Fast As You Can: Does Intermittent Fasting Work?

Intermittent fasting (IF) involves limiting eating to certain times of the day or week. The idea is that IF can reduce caloric intake and even boost overall metabolism.

There are many ways to structure IF. You can follow one of the popular pre-set formats or blaze your own trail (preferably under the guidance of a registered dietitian).

Most IF plans fall into one of four main categories:

• Time-restricted feeding (TRF): This common version of IF stipulates eating during specific hours of the day, then fasting for the remainder of the day. Popular variations include the 16:8 plan (fasting for 16 hours, then eating within the next 8 hours) and the 14:10 (same concept, different hours).
• The 24-hour fast: Known as “eat, stop, eat,” this IF method involves fasting for an entire 24 hours once or twice a week. It’s also called periodic prolonged fasting (PF) or intermittent calorie restriction (ICR).
• The 5:2 method: Here, we have five days per week of regular, unrestricted eating followed by two days of eating 500–600 calories per day.
• Alternate day fasting (ADF): As its name implies, this one involves fasting (albeit in a modified way) every other day. For example, eat what you want for a day, then stick to 500 calories the next day, then go back to usual for the third day… and so on.

Unlike some complicated diets that pair specific foods or tweak your macros, it’s easy to understand how IF works for weight loss. When you set time limits on your eating, you tend to eat less.

“For many people, creating a smaller eating window during the day will restrict calories enough to achieve weight loss,” says dietitian Shannan Bergtholdt, MS Ed, RDN, of Revolution RD.

A 2015 systematic review of 40 studies also found IF effective for weight loss. Folks who followed an IF plan lost an average of 7–10 pounds over 10 weeks — but we don’t know if the weight stayed off afterward.

Another 2020 review found that participants in 27 studies lost 0.8–13% of their baseline weight while participating in IF. Again, the lack of follow-up doesn’t tell us if folks gained the weight back later.

It’s not super clear whether IF is better than general calorie restriction when it comes to weight loss. A 2017 study with 100 participants found that alternate-day fasting didn’t produce better results than limiting daily calories through portion control or calorie counting.

So, is IF effective for anything beyond calorie restriction?

“There is evidence of metabolic benefits, such as improved insulin sensitivity, particularly if meals are eaten earlier in the day,” Bergtholdt says.

And some research suggests that IF *might* lower the risk of cardio-metabolic disease by decreasing your:

• oxidative stress (the kind that causes systemic inflammation)
• blood pressure
• “bad” LDL cholesterol
• symptoms of gastroesophageal reflux disease (GERD)

Of course, IF is no golden ticket to good health. Someone following IF could technically nosh on cheesy fried nachos for 8 hours straight. In general, the best way to support your health is to eat an array of whole foods with nutrients like fiber, protein, and healthy fats.

How long before I see results?

Big-picture benefits like disease prevention are a long game. But some folks drop lbs (if that’s the goal) within weeks.

“Most people start to experience weight loss between 2–10 weeks after starting,” says weight loss dietitian Melissa Mitri, MS, RD. “These results can vary depending on your fasting schedule, what you’re eating, your exercise regimen, and other lifestyle factors like sleep and stress.”

Remember, every body is different. As Mitri said, your potential for weight loss is also highly influenced by other lifestyle factors.

How to make an intermittent fasting schedule

Your IF schedule will depend on which form of fasting works best for you. Consider everything from your work schedule to your personality type. Some folks prefer an all-or-nothing approach (24-hour fast, anyone?), whereas others like a less rigid schedule that allows 500–600 calories even on fasting days.

When in doubt, start small.

“I always recommend starting with the least restrictive fasting schedule,” says Mitri. “For example, if they’re new to fasting, we’d start with a 12-hour fasting schedule where they only eat from, say, 8 AM to 8 PM. As their body adapts, we would gradually reduce the eating window time down to 10 hours, 8 hours, etc., to find the best fit.”

PSA: It’s *always* best to consult a dietitian or doc before going all-in on fasting. Removing your body’s fuel source for a significant length of time is *kind of* a big deal, especially if you have underlying health conditions!

Does sleeping count as fasting?

Yes, sleeping counts as fasting. Unless you possess powers beyond most mere mortals, you’re not eating while snoozing. (Right?)

Does fasting burn belly fat?

“Some research has shown intermittent fasting can lead to weight loss, including belly fat loss,” says Mitri. She points to a recent study that suggests IF yields a smaller waist circumference.

Still, it’s not an open-and-shut case. More research is needed to determine whether IF is useful for targeting abdominal fat specifically.

Can you drink coffee while intermittent fasting?

That depends on your health goals.

“When fasting for complete gut rest or metabolic rest … black coffee would break a fast. There is a negligible amount of calories in coffee, but caffeine does stimulate metabolism,” says Bergtholdt.

On the other hand, if you’re fasting for weight loss, calorie-free black coffee isn’t a dealbreaker, she says.

Intermittent fasting isn’t right for everyone. If you tend to turn into a raging, Hulk-like monster if you don’t eat for a few hours, consider other eating plans.

But if you’re OK with putting down your fork for a prescribed window of time each day or week, IF may be an effective, uncomplicated route to weight loss, blood sugar management, blood pressure reduction, and more.