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Intermittent fasting is one of those diet trends that only seems to be getting bigger and bigger. So by now you probably know at least a few people who’ve given it a try and seen results. But how does it actually work?
It’s a legit question, of course. Intermittent fasting is all about limiting your eating to certain times without worrying about calories or cutting out certain foods. How does something as seemingly simple as paying attention to the clock potentially help with weight loss?
As it turns out, intermittent fasting can trigger some significant changes in the way your body processes energy, which can help you lose fat and potentially reap some other big benefits.
Here’s a look at how intermittent fasting could lead to weight loss and what happens to your body when you give it a try.
Intermittent fasting (IF) is a style of eating that alternates periods of fasting with periods of eating.
During fasts, you avoid all food and calorie-containing beverages, but water, black coffee, and tea are OK. During eating periods, you basically just eat — there’s no restricting your calorie intake or avoiding certain foods.
Many people say IF helps them lose weight and spend less time thinking about food overall. But that’s not the only potential benefit. A growing body of evidence is also showing that an IF-style diet could help protect against certain diseases and improve overall health.
How long do you not eat on intermittent fasting? That depends on the IF type you try. There are a few different styles, and each one works a little differently.
This type of fasting involves limiting your eating to a certain number of hours within the day, every day. People generally aim to fast for at least 12 hours, with most of that time occurring during sleep.
Time-restricted fasting for 12 hours, for instance, might mean stopping eating at 7:00 p.m. and not eating again until 7:00 a.m. the next day.
But research suggests longer fasts might come with bigger benefits. A recent study found that participants who fasted for 18 hours per day burned more fat and felt less hungry than those who fasted for 12 hours.
This type is just what it sounds like: You alternate days of fasting (water, black coffee, and tea are OK) with days of eating the way you normally would.
Research has shown it’s about as helpful for weight loss as following a low calorie diet every day.
In fact, a small 2005 study found that participants who did alternate-day fasting for 22 days lost 2.5 percent of their body weight and 4 percent of their body fat. Their fasting insulin levels also decreased by 57 percent.
5:2 is a modified type of fasting where you alternate days of regular eating with days of eating a super limited number of calories — usually no more than 500. Each week you’ll eat normally for five days and limit calories for two.
This type of fasting also seems to help people lose weight about as well as traditional low calorie diets. But research shows that after 6 months, 5:2 fasting may lead to greater reductions in waist circumference and increased insulin sensitivity.
Limiting your eating to certain periods can easily set you up for eating less overall without having to track calories or avoid certain foods.
For example, if you stop eating after dinner and don’t eat again until morning, you might save a few hundred calories by not snacking before bed.
But science suggests there’s likely more to it. Every time you eat, your body makes insulin to move the energy from food into your cells. And any energy you don’t need to use ASAP gets stored in cells as fat.
Eating less often can help keep your body’s insulin levels lower, which can encourage fat burning. When there’s not a steady source of ready-to-use energy in your bloodstream waiting to be transported into cells, your body has to rely on the energy that’s already stored in fat cells.
And contrary to what you might think, going for longer periods without eating might actually make you less hungry. One study found that people who fasted 18 hours a day had lower levels of the hunger hormone ghrelin and reported having less of an appetite.
Fasting seems to help the body in other ways too. A recent review found that forcing cells to rely on existing energy stores seems to make them more resistant to oxidative stress and less prone to inflammation. That could potentially help improve overall health.
Got more burning questions about intermittent fasting? We’ve got all the answers.
Does intermittent fasting work for fat loss?
The answer seems to be yes. A 2019 review of 11 studies concluded that IF seems to be as beneficial for weight loss as eating a low calorie diet. What’s more, no one type of fasting seemed to lead to more fat loss than the others.
Other reviews back this up. One published in 2018 found that nearly all studies looking at IF have found that it can be helpful for fat loss, helping people drop up to 9.9 percent of their body weight.
How long does it take to lose weight on intermittent fasting?
There’s no one-size-fits-all answer, but in general, research suggests that the time frame is probably similar to weight loss on a typical low calorie diet. A review from 2014 found that people doing IF lost between 3 and 8 percent of their body weight in 3 to 24 weeks.
That’s a big range. But when you think about it, it makes sense. The speed of your weight loss will depend on a ton of factors, including what style of fasting you do and what your eating is like in nonfasting periods, plus things like your age, size, and activity level.
Is intermittent fasting safe to do long-term?
The evidence seems to suggest that IF is safe and might even deliver bigger health benefits over time. But experts don’t have enough research to say this for sure, especially for older adults or those with other health conditions.
That’s all a fancy way of saying that we don’t know for sure how IF might affect health over time. So if you’re thinking about trying it, run it by your doctor first.
Can you combine intermittent fasting with a keto or vegetarian diet?
Of course, it makes sense to talk with a registered dietitian any time you’re cutting out whole food groups, whether you’re doing it in addition to IF or not.
Cutting out certain foods can put you at risk of falling short on some nutrients. An RD can review your eating plan, pinpoint any potential nutritional gaps, and help you find ways to fill them.
Will I end up overeating during my eating window and cancel out the benefits of my fasts?
Intermittent fasters don’t seem to go hog wild during their eating windows. In fact, some research shows that people actually end up eating less than they did before starting IF.
If you’re worried you might overdo it, try to go in with a mindset of moderation. Instead of focusing on the fact that you only have a certain amount of time to eat, just pay attention to your body, eating when you’re hungry and stopping when you’re satisfied.
Does intermittent fasting have other benefits besides weight loss?
The answer seems to be yes. While experts caution that more long-term studies on the topic are needed, evidence suggests that IF is tied to improvements in blood pressure, heart rate, cholesterol levels, and insulin resistance. It may also lower the risk for type 2 diabetes, Alzheimer’s disease, and cancer.
IF can be an effective way to lose weight and improve your health, but limiting your eating to specific windows might not be right for everyone.
Here’s an overview of the benefits and drawbacks to help you figure out if it’s worth trying.
Pros of intermittent fasting
- You might lose just as much weight as on a traditional low cal diet.
- You don’t have to count calories or track your food intake.
- You can enjoy all foods in moderation — there’s no need to cut anything out.
- You can do IF in combination with other eating plans, like a keto or vegan diet.
- It might offer extra health benefits on top of weight loss.
Cons of intermittent fasting
- Eating on a different schedule can take some time to adjust to.
- The different schedule might not work for your lifestyle long-term.
- You might feel hungry or irritable at first, especially if you’re used to eating frequently.
- Eating only at certain times might be harder in social situations or clash with the rest of your family’s schedule.