Intermittent fasting is not a diet in the conventional sense. It does not specify what to eat, only when. The most studied protocol is the 16:8 approach, in which all food is consumed within an 8-hour window each day. The remaining 16 hours allow insulin to fall to baseline, which enables fat oxidation, and activate cellular repair processes that are suppressed by the constant insulin signalling of continuous feeding.
Why the Fasting Period Matters Physiologically
After a meal, insulin remains elevated for approximately 4 to 5 hours. During this time, fat oxidation is suppressed and the body runs primarily on glucose. Only when insulin returns to baseline does the body shift to fat burning and activate the autophagy pathways responsible for cellular housekeeping. Adults who eat across a 14 to 16-hour daily window rarely reach full insulin baseline before the next meal, creating a state of continuous anabolism with minimal periods of cellular repair.
The Clinical Evidence for 16:8
A randomised controlled trial of 16:8 time-restricted eating in overweight adults published in Nutrition and Healthy Aging found significant reductions in body weight, waist circumference, LDL cholesterol, and blood pressure over 12 weeks, without any instruction to reduce calorie intake. A separate trial in adults with metabolic syndrome found improvements in fasting insulin, fasting glucose, and inflammatory markers with 8-hour eating windows aligned to morning hours.
- Start with a 12-hour window and reduce gradually over weeks
- Black coffee and plain tea do not break a metabolic fast and help manage morning hunger
- Black water or sparkling water with citrus is acceptable during the fasting period
- Break the fast with a protein-rich whole food meal to maximise muscle protein synthesis
- Align the eating window with daylight hours: morning-to-afternoon windows produce better metabolic outcomes than afternoon-to-evening windows