Intermittent Fasting Pros & Cons {According to a Registered Dietitian}

Pros and cons to fasting

Last Updated on December 15, 2022 by Kayla

To begin, I would like to clarify this post discusses the fasting pros and cons for fasts lasting 24 hours or less {i.e., “intermittent fasts”} which includes various types of intermittent fasting such as time-restricted eating, alternate-day fasting, and 5:2 fasting. This blog post will not address fasts lasting longer than 24 hours {“extended” or “prolonged” fasts} here.

Table of Contents

Benefits of intermittent fasting

There is a multitude of fasting pros and cons. Let’s start by looking at the advantages of timed eating.


Intermittent fasting, or “timed eating,” as I like to call it, is FREE. But, unlike commercial weight loss programs on the market, you won’t be forking over your wallet or sacrificing next month’s rent just to try it. Some people save money when they follow timed eating. When you’re not stopping at the gas station for breakfast {and maybe lunch too}, you tend to spend less on food and drinks purchased outside the home.

You might also find yourself saving money on groceries. In this case, I suggest taking the extra savings and increasing the quality of your food. For example, opt for grass-fed butter, cage-free eggs, and wild-caught fish. I promise when you’re putting less in the tank, you want to make sure you’re giving your body premium fuel.


One of my favorite features of intermittent fasting is its versatility. Finding an eating regimen that works best for you will take time. Be patient. Flex your eating window to accommodate weekend brunch with the family or happy hour with co-workers.

There are no hard and fast rules for timed eating. The key is to find a flexible intermittent fasting routine that suits you best. Fasting should not be miserable or something that makes life more complicated.


Intermittent fasting is a simple way to add structure to your eating routine without overhauling what you eat. Instead, I recommend that an individual experiment with meal timing and then improve diet quality. Once you start feeling better, you’ll be looking for more ways to maximize your health, including what goes in your mouth.


Compared to calorie restriction for weight loss {think about sticking to 1200 calories per day – torture!}, intermittent fasting helps preserve and even increase metabolism.

Generally, when we restrict calories, our bodies catch on. They are smarter than you think! As a result, the body will compensate and reduce its daily caloric expenditure. This is called metabolic adaptation.

Over time, this change in metabolism means that we need to eat less and less to lose weight and maintain it.

For example – in theory – let’s say a person requires 1500 calories per day just to get by. So, what do they do? They reduce their intake to 1200 calories per day. The body starts to catch on and thinks, “Hey! You’re not giving me enough! You’re shorting me 300 calories a day, so I am going to compensate.”

Soon enough, this person continues to eat 1200 calories per day, and now, they go from burning 1500 calories daily to 1200 calories. Weight loss plateaus. Now what? They further restrict to 1000 calories per day, and the cycle continues.

How awful, right?! Unfortunately, this is the story we are told. Eat less and more move.

Well, the good news is that with intermittent fasting, you can eat more calories in a shorter window and still lose weight. Of course, this doesn’t grant permission to gorge the second your eating window opens, but hopefully, it gives you hope that you can lose weight without feeling deprived!

fasting pros and cons_lean mass retention

Lean mass retention

A hormone called HGH, or “human growth hormone,” is elevated in the fasted state. HGH not only helps preserve lean mass {muscle} but also helps increase the breakdown of fat for energy – specifically in the abdominal area.

HGH tends to decrease with aging. Other strategies to help increase growth hormone {outside of intermittent fasting, that is} include exercise {especially high-intensity interval training AKA HIIT} and getting adequate sleep.

Mental clarity

With time, some people find they have more mental clarity and energy while fasting. Personally, this is one of my favorite side effects of intermittent fasting. Some would even call it “euphoric.”

Insulin sensitivity

A primary goal of intermittent fasting is to reduce insulin. Insulin is a hormone that helps our cells take up glucose from the bloodstream for energy. Some foods impact our blood sugar, and thus insulin, more than others. Carbohydrates, for example, raise our blood sugar {and therefore insulin} more than other macronutrients like protein and fat.

In type II diabetes, the cells become resistant to insulin, and more and more insulin is released into the bloodstream. The problem with this is that insulin promotes fat storage and counteracts the breakdown of fat for energy. An obvious problem for anyone who is trying to lose weight!

With intermittent fasting, temporary breaks from food allow the body’s cells to recalibrate. The result is increased insulin sensitivity which is a good thing. When our cells are more sensitive to the hormone insulin, they react more efficiently at taking in energy from food {glucose} which helps maintain our blood sugar levels.

Keeping blood sugars within desired ranges results in many benefits and helps prevent cardiovascular disease.

For healthy individuals without diabetes, normal fasting blood sugar levels range from 70-99 mg/dL. However, a range of 100-125 mg/dL may indicate prediabetes, and fasting glucose greater than 126 mg/dL on more than one occasion may just get you an official diagnosis of diabetes.

Blood pressure

Because one of the main mechanisms of intermittent fasting is insulin management, blood pressure naturally follows. When insulin is low, the kidneys retain less sodium {meaning you pee out more salt when fasting}. This results in a lower fluid balance, and ultimately, lower blood pressure.

It’s essential to consider overall sodium intake and fast duration. In some cases, a person may benefit from a pinch of salt in their water or an electrolyte supplement to help prevent blood pressure from dropping too low.

fasting pros and cons autophagy


One of the most exciting health benefits of intermittent fasting is autophagy. The term “auto-phagy” quite literally means “self-eating.” When looking at our cell physiology, this is a good thing!

