What is 16:8 intermittent fasting and can it help you lose weight?

Intermittent fasting is a popular approach to dieting that focuses less on what you eat and more on when, but it isn’t one size fits all. There several versions of the eating plan, with one of the more well-known options being the 16:8 intermittent fasting method, which many people use to lose weight.

But first things first: Time-restricted eating isn’t about changing your diet or counting calories.

Instead, intermittent fasting requires you to fast for a period of time each day or week, per the Mayo Clinic. And because of the shorter eating window, by the end of the day or a week, you will likely be in a calorie deficit.

But how and when you fast can vary. Some people create and adhere to their own time-restricted eating pattern by stopping eating by 6:00 p.m., for example. If you're not sure where to start, there are four common methods for intermittent fasting: the 16:8 method, the 5:2 method, alternating the days you’ll eat and fast and time-restrictive intermittent fasting.

Here's what to know about the 16:8 intermittent fasting method in particular.

The main rule of the 16:8 intermittent fasting plan is fasting for 16 hours of the day and eating normally for eight.

It's not a diet, so unless you have individual dietary restrictions, you can eat whatever you want within the eight-hour time frame. But the plan will work best for weight loss if you are already making smart, nutrient-rich choices, said NBC News health and nutrition editor Madelyn Fernstrom, Ph.D.

For some people, restricting the amount of time they have to eat in a day naturally limits the number of calories they consume and therefore contributes to weight loss.

It might be better referred to as "intermittent eating," because it's not about deprivation, but rather about "boosting mindful eating and a new relationship with food," Fernstrom added.

The research on whether 16:8 intermittent fasting is good for weight loss is mixed.

A small 2018 study found people with obesity who followed the 16:8 fasting regimen for three months lost almost 3% of their body weight and lowered their blood pressure without feeling hungry or deprived.

The participants ended up eating 350 fewer calories a day compared to a control group simply because they couldn't squeeze in their normal food intake between 10 a.m. to 6 p.m., the prescribed eating window in the study, co-author Krista Varady, Ph.D., a professor of nutrition at the University of Illinois, Chicago, told TODAY.com. She has been studying fasting for 20 years.

Varady's latest study, published in June 2023, showed time-restricted eating without calorie counting was as effective as limiting calories and tracking them for weight loss. For the trial, people with obesity were asked either to eat only between noon and 8 p.m., or to eat whenever they wanted but count calories and reduce the amount of food they normally ate by 25%. They followed those routines for six months.

Both groups generally maintained the weight loss after their diets ended and lost 5% of their body weight over the course of a year, Varady told NBC News.

A 2020 systematic review of 27 studies that involved different kinds of intermittent fasting, including the 16:8 plan, found participants lost between 0.8% to 13.0% of their initial weight with no serious adverse events. The authors concluded intermittent fasting "shows promise" for the treatment of obesity.

But research presented at the American Heart Association's Scientific Sessions in 2020 found obese adults who ate most of their calories by 1 p.m. for three months didn't lose more weight than those who followed a more typical eating pattern, including eating a big meal after 5 p.m.

"The bottom line is that how many calories you take in is really much more important than when you eat, and that when you eat probably doesn't impact your weight," Dr. Nisa Maruthur, lead author of the AHA research and associate professor of medicine at Johns Hopkins University in Baltimore, previously told TODAY.com.

And the results of a 2023 study on the timing of meals, published in the Journal of the American Heart Association, "did not support the use of time‐restricted eating as a strategy for long‐term weight loss," researchers wrote. The timing of meals was less important than the amount of food a person ate, they noted.

If you're ready to try 16:8 intermittent fasting, Varady has some tips to make sure you do it in a way that is healthy and works with your lifestyle.

Experts advise picking an eating window that lets you finish your meals fairly early, such as 10 a.m. to 6 p.m. or earlier, because the body is less efficient at processing sugar as the day goes by.

In an "absolute ideal world," people would eat breakfast, take in most of their calories during the first part of the day, have a very light dinner — if any at all — and then fast for the rest of the evening, Dr. Susan Cheng, a professor of cardiology and the director of public health research in the Smidt Heart Institute at Cedars-Sinai in Los Angeles, told TODAY.com.

