I have been playing around with intermittent fasting for the past two years, and have noticed that when I have been fasting for long periods (19+ hours), and also when I have not been fasting but instead have spent a couple of days limiting calories to the 1700-2000 kcal range, I get a mild shortness of breath and cannot suck in full lungfuls of air. It persists for the duration of the fasting period and ends upon breaking the fast, and also goes away upon eating a small meal on those occasions in which I'm running lower calorie, and comes back 10-12 hours later if I'm doing multi-day mild calorie restriction.
Do you have any medical issues that may be causing it? I am asthmatic and notice that I feel light headed and my heart beats HARD around the 18 hour mark of fasting. It will go away when I eat and I don't feel like that any other time.
I have noticed that my hay fever and asthma symptoms have been drastically reduced since following the Paleo lifestyle, so I will take the light headedness!
Heavy heart beats are often related to stress and elevated cortisol. You may be stressing out your body by fasting and eating in a calorie deficit. Maybe try shortening your fasts and see how you feel.
What are your goals? Weight loss? Elevated cortisol over a period of time can really inhibit fat loss. Just something to keep in mind :)
Could it be anxiety? It's hard to tell but, I think, worth considering. Shortness of breath is often associated with anxiety. What are your thoughts like around the time that you experience the shortness of breath? Are you thinking about not eating? Are there other stressors around you?
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