Click here to listen to the pronunciation of autophagy {aw-TAH-fuh-jee}. Look for the microphone above “Definition of autophagy.” It took me six months to feel confident saying “autophagy” aloud! 😅

Anyhow, autophagy refers to the “recycling” of cellular parts. When we take a break from eating, it gives our body a rest from digesting food, so it can take time to work on a bit of cellular “TLC.” This time off allows our cells to get rid of damaged or misfolded proteins.

Autophagy results in a myriad of benefits, including:

  • reduced inflammation
  • increased immune function
  • prevention or delay of neurodegenerative diseases (eg. Parkinson’s and Alzheimer’s disease)
  • potential to increase lifespan and longevity

Research about autophagy is ongoing. We are just starting to discover some of the revelations and benefits thanks to Japanese cell biologist Yoshinori Ohsumi who received the 2016 Nobel Prize in Medicine for his discoveries on the subject.

Future research will hopefully uncover at which fasting intervals autophagy is most prominent. So far, we know that cellular cleanup and repair are taking place at a basal level, and as fasting time increases {say, 2-3 days}, it’s thought that autophagy peaks. However, we still don’t know enough to “prescribe” fasting for disease prevention.

Disadvantages of intermittent fasting

Now that we’ve discussed the advantages of timed eating, let’s dig into the not-so-great aspects of intermittent fasting.

Food culture

Unfortunately, large food manufacturers heavily influence our food environment, and they don’t always have health at the top of their minds. Try watching television, for example, and see how long it takes to see a fast-food ad. We have 24/7 access to food, and generally, what’s most convenient is not the most nutritious.

On top of this, social cues and peer pressure can halt fasting goals. For example, you might hear “just try it,” “one bite won’t hurt,” or “you shouldn’t starve yourself!” In addition, when people around us eat, we are tempted to eat even though we may not be hungry. All of these circumstances can make fasting more difficult.


One of the primary concerns when learning how to start intermittent fasting is hunger. It can take time for your body to adjust to a new rhythm with meals. Check out this blog post to read more about preparing for your first fast and managing hunger.


Depending on the length of your fast, headaches can be a side effect of intermittent fasting. In addition, headaches can result from dehydration and electrolyte imbalances such as sodium depletion.

If you experience a headache while fasting, try a pinch of sea salt in your water or sip on electrolyte-infused water. This helps replenish extra sodium losses and other crucial electrolytes.

Electrolyte replacement may not be necessary for fasts lasting less than 24 hours compared to extended fasts lasting 2-7 days. If you follow a keto or low carb diet, though, extra salt can help maintain fluid balance and keep you feeling top-notch.

Weight gain

Intermittent fasting doesn’t automatically grant you access to a leaner, meaner physique. What you eat has a significant impact on your results. When it comes time to break your fast, focus on high-quality proteins, healthy fats, and (s)low carbs like those listed in my free grocery list.


Some people fear getting too hungry and anticipate bingeing after a fast. As you continue to read, you’ll see that intermittent fasting is not recommended for anyone with an active eating disorder. If you question whether or not you have a healthy relationship with food, proceed with caution.

Many people fear the feeling of hunger. Yet, today, there is no shortage of food in the United States anyways. Sadly, if you’re reading this, chances are you aren’t worried where your next meal will come from, yet we fear “giving up” certain foods or abstaining from food for a while.

Mindset plays a significant role in whether or not your fast is “comfortable.” While in some cases, I feel that intermittent fasting can help tune into physical hunger cues {i.e., stomach growling}, I also recognize that this transition can be triggering for some individuals.

Before starting intermittent fasting, take a deep dive into your relationship with food. Remember, fasting is a health and weight management tool. What works for one person may not work for another. The best intermittent fasting routine is one that you stick to!

Who should and shouldn’t fast?

Intermittent fasting is a powerful tool with various health benefits, including weight loss, but it’s not for everyone.

Consult with a physician if you have:

  • Type I or II diabetes
  • Low blood pressure
  • Electrolyte imbalances
  • Take certain prescription medications

Do NOT begin intermittent fasting if you:

  • Are pregnant or breastfeeding
  • Are underweight (BMI <18.5 kg/m2)
  • Have advanced kidney or heart disease
  • Were recently hospitalized
  • Under the age of 20 or still growing
  • Over the age of 70
  • Have an active eating disorder

While I strongly believe in the benefits of intermittent fasting, I am cautious and use my clinical judgment on whether it’s a good fit for an individual. For the general population, it can be a beneficial weight loss tool. However, people rarely come to me when they’re at their peak physique.

People tend to seek guidance from a dietitian after they’ve been newly diagnosed with diabetes, fatty liver disease, or are experiencing digestive issues. Or maybe they have tried to lose weight on their own and feel stuck.

Intermittent fasting is safe for the average, healthy-ish adult, but it is not one-size-fits-all. That’s why I am a proponent of flexible intermittent fasting and easing into a regimen.

I hope this post about the intermittent fasting pros and cons has helped you determine if it’s a good fit for you. To learn more about how to start intermittent fasting, check out this post.

Thanks for reading! Comment below about your experience with intermittent fasting. I love to hear from all of you!

Yours truly,

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