What is intermittent fasting? A look at evolution
Intermittent fasting is not a new invention, but rather a natural part of human history. Our ancestors did not have constant access to food and were often forced to get by for longer periods without eating. Fasting was therefore not a choice, but a necessity. These rhythmic fasting periods have become evolutionarily imprinted in the human metabolism.
In various cultures and religions too, fasting has a firm place: from Ramadan in Islam to Christian fasting and Buddhist abstinence – everywhere the connection between fasting and spiritual and physical purification becomes apparent.
The scientific background of fasting
Intermittent fasting has gained enormously in popularity in recent years, and for good reason: it has tangible, scientifically proven effects on the body. Here are some of the main mechanisms that take place during fasting:
- Ketosis: energy production from fat
After about 12-16 hours without food, the glycogen stores (stored carbohydrates) in the body decline, and it begins to use fat as its primary energy source. This leads to the formation of ketone bodies, which supply the brain and the muscles with energy.
- Autophagy: cells in a cleaning mood
During fasting the body activates autophagy, a process in which damaged cells and proteins are recycled. This "cellular housekeeping" is associated with a lower risk of diseases such as cancer and neurodegenerative disorders.
- Antioxidant and anti-inflammatory effects
Fasting reduces oxidative damage and inflammation in the body, which can be particularly beneficial with chronic conditions such as arthritis or diabetes.
- Relief of the digestive system
Fasting gives your digestive system a much-needed break. During fasting the gut flora can regenerate, and inflamed areas of the intestinal mucosa have the opportunity to recover. This break also helps to increase the efficiency of digestion and to reduce the risk of digestive problems such as bloating or a feeling of fullness.
- Cardiovascular benefits
Regular fasting can lower the resting heart rate and improve heart rate variability (an indicator of the heart's adaptability). This promotes overall heart health and reduces the risk of cardiovascular diseases.
- Hormones: insulin, leptin and ghrelin
- Insulin: fasting improves insulin sensitivity, which means the body can handle sugar more efficiently. However, it is sometimes argued that prolonged fasting can lead to blood sugar and insulin spikes, especially in untrained individuals.
- Leptin and ghrelin: leptin, the satiety hormone, and ghrelin, the hunger hormone, are regulated by fasting, which can help to perceive the natural feeling of hunger better.
The improvement of metabolic flexibility through intermittent fasting has a direct influence on hormonal processes in the body. "Metabolic flexibility" refers to the body's ability to switch efficiently, as needed, between energy production from carbohydrates and fats. A body that adjusts to producing energy efficiently from both carbohydrate and fat reserves often shows improved insulin sensitivity. This adaptation does require time, however: in the first weeks there can be a stronger feeling of hunger or energy dips, as the body is still accustomed to producing energy from carbohydrates. A consistent implementation over at least two months can ease this transition and stabilise it in the long term.
The different forms of intermittent fasting
There are different methods of intermittent fasting, which differ in the duration and frequency of the fasting and eating periods:
- 16:8 (Time-Restricted Eating): you fast for 16 hours and eat within an 8-hour window. This is the most common form, as it is easy to integrate into everyday life. It promotes metabolic flexibility and enables the body to produce energy more efficiently from fat reserves. A gentler version of it, ideal for beginners, is 14:10 or even 12:12, in which you fast for 14 or 12 hours respectively and eat within 10 or 12 hours and slowly increase the fasting time, with the goal of reaching 16 hours.
- 5:2 method: on five days of the week you eat normally, on two days you eat nothing at all or drastically reduce your calorie intake (approx. 500-600 kcal).
- Alternate-day fasting: you fast completely every other day or eat very little on those days.
- OMAD (One Meal a Day): a more extreme variant in which you have only one meal per day.
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Tips for beginners: how to succeed at getting started with fasting
If you want to try intermittent fasting, it is important to start step by step:
1. Start with shorter fasting periods: begin with 12 hours of fasting and slowly increase to 14 or 16 hours.
2. Plan your eating window: choose a time window that suits your everyday life, e.g. 10:00–18:00 or 12:00–20:00.
3. Stay hydrated: during the fasting period, drink water, unsweetened tea or black coffee.
During the fasting period it is important not to consume any calories. Even small amounts of sugar or milk in coffee, sweetened iced tea or soft drinks like cola interrupt the fasting process and make it harder for your body to get into fat burning and the metabolic switch. Instead, you should drink exclusively calorie-free beverages such as water, unsweetened tea or black coffee. Seemingly small "exceptions" can considerably diminish the positive effects of fasting – so stay consistent!
4. Listen to your body: do you feel weak or unwell during fasting? Then adjust your method.
Even though during the eating window you can in principle consume the same amount of calories as usual, it is helpful to pay attention to the quality and quantity of your foods. A conscious diet with unprocessed foods and a slight reduction in calorie intake (depending on body weight) can enhance the benefits of intermittent fasting. This combination of fasting and high-quality nutrition helps not only to regulate weight, but also to feel healthier overall and fitter.
The quality of your nutrition plays a key role in supporting hormonal balance during fasting. A healthy diet with unprocessed foods – especially fibre-rich vegetables, healthy fats and protein – helps to keep blood sugar stable and to avoid insulin spikes during the eating window. This has a positive effect on insulin sensitivity and the long-term regulation of leptin and ghrelin. At the same time, a conscious calorie reduction (depending on individual goals and body weight) supports not only metabolic flexibility, but also helps to reduce inflammation and to strengthen the antioxidant effects of fasting.
Once you have practised intermittent fasting consistently over a few months, you can make the method more flexible. For example, you can switch to the 5:2 method or stick to the 16:8 scheme only 2 days per week. Your body has already got used to the metabolic switch and continues to benefit from the fasting effects. Nevertheless, it is important that you plan the more consistent phase – roughly 16:8 for two months – two to three times a year, in order to maintain the long-term health benefits and to keep "reminding" your body again and again that it should stay flexible.
Differences between men and women
Intermittent fasting affects men and women differently, especially in women in the menstrual phase:
- Hormonal differences: women react more sensitively to calorie restriction, as the female body adjusts its energy needs more quickly in order to protect reproductive capacity.
- Menstrual phase: during the luteal phase (after ovulation) prolonged fasting could lead to increased stress. Here it is better to choose shorter fasting intervals or a flexible approach.
Contraindications: when caution is advised
Intermittent fasting is not suitable for everyone. Particular caution is advised in the case of:
- Pregnancy and breastfeeding
- Underweight or eating disorders
- Chronic illnesses (medical consultation required)
- Heavy physical strain (e.g. competitive sport)
Summary: intermittent fasting as a conscious lifestyle
Intermittent fasting is not a miracle cure, but a natural and scientifically founded approach to promote health. It is worth adapting the method individually and addressing your own needs. Whether you decide on 16:8, 5:2 or another variant – the most important thing is to listen to your body and find a balance.
If you are curious, simply give it a try – you might discover a new routine that does you good in the long term.
