Physical Symptoms of Fasting

Most times when I try to fast I end up shaking and unable to do anything but sit. I begin sweating and feel hot. Is this how the rest of you feel when fasting and I just need to push through? This is more than just hunger and the weakness that comes from it. I’m beginning to wonder if it’s a blood sugar issue. I realize my flesh is weak which is why I’ve always wondered if this is what others experience when fasting. Thank you for your help.

Yes, that sounds like blood sugar imbalance. You may want to get checked out on this issue. If you are eating a lot of carbs/sugar normally, and then try to fast, it can be very rough. On the other hand, if you are eating low carb, such as a keto diet or low carb diet, and in a fat burning state, fasting is actually quite easy after a few weeks. It is kind of remarkable. Anyway, fast with caution until you get checked to see if your blood sugar is out of whack. Many people are pre-diabetic without knowing it.


Before you fast, you should get medical advice from your doctor as to whether it is a good idea. Please see your doctor before you next wish to fast and tell them of your symptoms. They will be able to tell you if it is wise to “push through” or not.


Remember that fasting for spiritual reasons can be very small. For example, many people “can’t live” without cream in their coffee. Have a cup of coffee without cream and offer it up. That’s fasting. Eliminate salt from your food. Don’t go back for seconds. Skip dessert. All of these things are just fine for fasting!


Thank you both, very much. I will definitely go see my doctor about this. I was really struggling with a fast today and thought it’s time to stop being embarrassed to ask and just ask. I appreciate your kind responses.


How long are you fasting? Especially if it goes more than a day you really have to be careful.

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Before I took up fasting, I went to get clearance from my doctor.

She had me undergo a blood test and then cleared me based on the results.

You may want to see your doctor to see if you are fit to undergo fasting.

Fasting is not for everyone.

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Remember, the Catholic teaching on fasting does not mean to abstain from all nutrition. It is eating less than usual, and you need to check with your doctor to make sure cutting into one meal.per day will not harm you

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Whenever I fast, I get just a bit elevated, mood-wise. I suppose I ought to ask a doctor about that too. Once I get past that initial hunger, it’s kind of fun.

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St. Thomas Aquinas said fasting is not permitted when it damages health. I would consult a priest and doctor and go from there

He also said it is not permitted when it hinders duties


Or fast from things other than food. I for one find it very difficult to do without Netflix and social media in my down time.


Yes, I have trouble being on the internet too much.

I recommend you see a doctor, preferably one that specializes in blood sugar or nutrition.

It is not uncommon to have a bad reaction to fasting, especially if you’re not used to fasting or are fasting in a new way from whatever you previously did. Posts like yours is why I am always annoyed when someone promulgates an “extreme fast” as a wonderful holy thing to do and claims they themseves feel so much better when they do it.

My own reactions to fasting range from headaches, to feeling faint, to extreme fatigue, to none of the above, depending on how I manage my fast/ what kind of fast i do. I have in the past seen a nutritionist and also read up a lot myself on this sort of issue. At this point, there are fasts that I know I can do and others that I know simply will not work for me. And these are likely to be different from the guy at the next desk or the next pew.

So please go see your own doctor and get personalized advice.

I’d basically second what others have already said, and just add that something to be aware of in general regarding foreseeable difficulties fasting is ‘reactive hypoglycaemia’. It’s not a disease or anything, just a description for the way the body reacts to a sudden lack of sugar after it’s just dumped a whole bunch of insulin into the blood to deal with the sugars that were previously coming through. The body doesn’t like to have a whole bunch of insulin sitting in the blood with no sugar to deal with, so it freaks out a bit to ‘persuade’ you to ‘Just eat that apple, please! Think about those sweet, sweet sugars!’

Like another poster said, it’s a totally different experience to fast from an otherwise keto — protein based — diet, versus fasting from a regular high-carb western diet. At least that’s my experience and that of many others; it’s much easier to go without food when the most recent food eaten didn’t trigger the body to go crazy if receiving no follow up. I’m not suggesting you go keto, just noting this is a noticed benefit for those who do.

This may or may not be related to the specific issue you’re having though (that’s between you and your doctor).

And personally even though I’m suggesting you be aware of it as a general phenomenon (so maybe take notes of your usual diet before going in to see your doctor/nutritionist, in case they spot something helpful?), I doubt your issue is reducible to this. I don’t know why overheating would be associated with reactive hypoglycaemia. So definitely talk to your doctor, for sure. This doesn’t sound ‘normal’, to me.

I also get shaky when I fast, but not quite to the degree you are describing. I’ve learned by trial and error to fast in a way that leaves me hungry but not sick.

I follow the official fasting rule of one full meal and two snacks. I don’t try to go on just one or two small snacks or no food at all.

I have low sugar Gatorade as needed.

I make sure my snacks have lots of protein and some fat, not carbs. Tuna, boiled egg, cheese, avocado. This is what makes all the difference. It keeps your blood sugar more stable.

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