When it comes to weight loss, is timing everything?
While many weight loss plans involve tracking what you eat, intermittent fasting (IF) tracks when you eat.
IF consists of ‘eating windows’ — when you can theoretically have whatever and however much food you want — and fasting periods, when you’re only allowed to have water, coffee, and tea. The duration of your eating and fasting windows depends on which intermittent fasting schedule you follow.
Before we get into the different intermittent fasting diets out there (such as the 5:2 diet and the 16:8 diet), let's answer one of the most commonly searched questions around IF: Çan it really help with weight loss?
‘Some research has shown IF may result in weight loss, but the reason is unclear,’ says Cesar Sauza, a registered dietitian nutritionist at the National Coalition on Healthcare (NCHC).
Limiting your ‘eating hours’ may result in fewer calories consumed, but the benefits of intermittent fasting are still debatable since there's a lack of sufficient research on the topic, Sauza says.
There are also multiple kinds of intermittent fasting schedules, with the best one for weight loss being the one you can stick to the best, says Dr Nisha Parikh, a cardiologist at Northwell Health.
Meet the experts: Cesar Sauza, RDN, registered dietitian nutritionist at the National Coalition on Healthcare (NCHC). Dana White, RD, is a nutritionist based in Connecticut who specializes in culinary nutrition, recipe development, and sports nutrition. Amanda Baker Lemein, RD, LDN, is a Chicago-based nutritionist who works with clients to create meal plans and strategizes healthy eating approaches for those with busy schedules. Nisha Parikh, MD, MPH, is a cardiologist at Northwell Health. Emily Van Eck, RD, is the owner of Emily Van Eck Nutrition & Wellness. Maya Feller, RD, is the lead dietitian at Maya Feller Nutrition and author of Eating from Our Roots: 80+ Healthy Home-Cooked Favorites from Cultures Around the World.
Who is intermittent fasting for?
IF is not for everyone, and the last thing you want to do is start with a plan that's unsustainable (or doesn't work for your schedule).
It's definitely not a good idea for anyone with a history of disordered eating or diabetes and those who are pregnant or breastfeeding — even if weight loss is recommended, says Dana White, a nutritionist and recipe consultant.
Also, if your schedule is sporadic or you work out at various times of the day, you might want to reconsider IF because the strict windows might keep you from getting the fuel you need, according to Maya Feller, the lead dietician at Maya Feller Nutrition and host of Slate’s Well, Now podcast.
So, who should try it? If you tend to overeat at night, intermittent fasting may help prevent mindless eating throughout the day. It is also one way to try and create a calorie deficit if you've struggled in the past.
‘Practicing intermittent fasting may benefit those individuals who struggle to eat breakfast or prefer to eat later in the day,’ adds Sauza. ‘Ultimately someone should choose to practice intermittent fasting if it works for them, but most importantly if it makes them feel good.’
Here are six of the most popular IF approaches that people tend to follow for weight loss —and what research says about each one.
1. The 14:10 diet
This involves fasting for a 14-hour window and eating for 10. It's a little easier to stick to than 16:8 because you have a longer eating window. But that doesn't mean it's not effective.
In fact, those who followed the 14:10 diet while incorporating nutritious foods and consistent exercise lost more weight than those who did the 12:12 diet, a 2021 study in Nutrition & Diabetes found. They also showed a more significant improvement in blood glucose levels after eight weeks.
Given that the fasting period is short and pretty much follows the way people already eat, it may be hard for some people to achieve a calorie deficit with this diet. Still, it could be a good way to ease into eating on an IF schedule if you're new to it and want to see how you do when you have a specific timeframe for eating. It's also a great option if you like to get your workouts in between 8 a.m. and 10 a.m.
2. The 16:8 diet
The 16:8 method of intermittent fasting involves fasting every day for 16 hours and restricting your daily eating window to eight hours. For most people, this schedule means not eating anything after dinner and skipping breakfast. You might eat between, say, noon and 8 p.m.
It tends to be the most popular IF method, according to White. But new headlines raised a few questions. A 2024 analysis across 20,000 US adults found that people who restricted their eating to an 8-hour window had a 91 per cent increased risk of cardiovascular death, according to the American Heart Association.
But, there are a few limitations to the findings of this survey and you don’t need to abandon it completely if it’s what works for you, says Dr Parikh. The findings are preliminary since they come from an abstract, not a peer-reviewed study. While there was an association between an 8-hour eating window and cardiovascular death, other details like total calories consumed, kinds of foods consumed, and other factors were not known, and these all have a big impact on heart health too, according to experts.
How does the 16:8 method fare for weight loss? Not much better than other diets, according to a 2020 study published in Jama Internal Medicine. The 12-week randomized clinical trial found that restricting eating to an 8-hour window in the absence of other interventions did not promote weight loss more effectively than eating throughout the day.
