What is the keto diet? Mechanisms, menus, risks, and evidence

If you're looking for a clear and reliable definition, "What is the Keto Diet?" is an excellent starting point for understanding a very specific nutritional strategy: drastically reducing carbohydrates to force the body to produce "ketone bodies" and use them as fuel. The term "keto" comes from "ketosis," a measurable metabolic state. But beyond the hype, there's a physiological reality, historical medical indications, common uses in weight loss, and also limitations and risks to be aware of.

In this guide, you will learn what the keto diet is from a biological perspective (what changes in metabolism), a practical one (what to eat, in what quantities, how to get started), and a critical perspective (what science supports, what remains uncertain, and for whom it is not suitable). The goal is to provide you with a clear framework, without exaggerated promises, so you can make informed decisions.

You'll also learn how to distinguish a well-structured keto diet from a simple high-fat, low-fiber diet, how to avoid common mistakes, how to adjust your electrolyte intake, and how to assess whether this approach aligns with your priorities (weight, blood sugar, performance, digestive comfort). Additionally, if your primary goal is weight management, you can also consult our dedicated weight loss.

Why is everyone asking: What is the keto diet?

The popularity of the keto diet stems from a simple observation: many people struggle to lose weight or stabilize their appetite using conventional methods. By reducing carbohydrates, some experience a spontaneous decrease in hunger, an increased ability to limit snacking, and, in some individuals, improved blood sugar regulation. This fuels curiosity and research surrounding the question: What is the keto diet?

There is also a medical legacy. Nutritional ketosis has a long history of use, particularly in certain neurological conditions (for example, drug-resistant epilepsy, under medical supervision). This history gives keto a particular credibility: it is not just a "fad diet," but a documented dietary intervention in specific contexts.

Finally, keto is often confused with other approaches: moderate low-carb, Atkins, paleo, or simply "sugar-free." Understanding "What is the keto diet?" also means learning to differentiate it: the central characteristic is not avoiding sugar on principle, but aiming for a sufficiently low carbohydrate threshold to promote ketosis, with controlled protein intake and appropriate fats.

Strict definition: keto, nutritional ketosis and "low-carb"

What is the keto diet? Strictly speaking, it's a very low-carbohydrate, moderate-protein, and relatively high-fat diet designed to induce nutritional ketosis. Nutritional ketosis corresponds to an increased production of ketone bodies (mainly beta-hydroxybutyrate, acetoacetate, and acetone) by the liver, from fats.

It is crucial to distinguish between nutritional ketosis and diabetic ketoacidosis. Ketoacidosis is a medical emergency linked to an insulin deficiency (often uncontrolled type 1 diabetes) and is accompanied by dangerously high blood acidity. Nutritional ketosis, in a metabolically stable individual, remains within very different ranges and does not have the same clinical significance.

A "low-carb" diet can simply mean a reduction in carbohydrates (for example, 80–130 g/day), without necessarily reaching ketosis. Keto, on the other hand, generally aims for very low net carbs (often around 20–50 g/day, depending on the individual), which explains why the question "What is the keto diet?" comes up so often: it's a more structured approach than simply "eating less bread."

Origins and context: from a therapeutic diet to a mass-market strategy

Historically, ketogenic diets were developed to mimic some of the metabolic effects of fasting while providing calories. The idea was to benefit from the use of ketones as an alternative fuel, particularly for the brain, in specific medical contexts. This basis explains some of the scientific literature surrounding the question, " What is the keto diet?"

Over time, these principles have spread to the general public. The rise of personal measurement tools (ketone meters, macro apps) has made the approach more accessible. At the same time, debates have arisen: the quality of fats, effects on LDL cholesterol, sustainability, impact on the gut microbiota, and risks for certain individuals.

The transition from a medical framework to general use requires solid education: the same tool can be relevant in one context and counterproductive in another. Understanding "What is the keto diet?" also means understanding that "ketogenic" is not an automatic health label: everything depends on the quality of food, micronutrient intake, and the individual.

How keto alters metabolism: from glucose to ketones

To fully answer the question, "What is the keto diet?", we need to explain the fuel shift. In a "conventional" diet, carbohydrates are digested into glucose, which quickly fuels the tissues. When carbohydrates are drastically reduced, insulin (the storage hormone) tends to decrease, which facilitates the release of fatty acids from adipose tissue.