But she noted that many people derive a psychological benefit from sitting around the table at dinner. So it's important to pick a window of time that works for your schedule. If you're a very early riser, for example, it may be difficult to wait until noon to eat.

Most people get hungry about half an hour after they finish working out and may find it too hard to stick to their plan if they can’t eat anything at all afterward, Varady notes. That's why it's important to exercise before you eat.

If you’re on the 16:8 plan, exercise before your eating window, or make sure to finish your workout at least 90 minutes before your eating window ends for the day.

Fasting for 16 hours may sound tough, but if you're getting the recommended seven or more hours of sleep per night, you'll be asleep for about half of it.

It's common to feel low energy at times when following this eating plan. Drinking black coffee can improve concentration and energy, and it has no calories in it, Varady said.

Mindfulness and a bit of meditation can go a long way in helping to restore your energy during your fasting period.

Many people are used to grazing and nibbling all day long, but there's no snacking during the fasting periods of intermittent fasting.

Here are some ways to avoid getting "hangry":

• Eat high-fiber foods, such as nuts, beans, fruits and vegetables and high-protein foods, including meat, fish, tofu or nuts, during your eating window, Varady advised. Chewing high-fiber gummies can also help.
• Drink lots of water. People tend to think they're hungry when they are really just thirsty, Varady said.
• Opt for certain teas, like cinnamon or licorice herbal teas. These beverages may have appetite-suppressing effects, Varady noted.

“I know this sounds strange, but while you are watching TV, you are bombarded with dozens of ads for food. This can make you feel hungry, when in actuality you are not hungry at all,” Varady said.

Use peppermint to defuse cravings. Inhaling a peppermint scent every two hours helped people defuse cravings and eat fewer calories, a study found. The exact reasons are unclear.

Don't drink any alcohol during your fasting window since it's high in calories and has no nutritional value, Varady advised. During non-fasting periods, women should limit their alcohol intake to one drink a day; men shouldn't have more than two.

Weight loss or not, there appear to be health benefits to intermittent fasting.

It may protect the heart by controlling inflammation, according to the American Heart Association.

"The idea is that if you eat all of your calories within a relatively fixed time window ... it's better because it allows your body to do this metabolic catch-up during a fasting state when you're not eating," Cheng explained. "I would say the jury is still out, but there is a lot of compelling evidence to suggest that is going to be favorable for cardiometabolic health."

Studies and clinical trials suggest intermittent fasting has "broad-spectrum benefits" for health problems such as obesity, diabetes, heart disease, cancer and neurologic disorders, according to a review of research in humans and animals published in 2019 in the New England Journal of Medicine.

The powerful health effects appear to come from the body shifting away from using sugar as its main source of energy and instead converting fat for fuel when a person's stomach is empty. When a person switches between a fed and fasted state, it stimulates responses that boost mental and physical performance, plus disease resistance, the authors wrote.

In addition, intermittent fasting can be easy to follow, provide daily structure and doesn’t require any calorie counting. It also reconnects people with true, biological hunger and makes it easier to

Eating only within an eight-hour window can help people with Type 2 diabetes lose weight and manage their blood sugar, according to a study published in JAMA Network Open in October 2023.

The clinical trial involved 75 adults with Type 2 diabetes and obesity who were randomly divided into three groups for six months. One group could eat whatever and as much as they wanted but only between noon and 8 p.m. The second group was instructed to eat 25% fewer calories than normal, and the rest stuck to their usual eating routine.

The participants' weight and blood sugar levels were measured at baseline, and then regularly throughout the six-month trial.

"We found that the fasting group lost twice as much weight as (daily calorie counting), but both groups improved HbA1c, a key diabetes risk parameter, similarly," Varady, a co-author of the study, told TODAY.com.

HbA1c measures a person's average blood sugar levels over the past three months. It fell about 0.7% for both groups. The fasting group also lost about 10 pounds over six months on average, compared to less than 6 pounds for the calorie-counting group.

