Dr. Anthony Gustin is a board-certified sports chiropractor, functional medicine practitioner, entrepreneur, podcast host, and founder of Perfect Keto.
You likely know someone who’s tried fasting. Maybe you have too, or you’re interested in starting it. But you might be wondering: is it dangerous to fast? Can fasting harm your body or slow your metabolism? These are common questions, and for good reason. While fasting has some amazing benefits for health, doing so correctly (and choosing the right type) is important.
First, let’s look at the benefits of fasting and then address a few fasting concerns you might have.
Benefits of Fasting
Let’s get this out of the way first: overall, intermittent fasting can be great. It can help you:
- Lose weight (and fat) quickly: When food intake ceases, the body’s glycogen stores are depleted by around the 24-hour mark before it starts switching to fat stores for energy (this is ketosis). This speeds up weight loss, including water weight initially.
- Reduce emotional eating and cravings: Those who struggle with any unhealthy relationship with food may be able to “reset” their taste buds and cravings through fasting.
- Increase focus: Many report feelings of hyper-awareness, higher mental clarity, and more productivity when fasting.
- Getting into ketosis faster: Even though ketosis is of course possible just from eating a ketogenic diet, fasting can get you there much faster.
- Promotes autophagy: Also known as the body repairing itself. Fasting encourages the body to “clean out” any toxic materials that you might have built up through diet and/or environment, which can repair oxidative stress, encourage healthy muscle building, and even be anti-aging. In addition, longer fasts have been shown to help with health concerns like high blood pressure  and even patients going through chemotherapy .
- Support of overall health: Fasting done correctly can benefit metabolism, reduce inflammation, and support longevity.
Intermittent fasting and modified fasting or fast mimicking are great ways to improve your health and benefit from the natural advantages the human body obtains through not eating all the time. However, there are certain types of fasting that should give you a little more pause.
Intermittent vs Longer Fasting
There are different types of longer-term fasting, such as water fasting (drinking only water and not consuming anything else), broth fasting, or juice fasting (drinking only natural juices from fruits and vegetables and nothing else).
Juice fasting is not a good idea for those on a ketogenic or low-carb diet (or really anyone), since it’s little difference than drinking sugar water. Water fasts can cause even more of a concern due to concerns we mention below.
Not only do these typically last longer and are not intermittent (switching from shorter fasting periods to shorter eating periods), as in more than a day or two, but you also aren’t receiving any sorts of calories and crucial vitamins and minerals.
Dangers of Fasting: Is It Dangerous to Fast?
Now that we’ve covered some of the good things about fasting, let’s talk about when it can get dangerous and how to avoid these situations. The fasting we’re talking about here is those with absolutely no food, only water (or very little water), or juice fasts.
Electrolyte imbalances: Our bodies need electrolytes like sodium, calcium, potassium, magnesium, and phosphate to balance our fluid levels and keep our organs functioning normally. We of course usually get these from food, so fasts without any food can lead to deficiencies that cause serious problems. Supplementing with these minerals or drinking electrolyte water can help prevent them. Also, be sure to always stay hydrated.
Yo-yo dieting results: While weight loss does happen quickly with fasting, as we mentioned above, it can also make one more prone to gaining all of the weight back if they struggle with food. Fasting without proper knowledge of nutrition and a good diet to accompany it can not only lead to weight loss failure but also lead to more weight gain afterwards, which is obviously harmful for health. It’s also not a good idea if you struggle or have struggled with an eating disorder.
Fasting is also not a good time to exercise intensely for performance or building massive strength, as rest is important the more “intense” the type of fasting.
You might have been told that fasting can slow down your metabolism. This one is actually not true, at least not in the way many people think. In fact, intermittent fasting is actually good for your metabolism. It can more rapidly shift you into ketosis, which is the body’s desired metabolic state and can improve insulin sensitivity to prevent metabolic issues like type 2 diabetes.
People Who Shouldn’t Fast
It’s also very important to mention that there are some people who should not engage in fasting, including the elderly, kids who are still growing and developing, pregnant women, and people with serious illnesses and medical conditions.
No matter who you are, check with your doctor before starting any type of fasting protocol, especially if it’s long-term, to make sure you’re getting everything your body needs to be healthy and not suffer from it later.
Fasting through a Ketogenic Diet
Fads like water-only fasts, especially for extended periods of time, are not recommended because you’re likely missing out on important nutrients or could be causing more harm than good. So, are we point-blank saying fasting is dangerous? No, not at all actually! As you can see, the benefits of fasting can greatly outweigh the downsides and potential dangers.
That being said, smart fasting is the way to go, and the smartest way of fasting is through modified fasts (where you mimic fasting with a lower calorie intake—see Dr. Anthony’s modified fasting results here) or intermittent fasting plus “fasting” the body of carbohydrates through ketosis with a ketogenic diet. This type of fasting can work wonders.