The liver converts some of these fatty acids into ketone bodies. Ketones then become an important energy source, particularly for the brain and muscles, especially after an adaptation phase. This is sometimes referred to as "metabolic flexibility": the ability to switch from glucose to lipids/ketones depending on availability.

Glycogenolysis (the breakdown of glycogen, the glucose reserve in the liver and muscles) occurs initially. Because glycogen is stored with water, the first weight loss observed may be partly water loss, not solely fat loss. This is a common source of initial enthusiasm, but also of disappointment if this mechanism is not understood.

The role of protein: why "too much" can hinder ketosis

A common misconception: keto doesn't mean "high protein." Protein is essential (for muscles, enzymes, and immunity), but in excess, it can increase glucose production through gluconeogenesis (the process of converting amino acids into glucose). This process is normal and beneficial, but it can lower ketone levels in some highly sensitive individuals.

In practice, the goal is generally to consume enough protein to preserve lean mass, without turning keto into an "all-you-can-eat steak" diet. Hence the importance of thinking in terms of overall structure: low carbohydrates, moderate protein, and fats adjusted according to the objective (weight loss vs. maintenance vs. performance).

What we actually eat: staple foods and foods to limit

In a well-structured keto diet, the foundation consists of low-carb vegetables (raw vegetables, leafy greens), quality fat sources (olive oil, avocado, nuts), and complete proteins (eggs, fish, meat, tofu/tempeh as tolerated). Ultra-processed "keto" products exist, but should not form the core of the diet.

Foods typically restricted include grains (bread, pasta, rice), most legumes (depending on the quantity), starchy foods (potatoes), and sweet fruits. Some dairy products can be used (cheeses, Greek yogurts), but digestive tolerance and quality are important.

The question "What is the keto diet?" also raises the question of fiber: it shouldn't disappear. Constipation is a common complaint when the diet becomes too low in vegetables, too high in cheese/cured meats, or insufficient in water and electrolytes. If you already have a sensitive digestive system, a look at our guide on digestion can help you make informed choices.

Typical macros: how many carbohydrates, proteins, fats?

The keto diet is often summarized as a "70–75% fat, 20–25% protein, 5–10% carbohydrate" ratio, but this percentage-based approach can be misleading. When trying to lose weight, fat intake can be reduced (while still remaining the primary source), as some of the energy then comes from the body's reserves.

A practical approach: set a net carbohydrate limit (total carbohydrates minus fiber), choose a protein target appropriate for weight and activity level, then supplement with fats according to hunger and calorie goal. Measuring blood ketones can help some, but isn't essential for everyone.

It's worth remembering that the answer is individual. Two people with the same "carb intake" can have different ketone levels depending on sleep, stress, activity, insulin sensitivity, and even the composition of their gut microbiota. That's why " What is the keto diet?" isn't a one-size-fits-all approach.

Table 1 — Keto, low-carb and balanced diet: key differences

Potential benefits: what some people feel (and why)

Many people turn to keto for weight loss. Part of the effect comes from the spontaneous reduction in calorie intake (fewer cravings, fewer blood sugar spikes) and a better dietary structure (fewer ultra-processed foods, more protein and vegetables). These benefits are plausible, but vary.

Another reported benefit is more stable energy throughout the day, especially when rapid glucose fluctuations were a problem. This may improve concentration in some people, although the adjustment period can be challenging.

Finally, some people experience an improvement in satiety. Fats and proteins slow gastric emptying and influence hunger hormones (such as ghrelin) and satiety hormones. This is a common explanation behind the interest in " What is the keto diet?".

Weight loss: be careful when interpreting the first few weeks

In the first few weeks, weight loss can be rapid, often due to a depletion of glycogen stores and the associated water retention. After that, progress depends primarily on the overall energy deficit, activity level, and the quality of the diet plan. Keto doesn't negate the laws of energy metabolism, but it can help some people adhere to them more easily.

If your goal is to lose weight sustainably, keep in mind that adherence (sticking to it over time) is a key factor, sometimes more important than the theoretical "superiority" of a diet. Our weight loss can help you compare different strategies in this regard.

