May 11

Intermittent Fasting: 10 Best Dietitians Share Their Expert Opinions

Reading time 10 min.

Intermittent fasting is the latest health trend people are buzzing about, but does it really contain all the magical health benefits we’re being told it does? 10 registered dietitians share their expert opinions on the topic.

First of all, what IS intermittent fasting?

Intermittent fasting involves alternating periods of fasting and eating. Intermittent fasting doesn’t tell you what to eat, but rather it tells you when to eat.

If you’re anything like me, the word “fast” is enough to make you stop reading altogether and run to the pantry for your favorite snack…

How do you practice intermittent fasting?

There are several ways to practice intermittent fasting, but the most popular form of intermittent fasting is time-restricted fasting.

16⁄8 and 14⁄10 methods are common examples of time-restricted fasting.

This means restricting for 16 hours and eating within an 8-hour time-frame, or restricting for 14 hours and eating within a 10-hour time-frame. You’re “allowed” to drink water, tea, and coffee and take supplements during the fasting period, as long as they’re all non-caloric (AKA skip the cream and sugar). Putting it simply, you cannot eat anything during the fasting cycle.

What are the five types of intermittent fasting?

 • The 5:2 diet — This type involves eating 500-600 calories for two days of the week but eating normally the other 5 days.

 • Eat-Stop-Eat: This form of intermittent fasting is practiced with one or two 24-hour fasts per week.

 • Alternate-day fasting: Fasting every other day, either by not eating anything or only eating a few hundred calories.

 • The Warrior Diet: Eating only small amounts of vegetables and fruits during the day, then eating one huge meal at night.

 • Time-Restricted Fasting: Discussed above.

What 10 health experts have to say about intermittent fasting:

1. Should women practice intermittent fasting?

“I believe #intermittentfasting can wreak havoc on female hormones and there have only been 2-3 studies done on females, mostly rats. If you’ve had any form of orthorexia, intermittent fasting can only fuel that. Some studies show intermittent fasting can help with fasting blood glucose levels but other more sustainable ways of eating can also help that. So while I’m not saying you have to “eat within an hour of waking”, you also don’t need to be strict about an eating “window”.

— Deanna Wolfe, MS, RDN | Founder and CEO, Dietitian Deanna, LLC

2. What are the pros and cons of intermittent fasting for women?

“One of the benefits of intermittent fasting is that it promotes cell autophagy: the self-cleaning process that breaks down and recycles damaged cells. Another benefit is that it helps our body become LESS reliant on glucose as our main energy source, as we’re forced to dip into our energy stores.

The caveat is that MOST studies on intermittent fasting are done on men, and this because women’s hormones can be highly sensitive to it. If a women’s body goes into “starvation mode” too frequently or intensely, one of our first biological responses is to shut down reproduction. We’re telling our bodies to conserve energy for necessary biological processes like breathing, digestion, etc., so we’re going to shut down the unnecessary process, such as reproduction. And then that can cause our periods to become out of sync, which could then affect our bone density, fertility, etc.

Another danger is for men or women with a history of disordered eating because it can push them to become further out of tune with their body’s natural hunger cues.

I think HEALTHY fasting is about 14 hours, so maybe your eating “window” is from 9 am-7 pm. However, if you’re going to implement time-restricted eating, I think it’s also important to consider the time you’re eating from a circadian rhythm standpoint. And the easiest way to think of this is to reference how our grandparents ate (dinner at 6 pm!). Eating super late at night isn’t as ideal because our body is digesting/processing foods when it should be in more of a resting phase.”

— Jamie Vespa, M.S. R.D | Dishing Out Health

3. What does the research say on intermittent fasting for women?

“There is some emerging research on intermittent fasting that looks promising in terms of potential health benefits such as fat loss and blood sugar control; however, the bottom line is there is still a LOT of research to be done on the long term effects in humans. In addition, females may react differently to fasting than men due to differences in hormones, so it’s important to watch for changes in menstrual cycles or any other negative symptoms. We would definitely not recommend intermittent fasting for anyone who has a history of an eating disorder or disordered eating.

At the end of the day, each individual has to do what works for them and pay attention to what helps them feel their best. If fasting for 12-16 hours makes you feel awesome and you don’t have any issues with extreme hunger, headaches, lightheadedness, symptoms of low blood sugar, or preoccupation with thoughts about your next meal, then great! Go for it. Just know that it’s not a magic pill and it’s definitely NOT the only way to lose weight and/or improve health, despite what you might see on social media. If this pattern of eating isn’t realistic for your lifestyle, know that it’s not your only option. Listening to your body’s internal cues can help guide you as well.

