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Intermittent Fasting and Exercise: Should You Work Out Without Eating?

Intermittent fasting has been popularised thanks to books such as The Fast Diet and The 5:2 Diet. However, unlike many “fad” eating plans that have fallen in and out of favour with dieters (remember the “baby food diet?”) intermittent fasting has the backing of actual science. Better yet, its benefits go beyond weight loss.

The Potential Benefits of Intermittent Fasting

Studies indicate that as well as aiding weight loss, intermittent fasting may:

  • Lower blood sugar and reduce insulin resistance
  • Reduce inflammation
  • Improve heart health
  • Aid the growth of new nerve cells in the brain
  • Reduce the risk of cancer
  • Increase our lifespan

That’s an impressive list of credentials. Unfortunately intermittent fasting isn’t necessarily suitable for everyone, and for some of us, it could even be dangerous.

Some of these risks could be amplified if you workout during a fast.

The Risks of Intermittent Fasting

There is a risk that intermittent fasting could result in poor calorie or nutrient intake. While calorie counting and taking extra care to eat a variety of high-nutrient foods can mitigate this risk, women that are pregnant or breastfeeding should avoid intermittent fasting altogether.

Those that have a history of eating disorders should also avoid intermittent fasting. That’s because it can trigger an unhealthy relationship with food, which could prompt a relapse (or aggravate an existing eating disorder).

Some experts believe that intermittent fasting could affect our internal clocks, which have an important role to play in our overall health. Risks include hormone imbalances, sleep disruption, problems with digestion, and a compromised immune system.

Intermittent fasting can also increase cortisol levels (the “stress hormone”), so those affected by chronic stress or anxiety should probably steer clear. 

Lastly, since your blood sugar levels drop when you’re fasting you might feel lightheaded, shaky and nauseous. 

The longer you fast, the more likely this will occur (particularly if you workout during a fast) - but don’t panic. 

It’s very unlikely that your blood sugar will drop to dangerous levels (known as hypoglycemia) while following an intermittent fasting plan. That is, unless you have a medical condition that affects blood sugar levels. Diabetics - needless to say - should not fast.  

Should You Workout While Fasting?

The short answer to this is “it depends”.

It’s widely accepted that working out on an empty stomach helps accelerate weight loss. This is because your body uses stored fat for energy, instead of food.

However it’s likely that working out mid-fast will zap your energy pretty quick, and that eating a light meal before you hit the gym will allow you to keep up the pace for longer. Regardless, it’s advisable to keep workouts to under an hour. Strenuous, or endurance exercise should not be attempted while fasting.

Exercising while fasting can also cause your body to use stored protein for energy, which means while you might lose fat, you might lose muscle mass, too. This means cardio only when fasting. Do not attempt strength training on an empty stomach.

So should you workout on an empty stomach?

If you’re healthy and only doing cardio, you should have no issues working out while fasting - and it may actually help you lose more weight, faster.

However you should always pay attention to what your body is telling you.

If you feel yourself getting dizzy, stop, take a break, and drink some water. Have a small snack or a sports drink if you feel the need. 

If this doesn’t help, just head home - tomorrow’s another day; another workout.