60 minutes before training to support performance during intense sessions. The idea is that the carbohydrates are used directly during the workout and don't disrupt ketosis for the rest of the day. Targeted ketogenic diets are suitable for people who want to combine intense physical activity with keto.

High-Protein Keto

This variant increases protein intake to approximately 30 to 35 percent of calories, with a correspondingly reduced fat intake. It can be attractive for people who have difficulty consuming so much fat or who place particular importance on preserving muscle mass. The disadvantage: The increased protein intake can disrupt ketosis through gluconeogenesis in some individuals. Regular monitoring of ketone levels is recommended.

Therapeutic Keto

For neurological conditions such as epilepsy, a stricter form of the ketogenic diet is used, often with a fat-to-protein-to-carbohydrate ratio of 4:1. This medical application requires close medical supervision and is significantly more restrictive than the lifestyle versions. Research is also investigating therapeutic ketosis for other neurological conditions, cancer, and metabolic disorders.

The compatibility of a ketogenic diet and athletic performance is a controversial topic. The truth, as is so often the case, lies somewhere in the middle: Keto can offer advantages for certain sports and athletes, while it may be suboptimal for others.

The adaptation phase

During the first two to eight weeks of switching to a keto diet, a temporary decrease in performance is to be expected. The body has not yet fully developed the ability to burn fat efficiently, and the usual glycogen reserves are depleted. This phase can be frustrating for athletes – understanding that it is temporary helps with perseverance.

After complete 'fat adaptation' – typically after four to eight weeks – performance returns for most activities. Some athletes even report improved endurance and stable energy during long workouts.

Where Keto offers benefits

Endurance sports of moderate intensity often benefit from keto. Fat-adapted athletes can efficiently utilize their virtually unlimited fat reserves and are less dependent on a constant supply of carbohydrates. Ultra-marathon runners, long-distance cyclists, and triathletes frequently report positive experiences with keto—more stable energy, fewer gastrointestinal issues due to reduced gel and bar intake, and no glycogen walls.

Keto can also work well for recreational athletes who primarily train for health and well-being. If the goal is fat loss while maintaining fitness, keto supports both goals.

Where carbohydrates retain an advantage

During high-intensity, short-duration activities – sprints, explosive bursts of power, interval training at maximum intensity – carbohydrates are the more efficient energy source. Glycolysis (energy production from glucose) is faster than fat oxidation. For competitive athletes in sports with repeated sprints or explosive bursts, strict ketosis may be suboptimal.

The solution for these athletes: targeted or cyclical keto variations that allow for strategic carbohydrate intake around training or on competition days. This allows the metabolic benefits of ketosis to be combined with optimal performance when needed.

Practical recommendations

Begin your keto adaptation with a reduced training volume. Expect a temporary decrease in performance and plan accordingly. Electrolytes are even more important when exercising on keto – mineral loss through sweat adds to the already increased need. Once fully adapted, you can experiment to find the keto variation that best suits your athletic goals.

The ketogenic diet is a powerful tool, but it's not equally suitable for everyone. An honest assessment of your own situation, goals, and circumstances is important before embarking on this path.

Especially suitable for:

For people with significant excess weight: Keto can effectively mobilize and burn large amounts of body fat. The appetite-suppressing effect of ketosis makes it easier to stick to than many other diets.

People with insulin resistance, prediabetes, or type 2 diabetes: Drastically reducing carbohydrate intake can transform blood sugar control. Important: If you are currently taking medication, do this only under medical supervision, as the dosage will need to be adjusted.

People struggling with mental fog or energy fluctuations: The stable energy supply provided by ketone bodies can bring mental clarity and constant energy.

People with epilepsy: Keto is an established medical therapy for drug-resistant epilepsy, especially in children.

Possible, but with caution:

Athletes: Depending on the sport and goals, keto can work, but requires careful planning and possibly modifications (cyclical or targeted variations).

People with thyroid disorders: Some people experience changes in thyroid function while following a strict keto diet. Regular monitoring and, if necessary, a moderate rather than strict carbohydrate reduction are advisable.

Not recommended for:

Pregnant and breastfeeding women: The increased need for nutrients and potential risks to the fetus make strict ketosis unsuitable during this phase of life.

People with eating disorders or a history of eating disorders: The strict rules and focus on specific foods can reinforce problematic patterns.

Type 1 diabetics without close medical supervision: The risk of diabetic ketoacidosis (a dangerous condition to be distinguished from nutritional ketosis) requires careful medical monitoring.

People with certain metabolic disorders: Rare diseases such as pyruvate carboxylase deficiency or lipid metabolism disorders make keto contraindicated.

People with advanced kidney disease: Increased protein intake can be problematic.