In addition, there are many other factors that go into weight management and overall well-being, including the quality of your diet (not just the window in which you are taking in calories), your exercise routine, sleep, stress management, certain health conditions/medications, genetics, and more. A lot of individuals (even those not intentionally doing intermittent fasting) skip breakfast, then eat very little during the workday due to their busy schedules, and end up taking in the vast majority of their daily calories in the evenings. People like this may benefit simply from distributing their calorie intake more evenly throughout the day to help them eat smaller quantities in the evenings, and don’t necessarily have to follow a strict intermittent fasting eating pattern to see benefits. If you find yourself doing a lot of grazing/snacking in the evenings, ask yourself if you have eaten enough throughout the day and/or if you’re eating foods that actually satisfy you, as these factors can make a huge difference too! More on that in this blog post.”

— Shanna Hutcheson, R.D. | Wellness For The Win

4. Does intermittent fasting put too much stress on the body?

“My goal as a registered dietitian and certified intuitive eating counselor is to help my clients stress less about food, and feel better physically and mentally, and I don’t see intermittent fasting as something that is helpful for those goals. Intermittent fasting may be the latest weight loss diet trend, but just like all other diets that have trended before it, there’s zero research showing it can help more than a small number of people lose weight and keep it off permanently, and the majority will gain back more than they lost.

What I see in my practice are the side effects of intermittent fasting that isn’t usually talked about – obsessing about food (and the clock!), headaches, fatigue, hypoglycemia, dysregulation of hunger and fullness cues, binge eating, and mood swings. Remember, your brain is an organ that requires a lot of energy and regular input of fuel from food. And going long periods of time without eating often leads to overeating and binging from extreme hunger. Instead of intermittent fasting, I encourage clients to eat a meal or snack every 3-4ish hours, which will help keep you energized, stimulate digestion, and help regulate hunger/fullness cues.”

— Rachael Hartley | RD, LD, Certified Intuitive Eating Counselor; Rachael Hartley Nutrition

5. Does intermittent fasting help with weight loss?

I’m actually not against intermittent fasting but I don’t think it’s appropriate for everyone. I believe it could be a helpful tool for some people, especially when weight loss is a goal. The first thing you want to get down to is the WHY behind your reasoning for intermittent fasting. What are you seeking? Is it weight loss? Digestive relief? Blood sugar control? etc.

Overzealous proponents of intermittent fasting will tell you it’s a superior way of eating for weight loss, it increases autophagy, decreases inflammation and improves insulin sensitivity but the truth is, there’s still a LOT we don’t know about intermittent fasting.

Time-restricted eating can make it easier for some people to create a caloric deficit without feeling restricted. If you’re trying to lose weight and that’s true for you, by all means, do intermittent fasting. BUT, intermittent fasting isn’t a magical solution to weight loss. When calories are equal, fasting does not seem to be superior for weight loss (Seimon et al, 2015)

In terms of increasing autophagy, caloric restriction already increases autophagy so it likely has nothing to do with the timing of your meals. Weight loss in general also decreases inflammation.

As always, the most important factor to consider when deciding if a way of eating is right for you is: how easily can I adhere to this? People can easily become too obsessive with intermittent fasting and start creating hard rules for themselves. When they break those rules (like fasting for fewer hours than they set out for), they can get discouraged and let things spiral. If that’s you, this probably isn’t a good way of eating for you!

— Alix Turoff | RD, MS, CDN, CPT

6. How long should women fast for when intermittent fasting?

“I love intermittent fasting and I think it can be really effective for fat loss, energy levels, digestion, and detoxing. However, most women do not do well with long periods of fasting like 18 or even 16 hours every single day. Hormonally, our bodies are just not set up for it and I often see either no results from these longer fasts or worse – weight gain.

I love a 12-hour fast because it’s simple and doable for most people and it’s easy on our hormones. Your digestive system works so hard; you need to give it a break. When you do this, your body starts to dip into your fat stores to burn energy. It also majorly reduces bloating (makes sense right? Your digestive system isn’t working overtime). And bonus – it curbs nighttime snacking. Why would you shove a handful of Goldfish in your mouth at 10 PM if it means you can’t eat until 10 AM?