What science says: evidence, nuances and limitations

Research on low-carb and ketogenic diets often shows interesting results on weight and certain cardiometabolic markers in the short term, but the comparison depends heavily on the quality of the diets being compared, their duration, and adherence. It is therefore essential to avoid simplistic conclusions such as "keto is always better.".

Regarding blood glucose and insulin resistance, carbohydrate reduction can improve certain parameters in individuals with impaired glycemic control. However, close monitoring is important, especially when undergoing antidiabetic treatment, as medication requirements can change rapidly.

Regarding blood lipids, the response is heterogeneous: some people see an improvement in triglycerides and HDL cholesterol, while others observe an increase in LDL cholesterol. This can depend on genetic makeup, the type of fats consumed (saturated vs. unsaturated), and the weight loss itself. Understanding "What is the keto diet?" therefore means understanding that there is no single, universal effect.

EEAT and caution: why "miracle" figures are suspicious

You will sometimes see very aggressive, quantified promises (for example, "-10 kg in 2 weeks") or absolute medical claims. Without context, these messages are rarely reliable. A scientific analysis requires distinguishing between: short-term vs. long-term results, the population studied, the comparator, and the quality of micronutrient intake.

If you like to check for yourself, PubMed is a useful base for exploring the literature, especially via a general search on the topic: PubMed searches on the keto diet.

Starting without getting into trouble: adaptation phase and "keto flu"

In the first few days, some people experience fatigue, headaches, irritability, cramps, decreased performance, or sleep disturbances. This is often called the "keto flu," even though it's not an infection. Part of it is related to the loss of water and electrolytes (sodium, potassium, magnesium) when insulin levels drop.

To limit these effects, it's important to plan ahead: drink enough fluids, use salt appropriately (unless medically contraindicated), consume potassium-rich foods (avocado, green vegetables), and ensure adequate magnesium intake through diet. A gradual reduction in carbohydrates may also be better tolerated than a sudden change.

Sleep and stress influence the perception of this phase. High cortisol (the stress hormone) can make adaptation more difficult and increase sugar cravings. If this is the case for you, you can also work on recovery habits through our stress and anxiety.

Table 2 — Start-up Protocol (2 weeks): Steps and Checkpoints

What a "keto" day looks like: sample menus

A good keto menu should be simple: a base of vegetables + a protein + a source of fat + possibly a portion of fermented or fiber-rich foods. The goal is to avoid "liquid calories" and ultra-processed foods, which can reduce satiety.

Example breakfast: Herb omelet with spinach and mushrooms, olive oil, avocado. Lunch: Tuna/salmon salad, olives, raw vegetables, olive oil vinaigrette, a few walnuts. Dinner: Sautéed chicken or tofu with broccoli and zucchini, and a sauce made with olive oil and lemon.

Snacks: not mandatory. If needed: plain Greek yogurt (if tolerated) with a few seeds, or a small bowl of olives/nuts. Understanding "What is the keto diet?" also means understanding that the goal is not to eat fat constantly, but to create a framework that stabilizes appetite.

Measuring ketosis: urine, breath, blood… useful or a gimmick?

There are three main methods: urine test strips (which primarily measure excreted acetoacetate), breath analysis (acetone), and blood measurement (beta-hydroxybutyrate). Blood measurement is often the most accurate for estimating nutritional ketosis.

However, "more ketones" doesn't always mean "better results." When it comes to weight loss, adherence, the quality of intake, and comfort remain key. For some, measuring can be reassuring and help them understand the impact of certain foods. For others, it can be anxiety-inducing.

The most important thing is to clarify your objective: performance, glycemic control, appetite, digestive health, etc. The question "What is the keto diet?" then becomes practical: what level of restriction is necessary for your goal, without creating undesirable side effects?

Possible side effects: what is common, what should raise concern

Common side effects include constipation, cramps, bad breath (acetone), transient fatigue, decreased tolerance to intense exercise initially, and sometimes irritability. Many are related to adaptation, a lack of fiber or electrolytes, or a sudden drop in calorie intake.

Less frequent but significant side effects include reflux, nausea if meals are too fatty at once, menstrual irregularities in some women, or an unfavorable increase in certain lipid markers. Monitoring may be advisable if you have a history of cardiovascular disease or already have elevated LDL cholesterol.

