Fasting & Weight Loss

BEYOND THE SCALE

The Science of Fasting and Weight Loss

For many people, weight gain is not the result of a single bad habit, but a gradual trend that unfolds over decades. Physical activity often decreases with age, daily energy needs quietly fall, while eating patterns, portion sizes, and food choices remain largely unchanged. Add stress, convenience foods, and a busy modern lifestyle, and weight gain becomes the norm rather than the exception. Therefore, it is no surprise that weight loss is one of the most common reasons people explore fasting as an intervention.

Fasting, in its various forms, is not simply a matter of eating less. When practised correctly, research shows that fasting induces coordinated physiological adaptations that support both weight loss and longer-term weight regulation, as well as reducing risk markers associated with chronic disease. For this reason, fasting is increasingly recognised for its health-promoting and preventive potential.

This article focuses on long-term fasting, generally defined as fasting for more than four days. It explores what happens in the body during this process, and what kind of weight loss is realistic, helping to separate evidence from expectation.

What actually happens in the body during fasting?

In everyday life, most people rely almost entirely on sugar for fuel. Think of this as being permanently connected to the grid. Each time we eat carbohydrates, the hormone insulin is released. Insulin helps move sugar into cells to be used for energy. Any excess sugar is first stored short-term as glycogen, mainly in the liver and muscles. When these stores are full, additional energy is stored longer-term as fat. When we eat frequently throughout the day, insulin remains elevated and the body stays locked into sugar burning mode. In this state, access to body fat is limited. Fat burning becomes like an off-grid energy source, reliable and steady, but rarely used because the switch is never flipped. Fasting activates that switch. By extending the time without food, the body first uses up its readily available sugar stores (glycogen). As insulin levels remain low, stored fat becomes more accessible.

Fatty acids are released, and the liver converts part of this fat into ketone bodies, providing a stable fuel for the brain and muscles, a state known as ketosis.This shift, often referred to as the “metabolic switch”, allows the body to access its internal energy reserves more efficiently. Many people report that hunger diminishes after the first days and energy feels steadier. This adaptation distinguishes fasting from chronic calorie restriction, where hunger often remains persistent.

How much weight can you expect to lose?

Perhaps the most frequently asked question is “how many kilograms will I lose during a fast”? The answer depends on several factors including sex, starting weight, duration of fasting, age, and individual physiology. In the first days of fasting, weight loss is often rapid. This is mainly because the body uses up its glycogen stores in the liver and muscles. Glycogen binds water, so when it is broken down, that water is released, leading to an early drop on the scale. Natural bowel emptying also contributes to this initial change. Understanding this early phase helps set realistic expectations for what happens next. As for what you might expect during a fasting period, large observational datasets from Buchinger Wilhelmi provide useful reference values.

In an analysis of 1,422 participants following the Buchinger method, weight loss increased predictably with fasting duration [1]. After five days, women lost on average 2.7 kg, while men lost 3.9 kg. After twenty-one days, the average reduction was 7.3 kg for women and 11.1 kg for men. These differences are not surprising. Men often lose more weight initially, and those with a higher starting BMI tend to lose more in absolute terms, largely due to higher energy expenditure and greater energy reserves. However, weight loss is not uniform and varies with age, sex, starting BMI, and fasting duration. The table below provides more granular reference values derived from our clinical data. Importantly, these numbers reflect total body weight change. They do not tell the whole story.

The data represents mean total body weight change in kilograms based on clinical data collected at Buchinger Wilhelmi. Sample sizes vary across age, sex, BMI category and fasting duration. For a personalised estimate based on your individual characteristics, please refer to our customised weight loss estimator here [2].

The scale is imperfect

Body weight fluctuates for many reasons, particularly during fasting. As discussed earlier, the initial drop on the scale predominantly reflects fluid shifts and natural clearance of intestinal contents. This is normal physiology and does not represent fat loss alone. For this reason, waist circumference is often a more informative marker of progress than total body weight. A reduction around the abdomen reflects a decrease in visceral fat, the type of fat most strongly linked to chronic disease and therefore the tissue we most want to reduce.

