What happens to your body after 24 hours without food?

A growing number of people are embracing 24-hour fasting as part of weight management and metabolic health strategies. While short-term fasting can trigger several beneficial physiological adaptations, medical experts caution that its effects vary considerably among individuals and may pose significant health risks for vulnerable groups without appropriate medical supervision.

According to health specialists, the body undergoes a sophisticated metabolic transition during a prolonged fast. Initially, it maintains blood glucose concentrations by mobilising glycogen reserves stored in the liver. Once these stores become depleted, metabolism progressively shifts towards breaking down adipose tissue, producing ketone bodies as an alternative energy substrate. Concurrently, insulin secretion declines, while glucagon activity intensifies to preserve energy homeostasis. In healthy adults, these physiological adaptations generally occur without major complications, although individuals unaccustomed to fasting may experience hunger, fatigue, dizziness, headaches, irritability, diminished concentration and transient weakness.

Experts emphasise that, for most healthy individuals, a 24-hour fast does not ordinarily precipitate dangerously low blood glucose levels because the liver continues to synthesise and release glucose as required. However, prolonged fasting may be hazardous for people living with diabetes, those taking insulin or glucose-lowering medication, pregnant or breastfeeding women, children and frail older adults. These groups are strongly advised to seek medical guidance before undertaking any extended fasting regimen to mitigate the risk of serious metabolic complications.

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Emerging scientific evidence suggests that intermittent fasting may enhance insulin sensitivity, improve metabolic function, and contribute to reductions in excess body fat, blood pressure, and adverse lipid profiles. A 2023 study also indicated that fasting could potentially lower the risk of metabolic disorders, while experimental animal research has hinted at a possible role in slowing the progression of neurodegenerative conditions such as Alzheimer's disease and Parkinson's disease. Nevertheless, researchers underscore that robust human evidence remains limited. They further note that headaches during the early stages of fasting may result from declining blood glucose, dehydration, abrupt caffeine withdrawal, or normal physiological adaptation. Conversely, excessively prolonged fasting may elevate cortisol levels, increasing the likelihood of muscle catabolism and impaired insulin responsiveness. Consequently, healthcare professionals recommend that extended fasting should be undertaken judiciously and preferably under qualified medical supervision, particularly by individuals with pre-existing health conditions.