72-Hour Fasts: Who Should and Shouldn’t Consider Them
The ketogenic diet primes the body to use fat for fuel, making extended fasts like 72-hour fasts more manageable for some. But fasting for three consecutive days requires careful consideration of individual health circumstances.
How 72-hour fasting works
After 24-48 hours without food, glycogen stores deplete and ketone production increases significantly. By 72 hours, the body relies almost entirely on fat metabolism. A 2013 study in the British Journal of Nutrition found very-low-carbohydrate diets enhance fat adaptation, which may make extended fasting easier (Bueno et al., 2013).
the keto adaptation timeline varies between individuals. Those already following a ketogenic diet often transition into fasting with fewer side effects like headaches or fatigue.
Potential benefits
Research suggests extended fasting may:
- Support cellular repair processes like autophagy
- Improve insulin sensitivity
- Reduce inflammation markers
The key mechanism appears to be metabolic switching – when the body shifts from glucose to fat and ketone metabolism. This process differs from intermittent fasting windows under 24 hours.
Who might benefit
Healthy adults already fat-adapted through ketogenic eating may tolerate 72-hour fasts well. Those with:
- Stable blood sugar regulation
- No history of disordered eating
- Normal blood pressure
could consider trying under medical supervision. At Tesco, electrolyte supplements like magnesium citrate cost £5 for 30 tablets – often helpful during extended fasts.
Who should avoid 72-hour fasts
Several groups should not attempt prolonged fasting: 1. People with type 1 diabetes 2. Anyone with a history of eating disorders 3. Pregnant or breastfeeding women 4. Those under 18 5. People taking certain medications
The NHS advises against extended fasting for these populations due to potential risks like hypoglycaemia or nutrient deficiencies.
What this means in practice
For UK adults considering a first 72-hour fast:
- Start with shorter fasts (16-24 hours) first
- Stay hydrated with mineral water (£1.20 for 2L at Sainsbury’s)
- Monitor energy levels closely
- Break the fast gently with bone broth or scrambled eggs
Autumn and winter months may be preferable to summer for extended fasting, as heat increases hydration needs.
Frequently asked questions
Is a 72-hour fast safe for weight loss?
While some weight loss occurs, primarily from water and glycogen depletion in early stages, extended fasting isn’t a sustainable weight management strategy. The Sumithran study (2013) found appetite hormones spike after prolonged fasting, potentially leading to rebound eating.
Can you exercise during a 72-hour fast?
Light activity like walking is generally fine, but high-intensity workouts should be avoided. Listen to your body – dizziness or weakness means stop immediately.
How often can you do 72-hour fasts?
No established safety data exists for frequent prolonged fasting. Most experts suggest no more than monthly, if at all, for healthy individuals.
The bottom line
72-hour fasts may offer metabolic benefits for some healthy, fat-adapted adults when done cautiously and infrequently. They’re not suitable for everyone and carry risks if undertaken without proper preparation. If you’d rather not do the macro maths yourself, the Keto Dieting app does it for you on Google Play and the App Store.
References
- Bueno NB, de Melo IS, de Oliveira SL, da Rocha Ataide T (2013). Very-low-carbohydrate ketogenic diet v. low-fat diet for long-term weight loss: a meta-analysis of randomised controlled trials. British Journal of Nutrition. https://doi.org/10.1017/S0007114513000548
- Sumithran P, Prendergast LA, Delbridge E, et al. (2013). Ketosis and appetite-mediating nutrients and hormones after weight loss. European Journal of Clinical Nutrition. https://doi.org/10.1038/ejcn.2013.90
