What Is Intermittent Fasting? Here's the Science Behind the Trend

Discover what science says about intermittent fasting for weight loss and find out if this style of time-focused eating is worth switching up your routine.

What we eat is important—but when we eat may deserve equal attention. Whether you are considering the best time to eat around exercise, work, or your kids' insane schedules, carving out proper timeframes for meals can impact your health. Intermittent fasting (IF) is a general term used to describe various methods of manipulating meal timing to create large windows of time without eating. Theoretically, this can help lower caloric intake and modify metabolism, but some science to back this up would be nice. IF has continued to gain popularity and unlike many other fad diets, there is some research to evaluate its effectiveness.

woman checking time on her watch while drinking a homemade smoothie
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What Is Intermittent Fasting?

There are a few widely accepted methods of IF.

Alternate Day Fasting

For Alternate Day Fasting (ADF), dieters participate in fasting days where no calories are consumed, cycled daily with non-fasting days throughout the week.

  • Modified Alternate Day Fasting: follows the same format, but allows for 25% of caloric needs to be consumed on fasting days. (That's 500 calories on a "fasting" day for those following a standard 2,000-calorie eating plan.)
  • 5:2 Intermittent Fasting: involves 5 days of normal eating, broken up with two, nonconsecutive fasting days where only 25% of calorie needs are consumed.

Time-Restricted Eating

But wait, there's more! Time-restricted eating (TRE), also known as time-restricted feeding (TRF), is an IF method where dieters fast daily for 16 hours and only consume calories in an 8-hour window; this "16:8" method is one of the most popular ways people participate in intermittent fasting.

Contrary to the typical fad diet protocol, IF regimens lack meal plans, or other food "rules." Dieters are free to eat what they choose as long as it is within their set timeframe and calorie allotments.

Science Behind Intermittent Fasting

Research on IF for weight loss remains in its infancy according to Felicia Steger, PhD, RD, Assistant Professor at the University of Kansas Medical Center, "The science behind IF is still young and we don't yet know if certain forms of IF are helpful long term." Nutrition research like this can be tricky considering there are so many variables to control for. "We also don't have much comparative data at all" adds Steger. "In other words, the types have not been tested against one another, nor have they been tested against other diet goals such as improving food quality."

Several studies have used rodents as subjects or have a small number of human participants which tells us more studies are needed before we can draw solid conclusions about IF. However, some studies point to beneficial results.

According to the literature, subjects following IF regimens often lose weight and body fat but it is unclear whether this is attributed to the structure of the IF program or simply a reduction in calories. A study published in 2016 suggests that age and ethnicity may play a role in who benefits from IF. Another study from 2019 found that subjects following TRE reported lower hunger levels, indicating that IF may affect hormones and metabolism somewhat differently than calorie cutting alone. On the downside, there is also data to suggest that IF can lead to muscle loss—not the fat loss most dieters are looking for. More research is also needed to determine if results can last long term — an area where many diets fall short.

How to Decide If Intermittent Fasting Is for You

It's easy to see the appeal of IF and as Dr. Steger points out, many folks want a break from low carb, Paleo, and other strict, micromanaged diets. "One reason I think so many people are attracted to intermittent fasting is because the goal of meal timing is a big shift from dietary approaches that focus solely on food choices." For this reason, Steger suggests serial dieters may find IF downright refreshing. "Some people have tried for decades to adhere to various food patterns (low fat, low carbohydrate, Mediterranean, etc.) and so a shift to thinking instead about food timing seems like a breath of fresh air."

If you are looking for boundaries, IF may be a helpful way to create them but this style of eating is not for everyone. Long bouts of fasting and strict timing guidelines may not benefit some people's lifestyles. Those with preexisting medical conditions or who are pregnant and breastfeeding are urged to check with their healthcare provider before embarking on an IF journey.

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  1. Varady, Krista, et al. "Determinants of Weight Loss Success With Alternate Day Fasting." Obes Res Clin Pract., vol. 10, no. 4, 2016, pp. 476-480, Asia Oceania Association for the Study of Obesity, doi:10.1016/j.orcp.2015.08.020

  2. Ravussin, Eric, et al. "Early Time-Restricted Feeding Reduces Appetite and Increases Fat Oxidation But Does Not Affect Energy Expenditure in Humans." Obesity (Silver Spring), vol. 27, no. 8, 2019, pp. 1244-1254, Obestity Society, doi:10.1002/oby.22518

  3. Lowe, Dylan A., et al. “Effects of Time-Restricted Eating on Weight Loss and Other Metabolic Parameters in Women and Men With Overweight and Obesity: The TREAT Randomized Clinical Trial.” JAMA Internal Medicine vol. 180, no. 11 , 2020, pp. 1491-1499, JAMA Network, doi:10.1001/jamainternmed.2020.4153

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