LONG-TERM FASTING
Fasting and weight loss, a practical science-based guide
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. It is therefore no surprise that weight loss is one of the most common reasons people explore fasting as an intervention.
Fasting, in its various forms, interrupts this pattern in a way that goes beyond simply eating less. Research increasingly shows that fasting triggers metabolic adaptations that support weight loss and long-term weight maintenance. In doing so, it can reduce disease risk and help the body reset how it uses and stores energy over time. For this reason, fasting is increasingly recognized for its profound health-promoting and disease-preventing potential.
As part 1 of our fasting and weight loss series, this article focuses on long-term fasting, generally defined as fasting for more than three days. It explores what happens in the body during this process, why the resulting weight loss differs from conventional dieting, and what people can expect from a medically supervised fasting experience at Buchinger Wilhelmi.
Why fasting is considered therapeutic
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 or stored for later. Its primary role is storage.
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 creates that switch. By extending the time without food, the body first uses up its readily available sugar stores (glycogen). As insulin levels are kept low, stored fat becomes accessible. Fatty acids are released, and the liver converts part of this fat into ketone bodies, providing a clean and efficient fuel for the brain and muscles, a state known as ketosis.
Many people are surprised to find that hunger decreases during this phase and that energy levels feel more stable. This is one of the key reasons fasting feels fundamentally different from simple calorie restriction, where hunger often remains constant and energy fluctuates throughout the day.
What happens during long-term fasting
In the first days of fasting, weight loss is often rapid. This is largely due to the mobilization of glycogen stores (the storage form of glucose in the liver and muscles), which bind significant amounts of water. As these stores are used up, the associated water is released, leading to a noticeable drop on the scales. Understanding this early phase, which also includes the physiological emptying of the bowels, is important, as it helps set realistic expectations for what follows.
As fasting continues, the body increasingly relies on stored fat for energy. Ketone production rises, hunger signals tend to soften, and many people report a steadier energy level and improved mental clarity. At this stage, weight loss usually slows, but it becomes more targeted as the metabolism shifts almost exclusively to fat burning. Research shows that this sustained shift towards burning fat and ketones, rather than sugar, is associated with lower levels of inflammation and more stable energy regulation.
What the weight loss actually consists of
The amount of weight lost during fasting varies considerably from person to person. Factors such as age, sex, starting weight, physical activity, and individual physiology all play a role.
Studies conducted at Buchinger Wilhelmi show that weight loss during long-term fasting is made up of several components. A meaningful proportion comes from fat stores, particularly from the abdominal region. Some of the loss reflects water released as sugar stores are depleted. A smaller fraction comes from the breakdown of older or damaged tissue through autophagy (a cellular recycling process), while the body actively works to preserve healthy muscle as much as possible. This explains why the pace of weight loss slows after the initial days. The body becomes more selective, prioritising quality and sustainability over speed.
Why abdominal fat matters most
Abdominal fat is not just an aesthetic concern. It sits deep within the abdomen, surrounding vital organs, and is closely linked to disrupted blood sugar control, fatty liver changes, raised blood pressure, and a higher risk of cardiovascular disease and diabetes.
Compared with many conventional weight loss approaches, long-term fasting leads to particularly consistent reductions in waist circumference. This makes it a far more meaningful indicator of improved health than body weight alone, as it reflects changes in the type of fat most strongly associated with disease risk.
Long-term fasting as a reset
Crash diets typically rely on restriction and willpower, which is why weight regain is so common. Long-term fasting works differently. It creates a pause, both physically and psychologically, that allows people to reconnect with genuine hunger, taste, and food choices.
When supported by medical supervision, gentle movement, rest, and reflection, fasting becomes more than a weight loss intervention. It becomes a structured reset that can help break entrenched patterns and support lasting change, rather than a short-term fix.
A final thought
While intermittent fasting and fasting mimicking programmes may be more practical for some people, long-term fasting offers a depth of therapeutic benefit that extends beyond weight loss alone. It is not always easy to integrate into everyday life, which is why a clinic-based setting can be particularly supportive.
By training the body to rely more effectively on fat as a fuel source, long-term fasting supports healthier weight regulation over time. It also helps stabilise processes linked to chronic disease and supports both physical and mental wellbeing. For many, the weight loss that occurs is not the goal in itself, but a sign of deeper, lasting changes taking place beneath the surface.
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