What does the weight loss actually consist of?

During fasting, the body gains access to its internal energy reserves, including stored fat. Fat loss is therefore an expected and central component of weight reduction. However, total body weight is composed of multiple compartments, and the physiological changes during fasting are more nuanced than simply “burning fat”.

Detailed metabolic research allows us to examine this more precisely. In a controlled study, healthy men fasted for ten days, and the average weight loss was approximately 5.9 kg [3]. This consisted of:

• ~2.4 kg fat mass
• ~0.5 kg glycogen
• ~1.6 kg associated water
• ~1.5 kg lean mass

However, lean mass is not the same as muscle loss. The term lean mass often causes confusion. Lean mass is not synonymous with skeletal muscle. It includes water, connective tissue, organ mass, and structural proteins. MRI-based analyses conducted at the clinic demonstrate that although lean mass decreases modestly, skeletal muscle tissue is largely preserved, and muscle performance is maintained [4]. Although some protein is used for glucose production during fasting, it is not drawn exclusively from muscle. Other tissues, together with autophagy, contribute to protein turnover and help spare muscle. This distinction is important. Fasting does not equate to muscle destruction. More importantly, imaging studies show a significant and preferential reduction in visceral fat mass. Follow-up MRI assessments indicate that reductions in visceral fat can persist for several months after fasting, suggesting sustained improvements in body composition rather than temporary weight change. In other words, fasting does not simply reduce total weight. It alters fat distribution.

The question of the yo-yo effect

A common concern with any weight loss intervention is whether the weight will return. There is no universal guarantee that weight loss will persist, as long-term outcomes depend largely on lifestyle after fasting. However, fasting appears to position individuals differently compared with chronic calorie restriction.

Physiologically, appetite regulation shifts. Many participants report greater sensitivity to hunger and satiety cues following fasting. Portion sizes often decrease naturally, and preferences frequently shift towards simpler, less processed foods. Because muscle mass is largely preserved, resting energy use does not drop excessively. This may help reduce some of the slowing in metabolism that is often seen with restrictive dieting. Psychologically, fasting interrupts habitual eating patterns and creates distance from automatic behaviours [5]. This behavioural reset may contribute meaningfully to longer-term outcomes.

Long-term observational data provide reassurance. And this is not new for us. Already in a retrospective analysis presented at the 5th European Congress on Obesity in 1993, we presented data from 372 individuals who had fasted more than ten times in our clinics. Contrary to the assumption that repeated fasting promotes progressive weight gain, the average BMI at the first fast (28.7 ± 4.7) was statistically similar to BMI at the tenth fast (28.3 ± 4.6). In other words, repeated fasting did not lead to cumulative weight gain.

A final thought

Long-term fasting offers a depth of therapeutic benefit that not only allows you to lose weight, but also provides you with the tools to sustain this weight loss in the long run. Weight loss is often measurable and, particularly in those with a higher starting BMI, substantial. Yet the most meaningful change is not simply the number of kilograms lost, but the deeper physiological shifts taking place beneath the surface.

By training the body to rely more effectively on fat as a fuel source, long-term fasting supports healthier weight regulation over time, reduces visceral fat, improves body composition, recalibrates appetite regulation, and improves several cardiometabolic risk markers. Taken together, this constellation of effects is difficult to replicate through a single isolated intervention, making long-term fasting a uniquely comprehensive and preventive strategy for sustainable weight loss maintenance. As the period following fasting is critical for long-term success, we are currently developing additional support for this phase through the FastForward programme. This programme combines medical guidance, personalised nutrition support, and tools such as continuous glucose monitoring to help patients navigate their return to everyday eating.

References
1. https://pmc.ncbi.nlm.nih.gov/articles/PMC6314618/
2. https://staging.buchinger-wilhelmi.com
3. https://pmc.ncbi.nlm.nih.gov/articles/PMC8718030/
4. https://www.buchinger-wilhelmi.com/en/fasting-muscle-loss/
5. https://www.buchinger-wilhelmi.com/en/new-year-new-rhythms-interview-with-dr-lisa-green/