3. The 5:2 method
WH doesn’t recommend the 5:2 diet method, which involves eating normally five days a week and cutting back to 20 per cent of your normal daily calorie intake for the other two.
Note: Women are supposed to have about 500 calories on fasting days, while men have about 600, and this simply isn’t enough calories to get all the vitamins, minerals, and energy you need. In fact, 1,200 calories (more than double what you’d consume on this diet) is what a moderately active three-year-old girl should consume.
This IF method results in just as much weight and fat loss as those who followed traditional dieting methods, according to a 2021 study published in PLOS ONE.
4. Alternate-day fasting
As the name implies, this diet involves fasting every other day — which we also don’t recommend because it won’t get you enough calories or nutrients. There are several different versions of this plan, with some of them allowing about 500 calories on the fasting days, and some encouraging that you eat even fewer or close to zero calories on fasting days.
Much of the weight loss-focused research hasn't been conclusive. And this style of intermittent fasting was deemed not superior to a calorie-restricted diet in terms of weight loss or maintenance in a 2017 JAMA study.
Plus, IF schedules that require 24 hours of fasting tend to lead to more side effects, including mood swings and hunger, according to White.
5. Eat-stop-eat diet
For the same reasons as alternate-day fasting, the eat-stop-eat method of intermittent fasting is not recommended. It involves a full fast for 24 hours once or twice a week. For example, you may eat dinner at 6 p.m. and then fast until 6 p.m. the next day, and you would do this one or two times per week — but not in a row.
Keep in mind that not eating for an entire day can be dangerous under certain circumstances and should not be taken lightly. Skipping full days of eating is not sustainable or safe long-term.
‘If you are knocking out two full days of eating, I would worry that the longer you do that, the greater at risk you would be for certain micronutrient deficiencies,’ says White.
6. The Warrior Diet
This diet is way different from the rest, with the majority of eating happening at night, and also might keep you from getting all the nutrients you need. The Warrior Diet was coined by fitness author Ori Hofmekler. It involves eating only small portions of raw fruits and vegetables during the day, then feasting on one huge meal at night within a 4-hour eating window.
There's no specific research on the Warrior Diet. While the fasting periods still allow for some food, the period when you can have heavier foods is very short, and the diet also involves focusing on paleo foods, so it's stricter than other forms of IF.
This option is also not sustainable long-term. ‘There's no way you're going to meet your nutrient needs eating this little food,’ explains White. ‘Your energy levels would suffer and you're basically asking to overeat. You're just going to hurt yourself if you go this route.’
Frequently asked questions...
What are the benefits of intermittent fasting?
Here are the potential benefits of IF based on recent studies.
IF may reduce insulin levels, fat mass, and the risk of developing age-related diseases in those who are overweight, according to a 2022 review in the Journal of Physiology and Biochemistry that investigated intermittent fasting in metabolic syndrome and type 2 diabetes.
IF could aid in the treatment and prevention of obesity, per Wageningen Academic.
What are the risks of intermittent fasting?
One risk of IF is not getting enough nutrients or fluids. During your eating windows, you’re going to want to make sure you get all your macronutrients and stay hydrated throughout the entire day, says Feller. While IF is about when you eat, focusing on what you eat is still the most important part of any diet, she says.
Fasting for more than 16 hours a day could also lead to gallstones, according to the National Institute of Health. This could be because fasting slows down the movement of the gallbladder, causing a build-up of bile and cholesterol, says Dr Parikh.
Another risk is disordered eating or an eating disorder. ‘Any type of very rigid diet like this creates excessive focus on when you can and cannot eat and put a lot of pressure around food,’ which could lead to disordered eating, says Feller.
For example, when dealing with any kind of restrictive diet, there’s a potential to trigger binge eating, Feller says. Anyone with a history of eating disorders should avoid this diet, and you should monitor how this diet makes you feel about your relationship with food too.
Which intermittent fasting schedule is best for weight loss?
In short, the one that's easiest for you to follow.
‘It is all about sustainability for the individual,’ says Amanda Baker Lemein, registered dietician and a Women's Health advisor. ‘If one of these diets work well for you and your lifestyle and preferences, then you will likely see some success. If not, then you'll likely not see the results you're looking for, as the sustainability piece will not be there.’
That means picking a diet that works best with all the lifestyle factors you need to consider, like your work schedule, family dynamic, living situation, commute time, and travel commitments, Lemein says.
Now, any weight loss that you experience from IF is most likely due to the basic fact that you're consuming fewer calories, not necessarily the timing of your eating, says Lemein.
And you can totally eat less, and lose weight without taking on fasting. Case in point: Intermittent fasting had similar weight loss results to a traditional calorie-restricted diet, a 2018 meta-analysis published in the JBI Database of Systematic Reviews and Implementation Reports found. So, unless fasting makes dieting easier for you, there's really no reason to choose IF over other diets.
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