What Does Science Say about Using Intermittent Fasting for Weight Loss?
Takeaways
Intermittent fasting (IF) involves alternating between fasting and eating on a regular schedule. There are three popular IF approaches: time-restricted eating, alternate-day fasting, and the 5:2 diet.
Intermittent fasting works for weight loss because it reduces overall calorie intake. IF will not lead to greater weight loss results than the standard calorie-restricted diet.
If may bring about uncomfortable symptoms. It’s also associated with a higher risk of eating disorders, muscle mass loss, and hyper- and hypoglycemia.
Before you try intermittent fasting, ask yourself if it’s sustainable in the context of your lifestyle. If it isn’t, you’re unlikely to see lasting weight loss results—and may be better off exploring other options (like NOVI Optimum).
For dieters sick of inflexible “eat-this-but-not-that” dietary rules and contradictory calorie-counting practices (“Should I weigh my foods raw or cooked?”), the appeal of intermittent fasting for weight loss is evident. By simply alternating periods of eating and extended fasting, a person could eat whatever and however much they desired—and still shed serious weight. Or at least … that’s what proponents of the dietary strategy say. Now, here’s the all-important question: are they right? Is intermittent fasting the “hack” for weight loss?
You know what they say: if it’s too good to be true, it probably is. Continue reading to find out why this proves especially true with regard to using intermittent fasting for weight loss.
What is intermittent fasting?
At its core, intermittent fasting means eating for a period and then not eating (or severely restricting food consumption) for another. There are three popular fasting approaches:
Time-restricted feeding (e.g., 16/8 and 14/10): Time-restricted feeding involves restricting food consumption to specific periods each day. Common examples within this category include the 16/8 method, which features a daily 16-hour fast and 8-hour eating window, and the 14/10 method, which features a daily 14-hour fast and 10-hour eating window.
Example of a free 16/8 intermittent fasting meal plan for weight loss: Limit your food intake to between 9 am and 5 pm.
Example of a free 14/10 intermittent fasting meal plan for weight loss: Limit your food intake to between 9 am and 7 pm.
Alternate day fasting: A type of intermittent fasting that cycles between days of fasting (where you’re typically allowed 500 calories) and normal eating.
Example of a free intermittent fasting weight loss plan: Eat on Monday, then fast on Tuesday—then repeat till Sunday.
5:2 diet: With the 5:2 diet, the goal is to eat per-normal five days of the week, then restrict calorie intake to 500-600 on the remaining two days.
Example of an intermittent fasting diet plan for weight loss: Eat as usual Monday through Friday, then fast during the weekend.
Is intermittent fasting an effective dietary strategy for weight loss?
Short answer? Yes. A large body of research shows that intermittent fasting works for weight loss. Take this 2021 umbrella review (note: umbrella reviews represent one of the highest levels of evidence synthesis currently available) published in Nutrition, Obesity, and Exercise, for instance.
After analyzing 11 meta-analyses comprising 120 randomized controlled trials (RCTs), the researchers found an association between intermittent fasting and a reduction in body mass index in healthy adults and individuals with obesity. In other words, their findings support the role of intermittent fasting as a weight loss approach.
How does intermittent fasting work for weight loss?
Don't get overly excited about divvying up your Google Calendar into periods of eating and non-eating just yet, though. While intermittent fasting works for weight loss, the unfortunate truth is that it isn't any more effective for weight loss than if you were to eat in a calorie deficit (i.e., fewer calories than your body burns). Yes, seriously.
If you’re staring at your screen with one raised eyebrow, this 2022 study published in The New England Journal of Medicine may help banish your skepticism. In this rigorous one-year study, researchers recruited 139 participants with obesity and randomly assigned them to two groups:
Group 1: Time-restricted eating (participants could eat only between 8 am and 4 pm) with calorie restriction
Group 2: Daily calorie restriction alone (i.e., participants could eat any time they wished)
To ensure compliance, the researchers requested participants to photograph every bit of food they ate and to keep food diaries. So, what did the researchers find after 12 months? Both groups lost weight—an average of about 6.35 to 8.16 kg (14 to 16 pounds)—but there was no significant difference in the weight lost with either diet strategy. There were also no significant differences between the groups in waist circumference, body fat, and lean body mass measures.
Why? Answer: the primary reason intermittent fasting works for weight loss is that it limits the number of calories you have an opportunity to consume each day. It essentially "tricks" you into eating less daily, nudging you toward a calorie deficit necessary for weight loss. (By the way: unsure how many calories you should eat daily to reach your weight loss goals? Check out our free weight loss planner for personalized guidance.)