Eating fewer calories is commonly prescribed as the first line of defense in Type 2 diabetes treatment, but many patients struggle with it, so time-restricted eating may offer a "refreshing alternative," the paper noted.

There were no serious adverse events reported during the trial, so the study shows intermittent fasting is safe in this population group, Varady said. But she advised people with diabetes to always check with their doctors before starting time-restricted eating.

The findings need to be confirmed by larger trials with longer follow-up, the authors noted.

Intermittent fasting isn't for people with Type 1 diabetes who take insulin because it may result in unsafe levels of hypoglycemia, or low blood sugar, during the fasting period, Mark Mattson, Ph.D, a leading researcher on intermittent fasting and adjunct professor of neuroscience at Johns Hopkins University School of Medicine, said.

If 16:8 intermittent fasting doesn't feel right for you, here are some other approaches to consider.

This plan means incorporating two non-consecutive fast days into your week, then eating normally during the other days. To follow this plan, eat as you normally would five days a week and consume fewer than 600 calories two days a week.

Eat was you normally would one day, then fast by eating fewer than 600 calories the next — and repeat throughout the week.

This intermittent fasting strategy allows people to eat for six hours a day, usually between 8 a.m. and 2 p.m., and fast for 18 hours. Research has shown that it can curb appetite by decreasing levels of hunger hormones and increasing levels of satiety hormones.

As intermittent fasting has become more popular, concerns have emerged due to rat studies that it has effects on hormonal cycles and fertility — and those fears have been amplified by social media.

"There have been a couple of animal studies that I don't think are very translatable to humans," Varady, said. "But the second they're up on Instagram, everyone finds out about them."

A 2022 study in the journal Obesity investigated the impact of intermittent fasting on women’s hormones. Two groups of women followed time-restricted eating plans for eight weeks. In the end, the findings were reassuring, Dr. Reshmi Srinath, director of the Mount Sinai Weight and Metabolism Program, told TODAY. Hormone levels weren’t drastically disrupted.

In postmenopausal women, there was no change in their levels of estradiol, estrone or progesterone. The only hormone that changed significantly during the study period was dehydroepiandrosterone, also called DHEA, which helps the body make male and female sex hormones, according to Mayo Clinic. DHEA, which can naturally rise and fall with weight changes.

Both groups of women in the study saw a statistically significant decrease in DHEA levels over the eight weeks they were on the time-restricted eating plan. But Varady emphasized that, even with the drop, participants' DHEA levels were still within the normal range — and they didn't show any worrying side effects in other tests.

“A lot of people who try to switch to intermittent fasting don’t realize it takes a while to adapt,” Mattson noted. They may experience hunger and irritability at first, but these usually disappear within a month.

To reduce side effects of intermittent fasting, ease into it, the experts advise. Start with an eating window of 12 hours, then gradually reduce it to 10 and then eight over several months.

Another reported drawback of intermittent fasting is a lack of flexibility, according to a small pilot study from 2018 published in the Journal of Nutritional Science. "Fasting diets are difficult to follow and may not always be compatible with family and social life," lead researcher Dr. Jonathan Johnston said.

If you want to try intermittent fasting but don't want to be locked into one schedule, try fasting a few non-consecutive days a week with a few days of time-restricted feeding.

Many people also think that intermittent fasting promotes disordered eating. However, Varady said the research shows that, unlike many fad diets, intermittent fasting doesn’t lead to eating disorders or slow down a person’s metabolism, Varady said. A 2022 review of studies she co-authored found intermittent fasting is generally safe and produces few gastrointestinal, neurological, hormonal or metabolic adverse effects.

Intermittent fasting, the 16:8 plan or otherwise, is not recommended for everyone. These groups include:

• Children or adolescents
• Women who are pregnant or lactating
• People with eating disorders
• Individuals who are underweight (have a body mass index below 18.5)
• People over the age of 70 because fasting may exacerbate muscle wasting in this population

Always check with your doctor or a dietitian before starting an intermittent fasting regimen.