Written by
Susan Sargeant, BSc (Biomedical Science)
Published March 28, 2026
In our goal-oriented society, fitness and optimum body image have become a sought-after and demanding pursuit, resulting in many of us wanting to find the best and shortest route to a defined and lean body, with maximum muscle mass and minimal fat. Different approaches to these goals have led to the popularity of targeted diets and fasting regimes.
The rise of intermittent fasting is one such approach, where prolonged periods of fasting are followed by shorter windows of eating. From a scientific viewpoint, this method changes how the body accesses and uses stored energy. Research highlighted by the Harvard T.H. Chan School of Public Health discusses how intermittent fasting may influence metabolic health and weight management.
Energy metabolism during fasting
During metabolism, nutrients from the diet are broken down into glucose, glycerol, and fatty acids to generate energy at a cellular level and maintain the body's normal processes. As recently consumed energy becomes less available, the body increasingly draws on stored glycogen and fat.
The Cleveland Clinic provides a useful overview of how the body transitions between fed and fasting states.
What happens when glycogen stores drop
As liver glycogen becomes depleted, the body must adjust its metabolic activity to maintain the glucose needed by tissues that depend heavily on it, including red blood cells and parts of the nervous system. The exact timing differs according to activity, previous meals, and the individual.
Two processes become increasingly important during this stage:
1. Gluconeogenesis
Gluconeogenesis involves the liver and kidneys generating glucose from compounds such as lactate, glycerol, and certain amino acids. This helps preserve essential glucose availability when dietary carbohydrate is not being consumed.
2. Ketogenesis
Ketogenesis is the production of ketone bodies from fatty acids. During longer fasting periods, these ketones become a more important fuel source for several tissues and reduce some of the body's dependence on glucose.
Scientific resources available through the National Center for Biotechnology Information provide more detailed discussion of these metabolic pathways.
Why intermittent fasting appeals to many people
Intermittent fasting gives people a clear structure for deciding when they eat. For some, that structure may make overall food intake easier to manage. Changes observed early in a fasting routine can include both changes in stored body water and changes in energy balance, so rapid scale movement should not automatically be interpreted as body-fat loss.
Guidance from the World Health Organization also reminds us that any dietary approach should be considered within the person's complete diet, health status, lifestyle, and nutritional needs.
A responsible approach
Intermittent fasting is not appropriate for everyone. It should be approached cautiously by younger users and by anyone who is pregnant, has diabetes or another relevant medical condition, takes medication affected by meal timing, has a history of disordered eating, or has high training and recovery demands.
A method that works comfortably for one person may not suit another. Anyone uncertain about fasting should discuss it with an appropriate qualified healthcare professional before making significant changes.