Warning signs: significant discomfort, persistent vomiting, confusion, severe abdominal pain, or symptoms consistent with hypoglycemia while on medication (tremors, sweating, palpitations). In these cases, medical advice is necessary. What is the keto diet? It should never mean "total independence" if you have a medical condition or are taking medication.

Contraindications and situations requiring medical advice

The keto diet is not suitable for everyone. Certain rare metabolic disorders (such as fatty acid oxidation disorders) are contraindications. Similarly, pregnancy and breastfeeding require increased caution and medical supervision if carbohydrate restriction is considered.

If you have diabetes being treated with insulin or sulfonylureas, a significant change in carbohydrate intake may require a rapid dose adjustment to prevent hypoglycemia. Close monitoring is therefore essential. The same applies if you are taking blood pressure medication, as the initial fluid loss can affect blood pressure.

Finally, in cases of a history of eating disorders, a very restrictive approach can be a factor in relapse. There are less strict strategies, sometimes psychologically healthier, that achieve the same goals. Understanding "What is the keto diet?" also means knowing when to choose an alternative.

Interactions: medications, supplements, alcohol

The most significant interactions involve diabetes medications (risk of hypoglycemia if carbohydrates are reduced without adjustment) and certain antihypertensive drugs (risk of hypotension, especially at the beginning). Any major dietary changes should be discussed with a healthcare professional if you are undergoing treatment.

Regarding supplements: some people use electrolytes, magnesium, or fiber. These can help, but unnecessary combinations should be avoided. "Exogenous ketones" (ketone drinks) do not replace diet-induced ketosis and can add calories. Their benefit is limited in most cases.

Alcohol: Tolerance can change. Alcohol is primarily metabolized by the liver and can disrupt ketone production, while also increasing the risk of hypoglycemia in some people. If you consume alcohol, opt for moderation and low-sugar choices.

Keto quality: fats, fiber, micronutrients, and microbiota

A "quality" keto diet prioritizes mostly unsaturated fats (olive oil, oily fish, nuts), while limiting excess saturated fats from ultra-processed foods. It includes a variety of vegetables to provide fiber, potassium, folate, vitamin C, and phytochemicals (protective plant molecules).

The gut microbiota (the collection of bacteria in the colon) feeds primarily on fermentable fiber. A keto diet too low in fiber can reduce microbial diversity in some people, potentially impacting digestive comfort. This point is often overlooked when discussing " What is the keto diet?".

A simple strategy: aim for low-carbohydrate vegetables at every meal, incorporate seeds (chia, flax), and choose minimally processed foods. If constipation occurs, adjust water/salt/fiber intake gradually rather than suddenly adding large amounts of fiber.

Common mistakes that cause failure (or make keto unpleasant)

Mistake #1: Removing carbohydrates without replacing them with a coherent dietary structure. Result: undernourishment, fatigue, irritability, and cravings. Keto requires a minimum of planning, especially at the beginning.

Mistake #2: Confusing "fatty" with "healthy." A ketogenic diet consisting mainly of bacon, cheese, and processed snacks can lead to micronutrient deficiencies and digestive problems. This is not what "What is the keto diet?" means from a health perspective.

Mistake #3: Neglecting electrolytes. Many initial discomforts stem from a lack of sodium, potassium, or magnesium. This is more important on a keto diet than on a standard diet, especially in the first few weeks.

Keto and sport: endurance, strength training, HIIT

For moderate-intensity endurance activities, some people adapt well to keto, as fat oxidation becomes more efficient. However, for explosive efforts (HIIT, sprints), glycogen remains a useful fuel source. A drop in performance is common at the beginning, but this may lessen depending on the individual.

In bodybuilding, the goal is to preserve muscle mass: sufficient protein, well-structured training, and sleep. Some people use hybrid approaches (targeting carbohydrates around workouts) rather than a constant strict keto diet. This illustrates the diversity of answers to the question, "What is the keto diet?"

Sleep is an often underestimated factor: a sleep deficit increases appetite and reduces recovery. If keto disrupts your sleep at first, it can be helpful to work on your sleep hygiene alongside it (see our sleep).

Table 3 — Keto vs alternatives: which approach for which objective?

FAQ: Short and concise answers

1) What is the keto diet? In a nutshell? It's a very low-carbohydrate, moderate-protein, and high-fat diet designed to induce nutritional ketosis (the production of ketone bodies) to increase the body's use of fat as fuel. It must be carefully planned to avoid deficiencies and digestive discomfort.

