What Is Intermittent Fasting?
Intermittent fasting (IF) is an eating pattern that cycles between defined periods of fasting and eating. It does not specify which foods to eat — only when to eat them. The most common protocols include 16:8 (fasting for 16 hours, eating within an 8-hour window), 5:2 (eating normally for 5 days, restricting calories to 500–600 on 2 non-consecutive days), and alternate-day fasting.
IF has attracted significant research attention over the past decade, and the evidence is more nuanced than the popular narrative suggests. It works for some people in some contexts — and is unnecessary or counterproductive for others.
"Intermittent fasting is a tool, not a magic protocol. For people whose lifestyle fits the eating window, it's an effective way to manage calorie intake. For people who train hard and need consistent fuelling, it can undermine performance." — Sarah Reyes, RD
What the Research Actually Shows
Fat loss
Multiple systematic reviews and meta-analyses confirm that intermittent fasting produces fat loss — but crucially, not more fat loss than continuous caloric restriction when total calorie intake is matched. The mechanism is simple: by restricting the eating window, most people naturally consume fewer calories without actively counting them. IF is a structure that makes caloric restriction easier for many people, not a metabolic magic trick.
Metabolic health
Beyond weight loss, IF has demonstrated benefits for insulin sensitivity, fasting blood glucose, blood pressure, and inflammatory markers in multiple studies. These metabolic improvements appear to occur independently of weight loss in some research, suggesting potential benefits beyond simple calorie restriction — though the evidence on this is not yet definitive.
Muscle preservation
This is where IF becomes more complicated for active people. Extended fasting periods can elevate cortisol and reduce muscle protein synthesis rates. Research on IF in resistance-trained individuals shows mixed results — some studies find no significant muscle loss, others show modest disadvantages compared to evenly distributed protein intake. If muscle preservation is a priority, the 16:8 protocol with high protein intake is better supported than more extreme fasting protocols.
The 16:8 Protocol: How to Start
16:8 is the most studied and most practical IF protocol for most people. A common structure: fast from 8pm to 12pm (sleeping through most of the fast), then eat between 12pm and 8pm. This requires skipping breakfast — a simple adjustment for people who are not naturally hungry in the morning.
| Time | 16:8 example schedule | State |
|---|---|---|
| 8:00 PM | Last meal of the day | Eating window closes |
| 10:00 PM | Sleep | Fasting (sleeping) |
| 7:00 AM | Wake up — black coffee or water permitted | Fasting |
| 12:00 PM | First meal (break the fast) | Eating window opens |
| 4:00 PM | Second meal | Eating window |
| 7:30 PM | Final meal | Eating window |
During the fasting window, water, black coffee, and plain tea are permitted — they do not meaningfully break a fast. Anything containing calories ends the fast.
Who IF Works Well For
- People who are not hungry in the morning and find skipping breakfast natural
- People who struggle with portion control but can adhere to a time restriction
- People with sedentary or lightly active jobs who train in the afternoon or evening
- People seeking a simple structure without calorie counting
Who Should Approach IF Carefully
- People doing intense morning training: Training fasted reduces performance on high-intensity work and may increase muscle catabolism
- People with a history of disordered eating: Restrictive eating patterns can exacerbate disordered relationships with food
- People who are pregnant or breastfeeding: Increased nutritional demands make extended fasting inappropriate
- People with diabetes or blood sugar regulation issues: Fasting significantly affects blood glucose — medical supervision is required
- People who become irritable or cognitively impaired when hungry: IF is a tool that should improve your quality of life, not degrade it
IF and Strength Training: The Key Consideration
If you are strength training, the most important adjustment to IF is ensuring your protein intake within the eating window meets your daily target of 2.0–2.4g per kg of bodyweight. Compressing all protein intake into an 8-hour window is achievable but requires deliberate planning — three high-protein meals spaced across the eating window is the most practical approach.
Schedule your training either at the end of the fasting window (training fasted for fat-loss focused sessions) or within the eating window (for performance-focused strength work). If training fasted, consume a protein-rich meal as soon as possible after the session to support recovery.
Intermittent fasting works — primarily by making caloric restriction easier, not through metabolic magic. It is a useful tool for people whose lifestyle accommodates an eating window, but it produces no superior fat loss compared to standard caloric restriction when total intake is equal. If you find it easy and sustainable, use it. If you find it miserable and performance-degrading, there are better approaches — including the strategies in our weight loss diet guide.