Intermittent fasting (IF) isn't really a diet - it's an eating pattern. Rather than changing what you eat, it changes when you eat. This simple shift has generated more research interest in the past decade than almost any other dietary approach.
But does timing your meals actually make a difference? Let's look at what the evidence shows.
What Is Intermittent Fasting?
Intermittent fasting involves alternating periods of eating and not eating (fasting). Unlike continuous calorie restriction - eating less at every meal - IF concentrates your food intake into specific windows.
The idea isn't new. Humans have fasted for religious, spiritual, and practical reasons throughout history. What's new is the scientific investigation of whether deliberate, regular fasting periods offer health benefits beyond simple calorie reduction.
Popular IF Methods
16:8 (Time-Restricted Eating)
The most popular method. Typically means skipping breakfast and eating between noon and 8pm (or similar window). Some research suggests eating earlier in the day may be more beneficial.
5:2 Diet
Eat normally five days per week, then consume only 500-600 calories on two non-consecutive days. Popularised by Dr Michael Mosley in the UK.
Eat-Stop-Eat
Complete 24-hour fasts (dinner to dinner) once or twice per week. More challenging but requires fewer restricted days.
Alternate Day Fasting
Alternating between normal eating days and fasting or very-low-calorie days. Effective but hard to maintain long-term.
Most Popular Choice
For most people, 16:8 offers the best balance of effectiveness and sustainability. It's essentially skipping one meal and avoiding late-night eating - changes many people find manageable long-term.
What Happens During Fasting
When you fast, several physiological changes occur:
- Insulin drops - After 12+ hours without food, insulin levels decline significantly. Lower insulin allows stored body fat to be accessed more easily as fuel.
- Human growth hormone rises - Studies show HGH can increase up to 5-fold during fasting, potentially supporting fat loss and muscle preservation.
- Cellular repair processes activate - Fasting triggers autophagy, a cellular "housekeeping" process where cells remove damaged components.
- Gene expression changes - Fasting affects genes related to longevity and disease protection, though the practical significance is still being studied.
These changes form the theoretical basis for why IF might offer benefits beyond simple calorie reduction. However, whether these mechanisms translate to meaningful real-world benefits is what the clinical trials actually test.
What the Research Shows
Weight Loss: Strong Evidence
Multiple systematic reviews confirm IF is effective for weight loss. A 2020 review in the Annual Review of Nutrition found IF produces weight loss of 3-8% over 3-24 weeks. However, when calories are matched, IF doesn't appear to be superior to continuous calorie restriction - it's simply an alternative strategy that some people find easier to follow.
Metabolic Health: Moderate Evidence
Studies show IF can improve insulin sensitivity, reduce fasting insulin, and lower markers of inflammation. A 2019 study in Cell Metabolism found that even without weight loss, early time-restricted eating (finishing eating by 3pm) improved insulin sensitivity and blood pressure in men with prediabetes.
Cardiovascular Health: Emerging Evidence
Some studies show improvements in blood pressure, resting heart rate, and cholesterol profiles. A 2021 meta-analysis found IF reduced total cholesterol, triglycerides, and LDL cholesterol. However, more long-term data is needed.
Longevity: Insufficient Evidence
Animal studies on fasting and longevity are promising, with calorie restriction consistently extending lifespan in various species. However, human evidence is limited. We don't yet know if IF extends human lifespan or healthspan.
Benefits and Drawbacks
Potential Benefits
- Simplifies eating decisions (fewer meals to plan)
- May reduce overall calorie intake naturally
- Can improve insulin sensitivity
- No specific foods required or forbidden
- May reduce late-night snacking
- Flexible - different methods suit different lifestyles
Potential Drawbacks
- Hunger, especially initially
- May conflict with social eating occasions
- Can lead to overeating in eating windows
- May worsen relationship with food for some
- Potential for low energy during fasting
- Not suitable for certain populations (see below)
Common Mistakes
People who struggle with IF often make these errors:
- Overeating in the eating window - IF isn't a license to eat unlimited food. If you consume the same (or more) calories in fewer hours, you won't lose weight.
- Choosing the wrong method - Not everyone suits the same approach. If 16:8 doesn't work, try 14:10 or 5:2. There's no single "correct" protocol.
- Ignoring food quality - What you eat still matters. Eating ultra-processed foods in a shortened window won't optimise health.
- Being too rigid - Occasional flexibility (a breakfast meeting, a late dinner) won't ruin everything. Sustainability beats perfection.
- Neglecting hydration - Water, black coffee, and tea are fine during fasts. Dehydration causes many of the symptoms blamed on fasting.
Who Should Avoid IF
Intermittent fasting is not appropriate for everyone. Those who are pregnant or breastfeeding, have a history of eating disorders, have diabetes (particularly type 1), are underweight, or are under 18 should not practice IF without medical supervision. If you're on any medications, consult your doctor first as timing may need adjustment.
Making IF Work For You
Getting Started
- Start gradually - try 12:12 before moving to 16:8
- Choose an eating window that fits your life (it doesn't have to be noon-8pm)
- Stay hydrated during fasting periods
- Expect 1-2 weeks of adaptation before hunger normalises
Optimising Results
- Prioritise protein and vegetables during eating windows
- Front-load calories (larger lunch, smaller dinner) if possible
- Maintain consistent sleep and eating schedules
- Consider your training schedule - fasted exercise works for some, not others
Is IF Right for You?
IF may suit you if:
- You naturally aren't hungry in the morning
- You prefer simple eating rules over calorie counting
- Late-night snacking is your main dietary weakness
- You've tried other approaches without success
- You don't have any contraindications listed above
IF probably isn't for you if:
- You have a history of disordered eating
- You need to eat around medications
- Skipping meals makes you irritable or unable to concentrate
- Your work or social life requires flexible meal timing
- You're an athlete with high energy demands
The Bottom Line
Intermittent fasting is a legitimate eating pattern with genuine research support. For weight loss, it's as effective as traditional calorie restriction - the question is which approach you can sustain. IF isn't magic, and it's not for everyone, but for people who suit it, it's a practical way to manage calorie intake and may offer additional metabolic benefits.
References
- Varady, K.A., et al. (2021). Clinical application of intermittent fasting for weight loss: progress and future directions. Nature Reviews Endocrinology, 18, 309-321. doi:10.1038/s41574-022-00638-x
- de Cabo, R. & Mattson, M.P. (2019). Effects of Intermittent Fasting on Health, Aging, and Disease. New England Journal of Medicine, 381(26), 2541-2551. doi:10.1056/NEJMra1905136
- Sutton, E.F., et al. (2018). Early Time-Restricted Feeding Improves Insulin Sensitivity, Blood Pressure, and Oxidative Stress Even without Weight Loss. Cell Metabolism, 27(6), 1212-1221. doi:10.1016/j.cmet.2018.04.010
- Harvie, M.N., et al. (2011). The effects of intermittent or continuous energy restriction on weight loss and metabolic disease risk markers. International Journal of Obesity, 35(5), 714-727. doi:10.1038/ijo.2010.171
- Anton, S.D., et al. (2018). Flipping the Metabolic Switch: Understanding and Applying the Health Benefits of Fasting. Obesity, 26(2), 254-268. doi:10.1002/oby.22065