Beyond helping an individual achieve a calorie deficit, there’s also evidence showing that intermittent fasting could lower insulin resistance—a known possible contributor to the development of obesity.
The risks and disadvantages of intermittent fasting
With all that said, the bottom line is that intermittent fasting works for weight loss, right? Are you free to choose between plain-and-simple calorie restriction and intermittent fasting? Unfortunately, things may not be as straightforward as they seem. That's because intermittent fasting isn't without its risks and disadvantages, including:
Uncomfortable symptoms: You’ll be going without food for extended periods. As a result, it’s likely that you’ll experience hunger, irritability, and a reduced ability to concentrate (imagine trying to get that Excel function to work at 3 pm with an incessantly growling stomach and hunger-induced mind fog!)
Sustainability and practicality: Socially, intermittent fasting can be very limiting. How do you respond to an 8 pm dinner invitation if that’s the start of your fasting window? Also, how would you explain that you can’t eat on a particular day when your manager requests a catch-up session at a café? Ultimately, research shows that the most effective diet is the one you can stick to while still living your best life. So, if you struggle with staying consistent with intermittent fasting, you'll likely see little success in weight loss.
You must be in a calorie deficit: Intermittent fasting won’t help you cheat the laws of thermodynamics. If you were to eat to your heart's content and, thus, bring yourself out of a calorie deficit, you would not lose weight. Always remember that.
Eating disorders: Intermittent fasting may promote and encourage an unhealthy relationship with food, increasing an individual’s risk for eating disorders like binge eating and bulimia. In fact, according to a 2022 study published in Eating Disorders, researchers found that men and women engaging in intermittent fasting scored significantly higher than community norms on all subscales of the Eating Disorder Examination Questionnaire (EDE-Q)—with 31.25% of participants’ scores being at or above the clinical EDE-Q cut-off.
Muscle mass loss: It’s normal to lose some muscle during weight loss. Worryingly, though, research suggests that a disproportionately higher amount of weight individuals shed through intermittent fasting will come from lean mass, which includes muscle, than body fat. Why is this so concerning? Well, that's because muscle provides many health benefits. For example, it protects against falls and disability as people age and is associated with lower mortality. It also increases metabolism and can help prevent rebound weight gain.
Hyper- and hypoglycemia (especially in individuals with diabetes): The most significant risk of intermittent fasting for individuals with diabetes is a sharp increase in blood sugar levels after breaking a fast (potentially causing hyperglycemia) and excessively low blood sugar levels when fasting (i.e., hyperglycemia). Both conditions can lead to serious diabetes complications, like seizures, fainting, and a higher risk of cardiovascular disease.
Should you consider intermittent fasting?
Sticking to a set eating and fasting schedule requires hard work and consistency. So, before you try it on for size, ask yourself the following questions:
Is it sustainable in the context of your lifestyle? Imagine the challenge of planning your eating and fasting windows around social gatherings, holidays, and parties. Then imagine doing it for a lifetime. Does the thought of it excite or overwhelm you?
Do you have any health conditions or mental disorders that may negatively interact with intermittent fasting? If you have any pre-existing health conditions (e.g., diabetes and cardiovascular disease) or a history of eating disorders, intermittent fasting may not be the wisest dietary strategy for you. Consider consulting your primary healthcare doctor before proceeding.
Don’t have any existing health conditions or mental disorders? Feel that restricting your feeding window is a much more enjoyable and sustainable way of controlling your food intake (i.e., managing your energy balance) than tracking every single calorie that ends in your mouth? Then intermittent fasting may be the right move for you. Here are two tips to get you started:
Focus on healthy foods: As tempting as it may be to cram in whatever you want during the eating window—regardless of nutritional value—in anticipation of the fasting period, that’s not a good idea. You should still build your diet around healthy foods, including whole grains, healthy fats, and protein (note: here’s how much protein you should eat for weight loss). Also, limit saturated fats and refined carbohydrates.
Make time for exercise: Beyond promoting better weight management, exercise (resistance training, in particular) could help you maintain or even increase your muscle mass. This counteracts the adverse effects intermittent fasting may have on your lean body mass.
“I don’t think intermittent fasting is right for me.” What now?
You know you’d like to lose weight. You know intermittent fasting may be effective. Still, when you look up “intermittent fasting weight loss plan” on Google, it all just looks like a little too much work than you’d like. Is there an alternative for you? Good news: there is.
Unlike intermittent fasting, NOVI Optimum doesn't require a complete overhaul of your dietary habits overnight. Instead, you'll get personalized, step-by-step guidance on how you could make tiny tweaks to your current lifestyle habits—from food choices to exercise regimen—for big, sustainable weight loss results.