2) How many carbohydrates per day are needed to be "in keto"? Many people aim for around 20 to 50 grams of net carbs per day, but the threshold varies. Activity, sleep, stress, and insulin sensitivity all affect the response. The key is consistency in the plan and individual tolerance, not a magic number.

3) Can you follow keto without measuring ketones? Yes. Practical markers include reduced sugar cravings, more stable hunger, and better meal regularity. Measuring can help with learning, but it's not mandatory. Good food quality and adequate electrolyte intake often matter more on a daily basis.

4) Is the "keto flu" inevitable? No, but it's common if the transition is abrupt. It's often linked to water and sodium loss. Drinking enough fluids, using salt properly, and consuming potassium-rich foods often helps. A gradual reduction in carbohydrates is sometimes better tolerated.

5) Does keto mean eating lots of bacon and cheese? No. A healthy keto diet emphasizes low-carb vegetables, fish, eggs, quality oils (like olive oil), avocado, nuts, and sufficient protein. An overly processed version can increase constipation, reflux, and lipid imbalances in some people.

6) Is it possible to lose weight without counting calories on keto? Sometimes, because satiety increases in some people and calorie intake decreases spontaneously. But it's not guaranteed. If fat intake becomes very high (cheese, oils), weight loss may plateau. The goal remains an overall energy deficit.

7) Is keto dangerous for the heart? It depends on the individual and the type of fats consumed. Many people see their triglycerides decrease and their HDL cholesterol increase, but some see their LDL cholesterol rise. Blood tests can be helpful, especially if there is a history of heart problems. Prioritizing unsaturated fats is a prudent strategy.

8) Can you follow the keto diet if you have diabetes? It can cause rapid changes in blood sugar levels, so it requires medical supervision, especially if you are taking insulin or other blood sugar-lowering medications. The main risk is hypoglycemia if the doses are not adjusted. Do not change your diet and medication on your own.

9) Keto and constipation: what to do? Gradually increase low-carb vegetables and water intake, and adjust your salt. Seeds (chia/flax) may help. Avoid replacing carbohydrates solely with cheese and processed meats. If the problem persists, your individual constitution and tolerance should be reassessed.

10) Can you follow a keto diet as a vegetarian? Yes, but it's more technical. You need to balance your protein intake (eggs, dairy products if tolerated, tofu/tempeh), fats (olives, avocado, nuts), and fiber (vegetables). Many legumes are higher in carbohydrates, so the quantities need to be adjusted.

11) Is keto suitable for athletes? For moderate-intensity endurance, some adapt well. For explosive efforts, performance may initially decrease. Targeted approaches (a small amount of carbohydrates around training) are sometimes preferred. The trial should be gradual, with monitoring of how you feel.

12) How long can you stay on keto? There's no universal duration. Some people do it in cycles, others long-term. The key considerations are nutritional quality, biological markers, digestive comfort, mental health, and social adherence. Regular checkups are advisable if you plan to continue.

Conclusion: Key takeaways

What is the keto diet? It's a nutritional strategy that aims for ketosis through a significant reduction in carbohydrates, with moderate protein and adjusted fat intake. When followed correctly, it can help certain profiles (appetite, weight, blood sugar), but it's neither a magic bullet nor a one-size-fits-all solution.

Its success depends on specific details: the quality of fats, fiber and micronutrient intake, electrolyte management, gradual adaptation, and compatibility with your lifestyle. If you are undergoing treatment or have a medical condition, medical supervision is essential.

Finally, if your main goal is overall health, keep one simple principle in mind: whatever approach you take, prioritize minimally processed foods, plenty of vegetables, sufficient protein, and a sustainable approach. This is often where the difference lies between a "diet experiment" and genuine improvement.

Sources & references

• PubMed – search: ketogenic diet
• PubMed – search: ketogenic diet epilepsy
• PubMed – search: very low carbohydrate diet weight loss
• Cochrane Library – systematic reviews
• Harvard Health – A Critical Look at the Keto Diet
• NCCIH (NIH) – Nutrition and Health Resources
• NIH – Health Information and Research
• WHO – Health Recommendations and Resources