The Science of Intermittent Fasting

Intermittent fasting has come into fashion surprisingly late, compared with the hundreds of dietary trends that have emerged over the past few decades. This is astonishing, since intermittent fasting is, in many ways, quite obvious. Ultimately, it simply means not eating all the time. And this should be made clear: Intermittent fasting is not a diet, because the aim is neither to eat less (even though this is often a side effect) nor alter what you eat. You can, of course, alter your diet while practicing intermittent fasting, but it’s not a must.

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There are numerous ways to practice intermittent fasting. You can adhere to an intermittent fasting period of 12, 14, or 16 hours. You could even choose to fast for two days a week. Furthermore, you could try only ingesting 500 to 600 kcal a day. Intermittent fasting is very easy for the body to adjust to; it is the most natural form of nutrition. A return to our prehistoric heritage, so to speak: Once upon a time, our ancestors ate only when food was available. When there was no food available, the body simply had to cope with hunger. That’s why intermittent fasting or periodic fasting doesn’t just seem natural, it’s also easy—and very healthy.

Almost all diets fail because it goes against our nature to eat very little over the course of many weeks. It’s no surprise, then, that when we eventually stop dieting or take a break from it, we end up gorging ourselves, leading to the fat reserves filling up again faster than we want. But not eating anything every now and again—that’s easily managed.

We don’t know exactly when humans began adhering to regular times for breakfast, lunch, and dinner. We know that in Germany it slowly became a habit to have not just two but three meals a day in the Middle Ages. This depended on the social class one belonged to—nobility, for example, thought it courtly to be served only two meals a day for a long time. The timing of meals, therefore, was connected to common customs and not to a certain necessity. One of the pioneers in the field of chronobiology, Satchidananda Panda, has found that 1200 genes are activated only in the state of fasting. And these are predominantly genes responsible for metabolic activity, as well as those that control the immune system—clear proof of our ancient biological program.

In 2018, I gave a lecture on fasting in Berlin. The speaker just before me was the director of Zoo Berlin, Andreas Knieriem. In the discussion that followed, he expressed how persuasive he found the concept of fasting in medicine. He mentioned the lions in his zoo, who ate 66 to 88 pounds of meat at a time and subsequently managed without any food for days. Living beings in the wild are all slender. There are no overweight wild animals. Domestic cats and many dogs, however, now have similar weight problems to humans.

Many years ago, when I began my research on intermittent fasting, I often had to put up with criticism from nutritionists who said that this was just another diet, and diets have been proven to be useless. There is a major misconception with this opinion because intermittent fasting is not a diet at all—all it means is turning back the clock and changing the way we think. We’ve been taught to feel guilty when we skip a meal—we’re meant to have regular meals. Societal norms are deeply anchored within our minds.

Intermittent fasting is not a new invention but rather the norm in the history of humankind. That’s why it is codified in our genes. Some people worry that they won’t be able to concentrate and stay active after several hours of fasting—but the opposite is the case. Generally, intermittent fasting is much easier than most people think it will be. The feeling of hunger does not increase. Instead, it fades. We also learn to recognize and evaluate the stomach rumble, often misinterpreted as hunger, as a temporary phenomenon.

The Origins of Intermittent Fasting

Many religious texts mention intermittent fasting. For example, Luke 18:12 in the Bible reads: “I fast twice a week.” Originally, fasting or partial fasting in Christian culture was done twice a week, in addition to Lent. Wednesday commemorated the betrayal of Jesus, and Friday the crucifixion. But this kind of remembrance and rite of humility disappeared almost completely over the course of the centuries. At my house, however, it was at least partly preserved—on Fridays our family did not eat meat. Instead, we ate only vegetarian dishes or fish (and nothing sweet).

Intermittent fasting is practiced during Ramadan, the fasting month of Islam. Followers of Islam will fast from sunrise to sunset and can eat and drink only before sunrise and after sundown. But people sometimes overindulge when they’re not fasting. This seems to be a contributing factor to why Ramadan fasting, for example, isn’t as medically successful as other forms of intermittent fasting in many studies. But overall, fasting during Ramadan does show health benefits. On average, bodyweight drops slightly, and blood lipid values and cholesterol levels improve.

Nevertheless, it’s difficult to examine Ramadan fasting scientifically because depending on the geographical location and the time of year, the daily fasting period can vary between nine and 20 hours. In our facility, we conducted research into a religious type of fast quite similar to Ramadan fasting, the fasting of the Baha’i religion, which has its origins in Iran. Under the direction of my colleague Daniela Liebscher, we made some interesting discoveries: There was a significant improvement in mood as well as a shift of the circadian rhythm of almost an hour and a half. Fasting can help readjust disrupted circadian rhythms.

Seventh-day Adventists in Loma Linda, California, form one of the Blue Zones. On average, they live seven to 10 years longer than other Americans who don’t belong to this church. The excellent health of the Adventists is mainly due to their vegetarian diet and their healthy lifestyle, but interestingly most of them have their last meal for the day in the afternoon. This essentially means that they practice TRE (time-restricted eating) with a prolonged night fast. Within the scope of the Adventist Health Study, data on this factor was unfortunately not gathered, but it’s possible that it contributes to a significantly longer life span.

Intermittent fasting has been known in the world of medicine for some time. Edward Dewey, born in 1839, was a pioneer of fasting as a cure in the nineteenth century. He himself didn’t have breakfast and described his improved mood and more energetic condition in his book The No-Breakfast Plan and the Fasting Cure. In the mornings, he only drank a cup of coffee, because he was convinced that breakfast was just a habit that weakened the nerves. He believed that having just one or two meals a day, eaten slowly, was a cure for chronic diseases. By fasting in the mornings, one would both lose weight and gain muscle strength.

These early observations are impressive because they were confirmed by scientific research 150 years later. Dewey formulated another intelligent dietary principle: You shouldn’t eat when you are tired—instead, you should rest. You’ve probably experienced this before. You arrive home exhausted, from a long day of work or some sort of strenuous activity. That’s when you might feel a sensation of hunger, so you eat voraciously and end up eating way too much. I second Dewey’s advice. Rest for 20 minutes after you get home. Eat afterward, and make sure to eat slowly. Once you’ve tried this, you’ll notice that you’ll eat in a more relaxed manner and therefore eat less.