Intermittent fasting (also known as intermittent fasting) and the ketogenic diet are popular biohacking methods for increasing energy levels and boosting fat burning. These are two different approaches: Fasting involves alternating between eating and fasting windows (e.g. 16/8 fasting or the 5:2 diet), while the keto diet is a permanent low-carb way of eating that puts the body into ketosis. Both methods aim to switch the metabolism - away from burning sugar and towards burning fat. Initial studies suggest that intermittent fasting can improve various health markers (e.g. weight, blood sugar, blood lipids) [1][2]. However, health claims about fasting and keto should be treated with caution: Many effects have not yet been confirmed by the EFSA and require further research.
What is intermittent fasting? - Basics and benefits
Intermittent fasting means eating during a certain time window and fasting during the rest of the period. A common model is 16/8 fasting: 16 hours of fasting, 8 hours of eating. For example, you only eat between 12 noon and 8 p.m. - this pattern is also known as time slot fasting. Alternatively, there is the 5:2 diet, where you eat normally on 5 days and greatly reduce your calorie intake on 2 days. There are also other variants such as alternate-day fasting (alternating fasting every other day) or the"OMAD" principle ("one meal a day", i.e. only one meal a day). Proponents report benefits such as increased energy, weight loss and mental clarity(intermittent fasting experiences). There is at least scientific evidence that intermittent fasting can help with weight loss - a recent meta-analysis found significant reductions in body weight, fat mass and insulin levels in overweight adults[5]. Interestingly, the long-term results were comparable to conventional continuous calorie reduction - intermittent fasting is therefore primarily an alternative strategy for achieving a calorie deficit. In addition, animal studies and cell studies suggest that fasting activates autophagy (cell cleansing)[3][4] - a process that may counteract ageing processes (although this has not yet been confirmed by EFSA; further studies required).
Fasting benefits: By taking longer breaks from eating, insulin levels drop, the body burns more fat reserves and many people experience more stable energy levels. Blood sugar stabilizes over the fasting period, which can be particularly beneficial for overweight people or people with insulin resistance (studies are underway, but official health claims are not available). Blood pressure and other metabolic parameters can also improve[1]. There are also indications of anti-inflammatory effects: In some studies, inflammation markers decreased in fasting individuals[1] (a possible benefit, although this needs further confirmation). Many people also appreciate the simplicity: you don't need a complicated fasting plan, you simply don't eat at certain times of the day. During the fasting period, calorie-free drinks are allowed - water, unsweetened tea or black coffee(fasting coffee). Coffee or green tea in particular can curb the feeling of hunger and slightly stimulate the metabolism without breaking the fast.
Mastering hunger during fasting: Of course, the question arises as to how to deal with periods of hunger. Many fasting people report that the feeling of hunger decreases after a few days as the body gets used to the rhythm(fasting adaptation). During the adaptation period, drinking enough fluids and mineral-rich drinks (e.g. water with a little salt or vegetable broth) helps to avoid circulatory problems. If you feel dizzy or nauseous while fasting, you should listen to your body - fasting is not a competition. Beginners in particular should start moderately, fasting for around 12 or 14 hours and then slowly extend this time window. It is also important to break your fast gently: A balanced meal that is not too sumptuous helps the digestive system to rev up again.
Ketogenic diet - permanently in ketosis
The ketogenicdiet (keto diet) is an extremely low-carbohydrate, high-fat form of nutrition. By greatly reducing carbohydrates (typically less than 50 g per day) and consuming sufficient protein, the body learns to use fats as its primary source of energy. In the liver, so-called ketones are formed from fat, which supply the organs with energy instead of glucose. This metabolic switch is called ketosis. To achieve ketosis, keto followers almost completely eliminate starchy foods (bread, pasta, rice, sugar) and focus on meat, fish, eggs, healthy fats (oils, butter, avocado, nuts) and some low-carb vegetables. After a few days to a few weeks of adaptation, the body begins to produce noticeable ketones - measurable, for example, with keto sticks in urine or special breathing devices. The keto diet also has medical origins: it was developed in the 1920s to treat epilepsy in children, as many patients experienced significantly fewer seizures under ketosis. Nowadays, however, healthy people also turn to keto, usually with the aim of losing weight or maintaining a stable energy level.
