The Physiology of Fasting
Intermittent fasting is a structured approach to meal timing. By creating dedicated periods for eating and fasting, it offers a simple framework that works with the body's natural daily rhythms rather than focusing on calorie counting or restrictive food rules.
Metabolic flexibility
The cellular transition
As the fasting period extends, the body gradually transitions from using dietary energy to drawing on stored energy reserves. This shift involves changes in fuel utilisation, including increased reliance on stored fat and, in some cases, ketone production.
Researchers refer to this adaptive capacity as metabolic flexibility—the body's ability to switch between available energy sources depending on metabolic demand.
Hour by hour
Physiological adaptations during fasting
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Early fasting
Energy source transition
Extended Fasting
During the hours after your last meal, digestion and nutrient absorption gradually come to an end. As circulating nutrients decline, the body begins transitioning away from recently consumed food as its primary energy source.
As fasting continues, the body increasingly relies on stored energy reserves. The balance between glucose and fat utilisation gradually shifts according to individual metabolism, activity levels, and dietary patterns.
During longer fasting periods, the body continues adapting to reduced energy intake. Research suggests that fasting may influence cellular maintenance processes, fuel utilisation, and metabolic regulation, although responses vary between individuals.
Safety Considerations
A thoughtful approach
Fasting is a personal practice and may not be appropriate for everyone. Prioritise your wellbeing and choose an approach that fits your individual circumstances.
Who it's for
When to pause
Generally intended for healthy adults interested in structured, time-based eating patterns.
If you feel unwell, experience concerning symptoms, or have underlying health considerations, seek guidance from a qualified healthcare professional before continuing.