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Should you try intermittent fasting? Here are the pros and cons

A registered dietitian's take on one of the most talked about weight loss trends.
Image: Woman on a scale
If you’re a fan of an all-or-nothing approach, or if you’re someone who likes defined rules to follow, intermittent fasting could be worth trying.Getty Images/Blend Images

For most people, the word fasting brings up the notion of prolonged periods without food or maybe even drink. Some people fast periodically for religious reasons or medical necessity (say, before a procedure). But a new type of diet has people fasting to lose weight, improve health, live longer (and healthier), boost mental clarity, and more. Here’s what you need to know about this fasting trend.

Intermittent fasting may be practiced in a few different ways, according to research on the subject:

  • Alternate day fasting is just what it sounds like. On alternating days, you either feast or fast.
  • Modified fasting, also known as 5:2 intermittent fasting, involves eating normally five days a week with food restricted to about 25% of your calorie needs on two non-consecutive days. (Think: around 500 calories or less on restricted days.)
  • Time restricted fasting limits food within specific time windows, say 8 PM to 10 AM. In this type of fasting, you go 12 to 16 hours restricting food.

Will you lose weight?

These forms of fasting do appear to promote weight loss, according to a review study published in August, 2017. However, the science is thin among humans and typical studies involve small sample sizes and limited durations. Still, the outcomes suggest that people do lose weight so if you want to give it a go, here’s what you should know.

How does intermittent fasting work?

There are a few things at play. Your circadian clock is actively involved in regulating your metabolism and many hormones, including the appetite-regulating hormones leptin and ghrelin, are subject to these day/night patterns. Studies have shown that eating the majority of food earlier in the day aligns more closely with our circadian rhythms, and disrupting these rhythms by eating late at night or say, eating in accordance with shift work, leads to a higher post-meal glucose response, prolonged insulin exposure, and a greater risk of type 2 diabetes and obesity.

Beyond this, there’s a behavioral component at play. Late night snacking can be a common practice (as any of us who have cozied up to a bowl of popcorn knows), and putting rules in place to offset this helps limit overeating.

Jamie I. Baum, PhD, Associate Professor of Nutrition Director, Center for Human Nutrition at the Department of Food Science, University of Arkansas, says that many plans, including intermittent fasting, are effective for weight loss, and that cutting calories (whether through intermittent fasting or another method) seems to be the main factor behind the slimdown. In fact, one study published in JAMA comparing people who participated in alternate day fasting to those who were just on a standard, reduced-calorie diet found that there was no difference in weight loss or weight maintenance over the course of one year between the two groups.

What are some of the other benefits of intermittent fasting?

According to Wayne B. Jonas, M.D., Executive Director of Samueli Integrative Health Programs, and author of the book "How Healing Works," restricting eating to daytime hours optimizes and extends your body’s natural cleansing process, which may have other benefits, such as improved alertness and attention, lower body-wide inflammation, a reduced risk of illness, and promoting a longer (and healthier) life. To optimize this process, you need at least 10 to 12 hours without food intake, he says.

It doesn’t matter whether you’re a night owl or a morning bird, he explains. The key is to stop eating a few hours before bed. “The food you eat before bed doesn’t have a chance to be used as energy so not only does it promote fat storage, it interferes with the body’s cleansing process.”

Dr. Jonas points to another benefit of intermittent fasting: eating more mindfully. There are so many social and environmental cues that prompt us to eat (and overeat) so adding some guardrails around when (and when not) to dine can help you reign in some of the grazing behaviors that aren’t serving you well.

Finally, poor sleep quality and getting insufficient sleep is a known risk factor for obesity and by restricting nighttime eating (and thus, reducing heartburn), you could get more sleep and the sleep you get may even be more satisfactory, which definitely brings health benefits.

What are some of the downsides of intermittent fasting?

Some studies find a higher drop out right among intermittent fasters, which suggests it might not be a sustainable approach. Plus, it might have other consequences. If, for instance, you’re doing alternate day fasting and all you can think about on your fasting days is food, that can interfere with focus, and potentially your on-the-job performance. Plus, there may be social implications to intermittent fasting. Are your friends gathering for dinner at 7 and you’re cut off from food at 6? You can see how this could interfere with your social life.

There may be medical concerns as well. In this year-long study, unhealthy LDL cholesterol had increased significantly after 12 months among the alternate fasting group. This might spell trouble for your heart.

Finally, I’ve heard of people using their non-fasting days to fill up on foods, like pizza, rich desserts, French fries and other fare that is low in nutrition and doesn’t fuel a healthy body. Though you may still lose weight due to the calorie deficits on other days, you won’t get the protective benefits of good nutrition.

Who can benefit from intermittent fasting?

If you’re a fan of an all-or-nothing approach, or if you’re someone who likes defined rules to follow, intermittent fasting could be worth trying. It also might be especially good for people who participate in mindless snacking at night since the hard stop will curb this behavior.

Who shouldn’t try intermittent fasting?

Do not attempt fasting if you’ve had an eating disorder or signs of disordered eating, which include (but aren’t limited to) binge eating, food obsession, misuse of laxatives and extreme food restriction. If you’re pregnant or breast feeding, you should not attempt intermittent fasting. Anyone being treated for diabetes (with medications) as well as anyone with cancer and those with a compromised immune system should also avoid intermittent fasting (or speak to their doctor before trying it).

What should know before you give it go

If you do try it, make your calories count by choosing wholesome foods that do a good job filling you up. Focus on a variety of veggies to make sure you get an array of healthy compounds, along with quality protein sources (such as eggs, skinless poultry, tuna and other seafood), which do a good job keeping hunger at bay. Balance out your meals with fruits and whole grains. If you’re doing an alternate fasting diet, you’ll want to pay really close attention to lower calorie foods that create volume — both on your plate and in your belly. Remember, as with any sensible eating plan, non-starchy vegetables are your friends.

Experts believe that the real key to lasting weight lies in how you handle the maintenance phase.

And don’t forget, it’s not a magic bullet. Experts believe that the real key to lasting weight lies in how you handle the maintenance phase. “It’s not easy, but a lot of people do successfully lose weight,” says Dr. Jonas. “You need to pay attention to what you eat, how much you eat, and how you’re supporting that process — either with a group or a friend or a healthcare professional to keep you accountable and help you manage the normal slip-ups that are a part of life.”


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