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Intermittent Fasting

Intermittent fasting re-examined: Can you fast too much?

by Barak Alon · July 05, 2021 · 3 minute read
can you fast too much

Intermittent fasting can help people lose weight and achieve a healthy metabolism. However, Lumen has uncovered a common pitfall among users that suggests they aren’t fasting and feasting the right way. Here we look at why over-fasting could potentially cause weight gain.

Can you fast too much?

There is such a thing as over-fasting- let’s let that rest for a minute. The typical 18:6 fasting regime is not for everyone. After analyzing over 1 million monthly metabolism measurements, we found that over 27% of users that fasted for more than 10 hours experienced a carb burn state (Lumen level 4/5).

when does the body start burning fat during fasting

This suggests the body overextended its fat-burning state and the stress response was triggered, contrary to our initial assumption which correlated more hours of fasting to more users entering fat burn.

The question is, does fasting burn fat? Read on

If you fast for too many hours it can trigger a counteractive stress response in the body, which results in carb rather than fat burn. This is not the best news, considering that we want to wake up in fat burn most of the time, as our bodies burn through our carb stores efficiently throughout the night, providing we didn’t overeat the night before.

What we can learn from the fasting habits of Lumen users

After analyzing the different fasting schedules of our users, we noticed there’s a lot to learn from their varying fasting windows.

best fasting schedule

Less than 10 hours

A total of 12.6% of Lumen users fast for less than 10 hours and reached fat burn. Your fasting window can be achieved by following eating habits such as having your last meal at 8pm and having a coffee with milk at 6am. Many of us probably unconsciously fast for this amount of time.

11-15 hours

The most common fasting window among users (64.9%) is 11-15 hours with a fat burn rate ranging from 37 to 42%. A popular method of intermittent fasting is the 14:10 ratio where you fast for 14 hours and consume all your meals in a 10-hour window.

16-20 hours

Our data shows 20.7% of users opt for a longer fast of 16 hours or more with a fat burn rate of 42-46%. The 16:8 ratio often involves skipping breakfast and not eating your first meal until around midday.

More than 20 hours

As our data science team predicted, the least popular method among Lumen users (1.8%) was the 20 hour+ fast where eat all your meals in four hours or less. With this many hours of fasting, our users didn’t seem to wake up in fat burn much more than those that fasted 16-18 hours.

Can you fast too much Lumen

So, what is the optimum fasting time?

There isn’t necessarily an ideal fasting time as we all have different metabolisms. Our data shows more than half of our users fast for between 11-15 hours but it is very much down to the individual. The key to reaching great results with intermittent fasting is to work out your fasting window so you can optimize your fast.

The best way to do this is to test your body with different hours of fasting and see how your body reacts. Use your Lumen to measure your metabolism at the start and end of your fast. If your level is higher than your morning measurement, it may indicate your body is going into ‘stress mode’. In this case, end your fast and the next time your fast, measure your metabolism one hour earlier.

Lumen why am I always burning carbs

When is it time to end your fast?

If you’re experiencing agitation, restlessness, or hunger; you might want to measure your metabolism at the end of your fasting window. In case your level is higher than your morning measurement, it may indicate that your body is going into ‘stress mode’.

In this case:

1.End you current fast.

2.Next time you fast, measure your metabolism 1 hour earlier.

You can try this spreadsheet to record your “end of fast” metabolism measurements to take smarter decisions regarding fasting routine.

Bottom line

Lumen encourages intermittent fasting but with a personalized and optimized approach that keeps metabolic health in mind. Taking a post-fast metabolic measurement using the Lumen device is a recommended solution to avoid canceling the many benefits of fasting. Intermittent fasters can also safely optimize their fat burn by becoming aware of the physiological indicators of a stress response, which include extreme hunger pangs, restlessness and agitation.

For additional articles by Lumen experts visit our Intermittent Fasting category


Barak Alon

Head of Data at Lumen