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Fasting is an age-old practice that is gaining speed in our modern-day world.

From intermittent fasting that can take place every few days or once in a while, to something that is a lifestyle, such as the one-meal-a-day, or OMAD, diet, fasting can take many forms.

Many formerly obese individual credit fasting for extreme weight loss. Others have said it improves overall health and wellness. And while the research backing up fasting regularly is mixed on all sides of the vein, the fact remains that when you don’t eat, things start to happen inside your body that affects your autonomic nervous system, and in turn, your heart rate variability.

Here is what happens to your body when you fast over a long period of time, and as a result, what role those changes play on your heart rate variability.

Your body will break down glycogen

In the beginning of your fast, your body will convert glycogen (sugar) into energy. This is entirely normal following a meal because it’s basically digestion (and your autonomic nervous system at its finest).  However, after about six hours, when you have “officially” begun you fast, your glycogen stores will begin to deplete, and you will become hungry.

Effect on HRV

Because of HRV levels being highly dependent on stress levels, in these beginning stages, your HRV could go high or low depending on your approach to the fast. If you are feeling stressed about being hungry, your HRV will likely be low. However, if you are feeling confident about the results of the fast, and even have the desired outcome, you are likely going to find that your HRV is high, indicating that you are handling the stress on your body quite well.

Your blood glucose level will rise

This may seem wrong because wouldn’t your body lose sugar if it doesn’t have the stores to break it down? And wouldn’t that mean that your blood sugar would go down?

You’d think so, but what actually happens when you fast, is that insulin levels start to drop, triggering a surge of hormones like including noradrenaline and growth hormone to fight against low blood sugar. This, in turn, concentrates the blood with sugar that it pulled from stored sugar that is usually in the liver.

Effect on HRV

According to research, high blood glucose concentration is associated with higher parasympathetic, but lower sympathetic CAM. This means that your body is under more stress to perform its normal functions of the nervous system. If you were to measure these using biometrics, you would likely find your HRV to be on the lower level.

Ketosis will begin

When your body doesn’t have the energy sources to break down new glycogen, it starts to starve and begin the hunt for other things to convert into energy. It will start breaking down fat into fatty acids in order to use them for energy rather than carbs. This is when those looking to use fasting for weight loss begin to see results.

However, due to the fact that the brain cannot use broken down fat for fuel, it turns to ketone bodies for energy. This works for a small time because ketone bodies can’t replace glucose. But after a few days, the ketone bodies build up and a volatile substance called acetone begins to form, lowering the pH of the blood. When this happens, a condition called acidosis develop and lead to coma or even death.

Effect on HRV

At this point — usually around the 48-hour mark —  your body is under stress as it searches for energy sources to survive.  Due to this, your HRV will lower. In fact, a study that took 16 young healthy female volunteers, and had them fast for 48 hours, found that parasympathetic withdrawal was induced with simultaneous sympathetic activation. These findings lead researchers to conclude that the changes in the women’s nervous systems appeared to reflect stress.

However, if your body is used to fasting, or if you have prepared yourself mentally and physically for the fast, the change may not be as significant as it could be otherwise.

If you do notice a significant drop in HRV and begin to feel considerably physical and mental stress, it might be best to abandon the fast at this point.

You’ll have cognitive function impairment

If you continue your fast, your body will be in the process of ketosis and quite possibly acidosis. During these stages, the body starts to break down protein to release amino acids that can convert into glucose. This is done to fuel your brain and suppress hunger.

For those who use fasting as a weight-loss measure, this is the next step that the body takes, and many experts — specifically as it relates to the keto diet —  say that ketosis is not entirely harmful. However, due to the strain on your brain, you may lose some simple brain functions that help you remember things, and carry out simple tasks.

Effect on HRV

The strain on many of your cognitive functions, and the continuing decline in your HRV levels will make it more difficult for your autonomic nervous system to work the way it needs to. You will be less alert and therefore unable to respond well to stressful situations

Fasting isn’t all bad …

The above may seem quite terrible and can be if taken to an extreme level. However, if you use fasting intermittently, your body will likely not have many or any of the negative side effects including those related to HRV.

Do your research on the right fasting approach for your health goals. And as always, check with a medical professional to make sure your body is able to handle the effects — whatever they may be — of a fasting regimen.

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