Intermittent Fasting: The Science and Practice of Time-Restricted Eating

Intermittent fasting (IF) — the practice of cycling between periods of eating and abstention — is one of the most studied dietary approaches in modern nutrition science. The concept is simple: it restricts the timing of food intake rather than its content. But the physiological effects are profound, and the evidence base has grown substantially over the past decade.

A landmark review published in the New England Journal of Medicine in December 2019 (de Cabo and Mattson, 'Effects of Intermittent Fasting on Health, Aging, and Disease') established the scientific framework: intermittent fasting ameliorates obesity, insulin resistance, dyslipidemia, hypertension, and inflammation, and produces cellular and metabolic adaptations that may delay aging and reduce the risk of chronic disease. Reference: [NEJM — Effects of Intermittent Fasting on Health, Aging, and Disease](https://www.nejm.org/doi/full/10.1056/NEJMra1905136)

The Practice

There are several established protocols, each with a different fasting-to-eating ratio:

**Time-Restricted Eating (TRE)** — confining daily food intake to a window of 6–10 hours (most commonly 8 hours, i.e. the 16:8 protocol: 16 hours fasting, 8 hours eating). This aligns food intake with the body's circadian rhythms, when metabolism is most efficient. A 2023 review published in PMC found that trials with an eating window of 6 to 10 hours were generally safe and produced health benefits including weight loss, improved insulin sensitivity, and reduced blood pressure.

**Alternate-Day Fasting** — alternating days of unrestricted eating with days of either complete fasting or very reduced intake (approximately 500 calories). The ADF protocol has been studied extensively by Dr. Krista Varady of the University of Illinois Chicago.

**The 5:2 Diet** — five days of normal eating and two non-consecutive days of reduced intake (500–600 calories) per week, popularised by Dr. Michael Mosley in his 2012 BBC Horizon documentary 'Eat Fast and Live Longer' and subsequent book 'The Fast Diet'.

The Physiology

During the fasting state, the body undergoes a series of metabolic changes, roughly in sequence:

**0–4 hours** — glucose from the most recent meal is the primary fuel source.

**4–12 hours** — glycogen (stored glucose in the liver) becomes the primary fuel. Insulin levels begin to fall.

**12–18 hours** — glycogen stores are depleted and the body shifts to burning fat (lipolysis), producing ketone bodies as an alternative fuel for the brain. This metabolic switch is central to many of the benefits of intermittent fasting.

**18+ hours** — autophagy is upregulated. Autophagy — literally 'self-eating' — is the process by which cells clear damaged proteins and organelles, a form of cellular housekeeping. The discovery of autophagy's mechanisms earned the Japanese cell biologist Yoshinori Ohsumi the 2016 Nobel Prize in Physiology or Medicine.

Who Should Be Cautious

Intermittent fasting is not suitable for everyone. The following groups should avoid it or consult a physician: pregnant or breastfeeding women, individuals with a history of eating disorders, those with type 1 diabetes or insulin-dependent type 2 diabetes (without medical supervision), and individuals with low body weight. The advice of a qualified physician is always essential before beginning any significant dietary change.

Reference Links:

- NEJM — Effects of Intermittent Fasting on Health, Aging, and Disease: nejm.org/doi/full/10.1056/NEJMra1905136

- NEJM — Calorie Restriction with or without Time-Restricted Eating: nejm.org/doi/full/10.1056/NEJMoa2114833

- PMC — Time-restricted eating: What we know: pmc.ncbi.nlm.nih.gov/articles/PMC10111287/

- Dr. Krista Varady (University of Illinois Chicago): uic.edu

- Michael Mosley — The Fast Diet: thefastdiet.co.uk

- Yoshinori Ohsumi — 2016 Nobel Prize: nobelprize.org/prizes/medicine/2016/ohsumi/

- National Institute on Aging: nia.nih.gov

- Academy of Nutrition and Dietetics: eatright.org