What is intermittent fasting?
It means cycling between periods of eating and periods of not eating (or eating very little). The main types are:
| Type | What it involves |
|---|---|
| Time-restricted eating | Eating within a set window each day, for example 8 hours (often called 16:8) |
| Alternate-day fasting | Alternating normal eating days with very low-calorie or fasting days |
| Whole-day fasting (for example 5:2) | One or two very low-calorie days a week, eating normally on the others |
This is different from religious fasting, such as during Ramadan or Lent, which many South Africans observe. If you have a medical condition, speak to your doctor before religious fasts too.
What does the evidence show?
In 2025, The BMJ published a systematic review and network meta-analysis of 99 randomised trials involving 6,582 adults, most with overweight or obesity. It found:
- All forms of intermittent fasting and standard calorie restriction led to weight loss compared with no dietary change.
- Alternate-day fasting led to slightly more weight loss than continuous calorie restriction (about 1.3 kg on average) and time-restricted eating (about 1.7 kg). These differences were smaller than the 2 kg the authors considered clinically important.
- No approach clearly improved blood sugar or HDL ("good") cholesterol more than the others.
- Most trials were short (average 12 weeks), and the certainty of evidence was low to moderate.
The authors concluded that intermittent fasting offers similar benefits to traditional diets and that longer trials are needed. An accompanying editorial suggested it should complement, not replace, other evidence-based nutrition approaches.
Why might it work for some people?
- A shorter eating window can reduce late-night snacking.
- Some people find clear rules easier than counting calories.
- It can simplify meal planning.
For others, it leads to overeating in the eating window, poor sleep, irritability or a stressful relationship with food. That's a sign it isn't the right tool.
Who should avoid it, or only fast with medical advice?
- People who take insulin or other medicines that can cause low blood sugar.
- People taking blood pressure medicines or diuretics, who may risk dehydration or dizziness.
- Anyone who is pregnant, trying to conceive or breastfeeding.
- Children and teenagers.
- Anyone with a current or past eating disorder.
- People who are underweight, frail or have certain medical conditions.
If you want to try it: tips for doing it well
- Focus on food quality during eating windows: vegetables, legumes, whole grains, fruit, healthy fats and protein.
- Eat enough protein to protect muscle, especially if you're also losing weight.
- Keep up strength training.
- Drink enough water, and be careful with caffeine late in the day.
- Start gently, for example a 12-hour overnight fast, before extending it.
- Stop if you feel unwell, dizzy, or notice an unhealthy preoccupation with food.
When to see a doctor
- You take medicines for diabetes, blood pressure or other long-term conditions.
- You feel dizzy, faint or very unwell while fasting.
- Your relationship with food or eating feels out of control.
Frequently asked questions
Is 16:8 better than a normal diet?
Current evidence suggests it works about as well as a standard reduced-calorie diet, not better, for most people.
Can I drink coffee while fasting?
Black coffee, tea and water are generally fine in most fasting approaches. Added sugar and milk break a fast.
Does fasting "reset" metabolism?
There's no good evidence it does. Benefits mostly come from eating less overall.
Will I lose muscle?
Any weight loss can include some muscle. Adequate protein and strength training help protect it.
References
- Semnani-Azad Z et al. Intermittent fasting strategies and their effects on body weight and other cardiometabolic risk factors: systematic review and network meta-analysis. The BMJ. 2025. Summary: https://bmjgroup.com/intermittent-fasting-comparable-to-traditional-diets-for-weight-loss/
- Intermittent fasting as a nutritional tool. Editorial. The BMJ. 2025. https://www.bmj.com/node/1111119
This article is for general educational purposes only, like a health newsletter. It is not medical advice, diagnosis or treatment, and reading it does not create a doctor and patient relationship. Please speak to your own doctor about your situation. In an emergency, call 10177 or 112, or go to your nearest emergency unit.