Fasting is back in fashion, often portrayed as a quick fix to regain energy or “reset” the body. But in most cases, it is talked about in a superficial way: two values on the scoresheet, some personal impressions and little else. In reality, going an entire week without introducing calories is something that affects every corner of the body, much deeper than we imagine.
A group of researchers from Queen Mary University and the Norwegian School of Sports Sciences tried to follow this process minute by minute. They monitored twelve healthy people as they underwent a seven-day water-only fast, tracking thousands of biological signals that changed over the course of the week. A work published in Nature Metabolism which, for the first time, allows us to observe more than just that What it happens, but When.
A gradual change
The scientists analyzed approximately 3,000 proteins per day. And it is thanks to this amount of data that we can clearly see: the first two days are uncertain terrain. The organism registers the absence of food, but reacts in a timid, almost prudent way. Then comes the third day. It is at this moment that the body changes strategy: more than a thousand proteins modify their behavior, some increase, others decrease, still others follow more complex movements. It is the point at which fasting stops being “absence of meals” and becomes a real reorganization.
The most interesting surprise concerns a protein called Tenascin-R, linked to the nervous system. Its variation during fasting suggests that communication between nerve cells may also be involved more than we thought. A track that researchers want to explore further.
Calorie deprivation doesn’t just change your metabolism: it changes the way your body communicates with itself. Leptin, which signals the brain about the level of fat stores, drops rapidly. The leptin receptor instead increases, as if the body was trying to “hear better” a message that is becoming increasingly weaker.
Meanwhile, the signals are growing that push towards the use of fats and ketone bodies as the main fuel. The organism understands that it must change its energy strategy and enters a phase of adaptation that involves multiple systems simultaneously. It is a complex but coordinated process, the result of millions of years of evolution.
What changes in the body
Over the course of the week, participants lost an average of around 5.7kg. Not just fat mass: a part of lean mass is also reduced, as expected in the absence of nutrients. The interesting fact is that, day after day, the body gradually reduces the loss of nitrogen in the urine, a sign that it is learning to save proteins, protecting essential tissues. It’s a surprising adaptation: a biological response that shows how the organism seeks balance, even in extreme conditions.
The change in fuel, from carbohydrates to fats, is not abrupt at all. The body proceeds gradually: in the first two days it consumes glucose reserves, then it relies on lipids and ketone bodies. The proteins measured in the study clearly show that this transition is not a switch, but a phase built step by step, with continuous adjustments.
This is not an invitation to fast
A seven-day fast remains an extreme practice, which requires medical supervision and should not be improvised. The volunteers were looked after by a team of professionals, and their data cannot be generalized to everyone.
The value of the study lies elsewhere: in the possibility of precisely observing how the body adapts to lack of food. This knowledge could help, in the future, to develop methods capable of replicating some beneficial effects of fasting without imposing a whole week of caloric abstinence.
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