Benefits and effects: In ketosis, you often have a stable feeling of energy without major blood sugar peaks and troughs. Many people report fewer food cravings and better mental focus. Studies show that a keto diet can lead to significant weight loss in the short term if you are overweight[6][7]. For example, a meta-analysis (2025) found that very low-carbohydrate diets (≤50 g carbohydrates daily) significantly reduced body weight and BMI over ≥1 month[7]. Beneficial effects on blood glucose in type 2 diabetics have also been observed (under medical supervision, a keto diet can improve blood glucose control; however, this link is not covered by an EFSA Health Claim and requires further research). In addition, keto often improves triglyceride and HDL cholesterol levels[6]. Nevertheless, a ketogenic diet is very restrictive and not suitable for everyone in the long term. It often lacks fiber, which can have a negative effect on intestinal health - this can be counteracted with low-carb, high-fiber vegetables (e.g. lettuce, broccoli).
Keto side effects and adaptation: Especially in the first few weeks of the transition, many people complain of the "keto flu" - symptoms such as tiredness, headaches, muscle cramps and irritability. These usually result from electrolyte and fluid losses, as water is stored in the body along with the carbohydrates. An adequate intake of keto minerals (sodium, potassium, magnesium) is therefore important: salt your food sufficiently, drink plenty of water and, if necessary, consider supplementing with electrolyte powders(keto nutritional supplements) to compensate for this loss. After about 2-4 weeks(keto adaptation), the body has usually adjusted to burning fat, and many people then report increased energy and less fatigue. However, a "cheat day" with lots of carbohydrates leads to ketosis being interrupted immediately - if you eat a large amount of sugar or bread, for example, the body switches back to glucose as fuel. After such a carbohydrate load, it usually takes 1-2 days of a strict keto diet to get back into ketosis.
Another possible side effect is digestive problems (constipation) due to the lack of fiber-rich carbohydrate sources. Green vegetables, linseed or chia seeds and sufficient fluids can help here. The keto diet is also very high in fat - the emphasis should be on "good" fats (unsaturated fatty acids from avocado, olive oil, nuts, fish). As the range of foods is limited, there is a risk of deficiencies in certain micronutrients (e.g. folic acid, some B vitamins from whole grains) if you stay on keto for a long time. A keto-specific multivitamin can be useful here (but be careful: always take all supplements as required and ideally in consultation with a doctor).
Some experienced keto users also rely on special aids. MCT oil (medium-chain fatty acids obtained from coconut oil), for example, is popular as the body can convert it into ketone bodies particularly quickly and it therefore serves as a fast source of energy. Exogenous ketones (ketone bodies in powder or drink form) are also available to temporarily supply the body with additional "ketofuel". However, such supplements are controversial - MCT oil can lead to digestive problems in high doses, and exogenous ketones cannot replace a consistent change in diet (although they provide ketone bodies, they do not relieve the body of the need to use its own fat reserves). For most users, these aids are not necessary to achieve success with keto nutrition.
Biohacking duo: combining fasting and keto?
Many people wonder whether it is possible to combine intermittent fasting and keto to achieve maximum results. In fact, the two approaches complement each other to a certain extent: Fasting causes glycogen stores to deplete faster, which can accelerate entry into ketosis. Those who already eat ketogenically will feel less hungry during the fasting hours, as the body is adapted to burning fat - this makes intermittent fasting easier. This combination - fasting in the morning and the first meal is in the afternoon, for example, consisting of ketogenic food - is practised by some biohackers to optimize mental performance and fat metabolism. There are indications that such a combination could lead to more pronounced autophagy (induced by fasting) and at the same time provide more constant energy via ketones. However, there is a lack of long-term studies on this in humans; most data on autophagy comes from cell cultures or animal models. So here too, much is not (yet) EFSA-confirmed and requires further scientific investigation.
If you want to combine, you should proceed cautiously: Start with either fasting or keto, do not radically switch to both at the same time. For example, eat keto for 2 weeks to establish ketosis and then add intermittent fasting (e.g. 14/10 and later 16/8). It is important to listen to your body - if you feel dizzy or very weak, adjust your strategy if necessary.
