Intermittent Fasting & Exercise: Is It a Good Fit for Active People?
Intermittent fasting (IF) has become one of the most talked-about health trends in recent years, praised for everything from weight loss to better blood sugar control. But if you’re someone who trains regularly—whether that’s running, cycling, lifting or fitness classes—does it actually support your goals, or could it do more harm than good?
Here’s what you need to know before trying intermittent fasting as someone with an active lifestyle.
What Is Intermittent Fasting?
At its core, intermittent fasting is about when you eat—not necessarily what you eat. The two most popular approaches include:
Time-Restricted Eating (TRE): Eating all your meals in a set time window (e.g. 10am–6pm), then fasting the rest of the day.
5:2 Diet: Eating normally for five days a week, and reducing calories significantly (or fasting) on the other two.
Claimed Benefits and What the Research Says
Some early studies and anecdotal evidence suggest intermittent fasting may:
Help with weight loss by reducing the time available to eat (and therefore overall calories).
Improve insulin sensitivity, which can support blood sugar control.
Support fat metabolism during lower-intensity exercise.
Potentially benefit gut health by increasing microbiome diversity.
However, most of these findings come from small studies—and many have been done in sedentary or overweight individuals, not active or athletic people. When researchers have looked at IF in people who train regularly, the benefits are less clear-cut.
What About Exercise Performance?
This is where things get interesting.
While intermittent fasting may help reduce body fat and preserve lean mass, multiple studies show it doesn’t translate to better performance:
In one study, elite cyclists lost weight on a time-restricted plan—but their power and endurance didn’t improve.
Runners following a 16:8 plan saw no gains in speed or VO₂ max, despite dropping weight.
A review of 46 studies found that intermittent fasting did not enhance sports performance.
Are There Risks for Active People?
Yes—especially if you’re training regularly or at high intensity. Key concerns include:
Low energy availability: Compressing your eating window can make it harder to meet your energy needs, especially if you’re doing long or intense sessions.
Poor recovery: Eating too far after training may blunt recovery and muscle repair.
Increased fatigue: If you’re not fuelling adequately, workouts can feel harder and leave you more drained.
Potential nutrient shortfalls: Less time to eat can mean less variety and missed nutrients.
Special Considerations for Women
Intermittent fasting may have a stronger physiological impact on women. According to research and sports scientists like Dr. Stacy Sims, fasting can:
Increase stress hormone (cortisol) levels
Disrupt menstrual cycles
Impair blood sugar control
Lower thyroid activity
This is especially risky for women who train hard or are already under stress (physically or emotionally). In these cases, intermittent fasting may do more harm than good.
Should You Try It?
You might consider intermittent fasting if you:
Are doing light to moderate training
Are more focused on weight loss than performance
Can plan workouts around your meals
Are not prone to low energy, hormone imbalances, or digestive issues
You should be cautious or avoid it if you:
Train intensely or for long durations
Struggle to eat enough during the day
Are female and training regularly
Are preparing for an event or race
Have a history of disordered eating
Tips for Active People Who Want to Try It
If you’re still curious about intermittent fasting, here’s how to make it work with your training:
Choose a longer eating window (e.g. 10–12 hours instead of 6–8)
Schedule workouts so they happen during or just after your eating window
Focus on nutrient-dense meals that include carbs, protein, healthy fats, and plenty of veg
Don’t skip post-workout recovery meals—these are key for strength and endurance gains
Pay attention to how you feel: fatigue, disrupted sleep, low mood, or constant hunger are signs it’s not working for you
Final Thoughts
For highly active people, intermittent fasting may not be the best approach—especially during heavy training blocks or when performance is a priority. While it may help some people manage their weight, it doesn’t appear to boost endurance, strength or recovery. And for many, the risks (fatigue, under-fuelling, hormonal disruption) may outweigh any potential benefit.
Instead, focus on fuelling your body consistently with regular meals and snacks, timed to support your workouts. That’s still the most reliable way to stay strong and healthy.