Does Intermittent Fasting Really Work?
by Carolyn Firestone
I’m NOT usually one to lecture anyone about their diet.
Unless it concerns a cake I spent 10 hours baking for a party where everyone is suddenly too full for dessert, I’m not about to get bossy about what you put into your body.
But, I do like to talk about different approaches to health. And health tends to include the food we eat, Or, in the case of intermittent fasting, when we eat it.
You may have heard of intermittent fasting (IF) from a circulating Instagram story or a friend who’s tried it out.
There are different ways to do it, but essentially all forms of IF involve eating only during certain designated timeframes and fasting the rest.
Many flock to this method for its promises of rapid weight loss or studies saying it could lengthen their lifespan. Others are just hoping to live out the fantasy of being able to eat as many scoops of Sea Salt Cream and Cookies ice cream as humanly possible between the hours of Noon to 8 PM and still look and feel ready to run a marathon the next morning. Just me?
Yet, if the reviews you’ve heard have been mixed and you’re confused about whether IF is worth our time and (seemingly inevitable) tummy rumbling, I hear you.
Here, we break down what you need to know about IF based on what the experts know right now. And by experts, I mean three incredibly body-positive, non-faddy, science-based Registered Dietitians who really know their stuff.
How does Intermittent Fasting work?
“Intermittent Fasting is an eating pattern that cycles between periods of eating and fasting. [It’s] essentially just a way to structure your eating schedule throughout the day/week,” said Alix Turoff, MS, RD, CPT.
The idea behind it is to use fasting periods to kick our bodies into ketosis. Ketosis is a metabolic process that occurs when we run out of glucose. Our system typically turns glucose to energy, so when it runs out it, we start to burn fat instead. This transition can occur after around 12 hours of fasting, so the idea of IF is basically to extend those fat-burning hours.
There are several ways to do intermittent fasting, but the most popular methods are:
Alternate Day Fasting (ADF): This involves a 24-hour fasting period followed by a 24-hour feeding period.
Whole Day Fasting (WDF) or Eat-Stop-Eat (ESE): This involves taking one or two days week where you fast for 24 hours.
Time Restricted Eating (TRE): This form of intermittent fasting is probably the one you’ve heard about. It’s the most popular, and tends to be a bit less intense than going a full 24 hours without food.
TRE involves a set daily fasting period that usually lasts between 12 and 20 hours followed by a “feeding window.”
Two of the most common timelines for TRE are:
The 16/8 method: Fast for 16 hours, then eat only within an 8-hour period.
The 14/10 method: Fast for 14 hours, then eat only within a 10-hour period.
Can I eat whatever I want within my “window” and still lose weight?
While the appeal of intermittent fasting for many might be that it focuses on when we eat rather than what we eat, the idea that we can consume whatever our hearts desire within our feeding window and still lose weight may not pan out the way we hope.
Each dietitian I spoke to pointed to research showing little if any difference in weight loss between intermittent fasting and any other calorie-restricted diet. Their consensus comes on the heels of a yearlong study showing that time-restricted eating was “no more beneficial with regard to reduction in body weight, body fat, or metabolic risk factors than daily calorie restriction.”
That doesn’t mean IF can’t work for some people, especially those who feel overwhelmed by dieting methods that require them to avoid certain foods or count every calorie they consume.
“I do believe IF can be a tool for weight loss if (and only if), it helps you to create a calorie deficit,” said Turoff. “There is nothing magical about having a specific eating window… BUT, if you’re spending less time eating, it may lead to a decrease in your overall calorie intake and, as a result, lead to weight loss.”
Is it easier to follow than other food plans?
As with any habit pattern, some people find IF easier to adjust to than others. They may find the time structure more appealing and less stressful than methods that focus on food itself. For them, fewer hours of eating paves the way to eating less.
“It creates rules and some people do well with that,” said Turoff. “For other people, stressing about intermittent fasting can actually make matters worse.”
As with any diet, tight regulations can yield results, but they can also cause problems when they fail to sync up with our body’s natural queues. You know, queues like literal hunger?
“Sometimes when we place rules and restrictions on what or when we are "allowed" to eat, it can make it more difficult to truly tune into our body's signals and actually notice what we need,” said Shanna Hutcheson, RD. “In addition, having set times you are allowed to eat may lead to feelings of deprivation and restriction, which for some, can ultimately lead to bingeing. It can be a vicious cycle that is hard to get out of, especially for those with a history of dieting.”
So is IF a safe and effective way to lose weight?
Right off the bat, it should be said that intermittent fasting is not recommended for certain groups of people.
Anyone with an active eating disorder, history of eating disorders, and/or disordered eating should avoid IF,” warns Hutcheson. Because this form of fasting can impact blood sugar levels, anyone with a diagnosis of diabetes or taking certain medications should consult their physician and very likely avoid IF altogether. Other groups for whom IF isn’t recommended include: pregnant, postpartum women, or women struggling to conceive as well as children and teens.
For people who haven’t struggled with disordered eating or certain health-related risk factors, it really depends on how IF impacts them BOTH physically and mentally. And that has to be weighed on a somewhat case-by-case basis.
If IF is making you feel good, nourished, and LESS obsessed with your diet, it may be a solid choice. If it’s leaving you tired, anxious, and reeeeallly hungry, that’s another story.
“Intermittent fasting is restriction,” said Sarah Eichenbaum, RD. “Because restriction can mean ignoring the body’s hunger signals, a lot of people end up failing.”
Her concern is one that’s echoed among many RDs about dieting in general. Getting out of sync with one’s own body (and natural hunger) can lead to things like disordered eating, anxiety, and feelings of failure when a plan inevitably feels too hard to follow.