One caveat – I do not, under any circumstance, recommend any sort of fasting for anyone who has disordered eating or an eating disordering OR someone recovering. It can be triggering. If you find yourself using it as an excuse to just not eat or if you are binging at lunch or later in the day intermittent fasting is not for you. “

— Megan Kober R.D., The Nutrition Addiction

7. Is intermittent fasting too restrictive?

Intermittent fasting restricts your eating window each day and the time frame varies depending on the plan followed. Overall, I’m not a fan of intermittent fasting personally, but I do think some people naturally eat in an intermittent fasting window, and it’s not restrictive for those people. Finding out what hours your body normally needs food to feel its best is better than jumping onto an intermittent fasting plan.

Nutrition should be individualized and you shouldn’t feel guilty if you’re eating a meal outside of a set time frame.”

— Claudia T. Felty, R.D. | The Diet Rescue

8. Does intermittent fasting help with digestion?

Intermittent fasting comes with a slew of potentially positive outcomes, such as being linked to weight loss, increases brain functioning and reduces inflammation. Ok, this may be true from the research perspective (and for a small percentage of the population that has the mental capacity to fast by choice) but in reality intermittent fasting is a tough diet to follow. For example, some people just can’t handle missing meals because it affects their performance, cognition levels, and energy.

It’s just not a good choice for everyone and is not recommended for adolescents, pregnant and lactating women, or elderly with metabolic diseases. In addition, studies show that men are more able to fast than women as their bodies are better at handling stressors. Intermittent fasting also comes along with digestion issues- some people may experience digestion problems with larger meals being consumed in a shorter amount of time.

Intermittent fasting may cause additional stress on your GI tract, leading to indigestion and bloating. Plus, since you only have a limited amount of time reserved for eating, some people may take the feasting period a bit too far and as an opportunity to eat more calories than they really need. Intermittent fasting is clearly not for everyone. It needs to work for your lifestyle of meal timing.

It is important that if you are doing intermittent fasting, you need to also eat healthy as calories still count, and food quality is still absolutely crucial. Make sure you consume a rainbow of vegetables, healthy fats, lean proteins, and whole grains while keeping sugar, refined carbohydrates, and saturated fat intakes to a minimum.

— Tracy Lockwood Beckerman, MS, RD, YouTube host of Well+Good’s “You Versus Food” and author of the “The Better Period Food Solution” | Tracy Nutrition

9. How does intermittent fasting impact women?

“The research has shown some potential health benefits to intermittent fasting, including improved cellular repair and digestive function. That said, much of the research is done in lab trials and it’s not conclusive or without risk. Potential risks include disordered eating behaviors, preoccupation with food, fatigue, difficulty concentrating, nutrient deficiencies, and hormonal imbalance and fertility loss in women.

Ultimately, intermittent fasting may work for you, but it’s definitely not for everyone. If it’s something you are keen to try, I would recommend starting with a 12-14 hour fast (i.e. dinner at 8 pm, breakfast at 8 am). If this feels good to you, then you may be able to extend it. But, if you feel sluggish, shaky, irritable, or preoccupied with food…skip it! Intermittent fasting is not the only way to achieve optimal health.

For women, in particular, I wouldn’t recommend going longer than a 12-14 hour fast as women are particularly sensitive to external stressors on our bodies. As I mentioned, prolonged periods of fasting can result in hormonal imbalances and even fertility loss. Other populations intermittent fasting isn’t for? Anyone who is pregnant, breastfeeding, have a past history of disordered eating, or who is insulin-dependent. If you’re taking any medications or have a pre-existing health condition, it’s also best to check with your doctor before starting.

In general, I don’t recommend placing restrictive rules on our eating habits, unless medically necessary. If intermittent fasting is something that is easily incorporated into your life and you feel awesome doing it, then that’s great! But if it negatively impacts your life and makes you feel worse, then you don’t have to do it. Our bodies are all unique; just because it’s worked for someone else, doesn’t mean it has to work for you. The best thing you can do is to honor that and find what works for your body and lifestyle.”

— Carrie Walder, MS, RD | Walder Wellness

10. The verdict

“While intermittent may help you to lose weight in the short term – the long term effectiveness and health benefits are yet to be understood. Personally, we do not like the idea of starving the body. It is important to understand that all diets essentially result in weight loss by ensuring that you eat less kilojoules than you spend – they’re just different means of doing so.

The main thing you need to consider is whether the diet is sustainable in the long term for YOU. Our approach is to adopt healthy eating habits that you can sustain for life and which nourish you rather than restrict you to certain foods. To figure out whether intermittent fasting is your match, see an Accredited Practising Dietitian.”

— Alexandra Parker, The Biting Truth

Have you tried intermittent fasting? What are your thoughts?

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