Sport and performance during fasting/keto
Keto & sport is a special topic. Since fatty acids play an important role in endurance sports, some athletes use the keto diet to maximize fat burning (keyword "fat-adapted"). In fact, research shows that keto increases the maximum fat oxidation rate - the body draws more energy from fat during moderate exercise[8][9]. However, there is a catch: performance can suffer during high-intensity exercise. A study with competitive athletes (walkers) showed that although fat burning was significantly higher after a few weeks of keto, the so-called economy of movement decreased at the same time - the athletes needed more oxygen at the same pace and were unable to improve their competition times like the carbohydrate group[8]. In other words: for high intensities (e.g. sprinting, HIIT or heavy strength training), a strict keto diet sometimes lacks "the last gear", as carbohydrates are the preferred fuel for maximum performance[9]. However, in amateur sports, many report that they do well on keto and/or fasted states (e.g. running in the morning before breakfast), especially for moderate endurance sessions.
The situation is similar when training in a fasted state (fasting, e.g. in the morning before the first meal): Moderate exercise is usually doable, and some even find that it further stimulates fat burning. However, you should watch out for warning signs - if you feel dizzy or very exhausted, you should either cut your training short or have a snack (e.g. a protein shake) beforehand. Important: When fasting, make sure you have sufficient hydration and electrolytes. Overall, the following applies: For peak performance or muscle building, targeted timing of carbohydrates around training despite fasting/keto can be useful - but this would be advanced techniques( calledcyclical keto or targeted keto - where either larger amounts of carbohydrates are planned cyclically (e.g. once a week) or specifically consumed around intensive training sessions to enable maximum performance despite a keto diet).
Tips for everyday life and conclusion
Intermittent fasting and keto require planning and discipline, but are perfectly feasible in everyday life. Here are a few practical tips:
- Build up slowly: start with 12 hours of fasting, for example, and gradually extend. Or start with moderate low-carb before eating strictly ketogenic.
- Be aware of the social aspects: Plan your fasting windows so that they fit in with your everyday life (e.g. skip a late breakfast if you're not hungry in the morning anyway). Communicate your plans to those around you - shared meals can be adjusted on fasting days if necessary.
- Fasting apps: Apps can motivate you by tracking fasting times and reminding you of your goal. Some apps (such as Zero, Fastic, etc.) also offer information and community features.
- Prepare keto meals: To avoid being haphazard during your eating window, create a simple fasting plan or keto meal plan. (Example: an omelette with spinach and feta in the morning, salmon with broccoli and butter sauce for lunch, a salad with chicken and avocado in the evening; snacks such as a few nuts or cheese cubes are suitable in between). Keep keto snacks (nuts, olives, cheese cubes) to hand in case you do get hungry.
- Use keto recipes: There are many creative recipes (e.g. cauliflower rice, zucchini noodles, "fat bombs" for dessert) so that your diet doesn't become monotonous. Variety increases the chance that you will stick with it in the long term.
- Make sure you get enough protein: Even when keto and fasting, your diet should contain enough protein to prevent muscle loss. Include a source of protein in every meal (e.g. eggs, fish, lean meat or plant-based alternatives such as tofu) so that your body is well supplied.
In conclusion, intermittent fasting and the keto diet are promising approaches to increase fat burning and perhaps also achieve health benefits. Studies show clear short-term success in losing weight[6], and many people report more energy and well-being. From a scientific perspective, some mechanisms are plausible, such as the stabilization of blood sugar and activation of autophagy in fasting mode[2][3]. However, not all long-term effects have been fully researched. Neither fasting nor keto are miracle cures - they work best when embedded in a healthy lifestyle with a balanced diet (despite keto, the diet should be nutrient-rich!), exercise, sufficient sleep and stress management. Sleep also plays a role - chronic sleep deprivation can affect metabolism and overall health[10]. Anyone who has medical conditions (diabetes, chronic illnesses), is pregnant, taking medication or suffers from an eating disorder should only approach fasting or extreme diets such as keto in consultation with a doctor. For healthy adults, however, intermittent fasting and/or a ketogenic diet can be exciting tools for "biohacking" in order to get to know your own body better and optimize it in a targeted manner. It is important to listen to your own body and to pull the ripcord at warning signs (persistent discomfort, dizziness, extreme weakness). When used correctly, fasting and keto can help you to make your metabolism more efficient - for more energy in everyday life and effective fat burning.
(Disclaimer: Some of the links between the diets mentioned and health effects have not yet been confirmed by the EFSA; further studies are required. This article does not provide medical advice. If you have any questions or pre-existing conditions, please consult a doctor).
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