In order to answer if IF is a healthy fit for any of us, we have to tune in to our body and mind and weigh how it’s affecting both.
“If someone genuinely enjoys intermittent fasting, feels great while they do it, and they don't feel that it is negatively affecting their energy levels, relationship with food, or ability to get adequate calories/nutrients in, that's great! I'll never tell someone to stop doing something they feel is working for them and that they genuinely enjoy. Everyone is different,” said Hutcheson.
“At the end of the day, 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?” said Turoff. “If you're worried about breaking your fast with toothpaste or find yourself struggling through your fasting window, then that’s an issue.”
What about IF’s other health benefits?
Most experts agree that more or better research is needed to answer certain questions about the health benefits of IF. For example, those studies I mentioned earlier about it leading to a longer life were mostly done on animals like rodents and fruit flies.
Nevertheless, there are a lot of fascinating findings in those animal studies that should certainly be tracked as they’re translated to human subjects. They include anything from benefits to cognitive function to reduced inflammation.
“One study did show some improvements in humans from time-restricted feeding, such as lower triglycerides, LDL [what we often call 'bad' cholesterol] and blood glucose, and a 12-hour fasting window was associated with favorable results in human subjects,” said Hutcheson. Yet, when it comes to other factors such as heart disease risk, Hutcheson said the benefits so far have looked similar to those seen with regular old calorie restriction.
“People tend to look at IF as a magic fix for everything, but the truth is, we don’t have enough research to support that,” said Turoff.
Are there any concerns about women and fasting?
By now, you’re probably sick of hearing the phrase “more research is needed.” Regrettably, that seems to be the case when it comes to measuring what (if any) differences exist between IF’s impact on men versus women.
There’s one small study (again on rats) that showed an unfavorable impact on glucose response in the furry female subjects but not the male. There’s also a discussion about whether IF can negatively impact female hormones.
“What we know is that being in too steep of a calorie deficit can impact hormone production and balance. If intermittent fasting results in too drastic of a calorie deficit, it may negatively impact female hormones, but there doesn’t seem to be a mechanism outside of that,” said Turoff.
“Shifts in hormones could impact libido, menstrual cycles, sleep, mood, and more,” said Hutcheson. Additionally, “if women are significantly restricting calorie intake, it could be difficult to get adequate nutrients which could have a snowball effect and lead to other health issues down the road.”
To help offset these risks, Hutcheson recommends women who are curious about IF try shorter fasting periods of 12-14 hours and not fast every single day.
So, is it worth a try?
IF could have some benefits for people who use their eating windows to actually enjoy the food they're eating and nourish themselves with nutritious meals. This is especially true if it’s helping them to stop focusing on their diet in an unhealthy way and reduce the number of rules and judgments they have around what they put on their plate.
Yet, most dieticians agree that women in particular need to get more in touch with their body’s natural circadian clock and hunger queues and less entrenched in diet culture.
“Technically, IF is another set of rules. A lot of people have a long list of rules they have either created themselves or implemented due to diet culture, and they feel that they need those in place to ‘control’ their eating,” said Hutcheson. “Unfortunately, that often doesn't work, because if it did, they wouldn't have to go on diet after diet after diet and feel guilty/shameful every time they ‘fail.’”
Hutcheson doesn't typically recommend intermittent fasting right off the bat for clients. She prefers helping them learn about nutrition and health in ways that help them feel their best and “most importantly, that [they] actually enjoy eating.”
While she does see a possible payoff for people who find intermittent fasting’s time-centered method not as “all-consuming” as other restrictive dieting approaches, she’s also seen clients struggle with IF long-term.
“IF is unique in that it doesn't dictate WHAT you eat, just WHEN you eat. In that sense, it does allow for some more freedom and flexibility with a variety of foods. However, in my experience working with clients, I have seen that it can make people over-restrict their intake, which can have a variety of negative long-term effects, such as impacting their relationship with food, but also their physical health.”
If someone wants to try IF, Hutcheson recommends being aware of possible downsides that include:
Very long fasting windows can make it difficult to meet overall nutritional needs - calories, protein, micronutrients. ‘IF doesn't necessarily focus on diet quality - just timing. It would be most beneficial to also make healthy food choices while implementing [it].”
Because of its restrictive nature, IF is not ideal for those with a history of eating disorders or disordered eating.
People may notice negative physical symptoms depending on which fasting method they are following and how much they are restricting calories. “If you notice that you are feeling dizzy, lightheaded, anxious, or any other symptoms, stop and consult your doctor.”
Pay attention if you’re not eating meals that satisfy and nourish you. This can lead to bingeing or feelings of failure when you inevitably feel hungry.
“At the end of the day, I think it's important to remember that it doesn't have to be all or nothing. If you are curious about IF and want to give it a shot, go for it! But you may consider starting with a fasting window like 12-14 hours and see how that feels,” said Hutcheson.
For anyone about to try IF, the most important thing seems to be staying sensitive to yourself and paying attention to how the diet affects you mentally and physically.
As with any approach to how you eat, you should keep asking yourself, “Is this making me more or less anxious?” Is it improving my relationship with food or making it worse? Am I nourishing myself with the meals I’m eating regardless of when I eat them, or am I falling into a pit of restriction?
“Instead of thinking about what I can take away, we should always be thinking about what we can add in terms of health,” said Eichenbaum. When it comes to diet, we have to think about what we’re gaining as much as what we’re losing.
Carolyn is a freelance writer and editor. Her favorite thing to do is to write about her favorite things, especially when they have even the slightest chance of making someone else’s something (mood, relationship, travel plans, or toiletry kit) a little better. You can find more of her articles here.
